Water tunnel flow visualization and wind tunnel data analysis of the F/A-18

- r - ..

Y

* -

- 1 '7'

NASA Co~trrodor Rep& 165859

,- . *v

. r

,-- - * r - ( : a 4, A b L m F - . l i L a 3 , t L t n I.. L h..

*= .

b F L n ? F/,-lu (;a;~t:.;c; c d r , . I - ~ a 2 + - --L

. . . - . nC A l , / = r ~

- .

5 a*- :. . ' . - ' ~ 1 9 . , 7 " % -,

- . Water Tunnel Flow

h ? *r -43,:

ct: '

+

- Tw.h Visudiza~on aftd Wind

0 %n.nel Data 'Analysis

of the F/A-18

O W l E R L 2";.:G,>.t.5

Gary E. Erickson

Northrop Corporation, -Aircraft Division

Hawthorne, CA 90250

Contract- v$S 1 -1 66l7 ' May t 982

m-

N ~ k ~ u ~ ~ S p m WmnrStrdrOn - LamgwRmlehCmbsr H3-n V17qfu-a 23555 . .

. . - .- . .- --~-..L---. . &--.A m NASA Contractor Report 165859

Water Tunnel Flow

Visualization and Wind

Tunnel Data Analysis

of the F/A-18

Gary E. Erickson Northrop Corporation, Aircraft Division Hawthorne, CA 90250 Contract NAS1-16617 May 1982 N$t;L)nal Aerondutlcs and 5 ~ ~ 0 Ad~~fl~StrdliOn

ORIO~NAL PACE P J T O S R A W

. . < . . . . . . . . . . . . . . . . . + . . ....... ,--I PRECEDING PAGE E L A M m m C o m N T s Page . . . . . . . . . . . . . . . . . . . . . .

SS:'ERIVENTAL METHODS . . . . . . . . . . . . . . . . . . .

Water Tunnel F a c i l i t y Water Tunnel Flow Visual.Lzat i o n

. . . . . . . . . . . . . . . . . . . . . . . of the PIA-19

Rase1 ine f dn/ df -35'10'; = -12O). . . . . . . . . . .

Douhle-Wi4th Forward . . . . . . . . . . . . . . . . . .

L F , Y S ~ O ~ S ( L E X ~ ~ ) .

Double-Width, Extended L e m t h Forward LRX S l o t e (LCX 12A) . . . . . . . . . . . . . .

LEX Lower Surface Fence - Oblique (Fence "B") . . . . . . . . . . . . . . . . . .

LRX Lower Surface Fence - . . . . . . . . . . . . . . . Streslawiee (Fence "A").

ORlGlNAL PAGE I S w ~ o o ~ QUALI-W

~ ~ N T S (Cent inucd) Page Closure of Forward L R X Rlec-4 S l o t s . . . . . . . . . . . . 102 Closure of A 1 1 LEX Rleed S l o t s . . . . . . . . . . . . . . 102 rorehodv S t r a k e s (Radial Position: M O O ) . . . . , . . . . 113 Forehodv S t r a k e s (Radial Positton: 0 ' ) . . . . . . . . . . 130 Forehodv S t r a k e s (Radial Position: -45') . . . . . . . . . 131 F l i g h t Test Nose Room . . . . . . . . . . . . . . . . . . 131 Wing Leading-Edge Snag and lipper Surface Fence . . . . . . 132 win^ Leading-Edge Extensions (LEXS) Off . . . . . . . . . 132 R e i n a t a l l a t i o n of Wing Leadiqt-Edge Extensions . . . . . . 145 Correlation of Water Tunnel Flow V i s u a l i z a t i o n R ~ R u ~ ~ s with 0.16-Scale F/A-IS tJind Tunnel Data T r e ~ d s . . . . . . . . 149 Double-Wtdth Forward LEX S l o t s (LEY 12). . . . . . . . . . 157 Double-Width, Increased Length Forward LEX S l o t s (LEX 12A) 163 LEY Lower Surface Fence (Fence " R " ) . . . . . . . . . . . . 168 LEY Lower Surface Fence (Fence "A"). . . . . . . . . . . . 170 LEX Planfonn Yodifications . . . . . . . . . . . . . . . . 170 W i q Leading-Edge Snag and Upper Surface Fence . . . . . . 179 Forvard LEX S l o t s C1oeed . . . . . . . . . . . . . . . . . 182 Forehodv S t r a k e s (Radial Location: +40°). . . . . . . . . 186 Forehodv S t r a k e s (Radial Location: +60°). . . . . . . . . 194 Forebody Strakea (Radial Locatlon: +30°). . . . . . . . . 194 Forebody Strakea (Radial Location: 0') . . . . . . . . . 199

CONTKNTS (Continued) Page Forehody Strakes (Radial Locations: - 1 5 ' and -30.) . . . 199 Forehodv Strakee (Radial Location: -45') . . . . . . . . 20 2 Plight Test Nose Boom . . . . . . . . . . . . . . . . . . 204 Flight Test Nose Boom and Forebody Strakes (#-+40~) . . . 204 "Wing Rock" Phenomena. . . . . . . . . . . . . . . . . . . 207 Anal ysi s of Scale-Model F/A-18 IJindTunnelData . . . . . . . . . . . . . . . . . . . . . . .

Raseline Configurations - Longitudinal Characteristics . . . . . . . . . . . . . . . . . . . . .

Raseline Configurations - Lateral/ Directtonal Characteristics . . . . . . . . . . . . . . .

Vertical Tail effect^ . . . . . . . . . . . . . . . . . .

Wing Leadin%-Edge extension (LBX) Effects . . . . . . . . . . . . . . . . . . . . . . . . .

Forehodv Effects . . . . . . . . . . . . . . . . . . . . .

Forehrniy Strake Effects . . . . . . . . . . . . . . . . .

CONCLUSIONS . . . e m . . . . . . . . . . . . . . o m . . . . . . .

RWCWMENDATIONS FOR FUTURE WIND TUNNEL TESTS . . . . . . .

RRFFRENCES . . o . . . . . . . . . . . . . . . . . . . ~ . . . . .

SUMMARY The Navy/Mc~onnell Douglas/Northrop F/A-18 operating at high angles of attack (a) is characterized by significant amounts of flow separation from the fuselage forebody and leading-edge extension (LEX)-wing surfaces. Sheets of distributed vorticity are shed from the slender forebody and LEXs, which roll up into concentrated vortices. Due to the size of the LEXs and their proximity to the forebody, a strong interaction can occur at high a's between the forebody and LEX vortices. A consequence of the coupling of the forebody and LEX flow fields is that LEX and/or £<,rebody geometry changee can promote large changes in the vortex jnteractire behavior. Particularly, small dis- turbances to the forebody flow development near the nose can be amplified downstream hy the more powerful LEX vortical motions. As a result., small perturbations to the forebody vortex behavior can, ur 'er certain conditions, dictate the wing stall and, hence, lateral stability characteristics near stall angle of attack. Similarl-y, subtle differences in the flow behavior on different F/A-18 test models can be responsible for large differences in lateral stability levels determined in wind tunnel cests at high angles of attack.

A detailed study has been made of the sensitivity of the F/A-18 vortex flow field behavior and low-speed wind tunnel data trends to LEX-wing and forehody geometry modifications. Flow field surveys in the Northrop 16x24- inch Diagnostic Water Tunnel of a 0.025-scale F/A-'-8 model and analyses of 0.06-, 0.07-, and 0.16-scale F/A-18 wind tunnel model data obtained in the NASA Langley Research Center 30x60-foot facility were made to assess the affects of (1) increased forward LEX boundary layer bleed slot width and length, (2) forward LEX slot closure, (3) closure of all LEX slots, (4) LEX lower surface fences located forward of the production break, ( 5 ) wing snag and fence combination, (6) forebody strakes at several radial positions, (7) flight test nose boom, and removal of (8) LEXs, (9) tv.n vertical tails, and (10) forebody. Emphasis was placed on variations of forebody and LEX vortex interactions and lateral-directional characteristics at low subsonip speeds with sideslip angle at angles of attack from 30 to 40 degrees. Note is made that the water tunnel snd wind tunnel Reynolds numbers based on maximum body width were all within the laminar regime.

The qua1 i t a t i v e r e s u l t s o b t a i n e d i n t h e w a t e r t u n n e l c o r r e l a t e d reason- a h l v w e l l w i t h 0.16-scale F/A-18 wind t u n n e l model d a t a t r e n d s . Wing s t a l l p a t t e r n 8 and t h e flow b e h a v i o r about t h e forebody and twin v e r t i c a l t a i l s a t h i ~ h a's were c o n s i s t e n t w i t h h a s e l i n e ( a l l LEX s l o t s open; 6,/ 6f=350/00; The d h a l 2') l n n ~ i tud t n a l and l a t e r a l - d t r e c t i o n a l aerodynamic d a t a t r e n d s .

F/A-1R h a s e l i n e w a t e r t u n n e l model developed a highly-asymmetric forebody v o r t e x svstem i n s i d e s l i p . The leeward body v o r t e x was s t r o n g l y e n t r a i n e d i n t o t h e leeward LEX v o r t e x v h e r e a s t h e windward body v o r t e x "sheared" away from t h e f u s e l a g e w i t h no a p p a r e n t i n f l u e n c e on t h e downstream flow behavior.

The flow f f e l d e f f e c t s of i n c r e a s e d forward LEX s l o t width and l e n g t h and LEY lower s u r f a c e f e n c e s , when c o n s i d e r e d i n c o n j u n c t i o n w i t h t h e low- speed wind tunnel d a t a , i n d i c a t e d t h a t such m o d i f i c a t i o n s reduced t h e "effec- t i v e " g e n e r a t i n g l e n g t h of t h e leading-edge e x t e n s i o n s . The LEX m o d i f i c a t i o n s l i m i t e d t h e amount of shed v o r t i c i t y a t t h e leading-edge a v a i l a b l e f o r f e e d i n g i n t o t h e primary v o r t e x , r e n d e r i n g t h e forward p o r t i o n of t h e LEX i n e f f e c t i v e R S a v o r t e x Renerator. The e a r l i e r wing s t a l l and r e d u c t i o n of LEX v o r t e x t&reakkfown asymmetry i n s i d e s l i p a t h i g h a's were i n q u a l i t a t i v e agreement w i t h reduced C (and a n g l e of a t t a c k f o r CL ) and i n c r e a s e d l e v e l s of l a t e r a l ~ A Y MAX s t ~ h l l l t v n e a r s t a l l a n g l e o f a t t a c k , r e s p e c t i v e l v , d e t e r m i n e d i n wind t u n n e l t e s t s .

W e n LEX b o u ~ ~ d a r y l a v e r bleed s l o t s promoted t h e f o r m a t i o n of two prima^; v o r t i c e s on each LEX due p r i m a r i l y t o forward s l o t flow entrainment which caused a s i g n i f i c a n t l o c a l r e d u c t i o n i n v o r t i c i t y rhed a t t h e LEX l e a d i n g edge near t h e p r o d u c t i o n break. Folward LEX s l o t c l o s u r e and c l o s u r e of a l l s l o t s rest11 ted i n a s i n g l e c o n c e n t r a t e d LEX v o r t e x . The i n t e r a c t i v e behavior of t h e forehociy and LEX v o r t i c e s a t filgh a's was s i m i l a r , however, r e g a r d l e s s of whether t h e s l o t s were open o r c l o s e d .

A wing leading-edge snag and upper s u r f a c e f e n c e combination promoted n e n r l r s p m e t r i c b u r s t i n g of t h e LEX v o r t i c e s i n s i d e s l i p a t h i g h a n g l e s of a t t a c k which appeared t o c o n t r i b u t e i n l a r g e p a r t t o t h e improved late.-a1 s t a b i l f t y c h a r a c t e r i s t i c s o b t a i n e d i n wind t u n n e l t e s t s .

The s e n s i t i v i t y of t h e LEX-wing flow f i e l d behavior t o changes i n t h e forebodv geometry was v i v i d l y demonstrated i n w a t e r t u n n e l s t u d i e s of nose s t r a k e e f f e c t s . Depending on t h e radome s t r a k e r a d i a l p o s i t i o n , enhancement o r d i s r u p t i o n of t h e forebody v o r t i c e s o c c u r r e d , w i t h a corresponding change i n t h e body v o r t e x i n t e r a c t i o n s w i t h t h e wing flow f i e l d . Nose s t r a k e s l o c a t e d 40 d e g r e e s above t h e maximum h a l f - b r e a d t h (MHB) were n e a r l y c o i n c i d e n t w i t h t h e p r i ~ a r y houndarv l a y e r s e p a r a t i o n l i n e s a l o n g t h e radome and :ere immersed i n a r e g i o n of r e l a t i v e l y low l o c a l a n g l e of a t t a c k due t o body vortex-induced downwash. S t r a k e s l o c a t e d i n t h i s manner were observed t o promote symmetric houndarv l a y e r s e p a r a t i o n l i n e s w i t h i n a l i m i t e d range of s i d e s l i p and t o shed d i s c r e t e v o r t i c e s up t o very high a n g l e s of a t t a c k which f e d d i r e c t l y i n t o t h e bodv primary v o r t e x system. A s a consequence, t h e forebody v o r t i c e s were h i g h l y r e s i s t a n t t o asymmetric o r i e n t a t i o n i n s i d e s l i p and, f u r t h e r m o r e , were s t r o n g l y coupled such t h a t powerful induced sidewash e f f e c t s c n t h e windward wing panel o c c u r l e d . Wing s t a l l was t h u s delayed t o h i g h e r a n g l e s of a t t a c k . The body v o r t e x behavior observed bv c o l o r e d dye e m i s s i o n s i n t h e water t u n n e l was s l i ~ h t l y o s c i l l a t o r y w i t h nose s t r a k e s i n s t a l l e d due t o a n a p p a r e n t "hydro- dynamic i q s t a b i l f t y " phenomenon. The s t r a k e e f f e c t s on forebody v o r t e x beha- v i o r , i n c l u d i n g t h e v o r t e x o s c i l l a t i o n s , were a l s o observed i n smoke flow v i s u a l i z a t i o n s t u d i e s of t h e 0.16-scale F/A-18 model i n t h e Langley wind t u n n e l .

The improved wing s t a l l c h a r a c t e r i s t i c s and t h e u n s t e a d y body v o r t e x behavior were r e f l e c t e d , r e s n e c t i v e l y , i n improved l a t e r a l s t a b i l i t y and modest wing rock a t high a's on t h e 0.16-scale model and f u l l - s c a l e f l i g h t t e s t s of t h e F/A-18.

-

Nose s t r a k e s a t 4 5 d e g r e e s b e l o w t h e MHB p r o m o t e d f l o w f i e l d c h a n g e s s f m i l a r t o , h u t much l e s s pronounced than, s t r a k e s a t 40 d e g r e e s above t h e HHR. Location of s t r a k e s a l o n g t h e MHB was found i n t h e w a t e r t u n n e l s t u d i e s t o c o m p l e t e l y d i s r u p t t h e f o r e b o d y p r i m a r y v o r t i c e s a t h i g h a's d u e t o a l o w - e n e r g y wake s h e d by t h e s t r a k e s and t o a d i s c o n t i n u i t y i n t h e v o r t e x s h e e t s emanating from t h e forebody s i d e s .

I n s t a l l a t i o n of a f l i g h t t e s t nose boom weakened t h e forebody primary

i

v o r t e x system due t o t h e wake shed by t h e hoom. The fundamental s t r u c t u r e of I: t h e v o r t i c e s was s i m i l a r , however, whether t h e hoom was on o r o f f .

Due t o minor changes i n t h e forebody and LEX geometries i n c u r r e d d u r i n g model changes, a s m a l l v a r i a t i o n i n t h e b a s e l i n e F/A-18 w a t e r t u n n e l model forebody v o r t e x o r i e n t a t i o n i n s i d e s l i p o c c u r r e d . T h i s e f f e c t was magnified d o w n s t r e a m , h o w e v e r , s u c h t h a t t h e w i n g s t a l l p a t t e r n s i n s i d e s l i p w e r e markedly d i f f e r e n t r e l a t i v e t o p r e v i o u s b a s e l i n e runs. A h i g h l y - f a v o r a h l a forebody vortex-induced e f f e c t on t h e windward wing panel was e v i d e n t .

To confirm t h a t t h e l a t t e r e f f e c t emanated from t h e nose r e g i o n , t h e forebody was removed from t h e water t u n n e l model. The vortex-induced e f f e c t s o n t h e winq p a n e l w i t h f o r e b o d y o n w e r e n o t i c e a b l y a b s e n t w i t h f o r e b o d y of c .

Large d i f f e r e n c e s i n l a t e r a l s t a b i l i t y l e v e l s n e a r s t a l l a n g l e of a t t a c k were r e v e a l e d i n wind t u n n e l s t u d i e s of 0.06-, 0.07-, and 0.16-scale FIA-18 models when t e s t e d i n t h e Langley 30x60-foot f a c i l i t y a t t h e same o r d i f f e r e n t Reynolds namhers. These t r e n d s were r e p e a t a h l e . Furthermore, a comparison of wind t u n n e l d a t a o h t a i n e d a t t h e NASA Ames Research C e n t e r 12-foot f a c i l i t y cnd i n t h e Langley f a c i l i t y on t h e 0.06-scale F/A-18 r e v e a l e d e x c e l l e n t agree- ment hetween t h e r e s p e c t i v e l a t e r a l s t a b i l i t y l e v e l s . I n s p e c t i o n of t h e models f a i l e d t o r e v e a l discernible d i f f e r e n c e s i n LEX geometry, p o s i t i o n , and i n c i d e n c e , hot! z o n t a l and v e r t i c a l t a i l p o s i t i o n s , and forebody c o n t o u r s , f o r example, a l t h o a g h t h e forebody c o n t o u r s were n o t i n s p e c t e d t o t h e d e g r e e t h a t now . onear9 warranted based on t h e p r e s e n t r e s u l t s . Furthermore, t e s t d a t a obte?ned i n the Langley f a c i l i t y i n d i c a t e d t h a t model s u p p o r t i n t e r f e r e n c e had o n l y a secondary e f f e c t on t h e wind-tunnel d - t a t r e n d s . Water t u n n e l flow f i e l d o b s e r v a t i o n s of t h e 0.025-scale F/A-18 and a n a l y s e s of 0.06-, 0.07-, and 0.16-scale F/A-18 wind t u n n e l d a t a su3gested t h a t t h e a p p a r e n t "model-scale" e f f e c t was a s s o c i a t e d w i t h s u b t l e d i f f e r e n c e s i n t h e forebody c o n t o u r s which promoted mnrkedSy different forebody-LEX v o r t e x i n t e r a c t i v e b e h a v i o r a t h i g h a's. A s s e ~ s m a n t of t h e r e l a t i v e e f f e c t s of removal of t h e LEXs a d twin v e r t i c a l t a i l s l e d i n a n i n d i r e c t manner t o t h e c o n c l u s i o n t h a t t h e v . l h - s c a l e F/A-1R model developed a more s e v e r e and p e r s i s t e n t LEX v o r t e x b r e a k d ~ w n asymmetry i n s i d e s l t p . The l a t t e r phenomenon appeared a t t r i b u t a b l e t o f o r e - body f l o w f i e l d d i f f e r e n c e s , f o r a comparison of small- and l a r g e - s c a l e model d a t a o b t a i n e d w i t h forebodv o f f r e v a a l e d e x c e l l e n t high-a, d a t a agreemew.

Arlditional evidence of d i f f e r e n t f o r e b d y v o r t e x behavior on t h e sub- s c a l e b a s e l i n e F/A-18 models was provided by d a t a o b t a i n e d w i t h nose s t r a k e s mounted 4 0 d e g r e e s ahove t h e maximum h a l f -breadt h. S t r a k e s p o s i t i o n e d i n t h i s manner o n t h e 0.07-scale model were d e s t a b i l i z i n g i n r o l l whereas t h e o p p o s t t e was t r u e f o r t h e 0.16-scale model.

A n e x c e l l e n t match of l a t e r a l - d i r e c t i o n a l c h a r a c t e r i s t i c s was obc . rled f o r t h e 0.06- and 0.16-scale models w i t h nose s t r a k e s mounted a t t h e NHR.

These r e s u l t s appeared a t t r i b u t a b l e t o t h e disruptiorn of t h e forebody v o r t i c e s w i t h s t r a k e s i n s t a l l e d and, c o n s e q u e n t l y , a1 l e r i a t i o n of t h e d i f f e r e n c e s i n hodv v o r t e x o r i e n t a t i o n s and i n t e r a c t i v e behavior w i t h t h e wings t h a t were e v i d e n t on t h e h a s e l i n e models.

The r e s u l t s of t h i s s t u d y i n d i c a t e d t h a t , f o r high angle-of-attack t e s t - ing of F-18-type a i r c r a f t f e a t u r i n g s t r o n ~ l g - c o u p l e d forebody and LEX v o r t e x f l o w s , model t o l e r a n c e s mav h a v e t o h e r e d u c e d t o e n s u r e s u h - s c a l e w!-nd tunnel model d a t a c o r r e l a t i o n . However, u n t i l experiments a r e conducted i n a s v s t e m a ~ i c way, o n e c a n n o t d e f i n e w h a t a r e a s o n a b l e t o l e r a n c e l e v e l i s .

It h a s long heen e s t a b l i s h e d i n wind t u n n e l t e s t s of s l e n d e r bodies t h a t free-stream t u r b u l e n c e , model support r i ~ i d i t y , model contour v a r i a t i o n s , Reynolds number, Yach number, e t c . can a l l c o n t r i b u t e t o changes i n body v o r t e x hehavior a t high a n g l e s of a t t a c k . Accordingly, a wind t u n n e l t e s t program h a s been proposed f o r t h e N A S A Langley Research Center VSTOL f a c i l i t y llstng t h e 0.06-, 0.07-, and 0.16-scale F/A-18 models. The proposed t e s t program would p r o v i d e f o r c e and moment r e s u l t s with which t o cclnpare t h e e x i s t i n g Langley d a t a . The f l o w v i s u a l i z a t i o n c a p a b i l i t i e s a v a i l a b l e i n t h e VSTOL t u n n e l would e n a b l e an assessment nf t h e flow b e h a v i o r i n t h e forebody and wing r e a i o n s of each model. The s e n s i t i v i t y of hlKil angle-of-attack l a t e r a l s t a b i l i t y t o a r t i f i c i a l t u r b u l e n c e induced by upstream s c r e e n s , down- stream o b s t a c l e s , model support r i g i d i t y , s l i g h t misalignment of t h e f u s e l a g e forchodv o r small forebodg c o n t o u r v a r i a t i o n s , e t c . could be addressed i n t h i s f a c i l f t v . The National Transonic F a c i l i t y (NTF) was suggested a s a s u i t a b l e f a c i l i t y f o r longer-term s t u d i e s t o a s s e s s Reynolds number and Mac0 numher e f f e c t s on h i ~ h l y - c o u p l e d forehody-wing v o r t e x f l o w s i n t h e extended angle-of- a t t a c k cegtrne.

RACKGROUND C u r r e n t and f u t u r e a i r c r a f t d e s i g n s a r e c h a r a c t e r i z e d by expanded f l i g h t e n v e l o p e s w i t h i n which. complex f l o w i n t e r a c t i o n s a r e f r e q u e n t l y encountered.

S i g n i f i c a n t ; i n c r e a s e s i n t h e maximum l i f t c o e f f i c i e n t and r e d u c t i o n s i n d r a g a t h i g h lift can r e s u l t from t h e separation-induced v o r t e x f l o w s shed f r o a s1ende.r f u s e l a g e f o r e b o d i e s , w i n g s , c a n a r d s , and wing l e a d i n g - e d g e e x t e n s i o n s (LEKS). Concurrent w f t h t h e s e l o n g i t u d i n a l b e n e f i t s , kowever, i s s u s c e p t i b i l i t y t o h i g h l y n o n l t n e a r l a t e r a l - d i r e c t i o n a l c h a r a c t e r i s t i c s a t h i g h a n g l e s of a t t a c k .

The LEX v o r t e x on t h e F/A-18 i s i l l u s t r a t e d i n t h e f l i g h t photograph i n F i g u r e 1 (from Reference 1 ) and i n t h e w a t e r t u n n e l photograph i n F i g u r e 2.

The v o r t e x flow was made v i s i b l e i n f l i g h t and i n t h e w a t e r t u n n e l by n a t u r a l condensation and dye i n j e c t i o n , r e s p e c t i v e l y .

The t h r u s t of t h e p r e s e n t s t u d y p e r t a i n s t o forebody and LEX v o r t e x i n t e r a c t i o n s a t high a n g l e s of a t t a c k on a "hybrid" f i g h t e r a i r c r a f t conf igu- r a t i o n . The l a t t e r terminology a p p l i e s t o c o n f i g u r a t i o n s such a s t h e Northrop F-5G arid t h e Navy/ McDonnell Douglas/Northrop F/A-18 which f e a t u r e s l e n d e r f o r e h o d i e s and highly-swept wing leading-edge e x t e n s i o n s . D i s t i n c t i o n s m w t h e made, however, between F-5 and F-18-type a i r c r a f t i n t h a t t h e d e g r e e of i n t e r a c t i o n between t h e forebody and LEX v o r t i c e s a t h i g h a n g l e s of a t t a c l d i f ' e r s c o n s i d e r a b l y .

The w a t e r t u n n e l photograph i n F i g u r e 3 ( a ) i l l u s t r a t e s t h e flow about a n F-5 model n e a r s t a l l a n g l e of a t t a c k . Although t h e r e l a t i v e l y s t r o n g forebody v o r t i c e s do i n f l u e n c e t h e wing flow f i e l d t o some e x t e n t , t h e body and LEX

-

v o r t i c e s a r e e s s e n t i a l l y uncoupled. One r e a s o n f o r t h i s is t h a t t h e F-5 LEX v o r t e x cannot p e r s i s t t o v e r y h i g h a n g l e s of a t t a c k due t o t h e low r ~ t i o of LEX e x p o ~ e d area-to-wing a r e a +O.O6). The dominant v o r t e x flow on t h e F-5 a t s t a l l and p o s t - s t a l l a n g l e s of a t t a c k is, t h e r e f o r e , developea a l o n g t h e f u s e l a g e forehody. The body v o r t e x f l o w can a l s o shed asymmetrically a t z e r o s i d e s l i p a s shown i n F i g u r e 3(b). Furthermore, t h e LEX s u r f a c e is n o t i n proximfty t o t h e forehody and t h e wing is i n a 1c.r p o s i t i o n , b o t h f a c t o r s c o n t r i h u t i n g t o t h e r e l a t i v e absence of fur&'imd~ LEX v o r t e x i n t e r a c t i o n .

ORIGINAL PAGE COLOR ?+*Sf OSRAPH

FIGURE 3 k WATER TUNNEL PHOTOGFIAPH OF FOREBODY AND LEX VoancEs ON A 0.025-SCALE F-5 MODEL NEAR STALL ANGLE OF ATTACK; P 0"

PREC€IMNG PBGC PLANK h w 1 7

C o n s e q u e n t l v , f o r e h o d y g e o m e t r y c h a n g e s w i l l i n f l u e n c e , p r i a a r i l y , t h e high-cr s t a t i c d i r e c t i o n a l s t a b i l i t y w i t h higher-order i n f l u e n c e o n s t a t i c l a t e r a l s t a b i l i t y , t h e m a l o r c o n t r i b u t o r t o t h e l a t t e r b e i n g t h e w i n g s .

i

S o n v e r s e l v , wing geometry changes w i l l b e r e f l e c t e d i n t h e l a t e r a l s t a b i l i t y

I

c h a r a c t e r i s t i c s a t s t a l l / p o s t - s t a l l a n g l e s of a t t a c k w i t h s e c o n d - o r d e r

. I

t .

e f f e c t s on d i r e c t i o n a l s t a h i l i t v .

' i

. , Thib t s n o t t h e c a s e f o r t h e F-18, however. Due t o t h e r e l a t i v e l y l a r g e r a t i o of LRX exposed area-to-wing a r e a (--0.14). t h e s l i g h t p o s i t i v e i n c i d e n c e a n g l e of t h e LEXs, and t h e proximity of t h e LEX s u r f a c e s t o t h e forebody, t h e LEX v o r t i c e s p e r s t s t t o v e r y high a n g l e s of a t t a c k and i n t e r a c t i n a s i g n i f i - c a n t manner wtth t h e f o r e b d v v o r t i c e s . T h i s flow phenomeno:~ is i l l u s t r a t e d i n t h e w a t e r tunnel photograph i n F i g u r e b(a). The forebody v o r t l c e s shed i n a symmetric manner a t z e r o s i d e s l i p . Due t o t h e powerful v o r t e x i n t e r a c t i o n s , however, t h e v o r t i c e s assume a h i g h l v asymmetric o r i e n t a t i o n a t s m a l l s i d e - s l t p a n g l e s , a s d e p i c t e d i n F i g u r e 4(b). The forebody and LEX v o r t i c e s a r e s t r n n ~ l y c o u p l e d and, a s a r e s u l t , s o a r e t h e l a t e r a l - d i r e c t i o n a l c h a r a c t e r - i s t i c s a t high a's. The F-18 flow f i e l d i s c h a r a c t e r i z e d , t h e n , bv m u l t i p l e v o r t e x i n t e r a c t i o n and v o r t e x breakdown, b o t h of which can occur i n a sym- m e t r i c o r a s p m e t r i c manner depending on t h e a n g l e s of a t t a c k a n d / o r s i d e s l i p , and highly-nonlinear l a t e r a l - d i r e c t i o n a l c h a r a c t e r i s t t c s . It ts t h i s c l a s s of v o r t e x flow i n t e r a c t i o n s t h a t is of primary concern i n t h e p r e s e n t i n v e s t i g a - t t o n .

PRECEDING PAGE PLANK NOT FILMED

zg TI-

PRECEDING PAM BUNK NOT FAMED

INTRODIJCTION On Decemher 21, 1978 t h e No. I F/A-18 a l r c r a f t on F l i g h t 16 e x p e r i e n c e d a mild vaw d e p a r t u r e . The a i r c r a f t a c h i e v e d a n a n g l e - o f - a t t a c k i n e x c e s s of 30 d e ~ r e e s and a s i d e s l i p a n g l e of 1 5 d e ~ r e e s a t Y 0.7 w h i l e a t t e m p t i n g a wind-up t u r n a t M 0.9 a t 40,000 f e e t t o a c h i e v e 4.5 g ' s (Mil power t h r o u g h o u t ) . An unbalanced r o l l i n g moment of a p p r o x i m a t e l y 1400 i t - l b d u e t o a r i g h t wing f u e l i n h a l a n c e of Rome 200 pounds may have c o n t r i b u t e d t o t h e i n t t i a l s i d e s l i p mot i n n .

Six-degree-of-freedom s t u d i e s were u t i l i z e d t o e x t r a c t a band of yawing ~ n d r o l l t n p moaent c o e f f i c i e n t s from t h e f l i g h t r e c o r d s . These were compared w i t h 0.06-scal e model d a t a o b t a i n e d i n t h e Arnold E n g i n e e r i n g Development C e n t e r (AEDC) 16T v i n d t u n n e l f a c i l i t y . The r e s u l t s , shown i n F i g u r e 5 , i n d i c a t e t h e f l i g h t test y a w i n g moment d a t a e x h i b i t a n i a p r o v e m e n t o v e r t h e wind t u n n e l d a t a t o n e a r - n e u t r a l s t a b i l i t y and a s i g n i f i c a n t r e d u c t i o n t n l ~ t c r a l s t ~ h i l t t v ( a ~ a i n t o a n e a r - n e u t r a l l e v e l ) . These d a t a are c n n s i s - t e n t w i t h t h e f l i g h t test r e s u l t s s i n c e t h e motion wara c h a r a c t e r i s e d by a r e l a t i v e l v s l o w d e p a r t u r e . F l i g h t tests i n Novemhtr 1980 r e p e a t e d t h e slow VRW d e p a r t u r e a t M 0.3.

h l v NASA Langley 0.16-scale model wind t u n n e l d a t a showed l e v e l s of l a t e r a l s t a h l l i t v similar t o t h e f l f g h t test r e s u l t s , a s shown i n F i g u r e 6 . Accordfngly, q e o m e t r i c m o d i f i c a t i o n s were i n v e s t i g a t e d commencing i n A p r i l lo79 on t h e 0.16-scale model tn t h e NASA Langlev R e s e a r c h C e n t e r 30x63-foot wind t u n n e l t o improve h i g h - a n g l e o f - a t t a c k ( (r-30-40 dep) l a t e r a l s t a b i l i t y .

Y c d i f i c a t i o n s t e s t e d i n c l u d e d i n c r e a s e d leading-edge f l a p d e f l e c t i o n t o 3 5 O (from ? 5 O ) : i n c o r p o r a t i o n of n o s e s t r a k e s ; widening of t h e forward LEX bound- a r y l a v e r h l e e d s l o t ; and i n c o r p o r a t i o n of a LEX lower s u r f a c e f e n c e .

I n a d d i t i o n t o t h e l a t e r a l s e n s i t t v i t y , a " m o d e l - s c a l e " e f f e c t w a s e v l d e n t which p r e c l u d e d model d a t a c o r r e l a t i o n . P r i o r t o t h e f i r s t f l i g h t t e s t i n c i d e n t , i n i t i a l e x p l o r a t o r v s t a t i c f o r c e a n d moment t e s t s i n t h e Lanu1 ev 3fM.3-faot wind t u n n e l u s i q t h e 0.16-scale F/A-18 mod21 r e v e a l e d anomalies i n t h e h i g h - a n ~ l e o f - a t t a c k l a t e r a l s t a b i l i t y c h a r a c t e r i s t i c s when

0RyiWAl. PAGE IS

apmRqoAUPI

STABLE OIRECTIONAL 8. DEG STAB1 LlTY STAPLE LATE HAL STABILITY 0.WSCALE WlND TUNNEL DATA d = 33 DEGREES iAEDC 16 T) M = 0.7 ,\\\\\\\\\\,'\\\\ APPRO X l MATE F L l G HT TEST FIGURE 5. COMPARISON OF LATERAL-DIRECTIONAL CHARACTERISTICS OBTAINED IN 0.06-SCALE HIGH REYNOLDS NUMBER WlND TUNNEL TESTS AND I N FULL SCALE FLIGHT TESTS.

J '/,/,'/APPROXIMATE , / .'I FLIGHT I : I .TEST (M = 0.7)

0.16 - SCALE (LANGLEY

I-.-

30 X 60 FT FST)

I

0.1 2 - SCALE (MCAI R

0.06 - SCALE (AMES

12 - FT PRESSURE TUNNEL)

I-

0.06 - SCALE (LANGLEY

30 X 60 - FT FST)

I--

I NOTE: MODEL TEST DATA FROM M , = 0.05 TO 0.20 FIGURE 6. COMPARISON OF SUdSCALE WlND TUNNEL MODEL DATA AND FULL-SCALE FLIGHT TEST ROLLING MOMENT VARIATION WITH SIDESLIP.

t h e r e s u l t s were compared with 0.06-scale model d a t a obtained a t t h e N A S A Ames Research Center 12-foot transonic pressure tunnel. The d i r e c t i o n a l s t a b i l i t y r e s u l t s obtained on the L a n g l e ~ 0.16-scale model were i n agreement with the ; r Ames 0.06-scale r e s u l t s . However, disagreement was evident t n the l a t e r a l s t a b i l i t y d a t a near , t h a t is, a t &= 35* t o 40°, t h e 0.16-scale r e s u l t s i n d i c a t i n g a l a t e r a l > .

A review of a v a i l a b l e l i t e r a t u r e on c o n f i g u r a t i o n s s i m i l a r both i n s i z e

,. / -

I and geometry t o t h e 0.16-scale model ( e . ~ . , t h e Northrop YF-17) revealed

comparable l e v e l s of l a t e r a l s t a b i l i t y .

The 0.06-scale high-Reynolds-number model was brought t o Langley and , t e s t e d i n t h e 30x60-foot wind tunnel a t ( 1 ) the same chordal Reynolds number 1 a s t h e 0.16-scale model, ( 2 ) the same Reynolds number a s t e s t e d i n t h e Ames 12-foot tunnel, and ( 3 ) t h e same free-stream dynamic pressure a s t h e 0.16- I - 1 s c a l e model. The l a t e r a l - d i r e c t i o n a l s t a b i l i t y r e s u l t s o b t a i n e d on t h e 0.06-scale model i n t h e Langley f a c i l i t y were found t o agree with t h e d a t a obtained a t Ames.

A CI.07-scale F/A-18 model was subsequently f a b r i c a t e d by Langley f o r . .

! t e s t s a t the Virgixia Polytechnic I n s t i t u t e (VPI) 6x6-foot curved flow wind tunnel and i n the Langiey 30x60-foot f a c i l i t y . R e s u l t s obtained on t h i s model 7 ' were i n agreement with t h e 0.06-scale d a t a , t h a t is, t h e s e r e s u l t s reveal I v a s t l y d i f f e r e n t l a t e r a l s t a b i l i t y l e v e l s r e l a t i v e t o t h e 0.16-scale model, a s , . can be seen i n F i ~ u r e 7.

The c l o s e l y - c o u p l e d f o r e b o d y / w i n g / ~ E X a r r a n g e m e n t on t h e FIA-18 i s conducive t o powerful I n t e r a c t i o n s between t h e v o r t i c e s shed from t h e f orebody and LEXe. I n a n e f f o r t t o d e t e r m i n e an e x p l a n a t i o n f o r t h e d i f f e r e n c e s between small- and 1 arge-scale model d a t a , tests were conducted i n Langley' s i 30x60-foot wind tunnel using t h e 0.07- and 0.16-scale models. Geometric . . parameters influencing t h e f o r e b o d y / w i n g / ~ ~ X vortex development, i n t e r a c t i o n , I Among these parameters were forebody o n / o f f , and s t a b i l i t y were investigated.

i i

T F X s o n / o f f , LEX boundary l a y e r bleed s l o t s openlclosed, and radome s t r a k e s on/ off .

The complex vortex i n t e r a c t i o n s were not well-understood during t h e wind tunnel t e s t s due t o a l a c k of adequate flow v i s u a l i z a t i o n c a p a b i l i t y . The

FIGURE 7. YAWING MOMENT AND ROLLING MOMENT VARIATIONS WITH SIDESLIP AT a = 350 ON 0.07 AND O.1MCALE FIA-18 MODELS TESTED IN THE NASA LANGLEY 30 X 60 FOOT WIND TUNNEL.

d i f f i c u l t y i n v i s u a l i z i n g i n a v i v i d manner highly-three-dimensional v o r t i c a l mottons i s t y p i c a l of low-speed wind t u n n e l s a t t h e p r e s e n t time. conse- q u e n t l y , t h e Northrop water t u n n e l f a c i l i t y was chosen t o p r o v i d e d e t a i l e d flow v i s u a l i z a t i o n of t h e F-18 v o r t e x f l o w f i e l d . A t t h e a n g l e s of a t t a c k of primary i n t e r e s t ((um30-40 d e g r e e s ) , s i g n i f i c a n t r e g t o n s of s e p a r a t e d flow e x i s t on t h e forebody, LEX, and wing s u r f a c e s r e g a r d l e s s of t h e v a l u e of Reynolds number. Consequently, t h e fundamental s t r u c t u r e of t h e v o r t i c e s w i l l he s i m i l a r whether t h e flow i s developed a t low Reynolds number i n a low-speed w a t e r t u n n e l o r a t h i g h e r Reynolds number i n a wind t u n n e l (References 2 and 3). I n t h i s manner, t h e r e s u l t s o b t a i n e d i n t h e water t u n n e l can be a p p l i e d t o o b t a i n a q u a l i t a t i v e u n d e r s t a n d i n g of t h e low-speed wind t u n n e l d a t a t r e n d s .

mean aerodynamic chord rolling moment coefficient lift coefficient pitching moment coefficient pawing moment coefficient normal force coefficient lateral stability parameter directional stability parameter wing root chord measured along wing-fusciage junction section suction coefficient maximum body wtdth free-stream Mach number Re- c Reynolds number based on mean aerodynamic chord Reynolds number based on maximum body width chordwise distance of vortex butst position measured from wing trailing edge free-stream speed free-stream dynamic pressure angle of attack angle of eideslip horizontal tail deflection angle leading-edge flap deflection trailing-edge flap deflection leading-edge w e e p angle forebody strake vadial position dimensionlere #pan station

EXPERIMENTAL METHODS Water Tunnel F a c i l i t y The Northrop water tunnel i s a closed r e t u r n tunnel used f o r high q u a l i t y flow v i s u a l i z a t i o n of complex three-dimensional flow f i e l d s . The water tunnel i s shown i n Figure 8. The test s e c t i o n i s 16 i n , by 24 i n , by 720 in. long and has w a l l s mad2 of t r a n s p a r e n t Plexiglas. The t e s t s e c t i o n i s oriented i n the v e r t i c a l d i r e c t i o n , which permits t h e model t o b e vieved from any angle.

A model is shown i n s t a l l e d i n t h e t e s t s e c t i o n i n Figure 9. The model i s accessed through t h e t o p of t h e tunnel by means of suspension cabics connected t o t h e model support system.

The model support system c o n s i s t s of a s t i n g and auto p i t c h ar.3 vaw mechanisms which a r e capable of p i t c h angles from -10' t o -.oncurrent with s i d e s l i p range of -15' t o 15'.

Test Procedure The flow v i s u a l i z a t i o n i n t h e water tunnel i s obtained by i n j e c t i n g colored food dyes having t h e same d e n s i t y a s water. T?le d e n s i t y of water i s 800 times t h a t of a i r , which g i v e s t h e dye e x c e l l e n t l i g h t r e f l e c t i n g c h a r a c t e r i s t i c s r e l a t i v e t o using smoke i n a i r . The dye i s introduced i n t o t h e flow f i e l d through small o r i f i c e s and 'dye t u b e s d i s t r i b u t e d a t s e l e c t e d o o s i t i o n s on t h e model a s shown i n Figure 9. The dye can a l s o be introduced through a dye prohe, which can be a c c u r a t e l y positioned clt any p o i n t i n t h e t e s t s e c t i o n by means of a t r a v e r s i n g mi.=haniw. This mechanism i s u t i l i z e d f o r t h e s o l e purpose of determining proper dye p o r t p o s i t i o n s on models f o r which tlie l a t t e r a r e not r e a d i l y evident. Use of t h i s e x t e r n a l dye probe app?ratus n e c e s s i t a t e s removal of t h e honeycomb flow s t r a i g h t e n e r which i s positioned d i r e c t l y above t h e v e r t i c a l test s e c t i o n . Absence of t h e honeycomb promotes undesirable flow c h a r a c t e r i s t i c e i n t h e test s e c t i o n . Consequently, i t bas become standard procedure t o fully-inetrument t h e t e e t models f o r b e s t flow v i e u a l i z a t i o n .

I n l e t C ~ O W S a r e simulated i n t h e water tunnel by applying s u c t i o n t o tubes connected t o t h e r e a r of t h e model's exhaust nozzles. The tubes a t e run

t o a water flow meter o u t s i d e t h e t u n n e l . Flow m e t e r s a r e used t o a c c u r a t e l y measure end R r t t h e i n l e t flow r a t e and any jet blowing r a t e s . The w a t e r t u n n e l i s operated a t a t e s t s e c t i o n v e l o c i t y of 0.25 foot/second which h a s been found t o produce d e s i r a b l e flow v i s u a l i z a t i o n r e s u l t s . T h i s v e l o c i t y c o r r e s p o n d s t o a R e y n o l d s n u s b e r of 3 x 1 0 / f o o t . The R e y n o l d s number based on mean aerodynamic chord f o r t h e 0.025-r c a l e F/A-18 model i s approxi- matelv 8700.

YODEL DESCRIPTION The g e n e r a l l a y o u t of t h e ~ / ~ - 1 8 i s d e p i c t e d i n F i g u r e 10. A planview photograph of t h e F/A-18 i n f l i g h t i s shown i n F i g u r e 1 0 ( a ) which v i v i d l y d e p i c t b t h e primary a i r f r a m e components. A 0.025-scale F/A-19 model. was used f o r flow v i s u a l i z a t i o n tests i n t h e w a t e r t u n n e l . The model was f u l l y - i n s t r u - mented w i r h i n t e r n a l and e x t e r n a l dye i n j e c t i o n o r i f i c e s . For exampie, t h e f u s e l a g e forebody f e a t u r e d 24 i n t e r n a l dye p o r t s : 4 l o n g i t u d i n a l rows of 6 p o r t s each w i t h 2 rows on t h e t o p and bottom s u r f a c e s ( t h e rows were posi- t i o n e d on e i t h e r s i d e of t h e model c e n t e r l i n e ) . A d d i t i o n a l p o r t s (6) i n t e r n a l t o t h e f u s e l a g e were i n s t a l l e d a l o n g t h e l e f t a i d e below t h e leading-edge e x t e n s i o n (LEX). The l e f t LEX was instrumented w i t h 18 i n t e r n a l dye r e l e a s e h o l e s : 2 chordwise row*q.of 6 each on t h e upper s u r f a c e and a s i n g l e row of 6 p o r t s on t h e lower s u r f a c e . Both l e f t and r i g h t LEXs f e a t u r e d e x t e r n a l dye l l n e s c o n s i s t i n g of small s t a i n l e s s s t e e l t u b e s e x t e n d i n g up t o t h e LEX apex and t h e LEX plarrform break. The L ~ X planfonn break i a d e p i c t e d i n F i g u r e 'LO(h). The l e f t wing f e a t u r e d 24 upper s u r f a c e dye o r i f i c e s : 6 chordwise p o r t s a t each of t h e 4 span s t a t i o n s . With t h e model inatrumented i n t h i J f a s h i o n , i t was p o ~ t s i b l e t o p r o v i d e d e t a i l e d v i s u a l i z a t i o n of t h e forebody, LEX and wing s u r f a c e flows and tbe forehody and LEX v o r t i c e s throughout t h e r a n g e s of a n g l e of a t t a c k and s i d e s l i p (0's (u 5 40'; -12°r/3s120). A p a i r of s u c t i o n t u b e s was i n s e r t e d i n t h e exhaust n o z z l e s t o p r o v i d e i n l e t s u c t i o n t o s i m u l a t e a r e a l i s t i c i n l e t maas f l e w r a t i o .

q e b a s e l i n e model featu;-q a l l LEX s l o t s open ( s e e F i g u r e s 1 0 ( a ) and 10(h)), leading-edge f l a p s d e f l e c t e d 35", t r a i l i n g - e d g e f l a p s undef l e c t e d , h o r i z o n t a l t a i l s d e f l e c t e d f u l l t r a i l i n g - e d g e up (-12'). Leading-edge f l a p d e f l e c t i o n a n g l e of 25' was n o t a a s e s s e d s i n c e t h e a s s o c i a t e d flow f i e l d changes i n t h e water t u n n e l , o p e r a t i n g a t low Reynolds number, a r e v i r t u a l l y u n d e t e c t a b l e on a wing of uuch low sweep a n g l e .

FIGURE 8. PJORTHRUP W X DIAGNQSTIC WATER fLINNE L FACILiTY FIGURE 9. ADVANCED FIGHTER MUDEL HWFALCECI Ihl THE WATER TUNNEL (DYE INJECTION f HROUGH INTERNAL AND EXTERNAL PORTS)

I A ~ FIA-18 I N F L I G H T F ~ G ~ ~ R E 10: F I A - I 8 OESCRlPTlON PRECEDING PAGF FUNK * ORIGINAL PAGE

I ~ o ~ o n ~ H o r p o a m

Ph'tCEiJlNG F'RGE DiANK NOT FICMCD

ORIGINAL. PAGE IS

OF POOR QUALrrV

INCREASED L E X SLOT W I D T H 8.1 111. ,FORWARD SLOT (LEX 12) PLANFORM BREAK INCREASE0 LEX SLOT WIDTH AND LENGTH L E X LOWER 1 SURFACE FENCE ! - - '' '".

- I

-*-

FENCE A

/ I

I FENCE 8 I

.\ I

-

55 \ I ) - /-/ FOREBODV STRAKES STRAKE , /- SECTION A - A (C) LEX A W FOREBODY MOOIFIWTIONS (ALL DIMENSIONS ARE FULLSCALE).

FIGURE 10. CONCLUDED.

S e v e r a l of t h e w a t e r t u n n e l moat.1 g e o m e t r y chnnp,cs i n v e s t i g a t e d i n t h e w a t e r t u n n e l test nropram a r e i l l u s t r a t e d i n F i g u r e 1 0 ( c ) . The model c h a n g e s t n c l u d e : d o u b l e - w i d t h f o r w a r d LEX h l e e d s l o t s (LRX 12:; d o u h l e - w i d t h , i n - c r e a s e d l e n g t h f o r w a r d s l o t s (',EX 12A); LEX l o w e r s u r f a c e f e n c e s o r i e n t e ' i ~ . a st reamwise and o b l iqrie manner ( F e n c e "A" a n d F e n c e "P", r e s p e c t i v e l y ) ; and f o r e h o d v s t r a k e s m o u v t e d a t r a d i a l v o s i t i o n s o f @ = + i O O , 0". a n d - 4 5 " .

A d d i t i o n a l model v a r i a t i o n s i n c l u d e d - f l i g h t test n o s e boom; wing s n a g and f e n c e c o m b i n a t i o n ; removal o f t h e LEXs; and remc) :a1 o f t h e f o r e b o d v . Th-- n n i n t a t which t h e forebociv 23s removed i s i n d i c a t e d i n F i g u r e lO(b). A l e n g t h o f 8.85 f t . ( f u l l - s c a l e ) was removed and r o p l e r n d w i t h a h e m * - m h e r i c a l c a p .

DISClTSSION OF RESULTS The s i e n i f i c a n c e o f t h e l e a d i n g - e d g e e x t e n s i o n (LEX) s l o t f l o w i s re- v e a l e d f n F i g u r e 1 1 , which d e p i c t s t h e s t r c ~ q c u r v a t u r e o f t h f 0 . 0 2 5 - s c a l e model f u s e l a g e s u r f a c e f l o w n e a r t h e LEX s t r u t s and s l o t s a t cr=Oo and j?=OO.

The entrainmen: of t h e b o u n d a r y l a v e r f l u i d i n t o t h e s l o t s i s e v i d e n t . The w a t e r t u n n c l p h o t o g r a p h s i n F i g u r e 11 p r o v i d e a p i c t o r i a l d e s c r i p t i o n o f t h c p u r p o s e o f t h e s l o t s : t o p r e v e n t i n g e s t i o n o f low-energv b o u n d a r y l a v e r f l u i d i n t o t h e s i d e m o u n t e d e n ~ i n e i n l e t s .

I t h a s b e e n e s t a b l i s h e d i n N o r t k r o p w a t e r t u n n e l f l o w v i s u a l i z a t i o n s t u d i e s a n d i n s m o k e f l o w v i s u a l i z a t i o n t e s t s a t N A S A L a n g l e y R e s e a r c h C e n t e r ' s 30x60-foot wind t u n n e l t h a t t h e F / A - ~ R LEX p l a n f o m w i t h s l o t s o p e n R e n e r a t e s a d u a l l e a d i n g - e d g e v o r t e x s v s t e m : o n e p r i m a r y v o r t e x o r i a i n a t i n l :

-

a t t h e LEX a p e x and a n o t h e r n e a r t h e p r o d u c t i o n b r e a k ( o r i n f l e c t i o n p o i n t ) i n tbe LEX p l a n f o r m . T h i s f l o w phenomenon is s k e t c h e d i n F i g u r e 12. W i t h b l e e d s l o t s c l o s e d , however, o n l v o n e p r i m a r v v o r t e x , o r i g i n a t i n g a t t h e LEX apex, i s e v i d e n t , a s s k e t c h e d i n F i g u r e 1 2 . As a c o n s e q u e n c e o f t h e s l o t f l o w , which r o l l s u p i n t o a v o r t e x o n t h e LEX u p p e r s u r f a t e and r o t a t e s i n a s e n s e o n p o s i t e t o t h e l e a d i n g - e d g e v o r t i c e s ( s e e F i g u r e 1 2 ) . less l o w e r s u r f a c e f l o w f s a v a f l a h l e f o r f e e d i n e i n t o t h e p r i m a r v l e a d i n g - e d g e v o r t e x . Due t o s l o t e n t r a i n m e n t e f f e c t s , t h e a n g l e and v e l o c i t v a t whlch t h e l o w e r s u r f a c e f l o w

d e n a r t s t h e l e a d i n e edge a r e r e d u c e d , a s d e p i c t e d i n F i g u r e 1 2 . The s t r e n g t h o f t h e leadit-g ed.qe v o r t e x i s d e p e n d e n t o n t h e d i f f e r e n c e hetween t h e v e l o c i - t i e s a t t h e o u t e r edge o f t h e l o w e r and u p p e r s u r f a c e boundary l a v e r s . The 9rea:er t h e v e l o c i t y d i f f e r e n c e , t h e g r e a t e r i s t h e v o r t e x s t r e n g t h , and v i c e v e r s a . C o n s e q u e n t l v , i n t h e v i c i n i t y o f t h e f o r w a r d LEX s l o t , t h e d i f f e r e n c e i n u p p e r and l o r e r v e l o c i t v components a t a t y p i c a l LEX c r o s s - s e c t i o n i s r e ~ i u c e d , w i t h a c o r r e s p o n d i n g r e d u c t i o n i n a p e x p r i m a r y v o r t e x s t r e n g t h i n t h i s r e ~ i o n .

4n F/A-IR-tvpe LEX p l a n f o r m i s c h a r a c t e r i z e d by a s e c t i o n s u c t i o n d i s - t r i h r r t i o n ( s e e R e f e r e n c e 4 ) a s s k e t c h e d i n F i g u r e 1.3. 4 d i s t i n c t b r e a k i s e v i d e n t i n t h i s a u a l i t a t i v e s u c t i o n d i s t r i b u t i o n n e a r t h e i n f l e c t i o n p o i n t .

I f t h e s u c t i o n ~ e a k n e a r t h e i n f l e c t i o n p o i n t i s s u f f i c i e n t l y s t r o n g , t h e e e n e r a t i o n o f a v o r t e x a t t h e i n f l e c t i o n n o i n t may r e s u l t ( i n t h e a b s e n c e of a h l e e d s l o t ) . V a t e r t u n n e l s t u d i e s a t N o r t h r o p h a v e c o n f i r m e d a t e n d e n c y o f srich p7anf:~rms t o d e v e l o p a second p r i m a r y v o r t e x . I n t h e e x t r e m e , a d o u b l e d e l t a win^ i s c h a r a c t e r i z e d hv R two-vortex system.

C o n s i d e r a t i o n o f t h e two f a c t o r s l u s t d i s z a s s e d , ( 1 ) r e d u c e d l o w e r s u r f a c e f l o w n e a r t h e f o r w a r d LEX s l o t a v a i l a b l e f o r f e e d i n g i n t o t h e l e a d - i . 1 ~ - e d g e v o r t e x and ( 2 ) i n f l e c t i o n p o i n t s u c t i o n p e a k , l e a d s t o t h e f o l l o w - i n g c o n c l u s i o n s : The p r o x i l ~ i i t p of t h e f o r w a r d LEX s l o t t o t h e p l a n f o n n i n f l e c t i o n p o i n t ( i n t e n s of b q t h l o n g i t u d t n a l and l a t e r a l s p a c i n g ) promotes t h e f o r i n a t i o n o f two p r i m a r v v o r t i c e s . C l o s u r e o f t h e s l o t r e s u l t s i n o n l y o n e p r i m a r v v o r t e x , s i n c e t h e F/A-18 LEK l o c a l sweep a n g l e v a r i a t i o n i s u n s u f f i c i e n t h v i t s e l f t o promote a second v o r t e x ( s e e F i g u r e 2, f o r example).

1,F.Y v o r t e x b e h a v i o r is e x p e c t e d t o b e s e n s i t i v e t o b l e e d s l o t Reometry (which d e t e r m i n e s s l o t e n t r a i n m e n t e f f e c t s ) , LEX p l a n f o r m v a r i a t i o n s , and t h e r e l a t i v e l o c a t i o n of t h e s l o t t o t h e LEX l e a d i n g edge. I t c a n h e s e e n , t h e n , t h a t anv m o d i f i c a t i o n o f t h e f o r w a r d LEX s l o t geometry o r LEX p l a n f o r m , o r more ~ e n e r a l l v , a n p means by which t h e v o r t i c i t v - f e e d i n g mechanism i s a l t e r e d V ' , , ~ I I t h i s r e s f o n l . w i l l I n f l u e n c e t h e F/A-18 LEX v o r t e x s y s t e m b e h a v i o r .

FIGUSE 11. FUSELAGE SURFACE FLOW PATTERNS ATcr-8-0"

LEX SLOT SLOT VORTEX LEX AFT PRIMARY VORTEX

\

(a1 SLOTS OPEN

ORIGINAL PAGE I S

OF m R Q U m

-

PRIMARY VORTEX LEX Ibl SLOTS CLOSED FIGURE 12. SKETCHES OF LEX LOWER SURFACE FLOW PAfTERNS.

FIGURE 13. SKETCH OF SECTION SUCTION DlSTAlBUTlON ON F/A-19TYPE LEX PIANFORM FIGURE 14. WATER f WNNEL PHOTOGRAPHS OF 0,025-SCALE FIA-18 BASELINE; &,idf 35°/00; A h = -12'

The LEX v o r t i c e s a t f f = l O O and 15' ( p = 0 " ) a r e d e p i c t e d i n t h e f l o w v i s u a l i z a t i o n p h o t o g r a p h s I11 F i g u r e 14. The f i r s t p o i n t o f i n t e r e s t i s t h a t t h e v o r t i c e s a p o e a r d i f f u s e , w i t h no d i s t i n c t c o r e b e i n g d e p i c t e d . The vor- t i c e s a r e e n e r g e t i c b u t , due t o t h e two-vortex s v s t e m on t h e F/A-18 LEX, t h e r o t a t i o n a l f l o w i s somewhat d i f f i c u l t t o d e f i n e i n a v i v i d manner. A t t h e s e a n ~ l e s of a t t a c k , t h e two p r i m a r y v o r t i c e s ; e r g e a s h o r t d i s t a n c e downstream of t h e planform i n f l e c t i o n p o i n t . Vortex b u r s t i n g a t t h e wing t r a i l i n g edge o c c u r r e d a t ff'15°, a l t h o u g h t h i s a n g l e o f a t t a c k i s c o n s i d e r e d low r e l a t i v e t o t h e e x p e c t e d v a l t l e a t h i g h e r Reynolds numbers i n a i r . Laminar boundary l a y e r s e p a r a t i o n a t t h e low Reynolds number z o n d i t i o n s i n t h e w a t e r t u n n e l promoted a l a r g e r r e g i o n of wing f l o w s e p a r a t i o n a n d , h e n c e , p r e m a t u r e b u r s t i n g of t h e LEY v o r t i c e s . Review O F NASA v i d e o t a p e s of smoke f l o w visualization o n t h e 0.16-scale F/A-18 i n d i c a t e s v o r t e x b u r s t i n g a t t h e wing t r a i l i n g e d g e a t a @ 20°. I;I s i c ' e s l i p , p=4O and 8", f o r example, v o r t e x breakdown asymmetry was e v i d e n t ( p h o t o g r a p h s n o t shown), w i t h t h e windward v o r t e x b r e a k i n g down n e a r t h e windward v e r t i c a l t a i l l e a d i n g edge and no e v i d e n c e o f leeward v o r t e x hreakdown o v e r t h e leeward wing p a n e l . T h e r e was a l s o a n o t i c e a b l e i n b o a r d and o u t b o a r d d i s p l a c e m e n t of t h e windward and leeward CEX v o r t i c e s , r e s p e c - t i v e l v .

The f l o w photograph ( p l a n v i e w ) i n F i g u r e 1 5 shows t h o LEK v o r t e x a t a = 2 0 , P m O O , where v o r t e x b u r s t i n g i s o b s e r v e d a t a p p r o x i m a t e l y X/CR '0.3. Here, X i s d e f i n e d a s t h e d i m e n s i o n a l d i s t a n c e o f t h e b u r s t p o i n t measured from t h e wing t r a i l i n g edge and C i s t h e wing c h o r d measured a l o n g t h e w i n g - f c s e l a g e R f u n c t i o n e x t e n d i n g from t h e trail in^ edge t o t h e wing l e a d i n g edge p r o j e c t e d t o t h e f u s e l a g e . T h e d e t e r m i n a t i o n o f t h e v o r t e x b x r s t p o i n t i s h i g h l y s u b j e c t t o t h e i n d i v i d u a l o b s e r v e r ' s i n t e r p r e t a t i o n . Breakdown i n t h i s r e p o r t is d e f i n e d a s t h e m i n t a t which t h e f i r s t noticeable r e g i o n o f s t a g n a t e d and r e v e r s e d f l o w a l o n g t h e v o r t e x a x i s was o b s e r v e d .

The two p r i m a r y v o r t i c e s on t h e LEX a r e a p p a r e n t i n t h e s i d e v i e w photo- s r a ~ h i n F i ~ u r e 15. The p o i n t of o r i g i n of t h e second LEX p r i m a r y v o r t e x is d i f f i c u l t t o d e t e r m i n e p r e c i s e l v b u t i s , a p p r o x i m a t e l y , s l i g h t l y f o r w a r d of t h c p l a n f o r m i n f l e c t i o n p o i n t .

'I: 4 1

PR€CEDINO PAGE B U M NOf flMED

7 : r .

Note i s made t h a t t h e p h o t o g r a p h s i n t h i s r e p o r t a r e i n a d e q u a t e i n p r o v i d i n g a complete d e s c r i p t i o n of t h e n a t u r e of t h e v o r t e x flows. For example, t h e a f t primary v o r t e x on t h e LEX i n F i g u r e 15 a p p e a r s a d i f f u s e , i l l - o r g a n i z e d mass of dye. Observation of t h i s v o r t e x i n s i t u , however, p r o v i d e s t h e f u l l three-dimensional n a t u r e of t h e f l o w and an a p p r e c i a t i o n f o r t h e s i g n i f i c a n t r o t a t i ~ n of t h e LEX v o r t e x which g r e a t l y i n f l u e n c e s t h e wing flow.

The r e s u l t s i n F i g u r e 16 show t h a t a t (r=20° t h e LEX v o r t i c e s do n o t e x h l h i t a l a r g e d i f f e r e n c e i n b u r s t p o s i t i o n s i n s i d e s l i p , a l t h o u g h t h e leeward v o r t e x does b u r s t f a r t h e r a f t . T h i s may h e due, i n p a r t , t o t h e nresence of t h e twin v e r t i c a l t a i l s . The twin v e r t i c a l s a c t a s downstream o b s t a c l e s which impost- an a d v e r s e p r e s s u r e f i e l d on t h e LEX v o r t i c e s , t h e r e b y reducing t h e p o t e n t i a l f o r l a r g e asymmetry i n h u r s t p o s i t i o n s . A second r e a s o n i s r e l a t e d t o v i s c o u s e f f e c t s . A t 0-20, t h e LEX v o r t i c e s a t e n o t v e t of s u f f i c i e n t s t r e n g t h t o dominate t h e f l o w f i e l d a t t h e low-Reynolds- ntimher c o n d i t i o n s i n t h e w a t e r t u n n e l . Consequently, l a m i n a r flow s e p a r a t i o n o n t h e cambered wing u p p e r s u r f a c e may t e n d t o r e d u c e v o r t e x breakdown asvmmetry. The primary d i f f e r e n c e s i n t h e LEX v o r t e x c h a r a c t e r i s t i c s l i e i n t h e c o r e p o s i t i o n s and t h e r o t a t i o n a l energy of t h e r e s p e c t i v e v o r t i c e s . The leeward TAX v o r t e x appeared more e n e r g e t i c ( t h i s assessment i s based on t h e " t i g h t n e s s " of t h e h e l i c o i d a l p a t t e r n d e p i c t e d by t h e dye t r a c e r s ) and was d i s p l a c e d outhoard. The flow photograph i n F i g u r e 16 shows t h a t a t h i g h e r s i d e s l i p a n g l e f P = R O ) t h e r e i s a g r e a t e r d i f f e r e n c e i n S u r s t p o s i t i o n s r e l a t i v e t o t h e p=4O r e s u l t due t o t h e d i s p l a c e m e n t of t h e leeward LEX v o r t e x f r o m t h e l e e w a r d v e r t i c s l t a i l s u r f a c e a n d / o r r e d u c e d l e e w a r d w i n g f l o w s e p a r c t i o n due t o t h e h i g h e r s i d e s l i p a n g l e . S t r o n g vortex-induced sweeping a c t i o n i s observed on t h e leeward wing p a n e l . The o u t e r e x t e n t o f t h e dye t r a c e r s e n t r a i n e d i n t o t h e v o r t e x d e f i n e s , a p p r o x i m a t e l y , t h e spanwise e x t e n t of t h i s sweeping a c t i o n . I n c o n t r a s t , t h e windward wing s u r f a c e e x h i b i t s s i g n i f i c a n t f l o w s e p a r a t i o n w i t h o u t r e a t t a c h m e n t ( n o photograph a v a i l a b l e ) .

(Note: S u r f a c e flows w i l l be d i s c u s s e d q u i t e f r e q u e n t l y . It i s recognized t h a t s u r f a c e f l o w c h a r a c t e r i s t i c s a t t h e low Reynolds number i n t h e w a t e r t u n n e l (Rec*8700) a r e n o t r e p r e s e n t a t i v e of t h e f l o w b e h a v i o r a t h i g h e r Reynolds numbers. The t r e n d s observed i n w a t e r , however, a r e i n s i g h t f u l and,

--

a s a r e s u l t , w i l l be u t i l i z e d where a p p r o p r i a t e . )

ORIGINAL PAGE C@LOP ?L'OTOGRA~IG FIGURE 15, WAlTR TUNNEL P H O T O G R H OF O.OZ6SCAI.E BASELINE FlA-18 DUAL LEX VORTEX SVSTEM

FLOW VIS;SALIZP.TION PHOTOGRAPHS OF BASELINE F/A-18 E N FIG'UkZ 75, S I D E S L l I ~ A T a = 20

PRECEDiNG PAGE BLANK NOT FILMED I n s i d e r l i p , t h e leeward LEX p r i m s r y v o r t i c e s merge q u i t e s t r o q g l y a m , shown i n F i g u r e 16 ( t h e apex v o r t e x e x h i h i t s a h e l i c a l p a t t e r n a b o u t t h e second p r i m a r y v o r t e x ) , w h e r e a s t h ~ windward LEX v o r t i c e s t e n d t o become i n d e p e n d e n t . A wing v o r t e x i s a1.m developed ( n o t d l s c e r n i h l e i n F i p u r e 1 6 ) , e m n n a t l n g f r o m t h e l e a d i n g - e d g e f l a p h i n g e l i n c r b g i o n . T h i s v o r t e x - J a s p a r t i c u l a r l y e v i d e n t o n t h e l e e w a r f i p a n e l d u e t o t h e " e f f e c t i v e " s w e e p i n c r e a s e a s s o c i a t e d w i t h s i f l e ~ l i p .

F i g u r e 17 p r e s e n t s p h o t o g r a p h s t a k e n a t 0!=25', @ = O D where v o r t e x break- Jown o c c u r r e d 3t appro::imately X/C 20.5. As v o r t e x s t r e n g t h i r . c r e a s e d w i t h R a r y l e of a t t a c k , t h e breakdown p o s i t i o n was somewhat more d i s c e r n i b l z . Of n o t e is t h a t t h e LEX apex v o r t e x e x h i b i t s Zest t e n d e n c y f o r merger w i t h + y e s e c o n d p r i m a r y v c r t e x a n d b;:akdown o f t h e a p e x v o r t e x c c z u r s . T h i s i s d e p i c t e d i n t h e p h o t o ~ r a p h s i n F i g u r e 17. The p r i m a r y d i f f e r m z c z s i n v o r t e x b e h a v i o r a t n o n z e r o s i d e s l i p a p p e a r t o be t h e v o r t e x s t r e n g t h and c o r e p o s i - t i ~ n s . The LEX v o r t e x b e h a v i o r o b s e r v e d !n smoke f l o w v i s u a l i z a t i o n o f a f i . 1 5 - s c a l e F/,4-10 m o d e l i n t h e L a n g l c y 3 0 x 6 0 - f o o t wind t u n n e l w a s q u ' t e s i m i l a r t o t h e xlti t e r t u n n e l r e s u l t s . Exami.iation of NASA v i a e o t a p e s r e v e a l e d r e a s o n a b l e t r e n d agreement of LEX v o r t e x b u r s t p r o g r e s s i o n s n d v o r t e x p o s i - t i o n s w i t h t h e low-Reyno1.l~-number hydrndynamic f l o w v i s u a l i z z t f o n .

A comparfson of upper s u r f a c e f l o w c h a r a c t e r i c , - i c : s ~ d e s l ! p a t a - 2 5 " i s provided i n F i ~ u r e 1 R . The leeward wing exhfhli.:, .,e. -$?r vertex sweepin2 a c t i o n , i n d i c a t e d hg t h e s p a n v i s e ~ r i e n t a t i o n of t h e s u r f a c c d y e , r e ' a t i v e t c t h e windward w i r . ..

The f o r e h o d y v o r t e x p a t t e r r s a t aa25* a r e showil i , , F i g u r e 1 9 . The v o r t i c e s a p p e a r no? t o b e a m a j o r f a c t o r i n t h e f l o w f i e l d a t t h i s a n g l e of

-

a t t a c k . The v o r t i c e s a r e weak ( n o " t i g h t " h e l i c o i d a l p a c t e r n ) a n d , upon t r a v e r s i n g t h e canopy, t h e v o r t i c a l m o t i o n s i n . , ~ d e s l i p t x h i h i t a marked I n s t a b i l i t y . T h z r e l a t i v e s t r e n g t h o f a v c r t e x i s a s s e s s e d i n a h i g h l y q u a l i t a t i v e f a s h i o n i n t h e w a t e r t u n n e l by o b s e r v i n g t h e number o f trlr~is c e r u n i t d i s t a n c e a l o n g t h e v o r t e x . The v o r t e x m o t i o n i r , .the w a t e r t u n n e l j.s so slow (Vm a.25 f t l s e c ) t h a t t h e p a t h of a d y e e l e m e n t c a n b e t r a c k e d a l o n g t h e v o r t e x . 'fie a n g l e s of a t t a c k of p r i m a r y i n t e r e s t , h o w e l e r , fn t h i s s t u d y

of F'A-IR v o r t e x hehavior a r e cr=30° t o 40°, i n c l u s i - r e . A t t h e s e a n g l e s of a t t a c k ~ o t e n t i a l l v s i ~ n l f i c a n t foreboriy-wing-LEX f l o w i n t e r a c t i o n s can a r i s e .

A t t Y = 3 0 ° , $ = O D , LEY v o r t e x breakdown o c c u r r e d a t a p p r o x i m a t e l y t h e leading-edge f l a n h i n q e l i n e a s shown i n F i g u r e 23. The r e s u l t s i a F i g u r e 20 i.:dicate t h a t d i f f e r e n c e s s t i l l e x i s t i n t h e leeward and windward wing s u r f a c e flow c ! - e r a c t e r i s t i c s i n s i d e s t l p . The LEX apex v c r t e x was obser-.ed t o b r e a k 80un s h o r r l v rt.ounstreaa of t h e second p r i ~ ~ a r v vot-tex p o i n t of o r i g i n a t i o n , a s deoicrer! i n F i q u r e 21. The f o r v a r d s l o t v o r t e x is a l s o i l l u s t r a t e d i n t h c 9hotop-avh i n F i g u r e 21..

k a l ! t a t i v e ohservatio:ls i n d i c a t e t h a t t h e b d v v o r t i c e s a t ( ~ = 3 0 O , shown In F i s u r e 23, a r e of i n c r e a s e d s t r e n g t h r e l a t i v e t o t h e r e s u l t s o b t a i n e d a t C Y = ? F o i s e e F i g u r e 19). T;le hadv v o r t i c e s a r e f e d bv v o r t i c i t v g e n e r a t e d w i t h i n t h e f u s e l a q e forebodv Soundarv l a v e r up t o t h e p o i n t of intersection of t h e L T and f u s e l a g e . The t i n e of s e p a r a t i o n or' t h e f u s e l a g e p r i n a r y boundary law: i s denoted i n F i g u r e 22. T h e r e a f t e r , t h e bol:- v o r r i c e s e x h i s i t essen- L i a l 1 v c o n s t a n t s D a c i n a b e ~ w e e n t - ~ r n s o f t h e d y e t r a c e r s , i n d i c a t i v e o f a p p r o s i a a t e l v c o n s t a n t wortex s t r e n ~ t h . The LEX v o r t i c e s rextiain dominant and t h e Forehodv y c r t i c e s a t z e r o s i d e s l i p a r e e n t r a i n e d i n t o t h e wing flow a s shcwn i n F i o u r e 22. The w i n t of e n t r a i n m e n t of t h e hody v o r t f c e s i n F i g u r e 22 i s a t aporoxi?latelv X / C = 0 . 5 where t h e bodo v o r t i c e s p a s s a n d e r n e a t h R ?he LCX v c - t f c e s and e x h i b i t a r a p i d d i f f u s i o n a s t h e y e n t e r t h e wing p r e s s u r e f i e l d .

111 7 S . i d e s l i ~ c o n d i t i o n , t h e bodv v o r t i c e s e x h i b i t a s t r o n g l y a s - p i n e t t i c o r i e n t a t i o n . F i q u r e 2 3 i s p r e s e n t e d f o r i l l u s t r a t i v e purposes o n l v which shows t h e leeward hodv v o r c e s e n t r a i n m e n t i n t o t h e lceward LEX-wing f l o w and t h e windward ?mdv v o r t e x d i s p l a c e m e n t upward and awav frols t h e wiildvard wing flcu. T h i s flow s i t u a t i o n a r i s e s from such f a c t o r s a s t h e c l o s e c o u p l i n g of t h e LEY and foreScAv and t h e forehodv c r o s s - s e c t i o n a l phape ( a n d , consequent- lv, t h e nanner i n which t h e forebod- primary boundarv l a y e r s s e p a r a t e a l o n g t h e f n s e l a ~ e s i d e s ) . There i s a s i g n i f i c a n t r o t a t i o n of t h e f o r e b d y primarv s e n a r a t i o n l i n e s w e t o s i d e s i i p , t S e leeward s e p a r a t i o n l i n e r o t a t i n g down- war,' w h i l e t h e windward s e p a r a t i o n l i n e r o t a t e s upward. Consequently, t h e body v o r t e x t r a j e c t o r i e s w i l l r o t a t e s o t h a t t h e l e e w a r d hody v o r t e x i s

i

I

FIGURE 17. LEX VORTEX FLOW OM BASELINE FfA-18

I

5 1 !

-- -- z # = < . - L

ORIGINAL PAGE m L O R PHOTOGRAPH

FIGURE 20. WATER TUNNEL PHOTOGRAPHS C f W E L I M E FfA-18 F L O W FIELD AT rr = 30"

ORIGINAL =OR PHOTOGRAW FIGURE 21. DUAL LZX VORTEX SYSTEM A T a = 30'; $ = 0"

I

FIGURE 22. FOREBODY VORf EX PATTERN A T a = 30", = 0" I : pR€CEOlNG PAGE BLANK NOT F!LMED =+ Q

I

- COLOR P!4OT@GQ3-

c l o s e r t o t h e f u s e l a g e a n d , hence, s u b j e c t t o s t r o n g e r i n t e r a c t i o n w i t h t h e leeward LEX v o r t e x . I n c o n t r a s t , t h e windward body vortex-LEX v o r t e x i n t e r - a c t i o n 1 4 reduced s i n c e t h e f o m e r i s d i s p l a c e d away from t h e windward LEX and wing.

A t a-33'. t h e windward and leeward wing s t a l l p a t t e r n s a r e q u i t e s i m i l a r ( s e e F i ~ u r e 24). Small s i d e s l i p changes produced no marked a l t e r a t i o n o f t h e LEY v o r t e x h u r s t p o s i t i o n s , which were o b s e r v e d n e a r t h e LEX-wing j u n c t i o n .

The q u a l i t a t i v e o b s e r v a t i o n s i n t h e w a t e r t u n n e l r e v e a l no a p p a r e n t i n f l u e n c e of t h e forebody v 3 r t i c e s o n t h e wing flow f i e l d . The s i d e v i e w and olanview photographs a t (Y-33O i n F i g u r e 25 show t h a t a t @=OO t h e body v o r t e x e n t r a i n m e n t p o i n t i s n e a r t h e LEX-wing j u n c t i o n . I n s i d e s l i p , t h e leeward body v o r t e x i s e n t r a i n e d forward of t h i s p ~ i n t i n t o t h e LEX v o r t e x and d i s - s i p a t e s r a p i d l v . The windward body v o r t i c e s i n F i g u r e 25 a r e s h i f t e d s o f a r o f f t h e hodv t h a t any induced e f f e c t s on t h e windward wing a r e consi-lered m a l l . F i g u r e 25 a l s o r e v e a l s t h e r o t a t i o n of t h e primary s e p a r a t i o n l i n e s due t o s i d e s 1 ip.

Q u a l i t a t i v e assessment o f t h e twin v e r t i c a l t a i l r e g i o n i n d i c a t e s t h a t t h e dynamic p r e s s u r e a t t h e t a i l s is reduced r e l a t i v e t o t h e f r e e s t r e a m v a l u e . T h i s c a n be i n f e r r e d from t h e photographs i n F i g u r e 25 which r e v e a l t h e low e n e r g y wing wake which " b l a n k e t s " t h e t a i l s .

A t a-35" and 40°, t h e leeward wing e x h i b i t e d a massive s t a l l . T h i s may he due t o t h e l a r g e a n g l e of a t t a c k normal t o t h e leeward LEX r e s u l t i n g from t h e c o c h i n a t i o n o f h i g h sweep-hack, a n g l e o f a t t a c k , a n d s i d e s l i p . The s u r f a c e flow p a t t e r n s i n F i g u r e 26 show a t ( ~ 1 3 5 ' . #?+do - t h a t t h e windward wing s t a l l I s somewhat less pronounced t h a n t h e leeward p a n e l .

R e p r e s e n t a t i v e photographs of t h e b a s e l i n e f orebody v o r t i c e s a t a-40" a r e p r e s e n t e d i n F i g u r e 27. Flow f i e l d o b s e r v a t i o n s s u g g e s t t h a t t h e leeward body v o r t e x e f f e c t o n t h e d o w n s t r e a m f l o w b e h a v i o r i s s m a l l , s i n c e i t b u r s t s a h r u p t l v upon e n t e r i n g t h e leeward LEX wing f l o w f i e l d . The windward body v o r t e x may p r o v i d e s m a l l b e n e f i c i a l e f f e c t s on t h e windward wing. Dye en- t r a i n e d a f t of t h e canopy i n t o t h e windward body v o r t e x was o h s e r v e d t o flow t!ownwards towards t h e f u s e l a g e and t h e n spanwise o v e r t h e windward wing. T h i s PRECEDING PAGE BLANK NOT FILMED

e f f e c t was n 3 t pronounced, however. Because of t h e i n c r e a s e d displacement of t h e windward body v o r t e x away from t h e f u s e l a g e with i n c r e a s e d a n g l e s of a t t a c k and s i d e s l i p , t h i s e f f e c t i s expected t o d i m i n i s h a c c o r d i n g l y .

Double-Width Forward LEX S l o t s (LEX 12) The e f f e c t s of t h e i n c r e a s e d forward s l o t wiCth a r e v i v i d l y i l l u s t r a t e d i n F i g u r e 28 a t ~ ~ 1 2 0 ~ . The double-width s l o t t e n d s t o decouple t h e LEX apex and a f t p r i m a r y v o r t i c e s , p a r t i c u l a r l y a t t h e h i g h e r s i d e s l i p a n g l e s i n F i ~ t u r e 28. Also, t h e a f t primary v o r t i c e s a r e q u i t e well-defined and appear -ore c o n c e n t r a t e d r e l a t i v e t o t h e b a s e l i n e . A comparison of b a s e l i n e and LEX 1 2 flows i s orovided i n F i g u r e 28.

The LEX lower s u r f a c e flow p a t t e r n s n e a r t h e forward s l o t l photog ray;^^ not a v a i l a b l e ) r e f l e c t t h e i n c r e a s e d s l o t flow entrainment w i t h LEX 1 2 and, In a d d i t i o n , s u g g e s t a f l o w s i t u a t i o n s i m i l a r t o t h a t developed on a LEX of reduced g e n e r a t i n g l e n g t h . By a p u r e l y q u a l i t a t i v e argument, t h e changes i n t h e leading-edge s u c t i o n c o e f f i c i e n t d i s t r i b u t i o n a r e shown i n F i g u r e 29 f o r a r e p r e s e n t a t i v e LEX p l a n f o r m . I n c r e a s e d f o r w a r d s l o t f l o w e n t r a i n m e n t produces a more marked b r e a k i n t h e s u c t i o n d i s t r i b u t i o n . S u f f i c i e n t widen- i n g of t h e s l o t r e s u l t s i n a s u c t i o n d i s t r i b u t i o n s i m i l a r t o t h e v a r i a t i o n of s e c t i o n s u c t i o n w i t h LEX span e x h i b i t e d by a s m a l l e r LEX. Due t o t h e s m a l l e r " e f f e c t i v e " g e n e r a t i n g l e n g t h , i t i s expected t h a t t h e LEX v o r t e x stability wil.1 be reduced a t a g i v e n a n g l e OF a t t a c k .

A 1 though photographs a r e n o t p r e s e n t e d , t h e p r o g r e s s i o n of LEX v o r t e x breakdown was more r a p i d w i t h a n g l e o r a t t a c k r e l a t i v e t o t h e b a s e l i n e . The Rve t r a c e r s a t a=30° shown i n F i g u r e 30 r e v e a l a more tightly-wrapped LEX a f t ~ r i m a r y v o r t e x and no d i s t i n g u i s h a b l e asymmetry i n b u r s t p o s i t i o n s i n side- s l i n . These r e s u l t s a r e r e p r e s e n t a t i v e of t h e flow f i e l d e f f e c t s a t h i g h e r ell ' s. The decoupling of t h e LEX apex and a f t primary v o r t i c e s i s e v i d e n t i n t h e sideview photographs i n F i g u r e 30.

ORIGINAL PAGE COLOR PHOTOG RAP^

O H ~ G ~ ~ A L PAGE M I SIMVlew w - F - FIGURE 25. FOFIEBODY AND LEX VORTEX FLOWS OY BASELINE FIA-18 AT 0 = 33" h 7

ORIGIFdAC PAGE PRECEDING PAGE D U N K NOT FiLMELb P H ~ T ~ 6 9

ORIGINAL PAGE GmD!? PHOTOGRAPH

FIGURE 18. LEX VQRTEX FLOW CHARACTERISTICS WITH LEX 12 AT a = M C 5 ~ I N G - ~ ~ E : BLANK

QW?%AL PAGE _*;7~ P H ~ T C G P ~ P ~ I B m fl (LEX 72)

m m -

I %

t€% A f t rnMkllV -

$ - lO (BASEUNEl

FIGURE 28. CWCLUDED. P R t E O l N G PnGE RLfiPJK NOT- 7 7 -

PRECECING PAGE BLANK NOT FILMES ORIGINAL PAGE I S

of POOR QuASltr

1 .o 11 = 0 (SMALL LEX)

LEXSPAN STATION 'I -

FIGURE 29. SKETCH OF LEX SLOT EFFECTS ON SECTION SUCTION DISTRIBUTION.

The s l o t m o d i f i c a t i o n a l s o a l t e r s t h e body vortex-LEX v o r t e x i n t e r - a c t i o n s . The forward s l o t f l o w d i s p l a c e s t h e apex v o r t i c e s upward, r e d u c e s he s t a h i l i t ~ r > f t h e LEX v o r t e x s y s t e m , and t h e r e b y r e d u c e - t h e body v o r t e x e n t r a i n m e n t i n t o t h e wing flow. The f l o w v i s u a l i z a t i o n resrilts i n F i g u r e 31 a t Q'=40° i n d i c a t e t h a t t h e bodv v o r t e x asymmetry i n s i d e s l i p i s reduced re1 a t i v e t o t h e h a s e l . i n e r e s u l t s .

Douhle-Width, Extended Length, Forward LEX S l o t (LEX 12A) The e f f e c t s o f douhle-width, i n c r e a s e d l e n g t h forward LEX b l e e d s l o t s (T,EX 12A) a r e s i m j l a r t o LEX 12 b u t c o n s i d e r a h l y more pronounced. F o r ex- ample, a t ~ ' 1 2 5 " LEX v o r t e x breakdown was o b s e r v e d a t a p p r o x i m a t e l y t h e l e a d - edge f l a p h i n g e l i n e , whereas breakdown a t t h e same p o s i t i r ~ n on t h e base- l i n e conf i ~ u r a t i o n d i d n o t o c c u r u n t i l @3O0. The forward s l o t m o d i f i c a t i o n , t h e r e f o r e , Dromotes e a r l i e r breakdown o f t h e LEX v o r t e x . F i g u r e 32 p r e s e n t s r e p r e s e n t a t i v e r e s u l t s a t Q'=40° which r e v e a l t h e d i s r u p t i o n of t h e LEX a p e x v o r t e x . The e f f e c t i v e g e n e r a t i n g l e n g t h of t h e LFX commences, e s s e n t i a l l y , a t t h e ~ r o d u c t i o n h r e a k .

TAEX 12A o r o m o t e s s i g n i f i c a n t c h a n g e s i n t h e f o r e b o d y v o r t e x p a t t e r n s and v o r t e x i n t e r a c t i o n s i n s i d e s l i ~ . The p l a n v i e w and s i d e v i e w r e s u l t s i n F i g u r e 3 3 i n d i c a t e t h a t t h e i n c r e a s e d forward s l o t f l c w a t a = & O O r e s t r i c t s t h e r o t a t i o n o f t h e bodv v o r t e x p a t h s i n s i d e s l i p . The hodv v o r t i c e s a r e high1 y- r e s i s t a n t t o asymmetric o r i e n t a t i o n a t P=+4'. The f o r e b o d y v o r t e x D a t t e r n s a r e remarkablv s i m i l a r t o t h e P = O 0 c a s e and s t r o n g e v i d e n c e of bodv v o r t e x - induced ef f e c t c o n t h e rdindward wing o c c u r s ( n o photograph a v a i l a b l e ) . The leeward and wfn14ward body v o r t e x p o s i t i o n s and r o t a t i o n a l s e n s e a r e s u c h a s t o p r o v i d e s t r o n g spanwise f l o w toward t h e windward s i d e , a s s k e t c h e d i n , . g u r e 34. The c o n l e c t u r e d f l o w nechanism is s u c h t h a t t h e two v o r t i c c ; a c t t o g e t h e r t o i n d u c e s t r o n g s i d e w a s h o n t h e w i n d w a r d w i n g . T h i s e f f e c t d i m i n t s h e s a t h i g h e r (y v a l u e s , s i n c e t h e asymmetric body v o r t e x o r i e n t a t i o n , shown i n F i g u r e 33, becomes more pronounced.

The primary q u a l i t a t i v e e f f e c t s a s s o c i a t e d w i t h LEX 12A a r e : ( I ) a d e c o u p l i n g of t h e LEX d u a l v o r t e x s y s t e m , ( 2 ) r e d u c t i o n o f v o r t e x s t a b i l i t y a t a g i v e n a n g l e of a t t a c k , ( 3 ) d e c r e a s e i n v o r t e x b u r s t asymmetrv due t o s i d e -

s l i p , and ( 4 ) r e s i s t a n c e of body primary v o r t i c e s t o asymmetric o r i e n t a t i o n s i n s i d e s 1 1 p. These r e s u l t s a l s o p r o v i d e p r e l iminarv i n d i c a t i o n of t h e poten- t i a l l v - s t r o n a c o u p l i n g of t h e F/A-18 forebody and LEX v o r t e x flows a t h i g h a's.

I,EX Lower S u r f a c e Fence-Oblique (Fence "B") The e f f e c t s of a f e n c e i n s t a l l & on t h e lower s u r f a c e of each LEX. n e a r t h e apex and o b l i q u e t o t h e free-stream (Fence "B"), a r e v e r y s i m i l a r t o t h e v o r t e x flow f i e l d changes a s s o c i a t e d w i t h LEX 12 and LEX 12A. T h i s s i m i l a r i ~ y i s n o t f o r t u i t o u s , s i n c e t h e corresponding flow mechanisms observed i n t h e water t u n n e l r e v e a l marked s i m i l a r i t i e s , a s d i s c u s s e d below.

R e l a t i v e t o t h e b a s e l i n e , t h e p r o g r e s s i o n of LEX v o r t e x b u r s t p o s i t i o n is s l i ~ h t l v more r a p i d w i t h LEX f e n c e s on, due t o t h e reduced " e f f e c t i v e " LEX g e n ~ r a t i n ~ l e n g t h . A t CY=?5O and 3 0 ° , f o r e x a m p l e , v o r t e x breakdown was ohserved a t approximately x / C = 0.7 and 1.0, r e s p e c t i v e l v , a s opposed t o R b a s e l i n e r e s u l t s of X / C R = 0.5 and O.S, r e s p e c t i v e l v .

The photographs i n F i g u r e 35 r e v e a l similar s t a l l p a t t e r n s on t h e wind- ward and l e e w a r d w i n g s a t (Y=25". T h i s c a n b e a t t r i b u t e d t o t h e n e a r l y - s:rmmetr!c LEY v o r t e x b u r s t p o s i t i o n s i ~ . - i d e s l i p due t o t h e lower s u r f a c e f e n c e s . Note a l s o i n F i g u r e 35 t h e h e t t e r - d e l i n e a t e d a f t primary v o r t i c e s .

A t h i g h e r a n g l e s of a t t a c k (CY=30°- 40°1 t h e LEX apex v o r t e x p e r s i s t s b u t , i n c e n e r a l , i n t e r a c t i o n w i t h t h e a f t p r i m a r y v o r t e x i s m i n i m a l . T h i s i s i l l u s t r a t e d i n F i g u r e 36 a t a= 3C0. The f e n c e s , i n a manner s i m i l a r t o t h e e f f e c t s Rue t o LEX 12 and LEX 12A, promot* a symmetry i n LEX breakdown posi- t i o n s a t h:gh a n g l e s of a t t a c k , s i m i l a r t o t h e e f f e c t s shown a t ~ 5 2 5 " i n F i g u r e 35.

The f l o w f i e l d s a s s o c i a t e d w i t h F e n c e " B " , LEX 1 2 , and LEX 12A a r e d i s s i m i l a r i n c e r t a i n r e s p e c t s . The LEX apex v o r t e x b e h a v i o r w i t h Fence "B" and LW 1 % a t h i e h a n g l e s of a t t a c k r e v e a l s a more c o p r e n t r a t e d apex v o r t e x w i t h t h e former. T h i s can be a t t r i b u t e d t o t h e d i s p r o p o r t i o n a t e amount of forward s l o t flow w i t h LEX 12A which d i s r u p t s t h e apex v o r t e x flow.

ORIGINAL PAGE aKOR PHOfOGHAPH FIGURE 31. FOREBODY VORTEX PATTERNS IN SIDESLI? WITH LEX 12 A T n = 4 0 -

-w& PAGE BlS$W NOT FILMED

. - 0 1

m-

e w -

~ ~ ~ w h t PAGE

PRECEDm PAGE DLANK NOT F I L M E D

COLOR ~ w o T o G ~ ~ ~ ~ 9 3 a

VORTEX

+ ? \ WINDWARD WING

FIGURE 34. SKETCH OF FOREBODY VORTEX EFFECTS ON WINDWARD WING.

ORlGlNAt PAGE COLOR PHOTOGRAPH FIGURE 3E LEX VORTEX BtHAVlOR IN SIDESLIP WITH LEX FENCE "B"; cy = 25" 9 7

PRECEDING PAGE BLANK NOT F!LKX Fence "R" s l i ~ h t l y a l t e r s t h e body vortex-LEX v o r t e x i n t e r a c t i v e b e h a v i o r I r e l a t i v e t o t h e b a s e l i n e v o r t a x behavior. The r o t a t i o n of t h e body v o r t e x !

p a t h s a t small s i d e s l i p a n g l e s i s r e s t r i c t e d , as d e p i c t e d i n F i g u r e 37 a t 0 1 3 5 ' . The e f f e c t s a r e n o t a s pronounced, however, a s t h e r e s u l t s shown i n F i g u r e 33 w i t h LEX 13.

LEX Lor r S u r f a c e Fence - Streamwtse (Fence "A")

- - Flow f i e l d o b s e r v a t i o n s w i t h LEX lower s u r f a c e f e n c e s o r i e n t e d i n a streamwise manner (Fence " A " ) r e v e a l t h e s e n s i t i v i t y of LEX f e n c e e f f e c t i v e - n e s s t o r e l a t i v e l y small p o s i t i o n changes. T y p i c a l r e s u l t s from t h e water tunnel s t u d i e s a r e shown i n F i g u r e 38 a t @ = 3 O 0 . The streamwise LEX f e n c e reduces t h e coupling of t h e LEX v o r t e x system, i n a manner s i ~ i l a r t o LEX Fence "B". However, marked d i f f e r e n c e s do e x i s t between t h e f e n c e arrange- n e n t s . Fence "A", t h e p o i n t of o r i g i n of t h e a f t primary v o r t e x is d i f f i c u l t t o a s c e r t a i n i? c o n t r a s t t o t h e f i x e d o r i g i n w i t h F e n c e " B " .

P a r t of t h e l o v e r s u r f a c e flow which s e p a r a t e s a t t h e l e a d i n g edge f e e d s i n t o t h e LEX apex v o r t e x w h i l e t h e remainder f e e d s I n t o t h e a f t v o r t e x . Close examination of dye t r a c e r s i n t h e water t u n n e l r e v e a l t h i s q u i t e c l e a r l y .

F o r e x a m p l e , d v e e m i t t e d o n t h e LEX l o w e r s u r f a c e n e a r t h e l e a d i n g e d g e s p l i t s a t t h e 1t j i n g e d ~ e , t h e r e b y d e l i n e a t i n g t h e flow which goes i n t o each It was e v i d e n t d u r i n g t h e tests t h a t LEX Fence "A" was l e s s e f f e c t i v e vortex.

i n i s o l a t i n ~ t h e LEX dual-vortex system. I n g e n e r a l , t h e a f t primary v o r t e x was .lore c o n c e n t r a t e d r e l a t i v e t o t h e b a s e l i n e and v o r t e x b u r s t p r o g r e s s i o n was s l i z h t l p more r a p i d ( s i m i l a r t o Fence "B" e f f e c t s ) . However, t h e wing flow f l e l d i n s i d e s l i p n e a r s t a l l a n g l e of a t t a c k suggested t h a t Fence "A" was 1..j e f f e c t i v e than Fence "3" i n promoting more symmetric s t a l l p a t t e r n s .

The flow mechanisms a s s o c i a t e d w i t h LEX 12, LEX 12A, Fence "B" and Fence " A " a r e s i m i l a r .

A l l m o d i f i c a t i o n s l i m i t t h e amount of v o r t i c i t y shed a t t h e LEX l e a d i n g edge i n t h e r e g i o n of t h e p r o d u c t i o n b r e a ~ . LEX 1 2 and LEX 1214 promote a " f l u i d fence" phenomenon, d i v e r t i n g LEX lowc- s u r f a c e boundary l a y e r ' f l u i d away from t h e l e a d i ? g e8eo s manner s i m i l a r t o a s o l i d f e n c e .

C l o s u r e of Forward LEX Bleed S l o t s Closure of t h e forward LEY, bleed s l o t s r e s u l t s i n t h e development of a s i n g l e c o n c e n t r a t e d leading-edge v o r t e x o n each LEX s u r f a c e , a s shown i n F i ~ u r e 34 a t ~ = 2 5 ~ . S l o t c l o s u r e r e s u l t s i n an i n c r e a s e i n lower s u r f a c e f l o w a v a f l a h l e f o r f e e d i n g i n t o t h e leading-edge v o r t e x . The flow f i e l d observa- t i o n s f n d f c a t e , t h e n , t h a t t h e g r a d u a l planform i n f l e c t i o n p o i n t on t h e F/A-18 LEX i s i n s i ~ f f i c i e n t t o promote a dual-vortex system. Also shown i n F i g u r e 39 is an accumulation of dye on t h e lower s u r f a c e of t h e LEX. The dye t r a c e r s a r e e n t r a i n e d i n t o a v o r t e x formed a t t h e j u n c t i o n o f t h e f u s e l a g e and LEX.

A s f a r a s can h e determined i n t h e w a t e r t u n n e l , t h e s i n g l e - v o r t e x system e x h i h i t s s l i g h t l y g r e a t e r v o r t e x b u r s t asymmetry i n s i d e s l i p r e l a t i v e t o t h e h a s e l i n e . R e c a l l , however, t h a t i t was d i f f i c u l t t o d e t e r m i n e p r e c i s e l y t h e h u r s t p o s i t i o n on t h e b a s e l i n e c o n f i g u r a t i o n due t o t h e d i f f u s e n a t u r e of t h e dual-vortex system.

The manner i n which t h e body v o r t i c e s i n t e r a c t w i t h the LEX v o r t e x f l o w s i n s i d e s l i ~ i s s i n i i l a r t o t h e b a s e l i n e r e s u l t s . The body v o r t e x :ore t r a j e c - t o r i e s dev'cted i n F i ~ u r e 40 a t cu=35O e x h i b i t s i m i l a r c h a r a c t e r i s t i c s t o t h e slots-open c a s e , a l though t h e leeward body v o r t e x appeared somewhat b e t t e r - d e f i n e d a t small p-values a s i t e n t e r s t h e wing f l o w f i e l d . T h i s i s c o n s i s t e n t w i t h t h e apparent i n c r e a s e i n leeward LEX v o r t e x s t a b i l i t y due t o forward s l ~ t closllre. In a d d i t i o n , t h e p o i n t of e n t r a i n m e n t o f t h e leeward body v o r t e x occurred somewhat f a r t h e r downstream w i t h s l o t s c l o s e d .

C l o s u r e of A l l LEX Bleed S l o t s C l o s ~ r e of a l l LEX boundary l a y e r b l e e d s l o t s r e s u l t s i n no s i g n i f i c a n t changes i n che high-cu v o r t e x f l o w f i e l d r e l a t i v e t o c l o s u r e of t h e forward s l o t s only. T h i s a p p e a r s r e a s o n a b l e s i n c e t h e forward s l o t , by v i r t u e of i t s l a r g e width-to-local LEX span r a t i o , i s i n a p o s i t i o n t o most a f f e c t t h e LEX v o r t e x behavior. T y p i c a l r e s u l t s w i t h a l l s l o t s c l o s e d a r e p r e s e n t e d i n F i g u r e 41 a t cu=30°. Fxamination of t h e L R X lower s u r f a c e p a t t e r q s (photographs n o t a v a i l a b l e ) n e a r t h e a f t s l o t s r e v e a l o n l y s m a l l v a r i a t i o n s

-

when t h e s l ~ t s a r e c l o s e d . Furthermore, a q u a l i t a t i v e assessment of t h e weak

FIGURE 39. SINGLE LEX VORf EX SYSTEM AND LOWER SURFACE FLOW SEHAVIOR WITH FORWARD SLOTS CLOSED; a = 25"

R E 40. FOREBODY VORTEX FLCW BEHAVIOR IN SIDESLIP WITH FORWARD SLOTS CLOSED; n = 35" m F e m m M P m m m ~ W 3 1 n Q

FIGURE 41. F/A-I8 VORf EX FLOW FIELD BEHAVIOR IN S l Q t S l l P WITH ALL SLOTS CLOSED AT a - 30"

e

PRECEDING PAGE BLANK NOT FILMED

c o n t r a - r o t a t i n g a f t lot v o r t i c e s which f o r n on t h e LEX upper s u r f a c e i n d i - c a t e s t h a t t h e s l o t f l o v h a s l i t t l e e f f e c t on LEX v o r t e x behavior and, a l s o , hody vortex-LEX v o r t e x i n t e r a c t i o n s . A q u a l i t a t i v e p e r s p e c t i v e of t h e r e l a - t i v e e f f e c t s of t h e forward s l o t flow a s opposed t o t h e a f t s l o t flows i s a s f a l l o w s . The " s l z e " of t h e forward s l o t f l o w and LEX v o r t e x n e a r t h e apex a r e of t h e same o r d e r of m a ~ n i t u d e . Consequently, changes t o t h e forward s l o t Reometrv a r e l i k e 1 y t o prcmote l a r g e changes i n LEX v o r i e x behavior. F a r t h e r downstream, however, t h e L5X v o r t e x s t r e n g t h and s i z e have i n c r e a s e d t o t h e e x t e n t t h a t t h e leading-edge v o r t e x dominates t h e a f t s l o t v o r t i c e s . There- f o r e , flow p e r t u r b a t i o n s emanating from t h e a f t s l o t s a r e l e s s l i k e l y t o have s i g n i f i c a n t g l o b a l e f f e c t s .

Forebody S t r a k e s ( R a d i a l P o s i t i o n : +40° ) Water t u n n e l r e s u l t s d i s c u s s e d t h u s f a r have r e v e a l e d s i g n i f i c a n t f l o w f i e l d changes a r i s i n g from modif i c a t i o n s t o , p r i m a r i l y , t h e LEX apex r e g i o n .

Du? t o t h e s t r o n g c o u p l i n g o f t h e F/A-18 f o r e b o d y and LEX f l o w f i e l d s , however, i t i s e v i d e n t t h a t forebody geometry v a r i a t i o n s a r e a p o t e n t i a l source of l a r g e g l o b a l flow f i e l d p e r t t i r b a t i o n s . The f o l l o w i ng d i s c u s s i ons w t l l a d d r e s s t h e e f f e c t s of forebody s t r a k e s on t h e high angle-of-attack v o r t e x flow behavior. The r e s u l t s o b t a i n e d i n t h e s e flow v i s u a l i z a t i o n t e s t s w f l l he shown i n l a t e r s e c t i o n s t o f i g u r e prominently i n t h e major c o n c l u s i o n s of t h i s study.

N o s i g n i f i c a n t flow c h a q e s a r e i n c u r r e d w i t h forehody s t r a k e s mounted a t 40' a b o v e t h e maximum h a l f b r e a d t h (@- +40° ) up t o a n g l e s o f a t t a c k o f approximatelv 25' (photographs n o t shown). The forebodv primary v o r t f c e s a r e weak w i t h i n t h i s a - r a n q e and, c o n s e q u e n t l y , nose s t r a k e s a r e n o t expected t o have any g l o b a l Impact. The flow photographs i n F i g u r e 42 i n d i c a t e t h a t a t CY= 25' t h e windward wing is s t a l l e d whereas t h e leeward wing e x h i b i t s s t r o n g v o r t e x a c t i o n . T h i s t r e n d is s i m i l a r t o t h e b a s e l i n e flow f i e l d r e s u l t s .

P r o g r e s s i o n of LEX v o r t e x breakdown p o s i t i o n w i t h a n g l e of a t t a c k a t @ - 0' is e s s e n t i a l l y i d e n t i c a l t o t h a t observed on t h e b a s e l i n e configura- t i o n . For example, a t CY * 2!i0 and 30°, v o r t e x b u r s t o c c u r r e d a t approximately TICR = 0 . 5 and 0.8, r e s p e c t i v e l y .

The water t u n n e l flow s u r v e y s i n d i c a t e d t h a t t h e s t r a k e s c a u s e an out- board and downward displacement of t h e windward body v o r t e x and a correspond- i n g inboard and upward displ.acement of t h e leeward body v o r t e x . T h i s a l t e r a - t i o n of t h e bodv v o r t e x p a t h s i n s i d e s l i p is of g r e a t import t o t h e wing s t a l l p a t t e r n s , a s w i l l now b e d i s c u s s e d .

The most d r a m a t i c s t r a k e e f f e c t s w e r e o b s e r v e d a t Q = 35" and 40'.

I J t i l i z i n g t h e nbmerous i n t e r n a l d y e p o r t s , s e v e r a l r e g i o n s o n t h e model were progressive1 v i n v e s t i g a t e d . For example, t h e wing upper s u r f a c e flow p a t t e r n s i n d i c a t e t h a t t h e leeward wing is completely s t a l l e d whereas v o r t e x a c t i o n i s p r e s e n t on t b : windward wing panel. T h i s is i l l u s t r a t e d i n t h e w a t e r t u n n e l photographs i n F i g u r e 4 3 a t Q = 40".

The flow v i s u a l f z a t i o n photographs (planviews) i n F i g u r e 44 i n d i c a t e t h a t t h e leeward LEX v o r t e x b r e a k s down a t a more forward p o s i t i o n on t h e LEX r e l a t i v e t o t h e wi3dward LEX v o r t e x . A v e r y i n t e r e s t i n g f e a t u r e o f t h e leeward flow f i e l d i n F i g u r e 44 i s t h a t dye o r i g i n a l l y e n t r a i n e d i n t o t h e leeward LEX v o r t e x i s swept o v e r t h e t o p of t h e f u s e l a g e and onto t h e windward wing.

The l a t t e r phenomenon s t r o n g l y s u g g e s t e d t h e p r e s e n c e of a powerful flow mechanism emanating from t h e forebody. Observation of t h e forebody v o r t i c e s , d e p i c t e d i n F i g u r e s 45 and 46 a t Q = 35' and 40°, r e s p e c t i v e l y , shows t h a t t h e s t r a k e s promote a more s y m n e t r i c shedding of t h e body v o r t i c e s i n s i d e s l i p and c r e a t e a v o r t e x p a t t e r n f e a t u r i n g s t r o n g c o u p l i n g of t h e windward and leeward f o r e h o d y v o r t e x f l o w s . A t (Y= 35' and 40' and P = 4', f o r e x a m p l e ( s e e F i g u r e s 45 and 4 6 ) , t h e forehody v o r t i c e s i n t h e s i d e v i e w s remain symmetric.

I n t h e planviews i n Q i q u r e s 45 and 46, however, t h e windward and leeward body v o r t i c e s a r e a c t u a l l v b i a s e d towards t h e windward s i d e . The r e s u l t of t h i s c o u p l i n g is s t r o n g vortex-induced s i d e v a s h o n t h e windward wing, a s sketched i n F i g u r e 47, and consr?quently improved windward LEX v o r t e x b e h a v i o r and r e d u c t i o n i n wing flow s e p a r a t i o n . The body vortex-induced sidewash a c c o u n t s f o r t h e flow phenomenon d i s c u s s e d i n F i g u r e 44. Note i s made t h a t s t r a k e e f f e c t i v e n e s s d e c r e a s e s a t h i g h e r s i d e s l i p a n g l e s s i n c e a n asymmetric body v o r t e x p a t t e r n is once a g a i n e v i d e n t ( s e e F i g u r e s 45 and 46).

ORIGINAL PAGE COLOR PW3taGRAPH

nRIG1NAi PAGE PPHT-

PRECEQIP(C PAGE m m rmnedr

OAElNAL PAGE .COLOR P H O -

ORIGINAL PAGE I S OF POOR QUALTTY LEEWARD FOR EBODY vORTFX

I i

i

i

FIGURE 47. SKETCH OF WINDWARD AND LEEWARD FOREBODY VORTEX COUPLING.

The present resrllts i n d i c a t e t h a t nose s t r a k e s a t 9 = +40° promote a

hodv vortex system which a c t s primarily t o enhance t h e windward wing f l w .

The q u a l i t a t i v e s t u d l e s i n t h e water tunnel i n d i c a t e t h a t under c e r t a i n ' ' o w conditions t h e forebody v o r t i c e s can b e of g r e a t s i g n i f i c a n c e i n influencing the w t ~ g s t a l l c h a r a c t e r i s t i c s a t high a n g l e s of a t t a c k . Depending on t h e o r i e n t a t i o n of t h e body v o r t i c e s r e l a t i v e t o t h e LEX and wing s u r f a c e s , wing flow separation (without reattachment) o r vortex-induced attached flow may n r e v a i l .

L o r s p e e d smoke f l w v i s u a l i z a t i o n s t u d i e s made i n t h e NASA Langley Research Center 30 x 60-foot wind tunnel of t h e 0.16-scale F/A-18 with s t r a k e s a t 9- +40° have yielded r e s u l t s i n e x c e l l e n t q u a l i t a t i v e agreement with t h e water tunnel ohservatfons. For example, observation of t h e smoke flow pat- t e r n s from a p o s i t i o n behind t h e 0.16-scale model revealed t h e strong coupling of t h e Forebodv primary v o r t i c e s and t h e tendency of t h e leeward body vortex t o pass 3elow t h e windward vortex, The agreement between flow v i s u a l i z a t i o n r e s u l t s i n water a t low Reynolds number and i n a i r a t higher Re:;nolds number is due t o t h e lw l o c a l Reynolds number along t h e r a d m e . The l o c a l cross- s e c t t o n is s u f f i c i e n t l y s m ~ ,l i n t h i s region t h a t t h e l o c a l Reynolds numbers remain within t h e laminar range. Consequently, t h e boundary l a y e r separation c h a r a c t e r i s t i c s w i l l be s i m i l a r on t h e r e s p e c t i v e test models. Furthermore, t h e nose s t r a k e p o s i t i o n i s n e a r l y coincident with t h e primar-. separatiorr l i n e and t h e s t r a k e vortex f l w s , which a r e reasonably i n s e n s i t i v e t o changes i n Reynolds number, feed d i r e c t l y i n t o t h e body primary v o r t i c e s . Therefore, t h e f l o w f i e l d e f f e c t s a s s o c i a t e d w i t h he n o s e s t r a k e s a t 4 - +40° a r e n o t s t r o n g l y lependent on Reynolds number, w i t h i n t h e range of test c o n d i t i o n s considered.

Porehody S t r a k e s (Radial Position: 0°) Alternate s t r n k e r b d l a l p o s i t i o n s were investigated t o a s s e s s t h e rela- t i v e e f f e c t s on body vortex development, vortex t r a j e c t o r i e s , and i n t e r - a c t ions.

A t a= 3S0 and 0- O0 t h e body flow f i e l d i n t h e presence of pose s t r a k e s a t the WlB (9 = 0°) is e s s e n t i a l l y an ill-organized wake-like t l t ~ w , a s shown

i n F i g u r e 48. Due t o t h e h i g h l o c a l a n g l e s o f a t t a c k , t h e s t r a k e s a t + = 0 " g e n e r a t e more of a wake-like, r a t h e r t h a n v o r t e x , flow. T h i s i s i n c o n t r a s t t o t h e += +40° s t r a k e p o s i t i o n where d i s c r e t e v o r t i c e s were observed even a t v e r y h i g h a n g l e s of a t t a c k . The l a t t e r phenomenon c a n be a t t r i b u t e d i n l a r g e Dart t o t h e body vortex-induced domwash which promotes lower l o c a l a n g l e s of a t t a c k i n t h i s r e g i o n .

0 0 I n g e n e r a l , a t CY= 35 and 4 0 t h e r e is no q u a l i t a t i v e evidence of favor- a b l e bodv vortex-induced e f f e c t s on t h e wing s t a l l behavior w i t h s t r a k e s a t + = O O .

Foreboa: S t r a k e s ( R a d i a l P o s i t i o n : -45O) Typical k e s u l t s o h t a i n e d w i t h nose s t r a k e s mounted a t 45O below t h e MHB ( 4 = -45') a r e shown i n F i q u r e 44 a t a = 35O. The s t r a k e v o r t i c e s a r e of c t ~ f f i c i e n t s t r e n g t h t o induce f l o w r e a t t a c h m e n t above t h e s t r a k e s . A s : r e s u l t , a p a i r of h d y primary v o r t i c e s form a l o n g t h e nose. These v o r t i c e s a r e r e a s o n s b l y symmetric a t s m a l l s i d e s 1 i p and, c o n s e q u e n t l y , c r e a t e a f l o w s i r . ; a t i o n analogous t o t h e Q = +40° c a s e . The e f f e c t s a r e l e s s pronounced, however, due t o t h e weakeqed body v o r t e x system. There is a s l i g h t b i a s a t s m a l l p - v a l n e s o f t h e b o d y v o r t e x s y s t e m t o w a r d s t h e windward s i d e which r e s u l t s i n downstream flow e f f e c t s s i n i i l a r t o t h e fl = +40° r e s u l t s .

k e t o an a p p a r e n t t r a n s i t i o n oc t h e leeward s t r a k e from a v o r t e x - g e n e r a t o r t o a wake-generator a t h i g h e r s i d e s l i p a n g l e s , a s sketched i n F i g u r e 50, t h e e f f e c t i v e n e r s of t h e nose s t r a k e s d i m i n i s h e s .

F l i n h t T e s t Nose Boom H e p r e s e n t a t l v e r e s u l t s from w a t e r t u n n e l s t u d i e s of f l i g h t t e s t nose boom e f f e c t s a r e p r e s e n t e d i n F i g u r e 51 a t 0 - 35O. The wake shed by t h e c o n s t a n t - diamc:er nose boom d i s r r ~ p t s t h e body v o r t i c e s t o a c e r t a i n e x t e n t . AJ.though t h e body v o r t i c e s a r e weaker r e l a t i v e t o t h e b a s e l i n e , t h e vorte:- p a t h s a r e v e r y s i m i l a r a s d e p i c t e d i n F i g u r e 51. A t h i g h e r a ' s (approximately 40°; t h e f l o w s h e d hv t h e n o s e boom and f o r e b o d y c o m b i n a t i o n e r - . i b i t s a s l i g h t l y o s c t l l a t o r y behavior.

Wing Lesding-Edge Snag and Upper S u r f a c e Pence A 17-percent l o c a l wing chord e x t e n s i o n , o r snag, p o s i t i o n e d a t q-0.5 i n c r e a s e d t h e wing camber near t h e l e a d i n g edge and, a s a r e s u l t , delayed f l w s e p a r a t t o n . I n combination w i t h a n upper s u r f a c e wing f e n c e (q=0.5), t h e most s i p n i f t c a n t e f f e c t s observed d u r i n g t h e flow s t u d i e s were: (1) t h e upper s u r f a c e f e n c e on t h e l e e w a r d wing p r e c l u d e d LEX v o r t e x - i n d u c e d s w e e p i n g a c t i o n a t wing s t a t i o n s outboard of t h e f e n c e (a weak wing v o r t e x developed outboard of t h e f e n c e , emanating from t h e leading-edge f l a p h i n g e l i n e ) , ( 2 ) an inhoard displacement of t h e leeward LEX v o r t e x p a t h o c c u r r e d , and ( 3 ) t h e leeward LEX v o r t e x s t a b i l i t y appeared reduced. R e p r e s e n t a t i v e r e s u l t s a r e presented i n F i g u r e 52 a c @= 30" which r e v e a l comparable windward and leeward wine flow f i e l d s .

Wing Leading-Edge E x t e n s i o n s (LEXs) O f f The 1,EXs were removed i n an e f f o r t t o enhance t h e u n d e r s t a n d i n g of t h e q u a l i t a t i v e e f f e c t s of t h e LEX s u r f a c e s on t h e F/A-18 f l w f i e l d .

A t a = 15"-20" both wing p a n e l s a r e s t a l l e d i n s i d e s l i p (no photographs a v a i l a h l e ) . m e flow f i e l d w i t h LEXs o f f is n o t vortex-dominated, hznce t h e water t u n n e l r e s u l t s a r e h i g h l y - q u a l i t a t i v e ( t h e massive wing flow s e p a r a t i o n i s u n r e a l i s t i c ) . The r e s u l t s w i t h LEXs o f f do confirm, however, t h e s i g n i f i - c a n t e f f e c t s a s s o c i a t e d w i t h t h e leading-edge e x t e n s i o n s , p a r t i c u l a r l y s i d e s l i p . For example, a t t h e same a n g l e s of a t t a c k w i t h LEXs on, a l a r g e d i s p a r i t y i n t h e wing s t a l l p a t t e r n s i n s i d e s l i p e x i s t s due t o asymmetries i n LEX v o r t e x breakdown p o s t t i o n , v o r t e x s t r e n g t h , and v o r t e x c o r e path.

R e p r e s e n t a t i v e r e s u l t s a t an a n g l e of a t t a c k of 40" a r e shown i n F i g u r e 53 where body v o r t e x asymmetry a t p- 0" i s apparent. A m u l t i p l e asymmetric.

v o r t e x system i s shed along t h e body l e n g t h . I n a n i n d i r e c t manner, t h e photographs i n F i g u r e 53 provide a n appreciatLon f o r t h e s i g n i f i c a n t i n f l u e n c e of t h e LEXs on t h e forebody v o r t e x flow behavior.

ORlGlNhi COLOR PHOTOCRAW FIGURE rC8. FIA-18 FOREBODY VORTEX FLOW EEHAWIOR WlTH NOSE STRAKES I& Q o ) AT a 35"

PRECEDING PAGE D U N K POT FILMED ORIGINAL PAGE COLOR PHOTOGRAPH LARGE SIDESLIP FIGURE Sr. SKETCH OF FOREBODY STRAKF FLOW AT HIGH ANGLE OF ATfACK.

PRECEDING W G E B U N K Wr S i i h t ' r 3 1 3 9

\ U W R SURFACE

FEW= ]

FIGU9E 52. FIA-18 LEX VORf EX FLOW BEHAVtCR WITH LEADING EDLC SNAG AND WING UPPER SURFACE FENCE; 0 = 3 0 '

ORlG l N 4L PAGE COLOR PWOTOGP4PH F!GURE 63, FlA-58 BQDY VPRTEX BEHAVIOR WITH ,EXe REMOVED; a = 40" ~ r E l x p l r l G Wtr;'; Dl.&% 1 4 3

R e i n s t a l l a ~ i o n of Wing Leading-Edge E x t e n s j o n s -

Repeat r u n s were made of t h e b a s e l i n e F/A-18 model (LEXs r e i n s t a l l e d ) t o a s s e s s any changes i n t h e v a r t e x flow f i e l d a t high a n g l e s of a t t a c k .

The forebody-LEX v o r t e x f l o w f i e l d s a t a = 4 0 ° shown i n F i g u r e 54 e x h i b i t d i f f e r e n c e s r e l a t i v e t o t h e I n i t i a l b a s e l i n e runs. The primary change is I n t h e p a t h of t h e windward body v o r t e x : i n s t e a d of s h e a r i n ? away from t h e F I s e l a g e forc:)ody when t h e model i s s i d e s ~ ' p p ~ . d , t h e v o r t e x r e m a i n s i n p r o x i m i t y t o t h e f u s e l a g e . A s a consequence, t h e windward r i n g p a n e l exhib- i t s a s u r f a c e flow p a t t e r n ind?.cative of t h e p r e s e n c e of a v o r t i c a l motion.

T h i s e f f e c t i s of p a r t i c u l a r s i g n i f i c a n c e a t 35' - 40' where t h e windward wing flow s e p a r a t i o n i s l o s s e x t e n s i v e when compared t o t h e leeward p a n e l .

The forebody v o r t e x p a t t e r n s p r i o r t o removal and r e i n s t a l l a t i o n of t h e I,l?Xs were v e r y r e p e a t a b l e . The l a t t e r two model changes appeared t o p r o d u c e s m a l l model g e o m e t r y v a r i a t i o n s n e a r t h e LEX a p e x and a l o n g t h e f u s e l a g e forebody. Although t h e g e o a e t r y changes were not s i g n , f i c a n t i n terms of a b s o l u t e dimensions, a l a r g e response was, nevertheless, t r i g g e r e d i n t h e v o r t e x f l o w i n t e r a c t i o n s .

Tie d i f f e r e n c e s ohserved i n t h e f s r e b o i y v o r t e x p a t t e r n s i n t h e two b a s e l i n e r u n s w i l l be u ; ? l i z e d l a t e r i n t h i s r e p o r t t o p r o v i d e d p l a u s i b l e e x p l a n a t i o n f o r t h e a p p a r e n t F/A-7.8 "model-scale" e f f e c t .

These r e s u l t s s e r v e t o p o i n t o u t , i n $ a r t i c u l a r , t h e s i g n i f i c a n c e of the VIA-1R wlndward forebody v o r t e x and i t s proximity t o t h e f u s e l a g e and wing w t r f a c e s . Although t h e r e l a t i v e change i n t n e windward body v o r t e x p s t h between t h e r e s p e c t i v e b a s e l i n e s was n o t l a r g e , t h e t r a j e c t o r y change i s s u f f i c i e n t t o t r i g g e r a l a r g e r e s p o n s e i n t h e o v e r a i l flow f i e l d . That is, t 4 e windward body v o r t e x was t r a n s f o r m e d f r o m a d e c o u p l e d v o r t i c a l f l o w !"Freew v o r t e x ) t o a motion t h a t is coupled t o t h e F/A-18 m u l t i p l e v o r t e x system. T h i s flow s e n s i t i v i t y i s analogpus t o t h e s e n s i t i v i t y of I v o r t e x t o minor p e r t u r b a t i o n s o u t s i d e t h e c o r e . Such p e r t u r b a t i o n s a r e g r e a t i y magni- f i e d w i t h i n t h e c o r e because of t h e s t l o n g c o u p l i n g of swirl and a x i a l velo- c i t y components.

PRECEDlNG PAGE BLANK NO? FILMED

CORRELATION OF WATER TUNNEL FLOW VISUALIZATION RESULTS WITH 0.16-SCALE F/A-18 WlNn TI.!L CATA TRENDS

Baseline Configurations ( &/ bf = 250/0°; d t , = -12O)

d , , / t& = 35"/0°; Two baseline F/A-18 configurations will be discussed featuring leading- edge flap deflection angles of 25" and 35". As a result, discussions of haseline '_a trends will also include effects of flap deflection angle.

Estimates of the angle of attack for LEX vortex breakdown at the wing

trailing edge ( a!BD-TE ) are 16" and 20°, respectively, for d,,= 2 5 " and

35". These estimates , r e made from the initial local lift-curve slope reduc- tions in Figure 55. In addition, estimates of angle of attack for vortex

Ir~rsting near the LEX-wing junction ( 'YBbLWJ ) are 36" and 40" for a= 25"

and 35O, respectively. Rased on Northrop water tunnel correlations of thin, flat-plate, sharn-edged LEX-wing planforms with low-speed wind tunnel results (Reference 5 ) , the angie of attack at which C is attained appears a k..

good approximation to @BD-LWJ The current test model, however, departs from the thin, uncamberec!

lifting surface assumed above. Water tunnel-determined values of a !

BD-TE

and a ! with a= 35" were, approximately, 1.5" and 35". The results

RD-LWJ in the hydrodynamic facility, then, reveal vortex instability at a given wing chord positton at approximately 5 degrees lower angle of attack relative to the wind tunnel estifiates due to viscous effects.

A review of NASA vide~tapes of smoke flow visualization studies of the 0.16-scale F/A-18 model confirm the similarity of LEX vortex behavior in water and air but, also, reveal more stable vortices in the wind tunnel. These trends are consistent with previous comparisons of water-to-air results obtained or! a LEXring planform with deflected leading- and trailing-edge flaps. The water tunnel coneiotently underpredicted the flap effects on LEX vortex ~tability, although the treqds were the same (Reference 6 ) .

ORIGINAL PAGE Is

3F POOR QlJAUFl

- I r

I

I 4, DEG

--- 36

w

---4-

+

--

- 1 h + + 9 FIGURE 55. EFFECT OF INCREASED LEIADING-EDGE FLAP DEFLECTION ON Ll FT CHARACTERISTICS OF THE BASELINE 0.16-SCALE FIA-18; ah = -12O; R%= 1.1 (191.

. ..

t

s

I t is e v t d e n t from t h e s e r e s u l t s t h a t d e f l e c t i n g t h e leading-edge f l a p s t o '35' .increases t h e l i f t a t h i g h e r a n g l e s of a t t a c k r e l a t i v e t o t h e 4- 25' c a s e . T h i s i s due t o delaved LEX v o r t e x breakdown and improved wing flow s e p a r a t t o n r h a r ~ r t e r i s t t c s . The r e d u c t i o n i n p o s i t i v e p r e s s u r e g r a d i e n t n e a r t h e wtnn l e a d i n g edqe e n a h l e s t h e LEX v o r t e x t o p e n e r r a t e somewhat f a r t h e r I n t o t h e wing flow f i e l d . Concurrent w i t h i n c r e a s e d f l a p a n g l e , however, i s a reductiorl t n v o r t e x s t r e n g t h a t a Riven a n g l e of a t t a c k .

F i q u r e 56 p r e s e n t s v a r i a t i o n s of l i f t and p i tchinp; moment c o e f f i c i e n t s wi Eh s i d e s l t p a t c o n s t a n t e n s l e of a t t a c k . The d a t a show l i t t l e v a r i a t i o n of C , , and C , , w l t h j 3 a t e l l a n g l e s of a t t a c k . T h i s is i n c o n t r a s t t o r e s u l t s o h t a t n d on arrow-wing planforms i n Reference 7 , where, a t h i g h a n g l e s o f a t t a c k , l a r g e v a r i a t i o n s i n C: o c c a r r e d . The l a t t e r planforms a c o n s i d e r - L a h l v more vortex-dominated, however, a t h i g h 0's r e l a t i v e t o t h e FIA-18. The p r e s e n t O.16-scaI e F/A-1R d a t a s u g g e s t , t h e n , t h a t t h e i n t e g r a t e d l i f t and n l t r h t n p . mnments on t h e wing p a n e l s i n s i d e s l i p a r e s i m i l a r and r e l a t i v e l v I n s e n s i t i v e t o s f d e s l i p v a r i a t i o n s .

The v a r i a t i o n s of r o l l i n g moment, yawing moment, and s i d e f o r c e c o e f f i - c i e n t s with a n ~ l e of a t t a c k a t P = -4" are p r e s e n t e d i n F i g u r e 57. RollCng moment c o e f f t c i e n t is i n c r e a s t n g l v s t a b l e up t o a; 20° a f t e r which o c c u r s a n r ~ n g t a h l e "hreak" I n t h e C vs. a curve. The l a t t e r can be a t t r i b u t e d t o t h e

e

appearance nf LFX v o r t e x hreakdown o v e r t h e wing p a n e l . The flow v i s u a l i z a - t f o n rhotograph i n F i g u r e 58 i l l u s t r a t e s t h i s e f f e c t a t a= 20' and @ = l O O ( a l l T Z X s l o t s a r e c l o s e d i n t h i s photograph). The u n s t a b l e break i n Cn v s . a i s a l s o t h e r e s a l t of v o r t e x b u r s t i n g . However, t h i s e f f e c t is e v i d e n t a t a lower a n g l e of a t t a c k ( a = 15') s i n c e v o r t e x b u r s t o c c u r s a t t h e windward v e r t i c a l t a i l p r i o r t o advancing forward over t h e v i n g panel ( s e e F i g u r e 5 8 ) .

A r e d u c t i o n i n s i d e f o r c e c o e f f i c i e n t a l s o r e f l e c t s t h e d e c r e a s e i n t a i l ef f ec t i v e n ~ o s comnenclng a t about a- 15'. I n c r e a s i n g v o r t e x burst asymmetry promotes u n s t a h l e v a r i ~ t i o n s o f r o l l i n g moment and yawing moment up t o 30' and 25O, r c s p e c t i v c l y . A f u r t h e r i n c r e a a e i n a n g l e of a t t a c k produces de- c r e a s e d v o r t e x breakdown asymmetry a n d , c o n s e q u e n t l y , u t a b l e v a r i a t i o n c ! of C

P

and C, w i t h a. The s t a b l e i n c r e m e n t s i n C , and C commencing a t a s 25' may

Y

ORIGINAL PAGE IS 3F POOR QUALITY

FIGUHE 57. VARIATION OF ROLLING MOMENT, YAWING MOMENT, AND SIDE FORCE COEFFICIENTS WITH ANGLE OF ATTACK; 0.16-SCALE F/A-18; p = -4O; hh = - 1 P ; 6,, = 25O; Reg = 1.1 (l@).

ORIGINAL PAGE W R PHOTOGRAPH M: ALL LEX S W T S W D I FIGURE 58. WINDWARD LEX VORTEX BREAKDOWN NEAR VERTICAL TAIL AT cu = XI0, /3= TO0 (NORTHROP WATER TUNNEL) PAGE BLANK NOT FILM= 1 5 5

he due t o a r e d u c t i o n i n a d v e r s e sidewash on t h e leeward v e r t i c a l t a i l assoc- i a t e d wfth a forward p r o g r e s s i o n of leeward LEX v o r t e x b u r s t p o i n t . A second u n s t a b l e break o c c u r s i n t h e yawing moment c u r v e a t a= 30- which i s apparent- l y t h e r e s u l t of immersion of t h e twin v e r t i c a l s i n an i n c r e a s i n g l y l a r g e r wake a s t h e wings approach s t a l l . I n g e n e r a l , t h e a n g l e s of a t t a c k a t which l.ar?e changes i r ~ t h e wind t u n n e l d a t a a r i s e a r e aboct 5' h i g h e r than t h e corresnonding a n g l e s f o r major f l o w f i e l d changes i n t h e w a t e r t u n n e l .

F i g u r e 5 9 p r e s e n t s r o l l i n g moment v a r i a t i o n w i t h s i d e s l i p a t a = 30'.

xO, and 40° f o r 6n = 25' and 35'. I n c r e a s e d d e f l e c t i o n ~f t h e wing leading- edge f l a p s from & = 25' t o & = 35' r e s u l t s i n l a r g e s t a b l e r o l l i n g moment increments a t Q = 30' and 35' a s a r e s u l t of delayed wing flow s e p a r a t i o n .

Flap e f f e c t s a r e n e g l i g i b l e a t a = 40°, however, s i n c e t h e a n g l e of a t t a c k i s t o o high t o e f f e c t i v e l y a l l e v i a t e flow s e p a r a t i o n near t h e f l a p h i n g e l i n e .

Vote is made i n F i g u r e 59 of t h e r e l a t i v e i n s e n s i t i v i t y of r o l l i n g moment c o e f f i c i e n t t o small v a r i a t i o n s about P = 0" a t t h e h i g h e r a n g l e s of a t t a c k ( a= 35' and 40'). A t t h e s e model a t t i t u d e s LEX v o r t e x breakdown o c c u r s n e a r t h e LFY-wing junction. I n a d d i t i o n t o t h e F/A-18 w a t e r t u n n e l o b s e r v a t i o n s , w a t e r t u n n e l t e s t s of L E X r i n g planforms, arrow wings, and d e l t a wings have i n d i c a t e d t h a r , n e a r s t a l l a n g l e of a t t a c k , t h e v o r t i c e s a r e r e s i s t a n t t o C h a n ~ e s i n b u r s t p o s i t i o n due t o s m e l l P - v a r i a t i o n s . These t r e n d s may be r e f 1 ected i n t h e 0.16-scale F/A-18 r o l l i n g moment i n v a r i a n c e a t cr = 35' and 40' hetween + 6 " . F u r t h e r s u p p o r t i v e evidence is a v a i l a b l e from Northrop

-

water tunnel and wind t u n n e l t e s t s (Reference 8) and NASA wind t u n n e l s t u d i e s (Reference 7 ) , which have shown t h a t a t brlgles of a t t a c k where LEX v o r t e x breakdown o c c u r s w e l l forward on t h e wing p a n e l s , t h e b u r s t p o s i t i o n s a r e i n s e n s i t i v e t o v a r i a t i o n s i n s i d e s l i p . At high-& r o n d i t i o n s , e x t e n s i v e wing flow s e p a r a t i o n o c c u r s and small s i d e s l i p p e r t u r b a t i o n s d o n o t a l t e r t h e wing p r e s s u r e f i e l d s u f f i c i e n t l y t o a f f e c t t h e s t a b i l i t y of t h e LEX v o r t i c e s .

Double-Width Forward LEX S l o t s (LEX 12) LEX 12 r e d u c e s C from about 1.79 t o 1.77 and, i n a d d i t i o n , lowers =MAX PRECEDING PAGE BLANK NOT FILMED

FIGURE 59. VARIATION OF ROLLING MJMENT COEFFICIENT WITH SIDESLIP; 0.16-SCALE FIA-18;hh :: -120; Re,-= 1.1 ( 1 0 ~ ) .

the s t a l l angle of a t t a c k from 4 0 ' t o 38'. approximately, a s shown i n Figure 6 0 . Water tunnel flow f i e l d observations suggest t h i s e f f e c t i s due t o the increased forward s l o t flow entrainment which l i m i t s t h e amount of lower s u r f a c e boundary l a y e r flow a v a i l a b l e f o r f e e d i w i n t o t h e LEX vortex system.

The r e ~ u l t i s a reduction i n t h e effective L2X generating length, s l i g h t l y f a s t e r p r o ~ r e s s i o n of vortex breakdown, and e a r l i e r s t a l l .

F i % u r e 61 p r e s e n t s r o l l i n g moment and y a c i n g momeat v a r i a t i o n w i t h s i d e s l i n a t a= 35' and 40'. The s l o t modification induces, i n general, s t a b l e r o l l i n g moment increments and u n s t a b l e yawing moment c o e f f i c i e n t increments.

Rased on water tunnel observations, t h e former e f f e c t can be a t t r i b u t e d t o a more symmetric breakdown of t h e LEX vortex i n s i d e s l i p , whereas t h e l a t t e r e f f e c t i s associated with a reduction i n dynamic p r e s s u r e a t t h e v e r t i c a l t a i l s due t o t h e l a r g e r wake shed from t h e wings. It i s noted t h a t , a s was t h e case with the b a s e l i n e d a t a , t h e water tunnel r e s u l t s tend t o r e v e a l flow f i e l d changes due t o t h e LEX mods a t about 5 degrees lower angle of a t t a c k than t h e corresponding a's a t which e f f e c t s a r e evident i n t h e wind tunnel d a t a .

It i s i n t e r e s t i n g t o ohserve t h a t reducing t h e leading-edge f l a p deflec- t i o n angle t o 2 5 O e l i m i n a t e s much of t h e favorable LEX 12 e f f e c t s a t a= 3 S 0 and 40°, a s shown i n Figure 62. Extensive wing flow s e p a r a t i o n occurs a t these angles of a t t a c k and t h e downstream influence of t h e LEX forward s l o t mod i s correspond i w l y decreased. This si t u a t i on is analogous t o s?anwise h l o w i n ~ e f f e c t i v e n e s s a t high a n g l e s of a t t a c k . Bloving e f f e c t i v e n e s s re- q u i r e s leading-edge flow s e p a r a t i o n . However, i f t h e degree of s e p a r a t i o n i s too extensfve, considerably higher blowing r a t e s a r e required t o achieve flow reattachment t o t h e wing surface. Similar r e s u l t s have been provided i n flow v i s u a l i z a t i o n tests of LEX vortex enhancement by blowing. A t high a n g l e s of a t t a c k , a s l i g h t forward s h i f t of t h e vortex b u r s t p o i n t due, say, t o decreased leading-edge f l a p d e f l e c t i o n angle, r e q u i r e s d i s p r o p o r t i o n a t e l y lif,o,lier blowing r a t e s t o provide t h e same enhancement of t h e vortex achieved with a g r e a t e r f l a p angle.

ORIGINAL PAGE I S

OF POOR QUALITY LEX 12 FIGURE 80. EFFECT OF LEX 12 ON LIFT COEFFICIENT; 0.1MCALE FIA-18; 6,- 360;6,,=-1P; Reg= 1.1 (108).

BASELINE LEX 12 FIGURE 61. EFFECT OF L.EX 12 O N VARIATION OF ROLLING MOMENT AFJn YAWING MOMENT C0EFF:CIENTS WITH SIDESLIP; O.1GSCALE FIA-IS; 6, = 350; 6h = -120; ReZ = 1.1 (1061,

OR\GINAL PAGE IS OF POOR Q U -

Double-WiAth, I n c r e a s e d Length Forward LEX S l o t s (LEX 12A) The e f f e c t s of forward LEX s l o t m o d i f i c a t i o n s a r e more v i v i d l y i l l u s - t r a t e d w i t h LEX 17A, o r t h e double-width, i n c r e a s e d l e n g t h forward s l o t s . The flow ~henomena a s s o c i a t e d w i t h LEX 12A a r e s i m i l a r t o t h e c o r r e s p o n d i n g flow s i t u a t i o n ~ i t h LFX 1 2 . However, t h e e f f e c t s c n l o n g i t u d i n a l and l a t e r a l - directional c h a r a c t e r i s t i c s a r e e v i d e n t o v e r a broader a n g l e of a t t a c k r a n e e ant1 t o a g r e a t e r d e g r e e .

L i f t t o s s e s r e l a t i v e t o t h e besp f n e d a t a a r c a p p a r e n t beginning a t

C Y S :?no a s shown i n F i g u r e 63. I n a d d i t i o n , C (1.70 w i t h LEX 12A a s

n n n o s e n t o 1 . 7 s f o r t h e b a s e l i n e ) i s achievedh& 01" 34.5' r e l a t l v e t o a ! hOo f o r t h e b a s e l i n e .

V a r i a t i o n of C and Cm w i t h /3 a t 01 35' and 40' i n F i p l r e 64 r e v e a l N s i m i l a r e f f e c t s due t o LEX 12A. The e a r l i e r s t a l l due t o L G X 12A i s r e f l e c t e d i n nose-down p t t c h i n g moment increments. Thus, t h e premature b u r s t i n g of t h e TAY v o r t i c e s due t o LEX 12A observed i n t h e w a t e r t u n n e l i s borne o u t i n t h e wrnd t u n n e l d a t a .

The wind t u n n e l d a t a i n F i g u r e 65, which show r o l l i n g and yawing moment v a r i a t i o n v i t h s i d e s l i p a t a = 35' and 40°, i n d i c a t e t h a t LEX 12A promotes a more symmetric s t a l l p a t t e r n i n s i d e s l i p . Consequently, l a r g e , s t a b l e C t - i n c r e m e n t s a r e a c h i e v e d a t t h e s e a n g l e s o f a t t a c k . The u n s t a b l e yawing moment i n c r e m e n i s a s s o c i a t e d w i t h LEX 12A cap b e perceived from a q u a l i t a t i v e s t a n d p o i n t by w a t e r t u n n e l t e s t r e s u l t s . Dye e n t r a i n e d i n t o t h e massive wake shed t h e wlng s u r f a c e s t r a v e r s e s a f t t o t h e v e r t i c a l t a i l r e g i o n . The r e d u c t i o n i n Local "q" a t t h e tails was e v i d e n t by comparison w i t h t h e f r e e - stream flow.

The r e s u l t s d i s c u s s e d t o t h i s p o i n t s e r v e t o i l l u s t r a t e t h e s i g n i f i c a n t i n f l u e n c e of t h e LEX f l o v forward of t h e p r o d u c t i o n break. Large improvements i n l a t e r a l s t a b i l i t y are a c h i e v a b l e by j u d i c i o u s m a n i p u l a t i o n of t h e v o r t e x feed in^ mechanism i n t h i s r e g i o n . For example, LEX 12 and LEX 12A reduce t h e a v a i l a b l e lower s u r f a c e v o r t t c i t y which i s shed a t t h e LEX l e a d i n g edge and

; LEX 12A L : I . .

a, deg FIGURE 63. EFFECT OF LEX 12A ON LIFT COEFFICIENT; st, = 350; 6h = -1P; ReE= 1.1 (I@).

1 6 4

I-' ---

FIGURE 65. EFFECT OF LEX 12A O N VARIATION O F ROLLING MOMENT WITH SIDESLIP; 0.16-SCALE FIA-18; 6 , = 35O; 6h = -120; Re,- = 1.1 (106).

f.eeds i n t o t h e LEX v o r t e x system. Concurrently, h w e v e r , maximum l i f t cap- n h i l i t y is compromised depending 011 t h e e x t e n t t o which 1.he v o r t e x f e e d i n g mechanism is r e s t r i c t e d . It has a l s o been observed i n t h e w a t e r t u n n e l t h a t trie body v o r t e x behavior is s u b j e c t t o p e r t u r b a t i o n s due t o LEX apex flow The following d i s c u s s i o n s p e r t a i n t o wind t u n n e l d a t a t r e n d s a s s o c i a t e d w i t h a l t e r n a t e means of a f f e c t i n g t h e LEX apex flow f i e l d .

LEX Lower Surf a c e Fence (Fence 3") E X Fence "B" is o r i e n t e d o b l i q u e t o t h e free-stream, terminating a t t h e )reduction break. T h i s l w e r s u r f a c e boundary l a y e r f e n c e l i m i t s t h e amount cf shed v o r t i c i t y along t h e LEX and, consequently, reduces t h e s t r e n g t h of t h e !EX v o r t e x i n t h e h i g h angle-of-attack range. Examination of t h e l i f t charac- t e r i s t i c s i n F i g u r e 66 r e v e a l s a r e d u c t i o n i n C f r m 1.79 ( b a s e l i n e ; d, = 3 5 " ) t o 1.75 a s w e l l as a d e c r e a s e i n s t a l l ang % e of a t t a c k t o 3 7 O from 4 i ! O . The w a t e r t u n n e l t e s t s r e v e a l e d t h e LEX v o r t e x system w i t h Fence "B" t o be s i m i l a r t o t h e flow s i t u a t i o n a s s o c i a t e d w i t h LEX 12A. The l a t t e r , by way of f l o w e n t r a i n m e n t i n t o t h e f o r w a r d s l o t , a c t s as a " f l u i d f e n c e , " d i v e r t i n g much of t h e l a v e r s u r f a c e spanwise flow i n t h e region of i n t e r e s t t o L. thordwise - 1 r e c t i o n . S i m i l a r l y , t h e f e n c e s e r v e s t h e same purpose, a l t h o u g h \ $ I t \ l e s s l i f t l o s s s i n c e LEX e u r f a c e a r e a h a s n o t been removed and t h e LEX apex v o r t i c e s a r e somewhat s t r o n g e r .

The g e n e r a t i n g l e n g t h of t h e LEX apex v o r t e x is confined t o t h e s m a l l rc:glon from t h e f i r s t p o i n t of i n t e r s e c t i o n of t h e f e n c e w i t h t h e l e a d i n g ed;;e t o t h e ? E X apex. I n t e r a c t i o n of t h e apex v o r t e x w i t h t h e a f t primary v o r t e x ir l i m i t e d and t h e former a c t s as a " f r e e vortex." An analogous flow s i t u a t .. ..n i s leading-edge v o r t i c e s shed from co-planar close-coupled canard- wi-g s u r f a c e s . The f e n c e s " f i x " t h e poirtc of o r i g i n o f t h e a f t p r i m a r y o r t i c e s even i n s i d e s l i p c o n d i t i o n s . T h i s i e i n c o n t r a s t t o t h e b a s e l i n e co11 f i g u r a t i o n where, d u r i n g w a t e r t u n n e l e t u d i e e , it appeared t h a t t h e leeward LEX v o r t e x b a s f e d by f l u i d from t h e LEX epex r e g i o n which, a t z e r o s i d e s l i p , was e n t ~ a i n e d i n t o t h e apex v o r t e x . T h i s f l a w s i t u a t i o n was not e v i d e n t on t h o windward LEX.

FIGURE 66. EFFECT OF LEX FENCE "B" ON LIFT COEFFICIENT: 0.16SCALE F!A-18; 6 , = 35O; 6h = -120; ReE= 1.1 (1061;

The LEX f e n c e s r e s u l t i n f a v o r a b l e r o l l i n g moment increments a t CY- 35' and 40" a s s e e n i n F i g u r e 67. The Cp-increments a s s o c i a t e d w i t h Fence "B" a r e comparable t o t h e LEX 12A r e s u l t s . However, wing s t a l l Is less p r o n ~ u n c e d w i t h t h e f e n c e s and, a c c o r d i n g l y , t h e u n f a v o r a b l e yawing moment increments i n F i g u r e 67 a r e l e s s . Consequently, LEX Fence "B" a p p e a r s more d e s i r a b l e i n r e d u c t ion.

terms of an a c c e p t a b l e compromise between C improvements and C

P

h x

LEX Lower Surf a c e Fence (Fence "A ") The e f f e c t i v e n e s s o f LEX l o w e r s u r f a c e f e n c e s i n i m p r o v i n g l a t e r a l s t a b i l i t y is s e n s i t i v e t o f e n c e o r i e n t a t i o n . The d a t a i n F i g u r e 68 i n d i c a t e t h a t f e n c e s o r i e n t e d i n a streamwise manner (Fence "A") a r e l e s s f a v o r a b l e r e l a t i v e t o Fence "B". A p l a u s i b l e e x p l a n a t i o n can be d e r i v e d from w a t e r t u ~ ~ n e l t e s t s i n terms of t h e e f f e c t i v e n e s s of t h e f e n c e i n decoupling t h e LEX a f t p r i m a r y v o r t e x f r o m t h e a p e x v o r t e x . The s t r e a m w i s e f e n c e d o e s n o t provide a d i s t i n c t break i n t h e v o r t i c i t y shed a l o n g t h e l e a d i n g edge which w i l l e f f e c t i v e l y i s o l a t e t h e a f t primary v o r t e x from t h e apex flow. The streamwise f e n c e does, however, d e l i n e a t e t h e lower s u r f a c e flow e n t r a i n e d i n t o t h e forward s l o t from t h e f l u i d which proceeds spanwise t o s e p a r a t e a t t h e leading edge. Because t h e p o i n t of o r i g i n of t h e a f t primary v o r t e x i s f r e e t o move w i t h c h a n g e s i n a n g l e o f a t t a c k and s i d e s l i p , h o w e v e r , t h e streamwise f e n c e i s n o t a s e f f e c t i v e i n promoting symmetric LEX v o r t e x break- down i n s i d e s l i p .

LEX Planform M o d i f i c a t i o n s

The LEX mods d i s c u s s e d up t o t h i s p o i n t - LEX 12, LEX 12A, Fence "A, "

Fence "B" - promote s i m i l a r changes i n t h e LEX v o r t e x flow f i e l d a t h i g h a n g l e s of a t t a c k . S p e c i f i c a l l y , t h e LEX primary v o r t e x developed i n t h e presence of t h e s e mods e x h i b i t s s t a b i l i t y c h a r a c t e r i s t i c s s i m i l a r t o a v o r t e x d e v e l o p e d on a LEX w i t h r e d u c e d g e n e r a t i n g l e n g t h . To c o r r o b o r a t e t h i s conclusion, low-speed wind t u n n e l d a t a a r e IIOW p r e s e n t e d t h a t f e a t u r e removal, t o v a r y i n g d e g r e e s , of LEX a r e a n e a r t h e apex. It w i l l be shavn t h a t t h e e f f e c t s 011 l a t e r a l s t a b i l i t y a t h i g h a n g l e s of a t t a c k a s s o c i a t e d w i t h LEX 1 2 ,

- - - L l X F F N C E "B"

L . l _ l - ~ -.--

FIGURE 67. EFFECT OF LEX FENCE "B" ON ROLllNG MOMENT AND YAWING MOMENT VARIATION WITH SIDESLIP; O.lr?SCALE F/A-18; 6" = 35O; 6 h = -120; Rec = 1.1 (1061.

c~~~;:!ji\L PAGE IS OF POOR QUALITY

- LEX F a - 'w

1 7 1 UT #I# "AL

FIGURE 68. COMPARISON OF ROLLING MOMENT VARIATION WlTH SIDESLIP FOR THE O.16SCALE FIA-18 WITH LEX FENCES "A" AND "B"; 6, = 350; 6 , = -120; me- 1.1 (lo6).

LEX 12A, Fence "A," and Fence "B" a r e similar t o t h e e f f e c t s a r i s i n g from removal of LEX a r e a t h a t corresponds approximately t o t h e r e g i o n of i n f l u e n c e of LEX 12, e t c . T h i s is shown s c h e m a t i c a l l y i n F i g u r e 69.

T r u n c a t i n g approximately 88" ( f u l l - s c a l e ) of t h e LEX back t o t h e pro- d u c t i o n b r e a k a s shown i n F i g u r e 69 g r e a t l y i m p r o v e s t h e r o l l i n g momeut c h a r a c t e r i s t i c s a t C Y - 35' and 40' a s shown i n F i g u r e 70. A t t h e s e a n g l e s of a t t a c k , t h e v o r t i c e s shed from t h e s m a l l e r LEXs ( d e s i g n a t e d LEX 2) a r e b u r s t - i n g o v e r t h e LEX s u r f a c e s and v o r t e x b e h a v i o r is r e l a t i v e l y i n s e n s i t i v e t o changes i n s i d e s l i p . Both wi.lgs have passed through C and t h e v a r i a t i o u of r o l l i n g moment w i t h s i d e s l i p i n F i g u r e 70 is h i g h l y Since t h e wings s t a l l at lower a's w i t h LEX 2, t h e v e r t i c a l t a i l s a r e a c c o r d i n g l y exposed t o a l a r g e r wing wake r e l a t i v e t o t h e b a s e l i n e c o n f i g u r a t i o n . A s a r e s u l t , u n s c a b l e yawing moment increments ( d a t a not s h o r n ) a r e developed a t t h e h i g h e r a n g l e s of a t t a c k .

The t r e n d s a s s o c i a t e d w i t h LEX 2 a r e q u i t e s i m i l a r t o t h e e f f e c t s ob- s e r v e d w?th LEX 12, LEX 12A, Fence "A" and Fence "B." S i n c e LEX 2 i n v o l v e s removal of l i f t i n g s u r f a c e folward of t h e p r o d u c t i o n break, t h e LEX modi- f i c a t t o n which is expected t o promote t h e most s i m i l a r e f f e c t s is LEX 12A.

(The l a t t e r produced a s i g n i f i c a n t l i f t l o s s a t h i g h a's due t o d i s r u p t i o n o f t h e LEX apex f lw f i e l d . ) A comparison of t h e r o l l i n g moment and yawing moment v a r i a t i o n s w i t h s i d e s l i p a t a = 3 5 ' and 40' a r e shown i n F i g u r e 71.

Note is made t h a t t h e leading-edge f l a p d e f l e c t i o n a n g l e s a r e n o t t h e same.

-

The d a t a t r e n d s a r e i n good agreement, however. Consequently, t h e c o n c l u s i o n t h a t LEX 12, LEX 12A, Fence " A , " and Fence "B" a r e , i n e s s e n c e , d i f f e r e n t means ( o f v a r y i n g e f f e c t i v e n e s s ) of s h o r t e n i n g t h e LEX "run l e n g t h " a p p e a r s s u b s t a n t i a t e d .

S i m i l a r , but l e a s f a v o r a b l e , e f f e c t s a r e a t t a i n e d w i t h LEX 7 and LEX 3 planforms, shown i n F i g u r e s 72 and 73, r e s p e c t i v e l y . LEX 7 f e a t u r e s a 69-inch ( f u l l - s c a l e ) t r u n c a t i o n of t h e LEX forward a r e a and a 55'-swept- forward apex. LEX 3 reduces t h e b a s e l i n e LEX a r e a t o a l e s s e r e x t e n t and f e a t u r e s a " g o t h i c " o r YF-17-type planform. D e s p i t e t h e planform d i f f e r e n c e s , t h e e f f e c t s on Ceand 5 v a r i a t i o n s w i t h s i d e s l i p a t n - 30°, 35', and 40' a r e

L E X 2 (TRUNCATED FIGURE 69. "EQUIVALENCE" OF LEX 12A AND TRUNCATED LEX (LEX 2).

LEX 7 1

--- LEX I

EZEl

FIGURE 72. EFFECT OF LEX 7 ON ROLLING MOMENT AND YAWING MOMENT VARIATION WITH SIDESLIP; 0.16-SCALE FIA-18; 6, = 25O; tih = -12O; Re,-= 1.1 (I@}.

r t ! t l . . , i L PAGE IS c."-'-'~- 77 pcca QUALlrV

very s i m i l a r . These r e s u l t s a r e , t h u s , c o n s i s t e ~ ~ t w i t h t h e t r e n d t h a t h a s emerged from t h e flaw v i s u a l i z a t i o n s t u d i e a aud wind t u n ~ l e l d a t s a n a l y s e s : s h o r t e n i n g t h e LEX promotes a shed v o r t e x system of reduced s t a b i l i t y ht g r e a t e r r e s i s t a n c e t o asymmetric b u r e t i n g I n s i d e s l i p a t a m 3 0 " - 4 0 ' .

By way of c o n t r a s t , r e s u l t s o b t a i n e d w i t h LEX 10, which h a s a forward e x t e n s i o u of ? h e b a s e l i n e LEX, a r e presented i c F i g u r e 74. The wind tulluel d a t a show u ~ l s t a b l e r o l l i a g moment +ud s t a b l e yawing moment iucremects a t (1 = ?9", 35". a11a 40" f o r s6', approximately. I n c r e s s i n g t h e LEX g e n e r a t - ing l e n g t h c a u s e s a more p e r s i s t e n t v o r t e x a t h i g h a n g l e s of a t t a c k . Vortex b u r s t asymmetry due t o s i d e s l i p is an u ~ l d e s i r a b l e f a l l - o u t , however. The more u o u l i u e a r v a r i a t i o n of Ct and C, v i t h fi a p p e a r s due t o t h e s e n s i t i v i t y of t h e l o n g e r LEX ~ L J v o r t e x b u r s t p o s i t i o n s t o s m a l l s i d e s l i p c h a n g e s . A t s i d e s l i p a n g l e s g e n e r a l l y g r e a t e r t h a n - +6" t h e s t a b l e r o l l i n g momellt coef f i- c i e u t s call be a t t r i b u t e d t o t h e r e d u c t i o n i n leeward LEX v o r t e x s t a b i l i t y , a c h a r a c t e r i s t i c i n h e r e n t t o s l e n d e r planforms a t h i g h a n g l e s of a t t a c k and l a r g e s i d e s l i p , The e f f e c t s or LEX g e n e r a t i n g l e n g t h on t h e aerody~lamic c h a r a c t e r i s t i c s call be summarized b r i e f l y i n t3rm.s of t h e s k e t c h e s i n F i g u r e 75. Maximum l i f t and s t a l l a u g l e of a t t a c k increasi. w i t h i n c r e a s e d LEX a r e a . Although t h e t r e n d s a s s o c i a t e d w i t h v o r t e x breakdm11 asymmetry end r o l l i ~ l p moment v a r i a t i o n w i t h s i d e s l i p a r e s i m i l a r r e g a r d l e s s 1 LEX s i z e , t h e a n g l e s of a t t ~ i k a t which major flow f i e l d changes o c c u r a r e q u i t e d i f f e r e n t . In t h e h i g h a n g l e of a t t a c k regime, t h e s m a l l LEX e x h i b i t s a recovery of l a t e r a l s t a b i l i t y due t o a r e d u c t i o n i n v o r t e x breakdown a s y m e t r y . The c u r v e s f o r t h e l a r g e LEX, however, d i s p l a y a s h i f t t c ? h i g h e r a n g l e s of a t t a c k . Accord- i n g l y , s i g n i f i c a n t v o r t e x breakcicwn asymmet r v a n d low l e v e l s o f l a t e r a l s t s b i l i t y p r e v a i 1.

W 11% Leading-Edge Snag and Upper S u r f a c e Fence R e s u l t s o b t a i n e d ri t h wing snag and f e n c e combil~at i n n s w a r r a n t d i s c u s - s i o ~ ~ s i n c e t h e b e n r ; f i c i a l e f f e c t s on l a t e r a l s t a b i l i t y i n c e r t a i n c a s e s a r e s i g n i f i c a n t .

OHLGhELAL PAGE I S OF PO02 QUALITY

SN1 and SN3 snags shown i n F i g u r e s 76 and 77 a r e f l a t - p l a t e , 17% l o c a l c h o r d e x t e n s i o n s f a i r i n g l i n e a r l y t o O X a t t h e w i n g t i p s , p o s i t i o n e d a t 50% and 38% semi-span, r e s p e c t i v e l y . The F1 and F3 u p p e r s u r f a c e f e n c e s , co;:responding t o t h e SN1 and SN3 s n a g s , r e s p e c t i v e l y , e x t e n d from t h e wing l e z d i n g edge t o t h e t r a i l i n g - e d g e f l a p h i n g e l i n e and a r e 8" ( f u l l - s c a l e ) i n h e i g h t . The f a v o r a b l e r o l l i n g moment increments i n F i g u r e s 76 and 77 due t o t h e snag-fence combinations can be a t t r i b u t e d t o ( 1 ) i n c r e a s e d leading-edge camber due t o t h e l a r g e s n a g s e x t e n d i n g along t h e d e f l e c t t leading-edge f l a p s and ( 2 ) s y l m e t r y o r LEX v o r t e x behav!or ' 3 s i d e s l i p . The l a t t e r f a c t o r i s t h e r e s c l t 3r t h s presence of u p p e r s u r f a c e f e n c e s , t h e g r t a c c r e f f e c t of which is on t h e leeward LEX v o r t e x . Water t u n n e l flow s t u d i e s r e v e a l a n o t i c e a b l e r e ? u c t l c n i n t h e s t a b i l i t y of t h e leeward LEX v o r t e x . The f e n c e r e s t r i c t s t h e vqrtex-induced spanwise flow on t h e wing, s h i f t s t h e v o r t e x inboard, and promotes a forward movement of t h e b u r s t p o s i t i o n r e l a t i v e t o t h e b a s e l i n e c o n f i g u r a t i o n .

It is noted t h a t t h e b e n e f i c i a l e f f e c t s i n r o l l of t h e snag (SN1) and f e n c e ( F l ) combination a r e g r e a t e r t h a n t h e a d d i t i v e e f f e c t s of a snag o r f e n c e in i s o l a t i o n . The d a t a i n F i g u r e 78 i n d i r a t e t h a t t h e fence-alone is e f f e c t i v e only a t C Y - 30". A t a = 3S0 and 40" t h e f e n c e i s submerged i n t h e s e p a r a t e d wing flow and, consequently, t h e f a v o r a b l e f e n c e e f f e c t s a r e l o s t .

The l a r g e s n a g , a n a l o g o u s t o i i ~ c r e a s e d l e a d i n g - e d g e f l a p d e f l e c t i o n , i s f a v o r a b l e a t a l l a n g l e s ( s e e F i g u r e 7 8 ) but w i t h d i m i n i s h i n g e f f e c t i v e n e s s w i t h i n c r e a s e d a. By reducing t h e s e p a r a t e d flow r e g i o n n e a r t h e wing l e a d i n g edge by a d d i t i o n of t h e snag, t h e f e n c e is nos a b l e t o e f f e c t a f a v o r a b l e flow f i e l d change a t a = 35" and 4 0 " . These e f f e c t s a r e s i m i l a r t o t h e t.rends a s s o c i a t e d 3 i t h LEX mods (LEX 12, e i c . ) . E x t e n s i v e flow s e p a r a t i o n on t h e wings was shown t o l i m i t t h e e f f e c t i v e n e s s , i n terms of improved Ct , of t h e P m o d i f i c a t i o n s . However, t h e combination of i n c r e a s e d f l a p d e f l e c t i o n and LEX mods was very e f f e c t i v e , i n g e n e r a l , u p t o and beyond s t a l l a n g l e of a t t a c k .

Forward LEX S l o t s Closed C l o s u r e of t h e f o r v a r d LEX. boundary l a y e r bleed s l o t s a l t e r s t h e s t r u c - t u r e of t h e LEX v o r t e x s y s t e E but does n o t a l t e r i n acy s u b s t a n t i a l manner t h e high-ci c h a r a c t e r i s t i c e . C l o s u r e of t h e s l o t s produces a s i n g l e primary v o r t e x

t - ' Q)

- .03 +

VI

--- FENCE F1

FIGURE 78. EFFECT OF SNAG AND FENCE ON ROLLING MOMENT VARIATION WITH SIDESLIP; 0.16SCALE FIA-18; 6 , = 250; 6h = -120; R b = 1.1 (106).

r a t h e r than t h e two-vortex system c h a r a c t e r i s t i c of t h e s l o t s - o p e n c a s e . A t a = 25"-70°, s l o t s c l o s e d a c t ~ l a l l v r e s c l t s i n a l i f t l o s s r e l a t i v e t o t h e hnsel i n e .?s shown i n F i g u r e 79. T h i s mav b e due t o t h e g r e a t e r l o c a l upwash nlonn t h e 1.EX anex r e g i o n w i t h s l o t s open, t h e r e b y promoting a s t r o n g e r v o r t e x system. At h i g h e r a n g l e s of a t t a c k , t h e s l o t s - c l o s e d c o n f i g u r a t i o n e x h i b i t s more p e r s i s t e n t v o r t i c e s and, a s a r e s u l t , l i f t i n c r e a s e s arc! e v i d e n t a t CY= 35' and 40 (wl t h c o n c u r r e n t nose-up p i t c h i n g moment i n c r e m e n t s ( n o t shown)).

Water tunnel r e s u l t s i n d i c a t e d t h a t t h e LEX v o r t l c e s w i t h s l o t s c l o s e d h r ~ r s t 111 a s l i ~ h t l v more asvmmetric f a s h i o n a t h i g h a's, t h e r e b y s u g g e s t i n g r ~ n f a v o r a b l e r o l l i n a moment e f f e c t s . The wind t u n n e l d a t a shown i n F i g u r e 8 0 d o n o t , i n g e n e r a l , h e a r t h i s o u t . Comparison w i t h h a s e l j n e r e s u l t s r e v e a l s l ! t t l e c h a n ~ e s i n C and C , v a r i a t i o n w i t h 0 a s s o c i a t e d w i t h s l o t c l o s u r e a t E cr= ?no-&no.

Forehodv S t r a k e s (Radial L o c a t i o n : +40°) F/A-18 m o d i f + c a t i o n s d i s c u s s e d t o t h i s p o i n t have f e a t u r e d changes t o t h e L E Y o r wing e e o m e t r v w h i c h d i r e c t l v a f f e c t t h e LEY v o r t e x and w i r . ~ f l o w behavior. The forehody v o r t e x system i s a l s o a f f e c t e d i n a n i n d i r e c t manner h e t o changes i n 1,F.X v o r t e x p a t h and s t a b i l i t y c h a r a c t e r i s t i c s and changes i n t h e wine p r e s s u r e f i e l d . The i n t e r a c t i o n of t h e forebody v o r t i c e s w i t h t h e lAF.X-w1n~ flow f l e l d i s a p o t e n t i a l s o u r c e of s i g n i f i c a n t l a t e r a l s t a h i l i t y e f f e c t s . The s t r e n g t h s and t r a j e c t o r i e s of t h e body v o r t i c e s can, under c e r t a l n c o n d i t l c n s , p l a y 3 prominant r o l e i n t h e s t a l l c h a r a c t e r i s t i c s of t h e w i w s . The m o 4 i f i c a t i o n s t o be d i s c u s s e d i n t h e f o l l o w i n g s e c t i o n s d e s c r i b e t h e e f f e c t s a s s o c i a t e d w i t h t h i n , highly-swept s u r f a c e s p o s i t i o n e d a l o n g t h e r a o e e forehodv s t r a k e s . It w i l l be shown t h a t t h e forebody v o r t i c e s can he a kev t o u n d e r s t a n d i n g t h e complex n a t u r e of t h e FIA-18 flow f i e l d a t ',ft?'-I a ' s .

9trqqkes of approximate1 y 55-inch l e n g t h and 2.34-inch maximum h e i g h t ( f u l l - s c a l e d i ~ ~ n a i o n s ) were mounted a l o n g t h e forehody a t 40' above t h e rrlaxtml~rn h a l f b r e a d t h . Strakes mounted i n t h i s manner produce i n s i g n i f i c a n t r h a n e e s i n t h e v a r i a t i o n o f n o r m a l € 0 - c e c o e f f i c i e n t w i t h s i d e s l i p a t a = 3 0 ° t n 4 0 " . n s d e p i c t e d i n F i g u r e 8 1 . W a t e r t n n n e l f l o w v i s u a l i z a t i o n

- SLOTS OPEN

SLOTS CLOSED FIGURE 79. EFFECT OF FORWARD SLOT CLOSURE ON LIFT COEFFICIENT; O.16SCALE F/A-18; 8, = 35O; gh = -12O: Ree = 1.1 (106).

ORIGINAL PAdE IS OF POOR Q U -

BAIELWE NOSE STRAKES a , D I G

....... a

-- 40

FIGURE 81. EFFECT OF FOREBODY STRAKES (9 = 400) ON NORMAL FORCE AND PITCHING MOMENT VARIATIONS WITH SIDESLIP; 0.16-SCALE F/A-18; 6, = 250; ah = -120; Ree= 1.1 (I$).

r e s u l t s s u g g e s t e d t h a t t h e s t r a k e s i n c r e a s e d t h e bcdy v o r t e x s t r e n g t h s a n d , h e n c e , bodv normal f o r c e . The i n c r e m e n t a l body v o r t e x s t r e n g t h i n d u c e s a correspond in^ downwash i n c r e m e n t o n t h e w i n g s , however, t h e r e b y r e s u l t i n g i n n e g l i g i b l e e f f e c t on n e t n o n n a l f o r c e . E v i d e n t i n F i g u r e 8 1 , however, a r e nose-up p i t c h t n g moment i n c r e m e n t s , which r e f l e c t t h e ". ~g moment arm of t h e s t r a k e l i f t .

Examination o f t h e b a s e l i n e and n o s e s t r a k e CL v s . a c u r v e s i n F i g u r e 8 2 i n d i c a t e s t h a t maximum l i f t and s t a l l a n g l e o f a t t a c k a r e e s s e n t i a l l y t h e sarTle.

The wind t u n n e l r e s u l t s i n F t g u r e 83 i n d i c a t e t h a t t h e n o s e s t r a k e s i n d u c e l a r g e s t a b l e r o l l i n g moment i n c r e m e n t s a t a= 3 5 ' and 49' and prcmote a m o r e l i n e a r v a r i a t i o n o f C w i t h 13 . The F/A-18 b a s e l i n e c o n f i g u r a t i o n i s

P

c h a r a c t e r i z e d bv a s t r o n g l y asymmetric f o r e b o d y v o r t e x o r i e n t a t i o n i n s i d e - s l f p : t h e leeward Sody v o r t e x i s e n t r a i n e d i n t o t h e l e e w a r d LEX v o r t i c a l f l o w w h i l e t h e windward hody v o r t e x i s r e l a t i v e l y u n c o u p l e d . Th.? body p r i m a r y v o r t e x p a t h s w i t h n o s e s t r a k e s O D , however, a r e e x t r e m e l y r e s i s t a n t t o t h i s asvmmetric o r i e n t a t i o n I n s i d e s l i p . Water t u n n e l and wind t u n n e l f l o w visual- i z a t i o n have confirmed t h a t t h e f o r e t 3y v o r t i c e s e x h i b i t a m u t u a l i n t e r a c t i o n s u c h t h a t t h e p r i m a r y v o r t e x - I n d u c e d e f f e c t s a r e e x p e r i e n c e d on t h e windward u l i n ~ . A s a consequence, s t a l l of t h e l a t t e r is del-ayed w h i l e p r e m a t u r e s t a l l o f t!.? leeward wing i s promoted. These r e s u l t s c o n f i r m t h a t u n d e r c e r t a i n f 1 ow c o n d i t i o n s t h e f o r e b o d v v o r t i c e s c a n f n t e r e c t i n a highly-f a v o r o b l e manner w i t h t h e wing f l o w .

A s s i d e s l i p a n g l e i s i n c r e a s e d , t e s t r e s u l t s i n F i g u r e 83 i ~ ~ d i c a t e reduced s t r a k e e f f e c t i v e n e s s . A t t h e h i g h e r s i d e s l i p a n g l e s t h e w a t e r t u n n e l f l c u 5 l r v e y s r e v e a l e d f n t d h o d y p r i m a r y boqindary l a y e r s e p a r a t i o n l i n e r o t a t i o : ~ t o s u c h a ? e x t e n t t h a t t h e s t r a k e s a r e l e s s a b l e t o i n d u c e symmetric v o r t e x s h e d d i n g a l o n g t h e n o s e r e g i o n . The wind t u n n e l d a t a i n F i g u r e 83 e x h i b i t a c o r r e s p o n d i n g r e d u c t i o n i n l o c a l s l o p e s ( C ) a t t h e h i g h e r s i d e s l i p a n g l e s .

P~ I n c r e a s i n g t h e l e n g t h o f t h e e t r a k e s p r o d u c e s a somewl~at more s t a b l e v a r i a t i o n o f r o l l i n g moment w i t h e i d e s l i p . R e s u l t s a t Q'=40° a r e shown

- - - FOREBODY STRAKES

FIGURE 82. EFFECT OF FOREBODY STRAKES (4 = 400) ON LIFT COEFFICIENT; O.1O-SCALE FIA-18; 6, = 3b0; 6h = -12O; Ra= 1.1 (lo6).

w

E - -

0 % =s: w u

$ g<

A

OF=

P t - r 23; 0 + I 1 CC 2 IU "0; -?? .-

8 I l k ~

g s:.; W O il 8 p o , fi aW,?

I = a s b k o m It > A og,c

-

0- .- m a - E g g a-2 g ; : ~ E O w o ?

:=L

u p 0

c r;';;NAL PAGE IS 2> POOR QUA1

i n F i g u r e 83. Comparison of t h e long- ' s h o r t - s t r a k e d a t e i n d i c a t e , how- e v e r , t h e s i g ~ l i f i c a n c e o f t h e f o r v a r d p c r t i o n o f t h e s t r a k e s . T h a t i s , a f f e c t i n g t h e i n i t i a l body v o r t e x development is a c r i t i c a l f a c t o r .

Col~currellt w i t h t h e b e ~ l e f i r l a l s t r a k e e f f e c t s on r o l l i n g momeut a r e d e s t s b l l i z i n g yawing moment iucremeuts, showu a l o o i n F i g u r e 83. The leeward s t r a k e v o r t e x i n d u r e s very low l o c a l p r e s s u r e s and call be p e r c e i v e d a s a f l u i d boundary l a y e r c o n t r o l d e v i c e which d e l a y s boundary l a y e r s e p a r a t i o n 011 t h e leeward f u s e l a g e s i d e . T h i s is d e p i c t e d i n t h e s k e t c h i n F i g u r e 84. Without t h e s t r a k e , t h e p r i m a r y b o u n d a r y l a y e r s e p a r a t e s below t h e maximum h a l f breadth. The i n c r e a s e d s u c t i o ~ l p r e s s u r e s due t o strake-induced a t t a c h e d flow along t h e leeward f u s e l a g e s i d e r e s u l t i n u n s t a b l e yawing moment increments.

The s i g ~ ~ of t h e s i d e f o r c e c o e f f i c i e n t increments i n F i g u r e s 8 3 and 84 due t o t h e s t re.kes is c o l ~ s i s t e n t w i t h i n c r e a s e d s u c t i o n p r e s s u r e s 011 t h e leeward f orebody s u r f ace.

Forebody S t r a k e s ( R a d i a l Location: +60°) The d a t a i n F i g u r e 85 show t h a t radome s t r a k e s l o c a t e d a t 6- +60° r e s u l t i n , r e l a t i v e t o t h e + = G O " c a s e , i n c r e a s i n g l y s t a b l e v a r i a t i o n of r o l l i n g moment w i t h s i d e s l i p a t s m a l l s i d e s l i p a n g l e s . W i t h i l l t h e approximate range -4' < $ ' 4'. t h e s t r a k e s i n c z e a s e t h e s t r o n g mutual i n t e r a c t i o n of t h e body v o r t i c e s w i t h a corresponding i n c r e a s e i n t h e f a v o r a b l e vortex-induced e f f e c t s 011 t h e windward wing pauel. S t r a k e e f f e c t i v e n e s s d r o p s o f f r a p i d i y , however, a t h i g h e r s i d e s l i p a n g l e s due t o r o t a t i o n uf t h e boundary l a y e r s e p a r a t i o ~ ~ l i n e s . I n b r i e f , t h e h i g h e r s t r a k e p o s i t i o n compresses t h e s i d e s l i p range w i t h i n which symmetric ( o r n e a r l y s o ) v o r t e x development n e a r t h e nose can be achieved.

Forebody S t r a k e s ( R a d i a l Location: +30°) The e f f e c t i v e ~ l e s s w i t h which t h e s t r a k e s promote a d i s c r e t e p a i r cf body v o r t i c e s which a r e l e s s prone t o asymmetry i n s i d e s l i p is h i g h l y s e n s i t i v e t o s t r a k e r a d i a l p o s i t j o n . With + = +30° much of t h e b e n e f i c i a l s t r a k e e f f e c t on l a t e r a l s t a b i l i t y is l o s t , a s shown i n F i g u r e 86 a t a- 35' and 40". The r e s u l t s i n F i g u r e 86 s u g g e s t an approximate band of s t r a k e r a d i a l p o s i t i o n s

FIGURE 84. SKETCH OF FOREBODY STRAKE EFFECT ON BODY VORTEX BEHAVIOR.

ORIGINAL PAGE I S OF POOR QUALITY FIGURE 85. EFFECT OF FOREBODY STRAKES (9 = 600) ON ROLLING MOMENT VARIATION WITH SIDESLIP; 0.16SCALE FIA-18; 6, = So; 6 t , = - 1 P ; Re, = 1.1 (lo6).

- - - FOREBODY STRAKES

(A) a = 36 DEGREES FIGURE 86. EFFECT OF FOREBODY STRAKES (@ = 30') ON ROLLING MOMENT VARIATION WITH SIDESLIP; O.16SCALE FJA-18; 6, = 35O; 6h = -120; Re, = 1.1 (1061.

ORIGINAL PAGE IS OF POOR QUALITY

- - - FOREGODY STRAKES

a - - FIGURE 86. CONCLUDED.

above t h e maximum h a l f b r e a d t h l i n e w i t h i n which f a v o r a b l e l a t e r a l s t a b i l i t y e f f e c t s a r e achieved: 30"s +560°.

Porebody S t rakes ( R a d i a l Location: 0'1 S t r a k e s l o c a t e d a l o n g t h e MHB, approximately 80" i n l e n g t h and 3 " maximum width ( f u l l - s c a l e dimensions) are g e n e r a l l y d e s t a b i l i z i n g i n r o l l , a s Shawn i n F i g u r e 87 a t a!= 35" and 40". Water t u n n e l flow v i s u a l i z a t i o n tests r e v e a l e d a d i s r u p t i o ~ i of t h e forebody v o r t e x flow f i e l d due t o s t r a k e s mounted i n t h i s manner. The s t r a k e s presented a d i s c o n t i n u i t y i n t h e body vortex-feeding mechanism and t h e s t r a k e s shed a vake-like, r a t h e r t h a n v o r t e x , f lw which impeded t h e development of a body primary v o r t e x p a i r .

The h i g h l y n o n l i n e a r r o l l i n g moment v a r i a t i o n w i t h s i d e s l i p i n F i g u r e 87 ( u n s t a b l e s l o p e a t small p ' s ; s t a b l e s l o p e a t h i g h e r p ' s ) is s i m i l a r t o t h e C vs. p v a r i a t i o n a t h i g h arsle of a t t a c k on a s l e n d e r wing. Vortex b u r s t

e

asymmetry is t h e s o u r c e of such v a r i a t i o n s on a cranked wing, f o r example.

W a t e r t u n n e l f l o w s u r v e y s o f t h e 4 - 0 " s t r a k e p o s i t i o n w e r e i n c o n c l u s i v e however, due t o t h e d i f f u s e n a t u r e of t h e body flow f i e l d .

Nose s t r a k e s a l o n g t h e maximum h a l f - b r e a d t h have g e n e r a l l y been a p p l i e d t o improve d i r e c t i o n a l s t a b i l i t y , u t i l i z i n g t h e windward s t r a k e f l o w t o g e n e r a t e s t a b l e yawing moment increments. Accordingly, t h e wind t u n n e l test r e s u l t s i n F i g u r e 87 show a s t a b i l i z i n g nose-strake c o n t r i b u t i o n t o yawing moment a t Q= 35Oand 40".

Forebody S t r a k e s ( R a d i a l Locations: -15" and -30") Radome s t r a k e s p o s i t i o n e d 15" below t h e MHB (+= -15") g e n e r a l l y show no improvement i n high angle-of-attack r o l l i n g moment c h a r a c t e r i s t i c e . A s shown i n Figure 88, however, radome s t r a k e s a t + -30" provide a l a t e r a l s t a b i l i t y i n c r e a s e a t ~ = 4 0 " , f o r example. S i n c e t h e flow mechanism a s s o c i a t e d w i t h s t r a k e s mounted i n t h i s manner is expected t o d i f f e r from t h e top-mounted st rake p o s i t i o n s , a more d e t a i l e d d i s c u s s i o n is p r e s e n t e d below.

FIGURE 37. EFFECT OF FOREBODY STRAKES (9 = 0°) ON ROLLING MOMENT AND YAWING MOMENT VARIATION WITH SIDESLIP; O.1tISCALE FIA-18; 6, = 25O; bh = -12O; Rec= 1.1 (106).

Forebody S t r a k e s ( R a d i a l Location: -4S0L S t r a k e s S45, S47, and S48, c h a r a c t e r i z e d by l e n g t h s of approximately 164", 102 ", and 56 " ( f u l l - s c a l e ) , r e s p k c t i v e l y , were mounted a l o n g t h e 0.16- s c a l e model forebody a t 45' below t h e MHB. Each s t r a k e was 3.13" ( f u l l - s c a l e ) maximum width t a p e r i n g t o z e r o a t t h e nose.

The wind t u n n e l r e s u l t s i n F i g u r e 89 show l a r g e i n c r e a s e s i n l a t e r a l s t a b i l i t y a t h i g h a n g l e of a t t a c k (Note: (r=40°, & = 4 0 ° ) , p a r t i c u l a r l y f o r t h e l o n g e r s t rake l e n g t h s . R e s u l t s a t n e g a t i v e s i d e s l i p show l i t t l e e f f e c t due t o t h e S48 s t r a k e , y e t at p o s i t i v e s i d e s l i p a n g l e s t h e same s t r a k e s induce a h i g h l y - s t a b l e v s r i a t i o n of r o l l i n g moment w i t h s i d e s l i p . F a c t o r s such a s st rake geomet ry and p o s i t i o n asymmet t i e s and d i f f e r e n t primary boundary l a y e r s e p a r a t i o n c h a r a c t e r i s t i c s a f t of t h e s h o r t s t r a k e s a r e p o s s i b i e s o u r c e s of t h e anomaly.

V i s u a l i z a t i o n s t u d i e s i n t h e w a t e r t u n n e l s u g g e s t e d a p l a u s i b l e flow mechanism a s s o c i a t e d w i t h t h e g i v e n s t r a k e s . The s t r a k e v o r t i c e s a r e of s u f f i c i e n t s t r e n g t h t o induce flow reattachment above t h e s t r a k e s . A s a r e s u l t , t h e body primary v o r t e x p a i r t e n d s t o d e v e l o p i n a more s ~ m m 2 t r i c manner i n s i d e s l i p . The f l o w ' s t u d i e s r e v e a l e d a weakening of t h e body primary v o r t i c e s r e l a t i v e t o t h e 4 - +40° c a s e s i n c e t h e s t r a k e s promote a d i s c o n t i n - uous boundary l a y e r s e p a r a t i o n l i n e p a t t e r n . Although t h e s t r a k e v o r t i c e s do n o t f e e d d i r e c t l y i n t o t h e body p r i m a r i e s , t h e s t r a k e s s e r v e much t h e same purpose i n l i m i t i n g t h e r o t a t i o n of t h e s e p a r c i o n l i n e s i n s i d e s l i p . A t h i g h e r sl l e s l i p a n g l e s , t h e leeward s t r a k e a p p e a r s t o shed a wake i n s t e a d of a d i s c r e t e v o r t e x . Consequently, t h e flow mechanism d e s c r i b e d above begins t o break down w i t h a r e s u l t i n g u n s t a b l e v a r i a t i o n of r o l l i n g moment w i t h s i d e - s l i p .

The i n c r e a s e i n d i r e c t i o n a l i n s t a b i l i t y a t h i g h a n g l e s of a t t a c k is a s s o c i a t e d w i t h t h e i n c r e a s e d s u c t i o n p r e s s u r e s a l o n g t h e leeward f u s e l a g e s i d e , i n much t h e same manner a s t h e 4 = +40° case.

I n summary, w a t e r t u n n e l flow v i s u a l i z a t i o n and wind t u n n e l t e s t d n t a a n a l y s e s i n d i c a t e t h a t t h e flow mechanisms and e f f e c t s on high-ru l a t e r a l -

I - B A S E L I N E

- - - FOREBODY S T H A K E S ( S 45)

FIGURE 89. EFFECTS OF FOREBODY STRAKES (4 = -45O) AND STRAKE LENGTH ON ROLLING MOMENT AND YAWING MOMENT VARIATION WITH SIDESLIP; a, = 400; a, = -120; R%= 1.1 (1081.

d i r e c t i o n a l s t a b i l t t v of forehodg s t r a k e s a r e dependent on s t r a k e r a d i a l l o c a t i o n and, t o a somewhat l e s s e r d e g r e e , on s t r a k e l e n g t h . I n terms of 131-era1 s t a h i l i t y improvements, l o c a t i n g t h e radome s t r a k e s i n a r e g i o n of low l o c a l anp,le of a t t a c k , a t + = +40° o r + = -45", f o r example, i s an e f f e c t i v e means of g e n e r a t t n g d i s c r e t e s t r a i e v o r t i c e s which s e r v e t o enhance t h e body v o r t e x s t r e n g t h s a n d / o r l i m f t t h e change i n body v o r t e x c o r e p a t h s due t o s i d e s l i p . S t r a k e s mounted a t t h e MHB a r e immersed i n a h i g h e r l o c a l a n g l e of a t t a c k flow. The s t r a k e v o r t i c e s a r e , t h e r e f o r e , l e s s s t a b l e a t high a's a n d , i n g e n e r a l , Dromote a l e s s - d e f i n e d f o r e b o d y f l o w f i e l d .

I t sclould be noted t h a t , i n a n o v e r a l l a s s e s s m e n t , t h e strake-induced l a t e r a l s t a h i l t t y improvements may be o v e r r i d d e n b y t h e c o n c u r r e n t yaw i n s t a - h i l i t y a t high a's. I n a d d i t i o n , e x c e s s i v e l a t e r a l s t a b i l i t y i s u n d e s i r a b l e s i n c e t h e a i r c r a f t w i l l be s l u g g i s h t o c o n t r o l i n p u t . P o s s i b l e d e g r a d a t i o n of r a d a r ~ e r f o r m a n c e and i n g e s t i o n of t h e s t r a k e v o r t i c e s i n t o e n g i n e i n l e t s ( a t low a' s) a r e a d d i t i o n a l c o n r i d e r a t i o n s .

F l i g h t T e s t Nose Room

-

Low-speed wind t u n n e l t e s t r e s u l t s a t a = 35' o b t a t n e d on t h e 0.16-scale FIA-18 morlel w i t h f l i ~ h t t e s t nose boom, shown i n F i g u r e 9 0 , r e v e a l sinall u n s t a h l e e f f e c t s i n r o l l a t s m a l l s i d e s l i p r e l a t i v e t o t h e b a s e l i n e . Water t u n n e l f l o w v i s u a l i z a t i o n i - d i c a t e d t h a t t h e wake s h e d by t h e n o ~ e boom Impeded t h e f o r m a t i o n of t h e b o d y p r i m a r y v o r t i c e s , a l t h o u g h t h e v o r t e x o r i ~ n t a t f o n r e l a t i v e t o t h e b a s e l i n e appeared n o t t o be a f f e c t e d .

F l i g h t T e s t Nose Boom and Forebody S t r a k e s ( = +40°) I n s t a l l a t i o n o f t h e f l i g h t t e s t nose boom i n c o n j u n c t i o n w i t h radome s t r a k e s mounted a t 40' above t h e MHR p r o v i d e s a d d l t i o n a l i n s i g h t i n t o t h e powerfrll forehody flow mechanism a s s o c i a t e d w i t h t h i c s t r a k e p o s i t i o n . It was observed t h a t t h e 4 = +40° s t r a k e s a r e e x t r e m e l y e f f e c t i v e L n enhancing t h e hodv v o r t i c e s and, a l s o , i n d i c t a t i n g t h e body v o r t e x i n t e r a c t i v e b e h a v i o r w i t h t h e wing f l o w . D e s p i t e t h e n o s e boom wake, t h e s t r a k e e f f e c t s a r e e s s e n t i a l l y unchanged a t (r=35O and 40' w i t h i n t h e s i d e s l i p a n g l e r a n g e of about -6'5 p 5 6', a s shown i n F i ~ u , r e 91. A t h i g h e r s i d e s l i p a n g l e s t h e nose

/ /-

N O # BOOM FIGURE 90. EFFECT OF FLIGHT TEST NOSE BOOM ON ROLLING MOMENT 4ND YAWING MOMENT VARIATION WITH SIDESLIP; 0.1G-SCALE F/A-18; 6, = 250; = -120; ReE= 1.1 (106).

I + $ Z L w w E -l o a s o u y

z z

-Id

6.2

$3 .

0 " ,r

z s s 02- 0 0 ' : :2; C O Y p o a z a 9 C .

+ o & C O N w > 7 + a I,

:s =

(3 w 7.

i K O , & , O N u .. II 0% c I--Iyo o w w W

ue

boom flow f i e l d e f f e c t s a r e evidenced by a r e d u c t i o n i n body v o r t e x s t a b i l i t y and a c o r r e s p o l i d i ~ ~ g u n s t a b l e v a r i a t i o n o f r o l l i n g moment w i t h s i d e s l i p .

'Wing Rock " Phe~lomena A ~ l a l y s i s of t h e w a t e r t u n n e l r e s u l t s o b t a i n e d ou t h e 0.025-scale F/A-18 model have r e v e a l e d s e v e r a l c o r r e l a t i o n s o f t h e v o r t e x f lw b e h a v l o r w i t h t h e t r e n d s observed i n l o r s p e e d wind t u l ~ n e l tests. The water-to-air c o r r e l a t i o n s have eve11 extended t o "wing rock" phenomena. S t u d i e s made a t N A S A Langley Research C e n t e r of t h e 0.16-scale F/A-18 model ullconstrained i n r o l l r e v e a l e d t h e following: No "wing rock" was e v i d e n t f o r t h e b a s e l i n e (&=25*, 35O) o r w i t h LEX m o d i f i c a t i o n s (LEX f e n c e s , i n c r e a s e d forward s l o t width and l e n g t h , e t c . ). W i t t i nose s t r a k e s mounted 40 d e g r e e s above t h e HHB, however, modest "wing rock" was observed a t (Y-35" t o 40" w i t h a maximum a m p l i t u d e of approxl- mately - + l o 0 ) .

Northrop w a t e r t u n n e l s t u d i e s of t h e 3.025-scale F/A-18 i n d i c a t e d t h a t t h e n o s e s t r a k e s promoted a s l i g h t l y o s c i l l a t o r y body v o r t e x p a t t e r n a t 0-35-40 degrees. T h i s appeared due t o a "hydrodynamic i n s t a b i l i t y " phenom- enon a s s c c i a t e d w i t h two v o r t e x c o r e s i n proximlty t o one a n o t h e r . A s t h e bod:. v o r t i c e s t r a v e r s e d t h e wing flow f i e l d , t h e leeward body v o r t e x showed a teildellcy t o pass u n d e r t h e windward body v o r t e x i n a p e r i o d i c manner. The b a s e l i n e and LEX mods r e v e a l e d o n l y s t e a d y body v o r t e x p a t t e r n s .

ANALYSIS OF SCALE-MODEL F/A-18 WIND TUNNEL D A T A During t h e course of t h e l o r s p e e d wind t u n n e l tests commencing i n 1979 i11 trle N A S A Langley Research C e n t e r 30x60-f o o t f a c i l i t y , comparisons of 0.06-, 0.07-, and 0.16-scale F/A-18 model d a t a r e v e a l e d l e r g e d i f f e r e n c e s i n l a t e r a l s t a b i l i t y l e v e l s st h i g h a n g l e s of a t t a c k iff =30°-40"). S p e c i f i c a l l y , t h e 0.06- and 0.07-scale b a s e l i n e models e x h i b i t e d highly-s t a b l e v a r i a t i o n s of r o l l i ~ ~ g moment w i t h s i d e s l i p . Conversely, t h e 0.16-eca l e F/A-18 r e v e a l e d a l a t e r a l s e ~ i s i t i v i t y a t s t a l l and p o s t - s t a l l a n g i e s of a t t a c k . The d i s c u s s i o n s t o f o l l o w w i l l a d d r e s s t h i s anomaly.

B a s e l i n e Cocf i a u r a t i o n s - L o n n i t u d i n a l C h a r a c t e r i s t i c s L o n g i t u d i n a l a e r o d y n a m i c d a t a a r e a v a i l a b l e o n l y f o r t h e 0.06- a n d 0.16-scale F/A-18 models. (The 0.07-scale model was o r i g i n a l l y f a b r i c a t e d f o r t h r e e - c o m p o u e u t ( l a t e r a l - d i r e c t i o n a l ) t e s t s a t t h e i i r g i n i a P o l . y t e c h n i c I n s t i t u t e 6x6-foot wind t u n n e l . ) A comparisoi~ of l i f t c h a r a c t e r i s t i c s of t h e 0.06- and 0.16-scale models o b t a i i ~ e d a t approximately t h e same Reynolds number based on r e s p e c t i v e mean a e r o d y u a m i c c h o r d s ( - 0 . 7 ( 1 0 ) ) is p r e s e n t . e d i n F i g u r e 92. The r e s u l t s w i t h f l a p s r e t r a c t e d ( & / d f = O O / O O ) a t P = O O i n d i c a t e a n e a r l i e r s t a l l of t h e l a r g e - s c a l e model. T h i s e f f e c t d i m i n i s h e s c o n s i d e r a b l y , however, w i t h lead- ing-edge f l a p s d e f l e c t e d t o 25'. Although t h e l i f t d a t a i n a s i d e s l i p coadi- t i o n (13' -4") a r e s i m i l a r , t h e l a r g e model does e x h i b i t a tendency t o s t a l l a t a s l i g h t l y lower a n g l e of a t t a c k . T h i s is r e f l e c t e d i n t h e p i t c h i n g moment d a t a a t @ = -4' i11 F i g u r e 93. The l a r g e model shows r s t a b l e s t a l l a t a lower l i f t coef f i c i e ~ i t r e l a t i v e t o t h e s m a l l - s c a l e F/A-18 model.

The r e s u l t s s u g g e s t t h a t t h e LEX v o r t e x b u r s t p r o g r e s s i o n on t h e 0.16- s c a l e model is s l i g h t l y more r a p i d r e l a t i v e t o t h e 0.06-scale F/A-18. Com- mects by NASA Lang l e y r e s e a r c h e r s in-rolved i n t h e F/A-18 t e s t program i n J i - cated t h a t d u r i n g smoke flow v i s u a l i z a t i o n s t u d i e s t h e s m a l l - s c a l e model LEX vorLices appeared s l i g h t l y more c o n c e n t r a t e d a t a g i v e n a n g l c of a t t a c k .

Although t h e s e o b s e r v a t i o n s a r e h i g h l y q u a l i t a t i v e , t h e y a r e c o n s i s t e n t w i t h t h e d a t a t r e n d s .

The d a t a d i f f e r e n c e s a r e g r e a t e s t i n t h e s t a l l angle-of-attack regime where s m a l l p e r t u r b a t i o i ~ s i n t h e e x t e r n a l flow f i e l d and s l i g h t d i f f e r e n c e s i n model c o n t o u r s can g r e a t l y a f f e c t s t a l l behavior. A s t h e a n a l y s e s p ~ o g r e s s , a p l a u s i b l e flow mechanism w i 11 emerge which a c c o u n t s f o r t h e model J i f f e r e n c e s a t h i g h a n g l e s of a t t a c k .

B a s e l i n e C o n f i n u r a t i o n s - L a t e r a l / D i r e c t i o n a l C h a r a c t z r i s t i c s The v a r i a t i o ~ ~ of r o l l i n g moment w i t h a n g l e of a t t a c k a t ,?=--4O, shown i n F i g u r e 9 4 , i s s i m i l a r f o r t h e 0.06- and 0.16-scale models. These r e s u l t s

FIGURE 93. COMPARISON OF 0.06- AND 0.16-SCALE FIA-18 PITCHING MOMENT CHARACTERISTICS; 0 = -4"; 6 , = 25"; hh = -12"

.*.. C..

., . , '.

, ... .

FIGURE 94. ROLLING MOMENT VARIATION WITH ANGLE OF ATTACK;

0.06- AND 0.16-SCALE FIA-18; 6 , = 25"; 6t, LI -12"; 0 = -4"

i n d i c a t e t h a t t h e aerodynamic flow phenomena a r e s i m i l a r , a l t h o u g h t h e d e g r e e t o which f l o w f i e l d changes o c c u r and t h e a u g l e s of a t t a c k a t which t h e l a t t e r art? e v i d e n t d i f f e r . An u n s t a b l e "break" i n r o l l i n g moment c o e f f i c i e n t o c c u r s a t an angle u i attack of about 20' f o r t h e 0.16- and 0.06-scale models.

A t a n g l e s of a t t a c k g e n e r a l l y g r e a t e r t h a n 20 d e g r e e s , t h e r a t e a t which r o l l i n g moment c o e f f i c i e n t d e c r e a s e s w i t h i n c r e a s e d a n g l e of a t t a c k is g r e a t e r f o r t h e 0.16-scale model. Furthermore, t h e o n s e t a n g l e of a t t a c k a t which r o l l i n g moment c o e f f i c i e n t a g a i n e x h i b i t s a s t a b l e v a r i a t i o n w i t h a n g l e of a t t a c k is approximately 5' h i g h e r ( a s 35') f o r t h e l a r g e model. These t r e n d s a r e ~ u d i c a t i v e o f a more p r o n o u n c e d a n d p e r s i s t e n t LEX v o r t e x breakdown asymmetry on t h e 3.16-scale FIA-18 i n t h e a n g l e of a t t a c k range of about 20" t o 40".

It s h o u l d be n o t e d t h a t t h e d i f f e r e n c e i n LEX v o r t e x b u r s t asymmetry between t h e models need not be g r e a t t o promote s i g n i f i c a n t v a r i a t i o n i n r o l l i n g moment c h a r a c t e r i s t i c s . F o r e x a m p l e , a d i f f e r e n c e o f windward v o r t e x b u r s t l o c a t i o n of j u s t a few p e r c e n t chord can g r e a t l y a l t e r t h e wing s t a l l behavior. Northrop low-speed wind t u n n e l t e s t s of a 0.10-scale F-5E (Reference 9 ) provide a good example of t h i s e f f e c t . S l i g h t l y below s t a l l a n g l e of a t t a c k (-26O) i t is known from w a t e r t , d n n e l t e s t s and wind t u n n e l w a t e r vapor r e s u l t s t h a t t h e I.EX v o r t e x c o r e - - d i s t i n g u i s h a b l e o n l y n e a r t h e LEX apex. D e s p i t e t h e f a c t t h a t windward LZX v o r t e x breakdown is s o f a r forward oa t h e LEX s u r f a c e , t h e vortex-induced e f f e c t s a r e s u f f i c i e n t t o d e l a y wing s t a l l . Should t h e b u r s t p o s i t i o n s h i f t forward s l i g h t l y , t h e windward wing passes through and t h e a b r u p t wing s t a l l promotes s u b s t a n t i a l reduct1011 i f i l a t e r a l The v i o l e n t n a t u r e o f t h i s l a t e r a l s t a b i l i t y l o s s a s s o c i a t e d w i t h asymmetric v o r t e x breakdown h a s a l s o been w i t n e s s e d i n Northrop wind t u n n e l L e s t s d u r i n g t h e development of t h e YF-17 (Reference 8). A sting-mounted model was o b s e r v e d t o o s c i l l a t e v i o l e n t l y a b o u t t h e body a x i s u n d e r t h e i a f luence of d i f f e r e n t i a l LEX v o r t e x breakdown p o s i t i o n s .

A s a n g l e of a t t a c k i n c r e a s e s from 30°, t h e F/A-18 LEX v o r t e x b u r e t p o i n t a t /3=0° approaches t h e L E X r i n g f u n c t i o n , a c o n d i t i o n which can be i n t e r p r e t e d

a s approximating C . I t is of i n t e r e s t t o n o t e t h a t v o r t e x breakdown

a t h i g h a n g l e s of has been d e t e c t e d by F/A-18 p i l o t s by a n i n c r e a s e i n n o i s e l e v e l o u t s i d e t h e c o c k p i t . T h i s is due t o t h e h i g h l y t u r b u l e n t n a t u r e of t h e b u r s t v o r t e x system. I n s i d e s l i p , phenomena s i m i l a r t o t h a t d e s c r i b e d f o r t h e F-5E c a n o c c u r . Due t o s m a l l p e r t u r b a t i o n s i n L E X v o r t e x b u r s t p o s i t i o n t h e wings t r a v e r s e C a t d i f f e r e n t a n g l e s of a t t a c k .

=MAX The " p i t c h sweeps" have been shown i n o r d e r t o r e v e a l i m p o r t a n t f o r c e and moment d a t a t r e n d s and t o h i g h l i g h t a r e a s of s e n s i t i v i t y . The fundamental q u e s t i o n , however, concerns t h e scale-model d a t a d i f f e r e n c e s a t a n g l e s of a t t a c k from approximately 30' - 40°, where t h e f u l l - s c a l e a i r c r a f t d u r i n g f l i g h t t e s t s h a s e x p e r i e n c e d a l a t e r a l s e n s i t i v i t y . Consequently, emphasis w i l l be made d u r i n g t h i s r e p o r t of "sideslip-sweeps" a t c o n s t a n t a n g l e of a t tack.

I t is n o t e d ? . h a t t h e a n a l y s e s c o n t a i n e d w i t h i n t h i s r e p o r t a r e n o t i n t e n d e d t o d e t e r m i n e which model d a t a a r e more c r e d i b l e . Indeed, t h e only model which h a s shown c o n s i s t e n t agreement w i t h t h e high- cr l a t e r a l s t a b i l i t y t r e n d s obsenred i n f l i g h t is t h e 0.16-scale c o n f i g u r a t i o n . The i n t e n t of t h e a n a l y s e s is t o provide a n u n d e r s t a n d i n g of p o t e n t i a l f l u i d flow v a r i a t i o n s which w i l l be of a i d i n f u t u r e t e s t programs designed t o a s s e s s t h i s problem i n d e t a i l .

R o l l i n g moment v a r i a t i o n w i t h s i d e s l i p oa t h e 0.06-, 0.07-, and 0.16- s c a l e models a r e p r e s e n t e d i n F i g u r e 95 a t a, =25O, 30°, and 35' w i t h leading- edge f l a p s u n d e f l e c t e d . H o r i z o n t a l t a i l d e f l e c t i o n a n g 1 . e ~ vary from O0 t o --12*, but t h e t a i l e f f e c t s on r o l l i n g moment e r e small enough a t t h e s e a n g l e s o f a t t a c k t h a t t h e c o m p a r i s o n s a r e v a l i d . A t a - 2 S 0 , a l l m o d e l s e x h i b i t u n s t a b l e r o l l i n g moment v a r i a t i o n s w i t h s i d e s l i p a t s m a l l s i d e s l i p a n g l e s , a l t h o u g h t h e s m a l l e r models a r e c h a r a c t e r i z e d by l e s s u n s t a b l e v a l u e s of r i l l i n g moment c o e f f i c i e n t . F o r comparison, 0.08-scale Northrop F-18L d a t a o b t a i n e d i n t h e Northrop 7x10-foot wind t u n n e l a r e shown, i n d i c a t i n g reason- a b l e agreement w i t h t h e l a r g e F/A-18 model a t a-25'. A t a = 3 0 ° , t h e s m a l l F/A-18 models (and t h e 0.08-scale P-18L) a r e i n f a i i - agreement, w h i l e t h e 0.16-scale model s t i l l e x h i b i t n an u n s t a b l e v a r i a t i o n of % w i t h p h t s i d e s l i p a n g l e s of about -6'50 5+ti0. The t r e n d c o n t i n u e 8 a t a-35' where t h e l a t e r a l

FIGURE 95. COMPARISON OF 0.06 -, 0.07 -, AND 0.16-SCALE F/A-18 ROLLING MOMENT VARIATION WITH SIDESLIP; 6, = 0'

sells i t i v i t y a t s m a l l s i d e s l i p of t h e 0.16-scale ~ / ~ - 1 8 is 111 marked c o n t r a s t t o t h e h i g l ~ l y - s t a b l e C v s . p v a r i a t i o n s e x h i b i t e d by t h e s m a l l m o d . 1 ~ .

Q

R o l l i n g momeut c h a r a c t e r i s t i c s a t a = 3 0 ° , 35', and 40" w i t h 4 ~ 2 5 ' a r e prrtirnted 111 F i g u r e 96 f o r t h e 0.06-, 0.07-, and 0.16-scale models. The d a t a t ~ - i . ~ i d s tend t o be c o i i s i s t e ~ l t w i t h t h e 6,=0° r e s u l t s . Consequellt l y , t h e model d a t a d i t t e rences c a ~ i u o t be a t t r i b u t e d t o leading-edge f l a p e f f e c t s ( a l t h o u g h account must be made of p o s s i b l e s l i g h t leading-edge f l a p a n g l e d i f f e r e l i c e s betweeii each model and, a l s o , on t h e same model). I n g e n e r a l , t h e 0.06- arid 6.07-scale F/A-18 models e x h i b i t h i g h l y s t a b l e r o l l i n g moment v a r i a t i o n s a t each a n g l e of a t t a c k w h i l e t h e 0.16-scale F/A-18 g e n e r a l l y shows a recuc- t i o i l i11 l a t e r a l s t a b i l i t y t o a n e a r n e u t r a l l e v e l . The wind t u n n e l clata provide 110 i i ~ d i c a t i o n t h a t Reyllolds number p l a y s a prominent r o l e i n t h e dpyareut "model-scale" e f f e c t . It is noted, though, t h a t t h e Reynolds numbers based 011 maximum body width a l l l i e w i t h i n t h e l a m i n a r range, according t o t h e procedure d e f i n e d ill Reference 10. Consequently, w i t h o u t correspoilding tlata i11 t h e t rails i t i oilal and fu l l y - t u r b u l e n t Reynolds number regimes, no conlllu- s i o n s can be made r e g a r d i n g Reynolds number e f f o r t s on t h e h i g h - a f or^ body-,-LEX v o r t e x i n t e r a c t ions.

The l a t e r a l s e n s i t i v i t y a t h i g h a n g l e s of a t t a c k is n o t confined t o t h e F/A-18, however. F o r comparison, Northrop 0.12-scale P-530 and Ci. 08-sc a l e F-18L d a t a a r e p r e s e n t e d I n F i g u r e 97. (The P-530 was a p r e c u r s o r of t h e Northrop YF-17. ) Any advanced f i g h t e r c o n f i g u r a t i o n which develops l a r g e amounts of v o r t e x l i f t is prone t o n o n l i n e a r b e h a v i o r n e a r s t a l l . F o r ex'lm- p l e , r e s u l t s from Reference 11 show s i m i l a r l e v e l s of l a t e r a l s t a b i l i t y a t h i g h a's 011 a 3-surface c a n a r d - d e r i v a t i v e of t h e F-15 and, a l s o , on an F-16 f i g h t e r model, both of which were c h a r a c t e r i z e d by powerful v o r t e x flows and v o r t e x i n t e r a c t i o n s .

The n a t u r e of v o r t i c e s is such t h a t t h e i r b e h a v i o r may n o t n e c e s s a r i l y be cons is t e n t from mode 1-to-mode 1 o r even on t h e same mode 1 d u r i n g r e p e a t runs.

For example, wind t u n n e l d a t a from Reference 12 shown i n F i g u r e 98 r e v e a l "model-scale " e f f e c t s , indepeltdent of Reynolds number, on t h e YF-17. The 0.03- and 0.08-scale models, both of which were b u i l t f o r high-speed t e s t i n g

FIGURE 96. COMPARISON OF 0.06 -, 0.07 -, AND 0.?6-SCALE FIA-18 ROLLING MOMENT VARIATION WITH SIDESLIP; 6 , = 2 5 ' ; 6,, = -12"

-

R

II a c o -

g

-

C C II IL 'a K LO d W

a

Y ' d

-

in the Northrop 2x2-foot transonic tunnel, develop different stall patterns near (Y=3O0. Consequently, the rolling moment variations with sideslip are quite different.

It is interesting to note that fighter pilots will often refer to a snecffic airplane within a fighter squadron since its behavior is distinguish- able from that exhibited by identical aircraft operating under the same flight conditions. This phenomenon appears similar to the "model-scale" effect and may he nttributable to subtle differences in aircraft geometry. For example, i t has heen established in flight tests of the Northrop F-5F that minute penmetrv differences in the nose region can, at very high angles of attack, cause one aircraft to nose slice right while another will slice left on a consistent basis. Furthermore, comments in Reference 13 indicate that a 0.10-inch "nick" (full-scale dimensions) in the LEX leading edge on an F-5E was the apparent source of large differences in lateral stability near stall angle cf attack in flight relative to the behavior of anotRer F-5E. These examples reveal the sensitivity of the forebody and ?,EX vortices at high nngles of attack to seemingly innozuo~.~s geometrv differences.

The F ! A - 1 8 yawing moment characteristics at ( ~ ~ 3 5 ' presented Ln Figure 99 ,qenerally shov reasonable agreement of all model data. In addition, the side force coefficient variations with sideslip at (Y=3!i0 in F i g ~ r e 99 are similar.

The w i d tunnel data comparisons show the "model-scale" sensitivity is tsolated to the rolling movent behavior with sideslip. The primary airframe contributor to sfatic la~eral stability is the wing. A check of the F/A-18 models at the NASA Lan~ley model shop using standard procedures revealed nc discernihl~ differences in LEX-wing positions and geometries. The remaining sections will provide evidence to support the conjecture that very subtle differences in the farebodg geometries are the source of model data disparity

-

due to the potentiallv-strong coupling of the F/A-18 forebody and LEX vor- tices. The model inspection cited above included an assessment of the fore- body contours. However, the inspection was not geared towards the identifica- tion of extremelv small model differences.

(A) YAWING MOMENT FIGURE 99. COMPARISON OF 0.06-, 0.07-, AND 0.16-SCALE F/A-18 YAWING MOMENT AND SIDE FORCE COEFFICIENT VARIATION WITH SIDESLIP; 6, = 25O; Sh = -12"; ry = 35"

FIGURE 99. CONCLUDED 2 2 ?

The d i s c u s s i o n s t o f o l l o w p e r t a i n t o t h e e f f e c t s of removing t h e twin v e r t i c a l t a i l s , t h e wing leading-edge e x t e n s i o n s (LEXs), t h e f u s e l a g e f o r e - horlv, and a d d i t i o n of forebody s t r a k e s . S i n c e t h e 0.07-scale F/A-18 was I n c o n s i s t e n t agreement w i t h t h e n.Q6-scale model, t h e former was t e s t e d a l o n g w l t h t h e 0. l h - s c a l e FIA-18 t o p r o v i d e comparative r e s u l t s t o a d d r e s s t h e "model-scale" e f f e c t .

V e r t f c a l T a i l E f f e c t s For i l l l l s t r a t i v e purposes, 0.07-scale F/A-18 r o l l i n g moment v a r i a t i o n w l t h a n g l e of a t t a c k a t P = - 5 O I s p r e s e n t e d i n F i g u r e LOO. 0.16-scale d a t a were nc;t a v a i l a b l e a t t h e same s i d e s l i p a n g l e . The t r e n d s , however, were s l a r F i g u r e 100 i n d i c a t e s t h a t removal of t h e v e r t i c a l t s i l s g e n e r a l l y promotes l a r e e u n s t a b l e r o l l i n g momer?: i n c r e m e n t s a t h i g h a's. The t a i l c n n t r l h u t i o n t o r o l l i n g moment a t low a n g l e s of a t t a c k i s of a d i r e c t n a t u r e .

Thc t a i l r e s u l t a n t l i f t a c t s a t a p o i n t above t h e moment r e f e r e n c e c e n t e r a n d , h ~ n c e , c o n t r i b u t e s s t a b l e , r o l l i n g moment increments. The C decrements a t

Q

h i g h 0's a r e a s s o c i a t e d p r i m a r i l y w i t h a n upstream i n f l u e n c e on t h e wing f l o w behavior drre t o removal of t h e v e r t i c a l s . I n t h e absence of t h e t a i l s , t h e L E Y v o r t i c e s a r e somewhat more s t a b l e , a l t h o u g h t h e v o r t i c e s b u r s t more asvmmetricallv i n s i d e s l t p . Removal of t h e v e r t i c a l t a i l s removes t h e a d v e r s e p r e s s u r e g r a d i e n t imposed on t h e f l o w f i e l d by t h e s e downstream o b s t a c l e s . A s a r e s u l t , t h e p o t e n t i a l f o r v o r t e x b u r s t asymmetry i n s i d e s l i p i n c r e a s e s .

4lthough t h e v e r t f c a l t a i l s a r e not t h e s o u r c e of s o d e l d a t a d i f f e r e n c e s , t h e r e s u l t s n o n e t h e l e s s r e v e a l t h e s e n s i t i v i t y of t h e F/A-18 LEX v o r t e x behavior t o downstream flow v a r i a t i o n s .

Recause t h e twin v e r t i c a l t a i l s impose a p o s i t i v e p r e s s u r e g r a d i e n t i n t h e flow f i e l d , t h e F/A-18 v o r t e x flow b e h a v i o r mav be somervhat more r e s i s t a n t t o s u p p o r t i n t e r f e r e n c e e f f e c t s . The r e s u l t s i n Reference 14 have shown a f t e r v o r t e x breakdown had b e e n e s t a b l i s h e d o v e r a d e l t a w i n g by i n s t a l l i n g a f l a t - n l a t ~ o b s t a c l e d o w n s t r e a m , v o r t e x s t a b i l i t y was i n s e n s i t i v e t o t h e i n t r o d u c t i o n of a p r e s s u r e probe i n t o t h e v o r t e x c o r e . Indeed, 0.16-scale F/A-18 t e s t s conducted i n t h e Langlev 30x60-foot t u n n e l usiiig :so v e r y i i f f e r - e n t sup^ 't arrangements s u g g e s t such e f f e c t s a r e , a t most, cecond o r d e r on t h e F/A-18.

FIGURE 100. ROLLING MOMENT VARIATION WITH ANGLE OF ATTACK; VERTICAL TAILS ON AND OFF; 0.07-SCALE FIA-18; 6, = 25'; ah = -12"; Re,-= 0.47 (lo6); P = - 5 O

Variations of rolling moment with sideslip were not available for the large model witilout vertical tails. The available 0.07-scale model results shown in Figure 101, however, provide interesting trends. At a=30°, 3 5 ' , and 40" the model with tails removed is characteri~ed by an unstable variation of rcllin~ moment with sideslip st small sldeslip angles. Indeed, the levels of lateral stahilitv are remarkably similar to the taseline 0.16-scale model behavior. These results support in an indirect manner the conjecture that the baseline 0.16-scale model develops slightly more asymmetric vortex bursting in sideslip rglative to the baseline 0.06- and 0.07-scale F , ' A - 1 8 models.

It will he shown that tne differences in LEX vortex burst asymmetry in sideslin can be attribured not to downstream flow variations but, instead, to different forebody-LEX vortex coupling mechanisms.

Wina Leadinrz-Edge Extension (LEX) Effects Removal of the wing leading-edge extensions results in large lift reduc- tions, parttcularly at the higher angles of attack where the winas without T.EX have stalled. This effect is illustrated for the 0.16-scale model in Figure 1 0 2 . Fven :bough LEY vortex bursting at a=30° - 4 0 ' has advanced far upstream on ';he wing, the LEX vortex-inrluced lift increments are very large. The LEX vortex flow field greatlv alters the wing spanwise lift distribution. Con- sequently, rlisturbances in the vortex behavior due, say, to sideslip can promote potentiallv-large changes in the spanwise lift distribution and, hence, rolling moment ~hcracceristlcs.

The wind tunnel result3 suggest, again in an indirect fashion, that the n.16-scale model with LEXs on develops a more asymmetric LEX vortex burst pattern relative to the 3.07-scale F/A-18. Figures 103 and 104 present rolling moment and yawing moment coefficient variations with sidesli,) for the 0.16- and n.07-scale models, respectively, at (r=30°, 3 5 ' , and 40". Larg:: stahle rolling moment inc-ements arise due to LEK removal on the large-scale F/P.-18 (see Figure 103). However, as shown in Figure 104, the 0.07-scale model shows onlv s all variation with LEXs off. Furthermore, very good data aqreerent is ohtained for the 0.07- and 0.16-scale models with LEXs off.

u ' ORfGrNXi PAGE i8 OF POOR QUALITY

FIGURE 102. EFFECT OF LEX REMOVAL ON 0.16-SCALE FIA-18 LIFT CHARACTERISTICS; 6, = 25'; 6h = - 1 2 ' ; Rec= 1.1 (lo6)

ORIGINAL PAGE IS OF POOR QUALITY

T l l e r e f o r e , removal of t h e LEX v o r t i c e s from t h e flow f i e l d and, h ,rice, e l i m l - n a t t o n of f orebody. T,EX v o r t e x i n t e r a c t i o n , promotes scale-model d a t a corr-.] - t i o n .

Forcbody E f f e c t s To t h i s p o i n t 11 t h e a n a l y s e s , i t h c s become e v i d e n t t h a t -1 fundamental f 1 ow f i e1A d i f f e r e n c e e x i s t .+ hctwaen t h e s m a l l (0.06- and 0 . 0 7 - s c a l e ) and l a r g e (n.ih-scale) model2 c a u s i . ~ p a s u b s t a n t i a l v a r i a t i o n i n l a t e r a l s t a b i l i , ~ l e v e l s near - t a l l a n g l e of a t t a c k .

Removal of t h e fuseLage forebody is expee.ted t o provide a r e a l i s t i c assessment of t h e incremental forebody e f f e c t s Qn i a t + 3 1 s t a 3 i l i t y a t t,lo,h a n g l e s of a t t a c k (3C0- 40"). The FIA-18 forebody geomztry ( c r o s s - s e c t i o n a l shape, f l n e n e s s r a t i o , e t r .) f a such t h a t st h i ~ h a ' s t h e c o n t r i b u t i o n t o yawing moment i s s m a l l ( d e s t a b i l i z i n g ) . Furthermore, fcrehody v o r t e x i n t e r a c - t i o n s w i t h t h e v e r t i c a l t a i l s u r f a c ~ s a r e minimal a c c o r d i n g t o 11ow v i s u a i l z a - - t i o n s t u d i e s i g t h e w a t e r t u n n e l . Absence of t h e f u s e l a g e forebody hou-~dary l a v e r should not a t f e c t t h e LrY v o r t e x b e h a v i o r t o any s i g n . ' f i c a n t e x t e n t .

For example, Vsrthrop w.ster t u n n e l s t u d i e s of f u l l - s p a n and h a l f - s p a n d e l t a wings have kndfcated t h a t t h i r e f l e r t i o n p l a n e b ~ u n d a r y l a y e r on t h e h a l f - s p a n wing model nad no o b s e r v a b l e e f f e c t on the vot :er s t a b i ! i t y c h a r n c t e r i s t i c s when compared t o t h e f u l l - s p a n wina v o r t e x b e h a v i o r . T;%e primary e f f e c t t o be r e v e a l e d by removal of t h e f o r e b o d y , t h e n , i s rhe d e g r e e of c o u p l i n g between t h e forebody and LEX-wing flow f i e l d s and, hence, t h e c o n t r i b u t i o n of the forehodv v o r t i c e s t o l a t e r a l s t a h i l i t y .

R o l l i n g moment and yawing moment v a r i a t i o n s w i t h s i d e s l i p a t f2=3C0 t o 40° a r e p r e s e n t e d I n F i g u r e 1 0 5 f o r t h e 0 . 1 6 - s c a l e F/A-18. M i n i m a l e f f e c t s a r e e v i d e n t due to absence of t h e forebody. From t h e s e r e s u l t s i t can concluded t h a t t h e forebody v o r t e x i n t e r a c t i o n s w i t h t h e vicy flow f i e l d h i g h a n g l e s of a t t a c k a r e not s i g n i f i c a n t on t h e l a r g e - s - , a l e model. These t r e n d s a r e c o n s i s t e n t w i t h t h e i n i t i a l w a t e r t u n n ~ i s t u d i e s of t h e 0.025-scale h a s e l i n e model which d i d n o t e x h i b i t d i s c e r n i b l e , s t r o n g body-LEX v b S , t e x i n t e r a c t t o n s .

cr~:Glf\lAL PAGE IS OF POOR QUALrfY

r

I I 7 9 rY 9 " 9 8

8: ' t o 1 oC ; 0 CY 9 0 7

f 0

Data obtained o i l the 0.07-scale model with forebody on and off indicate a measurahlt- dfffcrznre in lateral stability levels at 0=30° to 4 0 ° , as shown in Figure 106. Removal of the forebody promotes large unstable rolling moment increments. Thtse data stronglv suggest that the 0.07-scale model with forshndv on develops a favora5le flow interaction b e : w e e n the body and LEX vortices siqilar to the flov characteristics obscrved during repeat flow visualization runs of the b3sellne wafer tunnel model (after reinstallation of t ! ~ e LEKS). The latter low surveys snowed significant flow field changes, narticr~larlv in regards tc the wind-rard wing stall behavior, associated with a snail chenge in the forebody vortex pair orientation. Furthemcre, when the Forehodv vas removed, it was observed that the vortex-induced effects on th2 windward w i ~ p with forebodv on were noticeably absent. This is consistent with the results obtained on the O.n?-scale yodel. In addition, it is conjec- tt~red that similar flow phenomenz are developed on the 0.06-scale F/A-i8 nodel, si;,co the small-scale wind csnnel data were in ccnsistent agreeaent.

!he results show. t n Fjpures 105 and 106 support the hypothesis that the vortices develo?ed c n the forebody art 2 key to aderstanding thehigha"sca1e" of 'cc:s. F?r e x a m c ? ~ , coaparisorl of the f orebod"-of f roiling mcment and -.ad<ng rr.01~3t ;zri?ticns with stdeslip at c r = 3 S 5 , 3 5 O , and 40" indicates -:frv ~ o o k a ~ r o e m z ~ ~ or cho '3-07- m c i 3.f6-scale %ode1 data. In a manner siniiar t o :he LEKS-cfZ .:ese, elfnination of forebody-LEX vortex coupling at h i + z - . : i ~ ? e s of aftack ~romctes good ccrrelacioz of the sub-scale wind tunnel m.):leI tdat3. This aRreeTent t s an indication of different body vortex hehav- T-r .?a the !)aseif*~e F!A-lfi models resultinz from such factors as slight Fcrehndv s i s a ? i ~ ! i + t n + , silhtle forehortv cross-sectional ,hape variations, model s:1pDorc riqidi KV, e r r . Plausihi-12 flow necha2i sms are sketched in Figure 107.

'The different forehody vortex patternr pay be due to a slfght difference ic leewan? and windward prfnary boundary layer seqaration l ' . ~ . e locatiors &long :::e tes~ecttve fuselage forebody sides. The 0.07-scale model may feature a rotation of the fore87av vortex s y s t e m towards the windward side as sketched ir! F i e i l ~ - e i 0 7 . This rotation of the vortzx ~ a j r results i~ Ir.,:reased i~terac- tfon with the windw..cra ving and, consequently, sthble rolling moment incre-

ments. 111 contrast, 01e 3.16-scale FIc.-~~ develops a relatively uncoupled

n v r a 1eewa.-A hcdy vortex peir, the forrrler shearing away from the

fuselage. As a result, no flow mechanism exists that would provide favorable spanwise flow components on the windward wing.

Forebodv Strake Effects Further corroboration of the significance of the forebody vortex flow field in tnfl~~encing the high angle-of-attack lateral stability characteris- tics is provided bv comparisons of nose strake effects on the 0.06-, 0.07-, and 0.16-sca?e F/A-18 models. Direct comparisons are generally not possible due to, for example, different leading-edge flap deflection angles. Conclu- sions can still be made, however, regarding the relative effects of radome strakes mounted at varying radial positions.

Addition of nose strakes at 6 4 0 ° to the 0.07-scale model ( dn==2!i0) DroEotes unstable rolling moment increments at a=30°, 35O, and 40°, as shown in Figure 108. The adverse effect of the strakes mounted in this position at i h a's is indicative of a disturbance of the favorable forebody vortex inte -actions with the wing flow field. With strakes off, the body primary vortices assume an orientation in sideslip which results in strong vortex- in& : e r l effects on the windward wing panel. The vortices shed by the nose str,kes are not o~timally positioned, and as a result, tend to oppose the vortices formed by flow separation along t . . : :l,selage aides. The stability and trajectories of the body vortices are altered such that the windward wing exhihits more pronounced flow separation.

Differences in the primary boundary layer separation line locations on the n.n?- and O.lb-scale mxiels arc e~ldenced t y the highly-favorable effects shown ilr Figu.e 109 on the 0.16-scale model rolling moment variation with sideslip associated with stral-es mounted at +=+40°. The radome strakes are more nearly-aligned wj'h the separatlon lines in this region of the large model and tend to enhance, rather than disrupt, the forebody vortices. The vortex hehavior with strakes on becomes comparable to the flow about the small model without strakes. This is apparent in the reopective variations of rolling moment with sideslip at high angles of attack. Conversely, the variations of rolling moment with sideslip for the small model wilh str~kes and the large model wf t h strakes off are similar, which also suggests similar iorebody vortex flow field characteristics.

ORIGINAL PAGE IS OF POOR Q U A t W

Another example t o s n p n o r t t'le c o n t e n t i o n of d i - s s i m i l a r boundary l a y e r s e p a r a t i o n c h a r a c t e r t s t i c s on t h e emall and l a r g e models i s provided by d q t a o h t a i n e d on t h e 0.06-scale F/A-18 ( t e s t e d i n t h e NASA Ames ResearcLl C e n t e r I?-foot ~ u n n e l ) and 0.16-scale model ( t e s t e d a t NASA Langley) w i t h YF-17-type q a s e s t r a k e s mor~nted a ? m g t h e maximum h a l f h r e a d t h . R e c a l l t h a t water t u n n e l flow v i s u a l i z a t ion :esu its r e v e a l e d a d i s r u p t d o n of t h e body v o r t i c e s due t o s t r a k e s mounted a t t h e HIlh. Hence, i f d i F c e r e n c e s e x j s t i n t h e body v o r t e x s t r u c t u r e on t h e S U ~ J - s c a l e h a s e l i n e mrdels, s t r a k e s a t t h e WiR shc 1 reduce t h e flow ~ n o m a l l e s . T h i s , i n t u r n d e c r e e s e t h e d i f f e r e n c e s i n hodv-LEX- wing Flow i n t e r a c t i o n s and, c o n f e ~ ~ e n t l y , t h e r o l l i n g moment c h a r a c t e r i s t i c s .

? 0 2 l i n g moment and yaw in^ moF,pnt v a r i a t i o n w i t h s i d e s l i p a t ~ ~ 3 5 ' i n F i g u r e 110 ~ h ~ n e x c e l l e n t agreement between tF,e 0.06-scale Ames d a t a and t h e 0.16- s c a l e 1,anpIev r e s u l t s .

As a f i n s 1 example o+ t h e forebody flow s e n s i t i v i t y t o d i s t u r b a n c e s a l o n g t h e radome, 0.98-scale North-op F-18L d c t a a r e p r e s e n t e d i n F i ~ u r e 111 which show t h e e f f e c t s of r.ose gr:t ( # 4 5 c?rborundum g r i t ) and w i r e t r i p s (0.0625" df amcter (1~1ode1-scal e ) ) mouuLed a t 20" above t h e maximum h a l f b r e a d t h . A t = 7 5 ' , f o r example, nose g r i t n r o a o t e s a s i g n i f i c a n t i n c r e a s e i n s t a b l e r r : l l i n a moment c o e f f i c i e n t a t a l l s i d e s l i p a n g l e s , whereas w i r e t r i p s a r e h i g h l v - d e s t a S i l i z i n ~ . The r e s p e c t i v e flow mechanisms a r e d i s s i m i l a r , t h e former t e n d i n g t o enhance t h e forebody-wing i n t e r a c t i o n s by promoting a more svmmetric hody v o r t e x p a i r i n s i d e s l i p and t h e l a t t e r c a u s i n g a d e c o u p l i n g of t h e forehodv and wing flow f i e l d s by d i s r u p t i n g t h e forebody v o r t e x flows.

ILANGLEY 30 x W FT TINWEL) 0.06-SCALE U M E S 12 FT TUNNEL) FIG JRE 110. COMPARISOh! OF YAWING MOMENT AND ROLLING MOMENT VARIATION WlTH SIDESLIP WlTH FOREBODY STRAKES I$ = 0'); 0.06- AND 0.16-SCALE F/L,-18 MODELS; CY = 35"; 6, = 25"; tih = -12"

NOSE WIRE TRlPS 7

I SURE 11 1. EFFECTS OF WOSE GRIT AND NOSE WIRE TRIPS ON 0.08-SCAE T'-1BL.

FOLI.!NG MOMEMT VARlA ClON WITH SIDESLIP; 6 , = 25O; = -10'; HeE= 1.0 ( * ~ ~ l

CONCLUSIONS -

The F/A-18 operating at high angles of attack develops powerful vortices shed from the fuselage forebody and wing leading-edge extensions (LEXs).

Recause of the close-coupling of the forebody and LEX surfaces the flow field Is characterized by strong interactions betveen the multiple vortices and the occurrence of vortex breakdown. Pcrthermore, the global f l w field becomes w r e coaplex in sideslip due to the asymmetric lranner in which the forescd-. .rtices interact with the LEX vortical ~lotions.

Results obtained in this study contract have revealed the sensitivity of the F/A-1S forebody-LEX vortex interactions and vortex breakdown charac- teristtcs at high angles of attack to relatively small variations in model Reornet rv .

LEX geometry changes near the apex have been shown in the vater tunnel f l o w visualization studies of a 0.025-scale F/A-18 model to promote differen- ces in L?T vortex behavior sufficient to alter the wing stall patterns, in some cases, an? the manner ? n which the forebody vortices interact with the lift in^ surfaces. Specifically, the primary LEX modifications considered in this investi~ation vere, in every case, means by vhich the available vorticity shed at the LEX leading edge was reduced at high angles of attack. As a consequence, LEX vortex stability and vortex breakdown asymetry in sideslip were decreased.

The flcw field observations in the hydrodynamic facility were consistent wfth the premature wing stall and increased levels of lateral stability near stall associated with the LEX mods determined in law-speed wind tunnel tests in the NASA Langley Research Center 30x60-foot facility. A compromise appears required in terns of maximum attainable lift and lateral stability at high angles of attack. The former varies directly vith LEX area whereas the latter appears to vary inversely vith LEX sire.

The sens:tivity of vortices shed from slender forebodies at high atti- tudes to vl+tually undetectable model distortions, Reynolds number, Mach

nrlnher. f teest ream condl t t o n s , model s u p p o r t I n t e r f e r e n c e and s u p p o r t rip;id- i t v , anonp o t h e r F a c t o r s , h a s been i n v e s t i ~ a t e d i n d e t a i l bv many r e s e a r c h e r s .

I n wind tunnel tests of a x i s y m e t r t c b o d i e s where t h e above f a c t o r s were c a r e f u l l v c o n t r o l l e d , v i r t u a l l y i m p e r c e p t i b l e model d i s t o r t i o n s n e a r t h e nose t i p promote l a r ~ e v a r i a t i o n s i n f o r c e s and moments.

C o n s i s t e n t w i t h t h e s e s t u d i e s , r e s u l t s o b t a i n e d i n t h e p r e s e n t r e s e a r c h e f f o r t have revealed t h e v o r t i c e s shed from t h e F/A-18 forebody t o be s e n s l - t t v e t o smell model d i s t o r t i o n s . A s a consequence of t h e highly-coupled n a t a r e of t h e forehodv and LEY v o r t i c e s on t h e F/A-18, s l i g h t changes i n bodv v o r t e x o r i e n t a t t o n can d i c t a t e t h e manner i n which t h e wing p a n e l s s t a l l a t h l ~ h a n e l e s of a t t a c k and i n s i d e s l i p c o n d i t f o n s .

Corroborative evidence of t h e powerful mechanism a s s o c i a t e d w i t h f o r e - bdv-1.FK v o r t e x i n t e r a c t i o n s was provided i n w a t e r t u n n e l f l o w v i s u a l 1 z a t t o n tests of t h e h a s e l l n e F/A-1R which i n d i c a t e d t h a t t h e e x t e n t t o whtch f l o w s e p a r a t i o n o c c u r s on t h e d n d u a r d v i q panel is d i r e c t l v r e l a t e d t o t h e manner t n w\ich t h e forehodv ~ ~ o r t l c e s o r i e n t themselves i n s i d e s l i p and, p a r t i c u l a r - l v , t o t h e n r o x i n i t v of t h e windward forebody v o r t e x . The F/A-1R forebodv--LEX Peomecrv is such t h a t t h e windward body v c r t e x is n o t a s strongly-coupled w i t h t h e LEY-wtng flow f i e l d a s t t s leeward c o u n t e r p a r t . However, s m a l l forehodv c o n t o u r d i s t o r t i o n s can promote a windward body v o r t e x t h a t is c o n s i d e r a b l y more coupled w i t h t h e wing flow. I n t h e l a t t e r s i t u a t t o n , a greatly-magnified r e s p o n s e c a n b e t r i g ~ e r e d 11. t h e d o w n s t r e a m f l o w f i e l d c h i i r a c t e r i s t i c s .

A d d i t i o n a l s u p p o r t i v e d a t a was o h t a i n e d i n f l o w v i s u a l i z a t i o n s t u d i e s of m a l l s t r a k e s svwnetricallv-deploved a l o n g t h e radome. A d i r e c t r e l a t i o n - s h i p was observed between t h e s t r e n g t h and o r i e n t a t i o n of t h e body v o r t t x f l o w s ( d e p e ~ d e n t on s t r a k e t a d l a 1 p o s i t i o n ) and t h e d e g r e e of f l o w s e p a r a t i o n from t h e win^ s o r f a c e s .

The q u s l l t a t i v e d 6 t a o h t a i n e d on forebodv v o r t e x b e h a v i o r i n t h e hydro- Avnrrmic f a c l l t t v a r e t n c o n s i s t e n t agreement w i t h smoke f l o w v i s u a l i z a t i o n and low-speed wind t u n n e l d a t a t r e n d s o b t a i n e d i n t h e Lanpley 30x60-foot f a c t li t-: r l ~ i n ~ t h e 0.16-scale F/ 4-1A model. F u r t h e m o r e , t h e w a t e r t u n n e l

r e s u l t s used i n p a r a l l e l w i t h a n a l y s e s of 0.06-, 0.07-, and 0.16-scale F/A-18 d a t a , provide a b e t t e r u n d e r s t a n d i n g of t h e i ~ s n l i n e a r b e h a v i o r of t h e s t a t i c a e r o d y n a m i c c h a r a c t e r i s t f c s a t h i g h a n g l e s of a t t a c k . R e s u l t s I n d i c a t e t h a t g e n e r a t i o n of forebody v o r t e x f love r e s i s t a n t t o eoyranet r i c o r i e u t a - t i o i l i11 s i d e s l i p is conducive t o h i g h l e v e l s of l a t e r a l s t a b i l i t y n e a r C .

h x

Previous Northrop rlud NASA s t u d i e s of t h e F-5 have shown t h a t t h e mallner i u which t h e forebody v o r t i c e s o r i e n t themsefvee i n s i d e s l i p call be c o r r e l a t e d w i t h t h e high l e v e l of d i r e c t i o n a l s t a b i l i t y e x h i b i t e d by t h e a i r c r a f t a t h i g h ailgles of a t t a c k . The v o r t e x f l o v s i t u a t i o n is i l l u s t r a t e d i n t h e w a t e r tuilliel photograph i11 F i g u r e 112. R e s u l t s from t h e p r e s e n t illvest i g a t i o n i n d i c a t e t h a t t h e F / A - I 8 forebody v o r t e x o r i e l l t a t i o l r a t h i g h a's is c o l l s i s t e l ~ t w i t h t h e l a t e r a l s t a b i l i t y behavior due t o t h e coupled net- re of t h e F-18 f orebody and wing flow f i e l d s . The flow fienomeno11 i s d e p i c t e d i n F i g u r e 113.

T h e increased complexity of t h e F-18 v o r t e x flow f i e l d r e l a t i v e t o t h e F-5 is a l s o e v i d e n t from a comparison of F i g u r e s 112 and 113.

Evidence compiled w i t h i n t h i s r e p o r t s t r o n g l y s u g g e s t s t h a t t h e a p p a r e n t model-scale s e n s i t i v i t y encountered i n t h e Langley F/A-18 wind t u n n e l t e s t s is a s s o c i a t e d w i t h d i f f e r e n t primary boundary l a y e r s e p a r a t i o l l l i n e l o c a t i o l l s aloug t h e f u s e l a g e forebcdy s i d e s resulting from small v a r i a t i o u s 111 forebody c r o s s - s e c t i o n a l shape. A s a consequence, t h e manner i n which t h e forebody primary v o r t i c e s i n t e r a c t w i t h t h e L E X r i n g flow f i e l d a t h i g h a n g l e s of a t t a c k is d i f f e r e n t on t h e s m a l l and l a r g e models. An a s y e t u u r e s o l v e d and c o n f l i c t i ~ t g anomaly is t h a t t h e 0.06- and 0.07-scale F/A-18 models, which were f a b r i c a t e d a t d i f f e r e n t s i t e s and u s i n g d i f f e r e n t m a t e r i a l s , y i e l d e d c o a s i s - telrt agreement t h r o u ~ h o u t t h e t e s t program.

The lw-speed wind t u n n e l d a t a s u g g e s t t h a t t h e body vortex-induced effec:s on t h e windward wing p a n e l s of t h e 0.06- and 0.07-scale F/A-18 models a r e g r e a t e r t h a n t h e corresponding e f f e c t s on t h e 0.16-scale model. The small-scale wind t u n n e l models a r e c h a r a c t e r i z e d by windward body v o r t i c e s l e e s prane t o " s h e a r i n g avay" from t h e f u e e l a g e . A s a r e s u l t , t h e windward wing e x p e r i e n c e s f a v o r a b l e vortex-induced sidewash n e a r s t a l l a n g l e o f a t t a c k .

FIGURE t IT. F - 5 ~ FOREBODY VORTEX ORIENTATION IN SIDESLIP I 0 = 400: I?J= -1W. (NQROHROP WATER TUNNEL) r I

- +

F . f Lid€ ?I3 I * V ? T E ~ P r > T T E IS 2 N THE F A - I 8 IN S i 3 E S P P. r, = 320 , -100 I YORThtFOP W A T E R T l 'NNELI.

..

ORIGINAL PAGE - 6 - COLOR PHOTOGRAPH P m m PAGE mnm- a m - . .

PRECEDING PAGE BLANK NOT FILMED llctatled examination o f the flow field abnut the 0.025-scale water tunnel model indicates that removal of the forebody providos a realistic assessment of the direct forehody vortex-induced effects on the LEX-wing flow rharacteristlcs. This is significant in that superfluous flow changes are not introduced that would, otherwise, preclude such an assessment. The same logic was applted to enalysis of the low-speed wind tunnel data which revealed an excellent match of small- and large-scale F/A-18 model rolling moment varia- tion with sideslip with forehodies removed. This correlation supports the hvpothesis that a key to understanding the apparent model-scale effpct is the forebodv vortex hehavior. Furthermore, addition of YF-17-type nose strakes also promoted an excellent match. Flow field surveys indicate that this strak~ arrangement impedes development of the hody vortices and, consequently, sill sipnificantlv reduce any differences that exist in the scale-model bodv vortex behavior.

Rased on the present results, it appears necessary to reduce the model tolerances on F/A-18-type configurattons for high angle-of-attack testing due tc the sensitivity of closely-coupled forebodp and LEX vwtex flows.

A large model is then more desirable for such testing since model tolerances can he more easily satisfied. Until experiments are conducted in a systematic marlner, however, one cannot define what a reasonahle tolera..ce level is or, indeed, vhether thts is the pivotal problem in the apparent scale-effect.

ORIGINAL PAGE I S

OF POOR QUALITY RECOINENDATIONS FOR FUTURE WIND T U V L TESTS Tt is recommended that, prior to further testing of the FIA-18 models, a verv detailed inspection be made of the 0.05-, 0.07-, and 0.16-scale node! s . Specif ically, incnect ion is recommended of: LEX, wing, horizontal and vertical tail locations on the fuselage to assure that symetrv exists and, also, that LEX incidence angle, wing dihedrai, vertrcal tail cant angles, etc. are correct; wing leadlng-edge radius, wing camber and thickness, and wine; Flap defl.ection angles are consistent; T,EX boundary layer bleed slot Reometrv, t E X thickness and camber, ~ n d leadl~g-edge sharpness are the same on all models. Particular attention should be paid to the fuselage forebody contours, surface finish, and forebody alignwent.

-

Em~hasis shot-ld be placed on ieentifving even very suhtle variations tn model lines since such differences car. trigger greatly magnified effects on hodv vortex hehavior. Any discrepancies in mo.el geometries uncovered during the model inspection should be resolved before conducting furqher wind tunnel tests.

Sontingent on determination of even smal.1 model distortions and their suhsequent elimination, it is recommended that the baseline F/A-18 models he retested in the Langley 3Ox6n-foot wind tunnel. Agreement sf the 0.06-, n.07-, and 0.16-scale model rolling moment variations with sideslip would confirm that very small mod-1 tolerances are necessary for high angle-of- attack test in^ of a'rcraft models whlch develop powerful vortex flows.

If the apparent model-scale effect persists, however, then the following discussion applies, To address the sensitivitv of high angle-of-attack lateral stability characteristics to model scale, a suitable alternate wind tunnel facility 1 1 s t he f d c n t i T i , : d . This facility must feature: accommodation of the small- . . iarae-scale FIA-18 models without prohibitive blockage; operation at a sufficient ranse of free-stream dynamic pressure to enable force and moment

ORIGINAL PAGE IS OF POOR QUALITY data to he ohtained on each model at the same Reynold number; flow visuafi- zation capabilities enabllng a qualitative determination of forebody and T,EX vortex core stability and trajectory characteristics and surface flow patterns; and capability, as required, of investigating more subtle flow details involving the behavior of the boundary layer flow.

The proposed facility is the NASA Langley Research Center VSTOL vind tunnel. Force and moment data should be obtained on the 0.06-, 0.07-, and 0.16-scale FIA-1R models with which to compare the existing data obtained in the Langlev 30x60-foot facility. Should the 0.06-scale model not be available from McDonnell-Douglas, then it is mandatory that the 0.07-scale T.angley model be capable of accommodating a six-component balance. A proper assessment of the high- a ! characteristics necessitates acquisition of all six force and moment components on the 0.07- and 0.06-scale F/A-18 models.

Saseline data trends obtained in the VSTOL tunnel consistent with existlng results would require assessment of the effects of forebody geometry changes on rolling moment variations with sideslip to confirm that the source o f data discrepancies lies in the forebody region. It is recommended that studies be made of the effects of radome strakes (including asymmetric strake deployment) a . i d removal of the fuselage forebody on the high-cr, character- ist ics, augmented by flow visualization. Fuselage f orebody and wing sv-f ace pressure instrumentation would be helpful in assessing pressure distributions along the forehody, boundary layer separation on the forebody, and wing spanwise lift distributions associated with the presenze of the f orebody and LFX vortex flows. Alternate forebody geometriee are desirable, particularly forehody shapes which limit the primary boundary layer separation degree of freedom. Nose anC aft strain Rage balances are recommenaed in order to assess the influence of the forebody on vertical tail loads.

Flow ~j~ri31i~ation techniques involving helium-filled bubbles, tuft ? r i d s , laser vapor screen, and surface oil flow visualization would provide valuable information regarding the overall flow characteristics. The helium- huhble method, tuft grids suspended above the models, or laser vapor screen would enable an aseessment of forebody and LEX vortex positions. Detailed

ORIGiiqAL PAGE IS OF POOR QUALITY sc~rface oil or napthalene flow visualization along the fusela~e forebodies will provide information on primary houndary layer separation line locations, reslons of boundary layer transition (characterized hy a "kink" in the separa- t inn 1 ine), areas of vortex-induced reattached f iow, and positions of second- arv houndary layer separation lines. Any differences in the surface flow characteristics on each model would be indicative of variatfons in the body vortex paths.

Detailed forehody bc~ndarp layer measurements are required if the data differences are vet unresolved. The extensive data base on slender missile ccnfiguratlons has shown that subtleties in the boundary layer separation character1 stics are the source of large global flow field variations.

Oeterminatton of such 'ltferences on the small and large F/A-18 models does not bode well for f ~ t u r e testing of such configurations since extremely small model tolerances are, in general, unrealistic in wind tunnel investiga- :ions. Onlv when a salient edge of separation exists along the forebodv will the confidence level with which sub-scale model data are correlated increase.

Should the VSTOL facility baseline data do not concur with previous test results hut, instead, exhibit scale-model correlation, it is recommended that investination he made of the effects of such factors as free-stream turbu- lence, model support rigiditv, and support interference. It is noted, how- ever, that parameters which have been shown to be of major import to the behavior of slender body vortices at high angles of attack may not be so on the F/A-18. The reason for this being the F/A-18 forebody vortex hehavior is influenced in large part by the powerful LEX vortices. The latter are shed from relatively thin, slender, sharp-edged surfaccs and are less sensitive relative to vortices developed on surfaces with boundary layer separation llne degree of freedom.

The National Transonic Facility (NTF) at Ls~gley Research Center is a suitable facflitv in which to assess Reynolds number and Mach number effects on the behavior of highly-coupied body and wing vortex flows.

A fundamental studv of forebody and wing vortex interactions is desira- ble. Results from the present study indicate that knowledge of the proper integration of forebody and wing geometries to ensure desirable high angle-of- attack stability and control characteristics remains lacking. A definitive data base on the effects of forebody length, cross-sectional shape, wing planform, aspect ratio, location, etc. on vortex development and interactive behavior at high a's would assist in the design process of highly-maneuverable fighter aircraft. The advent of supersonic-cruise fighter designs, inherent to which are more slender wing planforms, requires a knowledge of the flow be- havior at off-design conditions where flow separation in the form of concen- trated vortices occurs. Use of diagnostic hydrodynamic flow visualization and low-speed wind tunnel facilities (featuring six-component force and moment capability and high-quality smoke flow visualization to complement the dye- tracer method in water) prior to more sophisticated and costly wind tunnel test in^ is one means of enhancing the understanding of effective utilization of organized separated flows (vortex flow?).

REFERENCES 1 . Aviation Week and Space Technology, Vol. 114, No. 25, 22 June 1981, p. 18.

2 . Werle, Henri: F l o w Visualization Techniques f o r the Study o ..ndh- Incidence Aerodynamics. AGARD Lecture Series No. 121 on High Anl?;':e-of- Attack Aerodynamics, presented at the von Karman Institute, Brussels, Belgium, 22-23 March 1982.

3 . Erickson, Gary.: Vortex Flow Correlation. AFWAL-TR-80-3143, January 1981.

4 , Lamar, John E . ; and Frink, Neal T . : Experimental and Analytical Study o f the Longitudinal Aerodynamic Characteristics of Analytically and Empirically Designed Strake-Wing Configurations at Subcritical Speeds.

NASA TP-1803, June 1981.

5 . Erickson, Gary E . : Water Tunnel Flow Visualization: Insight to Complex Three-Dimensional Flow Fields. AIAA Paper No. 79-1530 presented at the

12t h Fluid and Plasma Dynamics Conference, Will iamsburg , Virginia, Jrily

10-12, 1979.

6. Erickson, Gary E.: Water Tunnel Studies of Leading-Edge Vortices.

Journal of Aircraft, Vol. 19, No. 6, June 1982.

7 . Johnson, Joseph L . . Jr.; Grafton, Sue B . ; and Yip, Tong P . : Exploratory Investigation of the Effects of Vortex Bursting on High Angle-of-Attack Lateral-Directional Stability Characteristics of Highly-Swept Wings. AIAA Paper No. 80-0463, March 1980.

R . Gerhardt, Heinz. A . : T h e Aerodynamic Development of the Wing Root Leading Edge Extension of the P530 Airplace Configuration. NOR 73-71, Northrop Corp., Aircraft Division, 1972.

REFERENCES (Continued) 9. Headley, J a c k W.: A n a l y s i s of Wind Tunnel Data P e r t a i n i n g t o High Angle of Attack Aerodynamics. Vol. I, AFFDL-TR-78-14, 1978.

10. Lamont, P e t e r J.: The Complex Asymmetric Flow Over a 3.5D Ogive Nose and C y l i n d r i c a l Afterbody a t High Angles of Attack. AIM Paper No.

52-0053, January 1982.

11. Agnew, J. W.; L y e r l a , G. W.; and G r a f t o n , S. B.: L i n e a r and Nonlinear Aerodynamic8 of Three-Surface A i r c r a f t Concepts. J o u r n a l of A i r c r a f t , Vol. 18, No. 11, November 1981, pp. 956-962.

12. Pietzman, F. W.: Low-Speed Wind Tunnel I n v e s t i g a t i o n t o Develop High A t t i t u d e Wall C o r r e c t i o n s i n t h e Northrop 7 x 10-Foot Low-Speed Wind Tunnel. NOR-78-20, Northrop Corp., A i r c r a f t Div., May 1978.

13. Skow, Andrew M.: Panel Discussion. AGARD Symposium on High Angle of Attack Aerodynamics, AGARD-CP-247, October 1978.

14. Hummel, D . : Experimental I n v e s t i g a t i o n of t h e Flow on t h e S u c t i c n ie,?

of a Thin D e l t a Wing. 2. Flugwiss., J a h r g . 13, Heft 7 , J u l y 1965, pp.

247-252.