Analytical investigation of the landing dynamics of a large airplane with a load-control system in the main landing gear

-

NASA Technical Paper ,1555

AnalyticalInvestigation of the

Landing Dynamics of a Large

Airplane With a Load-Control

System in the MainLanding Gear

John R. McGehee and Huey D. Carden DECEMBER 19 7 9 . ~.. .

TECH LIBRARY KAFB, NM

NASA Technical Paper 1555

AnalyticalInvestigation of the

LandingDynamics of aLarge

Airplane With aLoad-Control

System intheMainLandingGear

John R. McGehee and Huey D. Carden La q l e y Resen rcb Cerz ter Harnpto~z, Virgirlia National Aeronautics and Space Administration Scientific and Technical Information Branch

L

SUMMARY T h i s p a p e r p r e s e n t s t h e r e s u l t s o f a s t u d y to e v a l u a t e t h e c a p a b i l i t y of a na c t i v el o a d - c o n t r o ll a n d i n g gear computer program (ACOEAG) f o r p r e d i c t i n g t h el a n d i n gd y n a m i c so fa i r p l a n e sw i t hp a s s i v ea n da c t i v em a i ng e a r s ,a n dt h e a p p l i c a t i o no ft h a tp r o g r a mi na na n a l y t i c a li n v e s t i g a t i o no ft h ed y n a m i c b e h a v i o rd u r i n gl a n d i n g of a l a r g e a i r p l a n e e q u i p p e d w i t h b o t h p a s s i v e a n d a c t i v e m a i ng e a r s .C o r r e l a t i o n sb e t w e e nc o m p u t e d data from ACOLAG, computed d a t af r o m a f l e x i b l ea i r c r a f tt a k e - o f fa n dl a n d i n ga n a l y s i s (FATOLA), and pre- l i m i n a r ye x p e r i m e n t a ld r o p - t e s t data f o r a m o d i f i e d a c t i v e c o n t r o l m a i n g e a r from a l i g h t a i r p l a n e i n d i c a t e t h a t ACOLAG is c a p a b l e of p r e d i c t i n g t h e l a n d i n g d y n a m i c so fa i r p l a n e sw i t hb o t hp a s s i v ea n da c t i v el o a d - c o n t r o lm a i nl a n d i n g g e a r s . A p a r a m e t r i cs t u d y of p a s s i v ea n da c t i v em a i ng e a r si n d i c a t e st h a tt h e a c t i v eg e a r is more e f f e c t i v ei nr e d u c i n gt h em a g n i t u d eo f t h e forces t r a n s m i t - ted to t h ea i r c r a f ts t r u c t u r et h a nt h ep a s s i v eg e a r . The e f f e c t i v e n e s s was most pronounced for those touchdownconditions which would r e s u l t i n l a r g e a i r f r a m e - g e a ri n t e r f a c e forces w i t h t h ep a s s i v eg e a r , t h a t is, l a r g ea i r p l a n e m a s s e s , h i g h t o u c h d o w n s i n k r a t e s , a n d u p h i l l r u n w a y slopes. The a c t i v eg e a r is also e f f e c t i v ei nr e d u c i n ga i r p l a n em o t i o n sf o l l o w i n gi n i t i a l impact. The r e d u c t i o n of c y c l i c forces a s s o c i a t e dw i t h t h e a c t i v eg e a rf u r t h e ri n d i c a t e s t h ep o t e n t i a l for s i g n i f i c a n tr e d u c t i o n si ns t r u c t u r a lf a t i g u e damage d u r i n g g r o u n d o p e r a t i o n s .

INTRODUCTION Ground loads imposed o na na i r p l a n e are i m p o r t a n tf a c t o r si nt h ed y n a m i c l o a d i n go f t h e a i r f r a m es t r u c t u r ea n d may r e s u l t i n s i g n i f i c a n t f a t i g u e damage.

The g r o u n d - i n d u c e ds t r u c t u r a lv i b r a t i o n s also r e s u l t i n crew andpassenger d i s c o m f o r t ,a n do nl a r g ef l e x i b l ea i r p l a n e s these v i b r a t i o n sc a nr e d u c e t h e p i l o t ' s c a p a b i l i t y t o c o n t r o l t h e a i r p l a n e . These problems of ground-induced v i b r a t i o n sh a v eb e e ne n c o u n t e r e d w i t h some c u r r e n t l y o p e r a t i o n a l t r a n s p o r t a i r - p l a n e s , as d i s c u s s e di nr e f e r e n c e s 1 and 2 . Such problems w i l l be m a g n i f i e d for s u p e r s o n i c - c r u i s ea i r p l a n e sb e c a u s e of t h e i n c r e a s e d s t r u c t u r a l f l e x i b i l i t y i n h e r e n ti nt h e i rs l e n d e r - b o d yd e s i g n , their t h i n - w i n gc o n s t r u c t i o n ,a n dt h e i r h i g h t a k e - o f f a n d l a n d i n g speeds. F o r e x a m p l e , i n v e s t i g a t i o n s o f t h e g r o u n d r i d e q u a l i t i e s of o n ep a r t i c u l a rd e s i g n of a s u p e r s o n i ct r a n s p o r tc o n d u c t e di n t h e s i x t i e si n d i c a t e de x t r e m e l yh i g hv i b r a t i o nl e v e l si n t h eU n i t e dS t a t e si n t h e crew c o m p a r t m e n td u r i n gt h et a k e - o f f r o l l ( r e f . 3 ) .

A n a l y t i c a ls t u d i e s ( r e f s . 3 to 5) have been conducted t o d e t e r m i n et h e f e a s i b i l i t y of a p p l y i n g a c t i v e load c o n t r o l s to t h ea i r p l a n em a i nl a n d i n gg e a r t o limit t h eg r o u n d loads a p p l i e d to t h ea i r f r a m e . The s t u d y reported i nr e f - e r e n c e 4 i n d i c a t e s t h a t a s h o c k s t r u t w i t h a h y d r a u l i c a l l y c o n t r o l l e d a c t u a t o r i n series w i t ht h ep a s s i v ee l e m e n t so f t h e s t r u t p r o v i d e d t h e most d e s i r a b l e dynamic properties. T h i s s t u d yu s e d a l i n e a r model a s a s i m p l i f i c a t i o n of a n a c t u a la i r p l a n es h o c k - s t r u ts y s t e mb u t recommended t h a t a n y f u t u r e s t u d i e s s h o u l di n c l u d et h en o n l i n e a rl a n d i n g - g e a rc h a r a c t e r i s t i c s .B o t hn o n l i n e a r

I '

s h o c k - s t r u ta n d t i r e c h a r a c t e r i s t i c s were i n c l u d e di nt h es t u d y of r e f e r e n c e 5, i n which a m a t h e m a t i c a l model of a s t i f f a i r p l a n e w i t h a s e r i e s - h y d r a u l i c , a c t i v el o a d - c o n t r o lm a i nl a n d i n gg e a r was d e v e l o p e d .

The purpose of t h i s paper is to p r e s e n t t h e r e s u l t s from a two-phase pro- gram to e v a l u a t e t h e c a p a b i l i t y of a na c t i v el o a d - c o n t r o ll a n d i n gg e a rc o m p u t e r program (ACOLAG) f o rp r e d i c t i n gt h el a n d i n gd y n a m i c so fa i r p l a n e sw i t hp a s s i v e a n da c t i v em a i ng e a r s ,a n d to a p p l y t h a t p r o g r a m i n a n a n a l y t i c a l i n v e s t i g a t i o n of t h e d y n a m i cb e h a v i o rd u r i n gl a n d i n go f a l a r g ea i r p l a n ee q u i p p e dw i t hb o t h p a s s i v ea n da c t i v em a i ng e a r s . The touchdown p a r a m e t e r sv a r i e di nt h es t u d y , f o rb o t hp a s s i v ea n da c t i v em a i ng e a r s ,i n c l u d e da i r p l a n em a s sa n dp i t c h mass moments o fi n e r t i a ,s u b s o n i ca e r o d y n a m i c s ,a e r o d y n a m i cp i t c hc o n t r o l( e l e v a - tors) , p i t c ha t t i t u d e ,g r o u n ds p e e d ,a n ds i n kr a t e ,i nc o n j u n c t i o nw i t ht h e u s e o fa c t u a l runway p r o f i l e s .

SYMBOLS The u n i t su s e df o rt h ep h y s i c a lq u a n t i t i e sd e f i n e di nt h i sp a p e ra r eg i v e n f i r s t i n t h e I n t e r n a t i o n a l S y s t e mo fU n i t sa n dp a r e n t h e t i c a l l yi nt h e U . S .

C u s t o m a r yU n i t s .M e a s u r e m e n t sa n dc a l c u l a t i o n s were made i n U . S . Customary U n i t s .

a r e ao fo p e n i n gi ns h o c k - s t r u to r i f i c ep l a t e , m2 ( f t 2 ) c r o s s - s e c t i o n a la r e ao fm e t e r i n gp i n , m2 ( f t 2 )

s h o c k - s t r u th y d r a u l i ca r e a( p i s t o n area) , m2 ( f t 2 )

s h o c k - s t r u tp n e u m a t i c area ( c y l i n d e r area) , m 2 ( f t 2 )

of v o l u m eb e t w e e ns h o c k - s t r u tp i s t o na n d c r o s s - s e c t i o n a la r e a c y l i n d e r , m 2 ( f t 2 ) f o r c e a p p l i e d a t composite mass c e n t e rd u e t o e l e v a t o r moment, N ( l b f ) r a t i o o f e l e v a t o r moment to a p p l i e d moment d i s t a n c eb e t w e e n composite mass c e n t e ra n de l e v a t o rh i n g ea x i si n body c o o r d i n a t es y s t e m , m ( f t ) a p p l i e d moment, N-m ( l b f - f t ) e l e v a t o r moment, N-m ( l b f - f t ) p i t c h i n g moment, N-m ( l b f - f t ) composite mass, kg ( s l u g s ) o n e - h a l fo ff u s e l a g e mass c o n c e n t r a t e d a t f u s e l a g ec e n t e ro fg r a v i t y , kg ( s l u g s ) mass a s s u m e dc o n c e n t r a t e d a t axle, kg ( s l u g s ) mh semispanwing mass assumed c o n c e n t r a t e d a t semispanwing c.g. on m W s p a n w i s ec h o r dc o n t a i n i n gw i n g - g e a ri n t e r f a c e , k g ( s l u g s ) h y d r a u l i cp r e s s u r ei ns h o c k - s t r u tp i s t o n , Pa ( l b f / f t 2 ) P1 p n e u m a t i cp r e s s u r ei ns h o c k - s t r u tc y l i n d e r , Pa ( l b f / f t 2 ) P2 f l o w rate t h r o u g hs h o c k - s t r u to r i f i c e , m3/sec (gal/min) QO f l a w r a t e o fh y d r a u l i c pump, m3/sec (gal/min) QPumP flow r a t e from h i g h - p r e s s u r e accumulator t h r o u g hs e r v ov a l v e t o Qsvl s h o c k - s t r u tp i s t o n , m3/sec (gal/min) flow r a t e f r o ms h o c k - s t r u tp i s t o nt h r o u g hs e r v ov a l v e to Qsv2 l a w - p r e s s u r e r e s e r v o i r , m 3 / s e c ( g a l / m i n ) S s h o c k - s t r u t stroke, m ( f t ) t elapsed time a f t e r touchdown, sec i n i t i a l volume of c h a r g i n gn i t r o g e ni nh i g h - p r e s s u r e accumulator,

Vac , i

m3 ( p i n t s ) t o t a l volume of h i g h - p r e s s u r ea c c u m u l a t o r , m3 ( p i n t s )

Vac , t

c u m u l a t i v e f l u i d v o l u m et r a n s f e r r e df r o ms h o c k - s t r u tp i s t o n t o Vc u m c y l i n d e r , m3 ( p i n t s ) volume of s h o c k - s t r u tp i s t o n , m3 ( p i n t s ) pneumaticvolumein s h o c k - s t r u t c y l i n d e r , m3 ( p i n t s ) volumebetween s h o c k - s t r u t p i s t o na n ds h o c k - s t r u tc y l i n d e r , m3 ( p i n t s ) l o n g i t u d i n a lc o o r d i n a t ea x i s t r a n s v e r s e c o o r d i n a t e a x i s v e r t i c a l or n o r m a lc o o r d i n a t ea x i s bulkmodulus of h y d r a u l i c f l u i d , N/m2 ( l b f / f t 2 ) r a t i o of specific h e a t of g a s a t c o n s t a n t p r e s s u r e t o t h a t a t constantvolume 1 1 g e n e r a l slope of runway, deg Subscripts : b body-axis system 9 gr avi ty-axi s s y s tem i i n i t i a l value Dots over symbols indicate differentiation w i t h respecttotime.

MODIFICATIONS TO ACOLAG The mathematical model of ACOLAG i n reference 5 was used i n thepresent investigation. Subsequent tothepublication of thatreference,modifications were made tothemathematical model of theseries-hydraulicactivecontrol gear,tothe aerodynamic model of theairplane, and t o t h e main-gear control logic. The equations have been programmed foroperation on a d i g i t a l computer and have a corerequirement of 72 000 octal words.

Series-HydraulicActiveControl Gear Model The mathematical model of theactivecontrol gear ( r e f . 5) assumes instan- taneouscontrolresponse and incompressiblehydraulic f l u i d ; however, i n real- i t y , delayedresponse and/or s t a b i l i t y problems can a r i s e w i t h an active gear as a consequence of hardware limitations and thecompressibility of hydraulic f l u i d a t h i g h operatingpressures.Therefore,themathematical model of the series-hydraulicactivecontrol gear was modified toincludeservo-valve

dynamics and compressible-fluidcharacteristics(ref. 6 ) . These modifications

changed thepressure and flaw equations of reference 5.

Figure 1 shows schematics of thepassive and active shock s t r u t s . The conservation of mass of thehydraulic f l u i d i n theshock-strutpiston,repre- sented by VI i n f i g u r el ( a ) , and consideration of theeffect of f l u i d com- pressibilityleadtoequations of the form and The servo-valve flow rates Qsvl and Qsv2 are functions of servo-valve geom- e t r y and spool displacement,instantaneouspistonpressure,controlsupply pressure, and controlreturnpressure(ref. 6 ) .

The p r e s s u r e - v o l u m er e l a t i o n s h i pf o rt h ep n e u m a t i cp r e s s u r ei nt h es h o c k - s t r u tc y l i n d e r (V2 i n f i g . 1 (a)) is where

V2 = V2,i - (A2 - A1 + A p ) S + (V3 - V 3 , i ) - VCm

w i t h t

vcum = $=, Qo dt

and V3 = V3, i -k A3S T h e main-gear s h o c k s t r u t of t h e simulated a i r p l a n e has a s n u b b e rv a l v ei n t h e o r i f i c e p l a t e t o r e s t r i c t f l o w from t h e s h o c k - s t r u tc y l i n d e r t o t h ep i s t o n d u r i n g s t r u t e x t e n s i o n . For a f e ws i m u l a t e dl a n d i n g s ,t h ep i s t o np r e s s u r e dropped b e l o w t h e f l u i d v a p o rp r e s s u r eb e c a u s e of t h eh i g hs t r u t - e x t e n s i o n r a t e c o u p l e dw i t ht h e restricted f l o w t h r o u g ht h es n u b b e rv a l v e .C o n s e q u e n t l yt h e p r e s s u r ee q u a t i o n s were modified t o limit t h e p r e s s u r e i n t h e p i s t o n t o vapor p r e s s u r ef o rt h e s ec o n d i t i o n s .F l u i dc o n t i n u e s t o f l o w from t h ec y l i n d e r t o t h e p i s t o n u n t i l t h e s h o c k - s t r u tp r e s s u r er e t u r n s to c h a r g i n g pressure or u n t i l t h eg e a r impacts t h e l a n d i n gs u r f a c e .

Aer odynam i c Mode 1 Theaerodynamic model p r e s e n t e d i n r e f e r e n c e 5 does n o ti n c l u d ee l e v a t o r s for c o n t r o l l i n gn o s e - g e a rt o u c h d o w nv e l o c i t y or t h e effect of s u c hc o n t r o lo n t h e l o a d i n g of t h em a i ng e a r .D u r i n gs i m u l a t i o n s of a i r p l a n el a n d i n g sa n d r o l l - o u t , nose-gear impact v e l o c i t i e sa n dr e s u l t i n gn o s e - g e a rf o r c e sg e n e r a t e d by t h es i m p l i f i e dn o s e - g e a rr e p r e s e n t a t i o n( l i n e a rs p r i n gw i t hn or e b o u n d ) were u n r e a l i s t i c a l l yl a r g e .C o n s e q u e n t l y , a simplified r e p r e s e n t a t i o no ft h ee l e v a - tors was added t o ACOLAG. T h ee l e v a t o rc o n t r o l (ELCON) w a s i n c l u d e d as a per- c e n t a g e of t h e o t h e r applied momentsandof opposite s i g n , as follows:

M, = K M a = ELCON * Ma

For example, a t t o u c h d o w nt h ea i r p l a n e is assumed to be a e r o d y n a m i c a l l y trimmed w i t h t h ea e r o d y n a m i c l i f t s u p p o r t i n gt h ea i r p l a n e mass. For t h i sc o n d i t i o n t h e e l e v a t o r moment is e q u a li nm a g n i t u d eb u t opposite i n s i g n t o t h e applied

moment ( K = 1 . O ) . The f o r c e applied a t t h e composite mass c e n t e rd u e to

e l e v a t o r moment is V a l u e so f K may b ei n p u t to s i m u l a t ec h a n g e si ne l e v a t o rc o n t r o ld u r i n gt h e impact p h a s eo f a landing.Duringnose-gear impact t h ee l e v a t o r moment and f o r c e are d e c r e a s e d as t h en o s e - g e a rf o r c ea n d moment i n c r e a s e .

Main-Gear C o n t r o l Logic The o p e r a t i o n o f t h e s e r i e s - h y d r a u l i c a c t i v e c o n t r o l gear is d e s c r i b e d i n r e f e r e n c e 5. Subsequent t o t h ep u b l i c a t i o no fr e f e r e n c e 5, it was f o u n dt h a t t h e c o n t r o l logic was n o tc o n d u c i v e to e f f i c i e n t o p e r a t i o n of t h e a c t i v e g e a r .

The c o n t r o l logic was, t h e r e f o r e ,m o d i f i e d as d e s c r i b e di nt h ef o l l o w i n g p a r a g r a p h s .

D u r i n gt h ei n i t i a lp o r t i o no ft h el a n d i n g impact, t h ee l e c t r o n i cc o n t r o l c o m p u t e st h ea i r p l a n ek i n e t i ce n e r g yi n t h e v e r t i c a l d i r e c t i o n f r o m t h e mea- s u r e ds i n k r a t e and t h e v e h i c l e mass, which is assumed t o r e m a i nc o n s t a n t .

T h i se n e r g y is a p p o r t i o n e d among t h em a i ng e a r sa n d i s compared w i t h t h e remaining w o r k c a p a b i l i t yo fe a c hm a i n - g e a rs h o c k s t r u t d u r i n g t h e s t r o k i n g process. The w o r k c a p a b i l i t yo ft h es t r u t is t h ep r o d u c to ft h ei n s t a n t a n e o u s v a l u eo f t h e f o r c e a t t h ei n t e r f a c eb e t w e e nt h e gear a n dt h ea i r f r a m ea n dt h e r e m a i n i n gs h o c k - s t r u t stroke. When t h e w o r k c a p a b i l i t yo ft h es h o c ks t r u t equals or e x c e e d st h er e m a i n i n gv e r t i c a lk i n e t i ce n e r g yo ft h ea i r c r a f t ,t h e c o n t r o la s s i g n st h a ti n s t a n t a n e o u sv a l u eo ft h ea i r f r a m e - g e a ri n t e r f a c ef o r c e as t h e limit f o r c ea n dt h el o a d i n g a t t h ei n t e r f a c e is c o n t r o l l e da b o u tt h i s v a l u ed u r i n gt h er e m a i n d e ro ft h e impact. The t r a n s i t i o n from t h e impact p h a s e t o t h e roll-out p h a s e is as d e s c r i b e di nr e f e r e n c e 5.

I ft h eg e a rf u l l ye x t e n d sa n dt h ep r e s s u r ei nt h es h o c k - s t r u tp i s t o n is less t h a nt h ec h a r g i n g pressure, t h e nt h ec o n t r o la d d sf l u i d t o t h e s t r u t u n t i l t h ep r e s s u r er e t u r n s t o c h a r g i n gp r e s s u r e or u n t i l t h e c o n t r o l is r e q u i r e d t o r e d u c e t h e a i r f r a m e - g e a r force. I ft h ea i r f r a m e - g e a rf o r c eb e c o m e sp o s i t i v e , t h e gear is s t r o k e d ,a n df l u i dh a sb e e nr e m o v e d ,t h e nt h ec o n t r o la d d sf l u i d to t h e s t r u t . The rate o fa d d i t i o no ff l u i d is p r o p o r t i o n a l t o t h ed i f f e r e n c e b e t w e e nt h ea s s i g n e d limit f o r c ea n dt h ei n s t a n t a n e o u sv a l u e so ft h ea i r f r a m e - gear f o r c e .A d d i t i o no ff l u i d ceases when c o n t r o l is a g a i nr e q u i r e d or when t h ef l u i d volume i nt h es t r u th a sb e e nr e t u r n e d t o t h ed e s i g nv a l u e .T h i s l o g i c m a i n t a i n s t h e s t r u t - f l u i d q u a n t i t y a n d pressure a t or n e a rp a s s i v e - g e a r d e s i g nv a l u e sd u r i n gc o n t r o li n a c t i v i t ya n dp r o v i d e s more e f f i c i e n t u t i l i z a t i o n o ft h ec o n t r o ls y s t e m .D u r i n gt h er o l l - o u tp h a s eo ft h el a n d i n g , a c o n t r o l b i a sr e t u r n st h e gear to t h e d e s i g n s t a t i c stroke i na p p r o x i m a t e l y 1 0 sec.

T h i sc o n t r o lb i a sd o e sn o td e g r a d ec o n t r o lp e r f o r m a n c ed u r i n go t h e rp h a s e so f o p e r a t i o n( r e f . 6 ) .

ANALYTICAL MODELING T h e p a s s i v e a n d a c t i v e s h o c k s t r u t s o f t h e m a i n l a n d i n g gears a n d t h e a s s u m e ds t i f f - a i r f r a m eg e o m e t r y of t h e l a r g e a i r p l a n e were modeled as shown s c h e m a t i c a l l yi nf i g u r e s 1 and 2, r e s p e c t i v e l y .

Main-Gear Shock S t r u t s P a s s i v eg e a r .- The passive-gear shock s t r u t is shown i n f i g u r e 1 ( a ) . The m a i n - l a n d i n g - g e a rs h o c ks t r u to ft h em o d e l e da i r p l a n e is a na i r - o v e r - o i l type.

T h em a i no r i f i c eb e t w e e nt h ep i s t o na n dt h ec y l i n d e r is e q u i p p e d w i t h a snubber v a l v e . The v a l v er e m a i n so p e nd u r i n g t h e c o m p r e s s i o np h a s e of gear o p e r a t i o n s b u t r e d u c e s t h e o r i f i c e area for f l o wf r o mt h ec y l i n d e r t o t h e p i s t o n d u r i n g gear e x t e n s i o n a n d t h u s i n c r e a s e s g e a r d a m p i n g . T h e p r e s s u r e - r e l i e f orifices i n t h e o r i f i c e t u b e permit p r e s s u r e e q u a l i z a t i o n b e t w e e n t h e o r i f i c e t u b e a n d t h eo u t e rp o r t i o no ft h ec y l i n d e r .S i m i l a r l y ,t h e orifices i nt h ep i s t o n wall permit p r e s s u r ee q u a l i z a t i o nb e t w e e nt h ec y l i n d e ra n dt h ea n n u l a rv o l u m e b e t w e e np i s t o na n dc y l i n d e r .

A c t i v e gear.- M o d i f i c a t i o n s to t h ep a s s i v e - g e a rs h o c k s t r u t t o accommodate t h es e r i e s - h y d r a u l i ca c t i v ec o n t r o l are shown i nf i g u r e 1 ( b ) . The c o n t r o l low-pr essur e h a r d w a r ei n c l u d e s a s e r v ov a l v e , a h i g h - p r e s s u r ea c c u m u l a t o r , a r e s e r v o i r , a h y d r a u l i c pump, a n da ne l e c t r o n i cc o n t r o l .I n add i t i o n ,t h e .it t h e c o n t r o l s i n g l e - w a l l o r i f i c e t u b e is r e p l a c e db ya na n n u l a rt u b e t o perm to operate on t h e f l u i d i n t h e s h o c k - s t r u tp i s t o n . A schematic r e p r e s e n t a t i o l of these m o d i f i c a t i o n s is shown by t h ed i a g o n a l l yl i n e d area i n f i g u r e 1 ( b ) .

a i r f rame-gear C o n t r o li n s t r u m e n t a t i o nc o n s i s t s of an accelerometer t o monitor a c c e l e r a t i o n , a p o t e n t i o m e t e r t o measure s t r u t stroke, and a p r e s s u r et r a n s - ducer t o measure f l u i d p r e s s u r ei n t h e p i s t o n . The e l e c t r o n i c c o n t r o l u t i l i z e s these d a t a i n a f e e d b a c k loop i n c o n j u n c t i o n w i t h t h e programmed l o g i c t o d r i v e t h es e r v o - v a l v e spool a n dc o n t r o lh y d r a u l i cf l u i dp r e s s u r ei nt h es h o c k - s t r u t p i s t o n a n d t h u s c o n t r o l t h e force applied t o t h e airframe.

Airplane Geometry and Mass D i s t r i b u t i o n A schematic r e p r e s e n t a t i o n of t h e geometric c o n f i g u r a t i o n of t h e s t i f f airframe and t h e mass d i s t r i b u t i o n is shown i n f i g u r e 2. A l l geometricdimen- s i o n s are shown w i t h r e f e r e n c e t o t h e a i r p l a n e composite mass c e n t e r( a i r p l a n e c e n t e r of g r a v i t y ,c . g . ) . The g e o m e t r i cl o c a t i o n s of a l l mass c e n t e r s are assumed to be t h e same f o r a l l mass c o n d i t i o n s .A l t h o u g h t h e g e o m e t r i c loca- t i o n of thehub mass c e n t e rd o e sc h a n g ew i t h stroke, its e f f e c to n t h e a i r p l a n e c.g. is small and is assumed to b en e g l i g i b l e .I nt h i ss t u d yt h e composite mass c e n t e r is located a t t h e most a l l o w a b l e a f t p o s i t i o n of t h e c e n t e ro f results i n t h e maximum l o a d i n g( d u e t o mass changes) g r a v i t y .T h i sl o c a t i o n of t h e m a i nl a n d i n g gears d u r i n gl a n d i n g impact a n dr o l l - o u t .

Control Hardware The h a r d w a r e r e q u i r e d f o r t h e s e r i e s - h y d r a u l i c c o n t r o l s y s t e m is shown s c h e m a t i c a l l y i n f i g u r e 1 (b) . The s i m u l a t e dc h a r a c t e r i s t i c so ft h eh a r d w a r e for t h i s s t u d y are t h e same as t h o s eo fr e f e r e n c e6 .T h es e r v ov a l v e is a t h r e e - s t a g e , i n d u s t r i a l - t y p e v a l v e w i t h a r a t e d f l o w of 0.01 26 m3/sec ( 2 0 0 gal/min)and a maximum flow rate of 0.01 51 m3/sec ( 2 4 0g a l / m i n )f o r a 6.9 M P a (1 .O k s i ) p r e s s u r ed r o p across t h ev a l v e . The c h a r a c t e r i s t i c s of t h e e l e c t r o n i c c o n t r o l as d e s i g n e di nr e f e r e n c e 6 are also u s e df o rt h el a n d i n g s i m u l a t i o n s made d u r i n gt h i ss t u d y . T h el o w - p r e s s u r er e s e r v o i r is assumed t o be a t a t m o s p h e r i cp r e s s u r e . The h y d r a u l i c pump is assumed t o s u p p l yf l u i d t o t h eh i g h - p r e s s u r ea c c u m u l a t o r a t a flow r a t e of0.0006 m3/sec ( 9 . 0g a l / m i n ) and a p r e s s u r eo f 20.7 MPa (3.00 k s i ) , a n dt h eh i g h - p r e s s u r ea c c u m u l a t o r is assumed to s u p p l y f l u i d t o t h e s e r v o v a l v e a t a c o n s t a n t p r e s s u r e of 20.7 M P a ( 3 . 0 0 k s i ) .

P a r a m e t e rV a r i a t i o n s I n t h i s s t u d y t o u c h d o w np a r a m e t e r sc o n s i s t e n tw i t hl a n d i n g - a p p r o a c hf l i g h t o p e r a t i o no ft h ea i r p l a n e are e s t a b l i s h e d . ACOLAG was used t o c o m p u t et h e a i r - p l a n ed y n a m i cl o a d sa n dm o t i o n sf r o mi n i t i a lt o u c h d o w no nt h er u n w a yt h r o u g h main-andnose-gear impacts f o l l o w e d by a p o r t i o no f the g r o u n dr o l l - o u tp h a s e .

Foreconomiccomputeroperation, it is d e s i r a b l e t o u s e t h e maximum time i n t e r v a l i n t h e i n t e g r a t i o n p r o c e d u r e t h a t s a t i s f i e s e s t a b l i s h e d e r r o r toler- a n c e sf o rt h ei n t e g r a t e dv a r i a b l e s . An e v a l u a t i o no ft h e maximum time i n t e r v a l and maximum e r r o rt o l e r a n c e so ft h ei n t e g r a t e dv a r i a b l e st h a t c o u l d be used w i t h o u t s i g n i f i c a n t l yc h a n g i n gt h ev a r i a b l e s was c o n d u c t e d . It was f o u n dt h a t a time i n t e r v a lo f 0.0001 sec, i nc o n j u n c t i o nw i t ht h e error t o l e r a n c e s shown i n t a b l e I, was t h e maximum time i n t e r v a l t h a t c o u l d be u s e dw i t h o u tn o t i c e a b l e c h a n g e si nt h ei n t e g r a t e dv a r i a b l e s .T h e r e f o r e ,t h el a n d i n gs i m u l a t i o n sf o r t h i s s t u d y were made u s i n g t h i s time i n t e r v a l a n d t h e s e error t o l e r a n c e s .

The t o u c h d o w np a r a m e t e rv a r i a t i o n si n c l u d et h r e ea i r p l a n em a s sc o n f i g u r a - t i o n s : small, medium, and l a r g e . The small m a s s c o n f i g u r a t i o n r e p r e s e n t s a minimum m a s sl a n d i n gc o n d i t i o n ,a n dt h e medium a n dl a r g e mass c o n f i g u r a t i o n s are s e l e c t e d as 1 . 4 and 1 .8 times g r e a t e r ,r e s p e c t i v e l y ,t h a nt h es m a l lm a s s c o n f i g u r a t i o n .S i n c et h ec e n t e ro fg r a v i t y is as f a r a f t as a l l o w a b l e ,t h e g r e a t e r p i t c h mass moments o f i n e r t i a f o r t h e l a r g e r m a s sc o n f i g u r a t i o n si m p o s e a g r e a t e r demand o nt h em a i n - l a n d i n g - g e a rc o n t r o ls y s t e m .

An a n a l y s i s of 2 3 8 5l a n d i n g so fc i v i la i r p l a n e si nr e f e r e n c e 7 i n d i c a t e s t h a t o n l y 1 l a n d i n g i n 1 0 000 had a s i n k r a t e e q u a l t o or g r e a t e rt h a n 1 . 5 m/sec ( 5 . 0f t / s e c ) .T h e r e f o r e ,t h e maximum d e s i g ns i n kr a t ea t touchdown for t h i ss t u d y is 1.5 m/sec ( 5 . 0f t / s e c ) . To encompass a r a n g eo fs i n k rates

a t touchdown,0.3 m/sec (1 . 0 f t/sec) and0.9 m/sec ( 3 . 0 f t/sec) were also

s e l e c t e d . An off-design (emergency) sink r a t e of 2.4 m/sec (8.0 f t / s e c ) was also i n v e s t i g a t e d .

P i t c h a t t i t u d e s a n d g r o u n d s p e e d s s e l e c t e d t o p r o v i d ec o n s t a n t - s i n k - r a t e touchdowns were 7.5O and 84.4 m/sec ( 2 7 7 f t / s e c )f o rt h es m a l l mass c o n f i g u r a - t i o n , 8.7O and91.4 m/sec (300 ft/sec) for t h e medium mass c o n f i g u r a t i o n ,a n d 1 Oo and 97.8 m/sec ( 3 2 1 ft/sec) f o r t h e l a r g e mass c o n f i g u r a t i o n .T h ee l e v a t o r d e f l e c t i o n (ELCON = 1.0 f o r ACOLAG) was i n i t i a l i z e d a t touchdown to m a i n t a i n t h e a i r p l a n e i n a trimmed c o n d i t i o n .

Landings were s i m u l a t e d for t w o o p e r a t i o n a l r u n w a y s w i t h d i f f e r e n t slopes a n du n e v e n n e s se l e v a t i o np r o f i l e s .T h ee l e v a t i o np r o f i l e s of t h e s er u n w a y s are p r e s e n t e di nf i g u r e 3 f o ru p h i l l slopes. F o rs i m u l a t e dl a n d i n g so nd o w n h i l l slopes, t h eu n e v e n n e s sp r o f i l e s are s u p e r p o s e do nt h er e v e r s e d slopes. Touch- down f o r a l l l a n d i n gs i m u l a t i o n so c c u r r e d a t t h e runwaythreshold(zerorunway d i s t a n c e ) .

RESULTS AND DISCUSSION The r e s u l t so f t h i s a n a l y t i c a l i n v e s t i g a t i o n are p r e s e n t e d to compare t h e performance of t h e a i r p l a n e w i t h t h e active c o n t r o ll a n d i n gg e a rw i t ht h e per- f o r m a n c ew i t ht h ep a s s i v el a n d i n gg e a r . Variables i nt h es i m u l a t e dc o n t r o l system are also p r e s e n t e d t o i l l u s t r a t e t h e l i m i t a t i o n s o f t h e c o n t r o l h a r d w a r e a n dl o g i cu s e di n t h i s i n v e s t i g a t i o n .

V a l i d a t i o n o f ACOLAG R e s u l t s are p r e s e n t e d i n f i g u r e s 4 and 5 t o d e m o n s t r a t et h ev a l i d i t yo f t h e ACOLAG a n a l y s i sa n dc o m p u t e rp r o g r a mf o rp r e d i c t i n gt h el a n d i n gd y n a m i c so f a i r p l a n e sw i t hb o t hp a s s i v ea n da c t i v el o a d - c o n t r o lm a i n - l a n d i n g - g e a rs y s t e m s .

P a s s i v e - g e a r mode.- Ccmparisons of computed d a t a from ACOLAG w i t h d a t a f r o mt h ev a l i d a t e df l e x i b l ea i r c r a f tt a k e - o f fa n dl a n d i n ga n a l y s i s (FATOLA, r e f .8 ) are made i n f i g u r e 4 t o d e m o n s t r a t et h ev a l i d i t yo f ACOLAG to compute l a n d i n gd y n a m i c sw i t hp a s s i v el a n d i n g - g e a rs y s t e m s .L a n d i n gd y n a m i c s of a l a r g e , stiff a i r p l a n e for a symmetric touchdownon a smooth f l a t runway were computed w i t h FATOLA and ACOLAG. T h es i n kr a t e was 1 . 5 m/sec ( 5 . 0 ft/sec), t h e p i t c h r a t e was -0.5 d e g / s e cn o s eo v e r ,a n d t h e c . g . v e r t i c a l a c c e l e r a t i o n was z e r o .C o m p a r i s o n sa r e shown i nf i g u r e 4 of c o n t r o li n p u t s ,p i t c h r a t e s , p i t c h a t t i t u d e s , and main-gear s t r u t f o r c e s and strokes f o rt h e simulated l a n d i n g .

E l e v a t o r d e f l e c t i o n s i n FATOLA and ELCON i n p u t s for s i m u l a t e de l e v a t o r c o n t r o li n ACOLAG a r e shown i n f i g u r e 4 ( a ) . The e l e v a t o r - c o n t r o li n p u tv a r i a - t i o n s f o r ACOLAG were d e f i n e db y t r i a l and error and a r ed e n o t e d by t h e d a s h e d resulted i n good agree- l i n ei nf i g u r e4 ( a ) .T h e s ee l e v a t o r - c o n t r o lv a r i a t i o n s m e n tw i t hp i t c h rates, pitch a t t i t u d e s , and time o fn o s e - g e a rc o n t a c to b t a i n e d from FATOLA, as shown i nf i g u r e 4 ( b ) . Subsequent to n o s e - g e a rc o n t a c t , some d i f f e r e n c e s i n p i t c h rates r e s u l tb e c a u s eo fd i f f e r e n c e si nn o s e - g e a rs i m u l a - t i o n s .I n FATOLA, t h e more r e a l i s t i c n o s e - g e a rs i m u l a t i o n permits p i t c h - r a t e c h a n g e sa s t h e n o s e - g e a rl o a d sa n du n l o a d s ,w h e r e a si n ACOLAG t h e p i t c h r a t e is set t o 0 deg/secand t h e n o s e - g e a rf o r c e is computed t o offset t h e moment a p p l i e db yt h em a i ng e a r s .T h ep i t c ha t t i t u d ec o m p u t e di n FATOLA is s l i g h t l y l a r g e r t h a n t h a t o b t a i n e d w i t h ACOLAG f o l l o w i n gn o s e - g e a rc o n t a c t .

Comparisons of main-gear strutforcesare shown i n figure 4 ( c ) . Strut forcesare i n good agreement up to nose-gear contact.Differences shown a t nose-gear contactresult from theelevator and nose-gear simulations used i n ACOLAG. Beyond nose-gearcontacttheforces still agree well; however, strut forces from A m L A G aregreaterthanthose from FATOLA. The differences between the two simulationsoccurbecause i n F A T O L A theangle of attack is s l i g h t l y greater than thezero-liftangle and i n ACOLAG theangle is s l i g h t l y lower than zero-liftangle. Consequently l i f t reduces main-gear loads i n F A T O L A and loads thegear s l i g h t l y i n A O L A G . Main-gear strokesforthe two simulations compare

well(fig. 4 ( d ) ) , u n t i l thedifferences i n the aerodynamic l i f t influencethe

results,as noted fortheshock-strutforces.

The preceding comparisons indicate that the ACOLAG computerprogram is validfor computing airplanelandingloads and motions for symmetric landings of airplanes w i t h stiff airframes and passive main landinggears.

Active-gear mode.- Preliminaryvalidation of theactive-gear mode of ACOLAG is accomplis~hed by comparing computed data w i t h experimentalresults obtained from thetest program of reference 6 . Comparisons of computed and experimentalairframe-gearforces and shock-strutstrokesarepresented i n f i g - ure 5 for a gear s i n k r a t e of 1 . 5 m/sec ( 5 . 0 f t / s e c ) . A s shown i n figure5(a), there was a force imbalance of 2.45 kN (550 l b f ) between theexperimentaldata (circular symbols) and the computed r e s u l t sa t time of touchdown. The imbal- ance resulted f r m d i f f e r e n c e s i n thesimulated l i f t force and t h e i n e r t i a forceactingatthestrutattachment i n the experiment. From impulse-momentum considerations,theintegral of theforce-timecurve from the 2.45-kN ( 5 5 0 - l b f ) leveltothe time of maximum gear stroke ( 0 . 3 3 sec)dissipates the t e s t mass velocity.Therefore, it is validto s h i f t theexperimentaldata t or e f l e c t a forcebalance a t touchdown a s w i t h the computed data. A comparison of the shiftedexperimentalforcedata (dashed l i n e i n f i g . 5 ( a ) ) w i t h the computed forcedataindicates good agreement i n both magnitude and variations. The good agreement between the computed and experimental strut strokes, shown i n figure 5 ( b ) , furtherindicatesthevalidity of shiftingtheforcedatafor purposes ofcompar ison.

These resultsindicatethatthe ACOLAG computerprogram is validfor pre- dictingthe dynamics of l i g h t airplanes w i t h activegearsduringthe impact phase of landings.

OperationalConsiderations Data arepresentedto show theoperation ofan activeload-control system i n the main landinggear of a largeairplane. The dataarepresentedas t i m e - historyplots of airframe-gearforces,shock-strutstrokes, and fuselage mass- center displacements along t h e Zg axisforairplanelandingsimulations w i t h both passive and activegears. For theactive-gearsimulations,hydraulic- f l u i d flow rates and volume of f l u i d transferredarealsopresentedto i l l u s - t r a t e theoperationalcompatibility of thesimulatedcontrol hardware and the modifiedlanding-gear shock s t r u t . A l l landingsimulations w i t h theactive gear were computed w i t h a constantpressure of 20.7 M P a (3.00 k s i ) i n the high- pressureaccumulator.

1 0 F o r m u l a t e dc o n s t r a i n t s . -T oa s s u r em e a n i n g f u lp e r f o r m a n c e of t h e a c t i v e l o a d - c o n t r o l m a i n l a n d i n g gears i n v e s t i g a t e d i n t h i s s t u d y t h e f o l l o w i n g opera- t i o n a l c o n s t r a i n t s were imposed: 1 . The controlmustneverremove a q u a n t i t y o f f l u i d f r o m t h e s t r u t g r e a t e rt h a nt h e volume of f l u i d i n t h e p i s t o n of t h e f u l l y e x t e n d e d s t r u t (0.010 m3 ( 2 1 . 8 p i n t s ) ) .

2. T h ec o n t r o lm u s tn e v e r add a q u a n t i t y of f l u i d to t h e s t r u t e q u a l to or g r e a t e rt h a nt h ei n s t a n t a n e o u sv a l u eo ft h e gas volume i n t h e s t r u t .

3 . The maximum s h o c k - s t r u t stroke e n c o u n t e r e dd u r i n ga n yl a n d i n g sim- u l a t i o ns h o u l dn e v e re q u a l or e x c e e dt h e allowable d e s i g n stroke (0.508 m ( 2 0 . 0 i n . ) 1 .

4. T h e c o n t r o l - h a r d w a r e c h a r a c t e r i s t i c s for a n optimum d e s i g ns h o u l d s u p p l y h y d r a u l i c f l u i d a t t h e flow rates r e q u i r e d by t h e dynamic b e h a v i o r of t h e s h o c k s t r u t .

5. A l l t h ea f o r e m e n t i o n e dc o n s t r a i n t sm u s t be met for t h er a n g e of d e s i g n touchdown parameters of t h e a i ' r p l a n e .

6 . The a c t i v el o a d - c o n t r o ll a n d i n gg e a rm u s t be a d a p t a b l e f o r accommodat- i n gg r e a t e rt h a nd e s i g nt o u c h d o w ns i n k rates which may be e n c o u n t e r e di n emer- g e n c y s i t u a t i o n s .

P e r f o r m a n c e w i t h i n c o n s t r a i n t s . - T o show o p e r a t i o n w i t h i n t h e i m p o s e d c o n s t r a i n t s , typical data are p r e s e n t e di nf i g u r e 6 for l a n d i n gs i m u l a t i o n so f a l l t h e a i r p l a n e mass c o n f i g u r a t i o n s a t a touchdownsink r a t e of0.9 m/sec (3.0 f t / s e c )o n runway A w i t h b o t h u p h i l la n dd o w n h i l l slopes. Data for t h e s e mass c o n f i g u r a t i o n sf o ro t h e rt o u c h d o w ns i n k rates, runway slopes, and runway unevenness are p r e s e n t e di n t h e a p p e n d i x .T a b u l a t e dr e s u l t s are shown i n t a b l e I1 for a l l a i r p l a n el a n d i n gs i m u l a t i o n so nr u n w a y A w i t h a c t i v e load- c o n t r o l m a i n gears. The data are p r e s e n t e d as follows: p e r c e n t a i r f r a m e - g e a r - force r e d u c t i o n so f t h e a c t i v e gear r e l a t i v e t o t h e p a s s i v e g e a r d u r i n g i n i t i a l impact andmain-gearresponse t o nose-gear impact; maximum flow r a t e of f l u i d from and i n t o t h e s t r u t ; maximum volume of f l u i d removed from or added t o t h e s t r u t ; maximum s h o c k - s t r u t stroke e n c o u n t e r e dd u r i n gl a n d i n gs i m u l a t i o n s ;a n d t h ea l l o w a b l ev o l u m e of f l g i dt h a tc o u l db ea d d e d t o t h e s t r u t .

The maximum v o l u m eo ft h ef l u i dr e m o v e d f r m t h e s t r u t d u r i n g t h e l a n d i n g s i m u l a t i o n s was 0.0081 m3 (1 7 . 2 0p i n t s ) ,w h i c h is a p p r o x i m a t e l y7 9p e r c e n to f t h ea l l o w a b l e volume, a n do c c u r r e df o rt h e medium mass c o n f i g u r a t i o n a t a s i n k rate of 1 .5 m/sec (5.0 f t/sec) o n t h e u p h i l l slope of runway A . The maximum volume of f l u i d a d d e d t o t h e s t r u t was 0.0072 m3 (15.10 p i n t s ) , which is a p p r o x i m a t e l y 56 p e r c e n t of t h ea l l o w a b l ev o l u m ea n do c c u r r e dd u r i n gt h e l a n d i n gs i m u l a t i o no f t h e large mass c o n f i g u r a t i o n a t a s i n k r a t e of1.5 m/sec (5.0 ft/sec) o nt h eu p h i l l slope of runway A. The maximum s h o c k - s t r u t stroke used by t h ea c t i v e gear was 0.480 m (18.9in.),which is 9 5p e r c e n to f allow- able stroke a n d o c c u r r e d d u r i n g i n i t i a l impact o f t h e l a n d i n g for t h e large mass c o n f i g u r a t i o n a t a s i n k rate of 1 . 5 m/sec (5.0 ft/sec) o nt h eu p h i l l slope ofrunway A.

The servovalve became saturated (maximum displacement of servo-valve spool, +O. 0025 m (kO.10 i n . 1 ) during the removal of f l u i d from t h e s t r u t a t i n i t i a l impact f o r a l l landingsimulations,indicatingthatthecontrol- hardware characteristics used were notfor an optimum design. However, t h i s nonoptimum design was s u f f i c i e n t t o s u b s t a n t i a l l y reducetheforces w i t h the active gearduring i n i t i a l impact.Therefore,theperformance of theactive load-controlgear has been demonstratedoverthedesignrange of touchdown parameters w i t h i n the imposed constraints.

The adaptability of theactiveload-controlgearto accommodate off-design s i n k rates is i l l u s t r a t e d by datapresented i n figure 7. Airframe-gear forces i n figure7(a)forlandingsimulations of thelarge mass configura- are shown tion w i t h both passive and activegears. Landings were made on theuphill

slope of runway A w i t h a s i n k r a t e of 2.4 m/sec (8.0 ft/sec) . The control

logic was the same asthat used w i t h design s i n k rates. Following i n i t i a l impact theairplane rebounded from the runway and sustained secondimpacts a t s i n k rates of approximately 1 . 2 m/sec ( 3 . 9 f t/sec) and 1 . 4 m/sec ( 4 . 7 f t/sec) forthepassive and activegears,respectively.Afterinitial impact the active-gearcontrol had transitioned from the impact limit force to a roll-out limit force of zero. The designedforce limits of k8.896 kN ( k 2 0 0 0 l b f ) were too l o w and theallowableshock-strutstroke was exceeded duringthe second impact for t h i s simulated emergency condition. Consequently, the present controllogicrequiresmodifications which w i l l either increase the roll-out force limits i f the touchdown s i n k r a t e is greaterthanthe maximum s i n k r a t e for which thecontrol was designed, or reset the control to the impact mode following rebound. For t h i s s t u d y theroll-outforce limits were increased from k8.896 kN ( k 2 0 0 0 l b f ) to k 1 3 3 . 4 5 kN ( k 3 0 000 l b f ) . A s a result of these largeroll-outforce limits, thecontrol was not requiredafterthe second impact and the maximum stroke capability of the shock s t r u t was notexceeded,

as shown i n figure 7 ( b ) . The forcereductionsafter nose-gear impact r e s u l t

from changes i n shock-strutpressure and f l u i d volume effected by thecontrol duringthe i n i t i a l and secondimpacts.

The performance of theactiveload-control main landinggearsimulated i n t h i s s t u d y has t h u s been demonstrated foroperation w i t h i n the imposed constraints.

Effect of accumulator pressure and pump capacity.- The airplanelanding simulations w i t h activeload-controlgears were made w i t h the assumption of a constant 20.7 M P a (3.00 k s i ) pressure i n the high-pressure accumulator. To maintain t h i s pressure w i t h the large flow rates encountered would require a more massiveaccumulatoror a hydraulic pump w i t h a large pumping capacity requiringexcessive power. A limited s t u d y was conducted t o determinethe effect of varyingaccumulatorpressure and pump capacity on the performance of t h e activegear.Results of thestudyarepresented i n figure 8 forairplane landingsimulations of thelarge mass configuration on theuphillslope of

runway A a t a touchdown s i n k r a t e of 1 .5 m/sec ( 5 . 0 f t/sec) . These conditions

requiredthelargesttransfer of f l u i d from thehigh-pressureaccumulator t o the shock s t r u t . (See table 11.)

Two accumulator-volume and ump-capacity combinations were investigated: an accumulator volume of 0.038 my (1 0.0 gal) and a pump capacity of 1 2 0.0006 m3/sec ( 9 gal/min) ; andan accumulator volumeof 0.01 9 m 3 (5.0 gal) and a pump capacity of 0.0032 m3/sec ( 5 0 gal/min) . A s shown i n figure 8 ( a ) , neitherambination of accumulator volume and pump capacity had any appreciable effect on theairframe-gearforce. However, figure8(b) shows thatthe shock- strutextension is less for these two combinations compared w i t h theextension fortheconstant accumulator pressureduringthehigh-pressure phase of opera- tion. The shock-strutextensionforthe 5-gal accumulator and the 50-gal/min pump is l e s s thanthatfor t h e 10-gal accumulator and 9-gal/min pump. These dataindicatethattheresults of theparametricstudy a t a constant accumu- latorpressureduringthehigh-pressure phase of controloperationare conservative.

Effect of braking.- An investigation of the effects of the combined opera- tion of theactiveload-controlgear and a simplifiedantiskidbraking system was conducted for the large mass configuration w i t h a s i n k r a t e of 0.9 m/sec ( 3 . 0 ft/sec) on theuphill slope of runway A . Airframe-gear-force and shock- strut-stroke time histories are shown i n figure 9 forbrakeapplicationafter nose-gear impact. Comparison of figures9(a) and 9(b) w i t h figures 6 ( i ) and 6 ( j ) , r e s p e c t i v e l y , i l l u s t r a t e s t h e e f f e c t of braking on theairplane landingsimulations w i t h passive and activegears. Following brake application w i t h passivegears,the magnitudes of theairframe-gearforces and shock-strut strokes increase. The activegear, however, controls the airframe-gear forces effectively duringbraking, b u t a t a higherfrequency of operation. Shock- strutstrokesfortheactive gear ( f i g . 9 ( b ) ) indicatethatthestrokereturns to the designed s t a t i c s t r o k e more rapidly w i t h brakingthanwithoutbraking

( f i g . 6 (j) ) . N o detrimentaleffects on the performance of theactive gear

coupled w i t h thesimplifiedantiskidbraking were encountered.Indeed, it is possiblethat improved braking performance could be realized,sincetheactive gearmaintains a more constantforceatthetire-runwayinterfacethanthe passivegear.

Comparison of Passive and Active Gear Results The typicaldatapresented i n figure 6 and thetabulateddata shown i n tables I1 and I11 are used to compare theresultsobtainedfortheactiveload- control gear w i t h thoseobtainedforthepassivegear. These comparisons are made forairframe-gearforces and shock-strutstrokes,fuselage mass-center displacements, and cyclicforces.

Airframe-gear forces and shock-strutstrokes.- The airframe-gearforces and shock-strutstrokes computed fortypicalairplanelandingsimulations w i t h both passive and active main gearsare compared i n figure 6 for the same land- ingconditions. The variousphases of thelandingsimulationsare shown i n figure6(a)for bothtypes of main gears. The i n i t i a l impact and rebound phase fortheairplane w i t h thepassive gear d i f f e r s from that w i t h theactivegear, sincetheactive gearreduces theshock-strutforce(atthe expense of

increasedshock-strutstroke,fig. 6 (b) ) . The reduced shock-strutforce

r e s u l t s i n smaller airframe-gear and ground forces. Nose-gear contact occurred a t approximatelythe same time forthelandingsimulations w i t h both passive and activegears,indicatingthattherotational impulse applied to theairplane was approximatelythe same w i t h bothpassive and activegears 1 3 duringthe impactphase and t h e rebound and rotation to nose-gear contact phase.Since t h e airframe-gear and ground forces were smaller w i t h theactive gearthanthey were w i t h thepassivegear, t h e moment appliedtotheairplane duringthe impact and rebound phase had to be sustainedfor a longerperiod of time. T h i s was thecase,as shown by thelongerperiod of decelerating (nega- t i v e ) airframe-gearforce(fig. 6 (a) ) and also by thegreaterperiod of ground- forceapplicationasindicated by thesustainedshock-strutstrokeforthe

active gear ( f i g .6 ( b ) ) . During nose-gear impact, theairframe-gearforcefor

t h e airplanelandingsimulation w i t h theactive gear was l e s s than that w i t h thepassivegear. The shock-strutstroke was onlyslightlygreaterthanthat

of thepassivegearforthesmall mass configuration(fig.6(b)) , b u t was s u s -

tainedat a greaterstrokefor a longerperiod of time. Subsequent t o nose- gear impact (duringtheroll-outphase),theairframe-gearforcefortheland- i n g simulation w i t h theactive gear was generallysmallerthanthat of the passivegear and theshock-strutstrokes were approximatelythe same forthe small mass configuration.

The results of theairframe-gear-force comparisons forthelanding simula- tions of the medium and large mass configurations ( f i g s . 6 (e) , 6 ( g ) , 6 (i) , and 6 ( k ) ) arethe same asthoseforthesmall mass configuration ( f i g s . 6 ( a ) and 6 ( c ) ) . The results of theshock-strut-stroke comparisons forthelarger mass configurationsarethe same asthose of thesmall mass configuration dur- i n g t h ei n i t i a l impact and rebound phase. However, duringtherotationto nose-gear contact,theshock-strutstrokesforthelarger mass configurations show that the main gear does not extendas much as it d i d w i t h thesmall mass configuration. T h i s shortenedstroke is indicative of reduced airplane motions. I n addition,duringthe nose-gear impact and roll-outphases,the shock-strutstrokesfortheactive gear were l e s s thanthose of thepassive gear because of thecontrollogic employed. However, asthe time duringroll- outincreased,theshock-strutstroke of theactive gearapproached that of the passivegear, which had attainedthevalue of strokerequiredtosupportthe s t a t i c mass of theairplane.

The tabulatedpercentforce-reductiondatapresented i n table I1 are used toillustratetheeffectiveness of theactiveload-control gear and toaid i n discussion of theeffects of airplane mass, touchdown s i n k r a t e , and runway slope on airframe-gearforces. The activeload-control gear was most effective i n reducingtheairframe-gearforcesduring main-gear response t o nose-gear impact,as shown by thelargevalues of percentforcereduction i n table 11.

Significantreductions were also achievedduringtheinitial-impact phase for a l l thelandingsimulationsinvestigated. The airframe-gear-force reductions generallyincreasedforthelarger mass configurationsforlandingsimulations a t the same touchdown s i n k r a t e and on the same runway slopeduringboth i n i - t i a l impact and main-gear responseto nose-gear impact. For the same mass configuration and runway slope, theairframe-gear-forcereductionsgenerally increasedforthelarger touchdown s i n k rates. The effect of runway slope was which theairframe-gear-force primarilydiscernibleduringinitial impact,for reduction was generallygreaterforthelandingsimulations on theuphill run- way slope atconstant mass and touchdown s i n k r a t e . T h i s result is to be expected,sincetheairframe-gearforcesfor t h e passive-gearlandingsimula- tionaregreater because of the added component of s i n k r a t e due toairplane horizontal motion intotheuphillslope. The airframe-gearforcesforthe J4 a c t i v e - g e a rl a n d i n gs i m u l a t i o n were e s s e n t i a l l y t h e same f o rl a n d i n gs i m u l a - t i o n so nu p h i l l or downhillrunway slopes. A similar e f f e c to fr u n w a y slope on airframe-gear-force r e d u c t i o n o c c u r r e d d u r i n g t h e r o l l - o u t p h a s e a n d is most p r o n o u n c e d f o r t h e l a n d i n g s i m u l a t i o n s of t h e large mass c o n f i g u r a t i o n ( f i g s . 6 ( i ) a n d6 ( k ) ) .

T h e f o r e g o i n g r e s u l t s i n d i c a t e t h a t t h e a c t i v e l o a d - c o n t r o l gear is e f f e c - t i v e i n r e d u c i n g t h e airframe-gear force r e l a t i v e to t h a t o c c u r r i n g w i t h t h e p a s s i v e gear d u r i n g a l l p h a s e so f a l a n d i n g .T h ee f f e c t i v e n e s s of t h e a c t i v e gear was most p r o n o u n c e df o rt h o s et o u c h d o w nc o n d i t i o n sw h i c hr e s u l ti n large a i r f r a m e - g e a rf o r c e sw i t ht h ep a s s i v e gear; t h a t is, large airplane masses, h i g h e rt o u c h d o w ns i n k rates, anduphillrunway slopes.

F u s e l a g e m a s s - c e n t e r d i s p l a c e m e n t s .- C o m p a r i s o n s o f a i r p l a n e m o t i o n s f o r l a n d i n g s i m u l a t i o n s w i t h b o t h p a s s i v e a n d a c t i v e m a i n gears are made w i t h t h e t y p i c a ld a t a shown i nf i g u r e 10. G r a v i t yv e r t i c a la x i s Zg d i s p l a c e m e n t s o f t h ef u s e l a g e mass c e n t e r ,w h i c h is l o c a t e d 3.05 m ( 1 0 . 0 f t ) f o r w a r do ft h e a i r p l a n e composite mass c e n t e r( a i r p l a n ec . g . 1 , are shown, r e l a t i v e t o its d i s p l a c e m e n t a t touchdown, as a f u n c t i o no f time. The d a t ap r e s e n t e d are for s i m u l a t i o n s o f t h e medium mass c o n f i g u r a t i o n o n t h e u p h i l l slope of runway A a t each of t h et o u c h d o w ns i n k rates i n v e s t i g a t e d . T h ev a r i o u sp h a s e s of t h el a n d - i n g s i m u l a t i o n s w h i c h a f f e c t t h e a i r p l a n e m o t i o n a r e i n d i c a t e d i n f i g u r e 1 O(a) .

S i n c ev e r y l i t t l e r o t a t i o n o c c u r s d u r i n g i n i t i a l impact, t h e s h o c k - s t r u t stroke is p r i m a r i l yr e s p o n s i b l e for t h e f u s e l a g em a s s - c e n t e rd i s p l a c e m e n t .S u b s e q u e n t t o i n i t i a l impact and prior t o n o s e - g e a rc o n t a c t , t h e fuselage mass-centerdis- placement is d u e to a i r p l a n er e b o u n da n dr o t a t i o n .F o l l o w i n gn o s e - g e a rc o n t a c t and impact, t h e d i s p l a c e m e n tr e s u l t s from t h ec h a n g ei nr u n w a ye l e v a t i o nd u e t o t h e u p h i l l slope.

T h ef u s e l a g em a s s - c e n t e rd i s p l a c e m e n t s for t h e l a n d i n g s i m u l a t i o n w i t h t h e a c t i v e gear are greater d u r i n g i n i t i a l impact t h a nt h o s eo c c u r r i n gw i t ht h e p a s s i v eg e a rb e c a u s eo ft h eg r e a t e r s h o c k - s t r u t stroke r e q u i r e d by t h e a c t i v e gear.However,duringrebound from i n i t i a l impact a n dr o t a t i o nt h r o u g hn o s e - g e a r impact, t h ed i s p l a c e m e n t s are s i g n i f i c a n t l yr e d u c e d by t h e a c t i v e gear compared w i t h those of t h ep a s s i v e gear. T h i s r e d u c t i o ni nd i s p l a c e m e n t r e s u l t sf r o m t h e reduced rebound of t h ea i r p l a n e w i t h t h e a c t i v eg e a rd u e t o c o n t r o l l e dv a r i a t i o n so ft h es h o c k - s t r u t stroke. Themagnitudeofthesecond- a r ym o t i o n s ,w h i c ho c c u r w i t h t h ep a s s i v e gear d u r i n g t h e r o l l - o u tp h a s e , is reduced by t h ea c t i v eg e a rt h r o u g hc o n t r o lo ft h es h o c k - s t r u tf o r c ea n d stroke.

T h e s er e s u l t s are typical of those o b t a i n e dd u r i n gt h ea i r p l a n el a n d i n g s i m u l a t i o n s for t h es m a l la n d l a r g e mass c o n f i g u r a t i o n s .T h e s er e s u l t si n d i - cate t h a t t h e a c t i v e gear is e f f e c t i v ei nr e d u c i n ga i r p l a n em o t i o n sf o l l o w i n g impact.

i n i t i a l C y c l i c forces.- The c y c l i c f o r c e s o n a n a i r p l a n e structure are i m p o r t a n t s i n c e t h e f a t i g u e damage s u s t a i n e d by t h e s t r u c t u r e is p r i m a r i l y a f u n c t i o n o f c y c l i cl o a d i n g sd u e to theground-air-ground loads, m a n e u v e rl o a d s ,g u s t loads, a c o u s t i cl o a d s ,a n dg r o u n do p e r a t i o n a ll o a d s .F o r most parts of a c o n v e n t i o n a l t r a n s p o r ta i r p l a n es t r u c t u r e ,t h ef a t i g u e damage s u s t a i n e df r o mc y c l i cg r o u n d o p e r a t i o n a l l o a d s is s m a l l compared w i t h t h e f a t i g u e damage s u s t a i n e d d u r i n g o t h e r phases of o p e r a t i o n .H w e v e r ,t h ea p p l i c a t i o n of a c t i v ec o n t r o l s t o I reduce aerodynamic maneuver and/or gustloads or increasedoperation of super- sonic cruise airplanes at altitudes where the number of gustsencounteredare considerablysmaller w i l l causethe ground operationalloadsto become propor- tionately more important.

A comparison of thetypicalairframe-gear-forcetimehistoriespresented i n figure 6 forthesimulatedairplanelandings w i t h passive and activegears shows thatthe magnitude of thecyclicforces w i t h theactive gear was substan- t i a l l y l e s s than t h a t w i t h thepassivegearduring a l l phases of thelandings.

An in-depth structural-fatigue analysis is notconsideredappropriateforthe analyticaldatapresented i n t h i s paper; however, toobtain an indication of theeffect of thereduction i n cyclicforces on fatigue damage, obtained w i t h theactivegear,thefollowingprocedure was used. The f a t i g u el i f e of a

structure(ref. 4) , for f u l l y reversedstress, may be expressed by

where A a constant for a given material and loading pattern N number of cycles to failure u peak-to-peak s t r e s s 5 is a typicalvalue used i n structural-fatigueanalysis and and the exponent is the value used i n t h i s s t u d y . Sincetheairplanestructure i n t h i s s t u d y is the same for l a n d i n g simulations w i t h thepassive and activegears,the term A is a constant and forcereductionsareequivalenttostressreductions.

Therefore,the number of cyclestofailurefortheairplanestructure can be expressedas A and where F represents the peak-to-peak forces and subscripts p and a repre- sent passive and active gears, respectively. Fatigue damage D can be defined astheinverse of t h e f a t i g u e l i f e , t h a t is, and Hence,the damage t o t h e s t r u c t u r e w i t h t h e active g e a rr e l a t i v e t o t h a t w i t h the p a s s i v eg e a r is Da Fa5/A Fa5 A Fa5 A FP5 FP5 or The damage o c c u r r i n gw i t ht h ep a s s i v eg e a r is assumed t o be u n i t y t o permit e v a l u a t i o no ft h er e l a t i v e damage o c c u r r i n gw i t h t h e a c t i v e g e a r .

Results of t h e a p p l i c a t i o n of t h i s p r o c e d u r e t o t h e c y c l i ca i r f r a m e - g e a r forces o b t a i n e df r o ml a n d i n gs i m u l a t i o n s of t h e l a r g e mass c o n f i g u r a t i o n w i t h p a s s i v ea n da c t i v eg e a r s a t a touchdownsink r a t e of 0.9 m/sec ( 3 . 0 f t / s e c )o n t h e u p h i l l slope of runway A are shown i nf i g u r e1 1 .N e g a t i v ea n dp o s i t i v e f o r c eb o u n d s for t h e p a s s i v e a n d a c t i v e g e a r s were o b t a i n e db ya v e r a g i n gt h e rms v a l u e so f t h e p e a k n e g a t i v ea n d p e a k p o s i t i v e f o r c e s m e a s u r e d d u r i n g t h e l a n d i n gs i m u l a t i o n s . Summing t h ea b s o l u t ev a l u e s of t h e force bounds results i nt h ea v e r a g eo ft h e rms v a l u e s of a l l p e a k - t o - p e a k forces e q u a l to 59.920 kN ( 1 3 471 l b f ) for t h ep a s s i v e gear FP and 14.13 kN ( 3 1 7 7l b f )f o r t h e a c t i v e g e a r Fa. A p p l y i n g t h e s e forces i nt h er e l a t i v e damage e q u a t i o ng i v e s Da (14.13 )5 " - = 0.0007 S i n c et h es t r u c t u r a l - f a t i g u e damage w h i c h o c c u r s w i t h t h ep a s s i v eg e a r is assumed to be u n i t y ,t h e d a m a g ew h i c ho c c u r r e dw i t ht h ea c t i v eg e a rf o r t h e same l a n d i n gc o n d i t i o n s was less t h a n 1 p e r c e n to ft h a tr e s u l t i n gw i t ht h e gear.

p a s s i v e The data o b t a i n e df r o ma p p l y i n gt h i sp r o c e d u r e t o a l l l a n d i n gs i m u l a t i o n s of t h i ss t u d y are p r e s e n t e di nt a b l e 111. The data are p r e s e n t e d as peak neg- ative a n dp o s i t i v e rms a v e r a g ef o r c e sa n d rms a v e r a g e p e a k - t o - p e a k forces (FP and Fa) f o r l a n d i n g s i m u l a t i o n s w i t h b o t h p a s s i v e a n d a c t i v e gears. The p o t e n t i a lo f t h e a c t i v eg e a r t o r e d u c es t r u c t u r a lf a t i g u e damage ( r e l a t i v e t o t h a to ft h ep a s s i v eg e a r ) is also shown f o re a c hl a n d i n gs i m u l a t i o n made d u r i n g t h es t u d y . The a v e r a g e value of t h i sf a t i g u e damage is 0 . 1 4 ;t h a t is, s t r u c - t u r a l f a t i g u e d a m a g ew i t ht h ea c t i v e gear was 86 p e r c e n t less t h a n t h a t w h i c h w o u l d o c c u rw i t ht h ep a s s i v e gear.

1 7 CONCLUDING REMARKS T h i s paper presentstheresults of a studytoevaluatethecapability of ACOLAG forpredictingthelanding dynamics of airplanes w i t h passive and active main gears, and results of theapplication of that program t o an analytical investigation of the dynamic behaviorduringlandingof a largeairplane equipped w i t h bothtypes ofmain gears.

Correlations betweencomputed data from ACOLAG, computed data from FATOLA, and preliminaryexperimentaldrop-testdata(for a modified activecontrol main gear from a l i g h t airplane)indicatethat ACOLAG is validforpredictingthe landing dynamics of airplanes w i t h both passive and activeload-control main landinggears.Results from theanalytical parameter s t u d y show thatthe activeload-control gearperforms w i t h i n formulated operational constraints.

The operation of a simplifiedantiskidbraking system was shown t o have no detrimentaleffects on the performance of theactivegear, and it is possible that improved braking performance could be obtained w i t h theactivegear. A comparison of thepassive- and active-gearresultsindicatesthattheactive gear is more effective i n reducingairframe-gearforcesthanthepassive gear €or a l l parametersinvestigated and during a l l phases of a landing. The effectiveness of theactive gear was most pronounced forthose touchdown con- ditions which would result i n developinglargeairframe-gearforces w i t h the passivegear,that is, largeairplane masses, higher touchdown s i n k rates, and uphill runway slopes. The active gear is alsoeffective i n reducingairplane motions following i n i t i a l impact. The reduction i n cyclicforcesresulting from use of theactive gear indicatesthepotentialforsignificantreductions i n structuralfatigue damage during ground operations.

LangleyResearchCenter NationalAeronautics and Space Administration Hampton, VA 23665 October 1 6 , 1979 1 8 APPENDIX ADDITIONAL DATA FOR AIRPLANE LANDING SIMULATIONS T h i sa p p e n d i xp r e s e n t s data f o r a i r p l a n e l a n d i n g s i m u l a t i o n s o f t h e small mass c o n f i g u r a t i o n a t a touchdownsink r a t e of 1 . 5 m/sec ( 5 . 0 f t/sec) o n t h e u p h i l la n dd o w n h i l l slopes o ft h e more uneven runway (runway B) . Data are also p r e s e n t e d f o r t h e a i r p l a n e l a n d i n g s i m u l a t i o n s o f a l l mass c o n f i g u r a t i o n s a t touchdownsink rates of 0.3 m/sec ( 1 . 0f t / s e c )a n d1 . 5 m/sec (5.0 f t / s e c )o n t h e u p h i l l anddownhill slopes of runway A.

1 9 h) " A c t i v e gear -50 X IO3 ""_ Passive gear

-200 -2401

4 -40

- 1 6 0 -

-120 -

- -20 Airframe- gear force, I bf I n ; ', .- -10

%

i H

X

1 Nosegear

Touchdown contact

80 f

$20 0 I 2 3 4 5 6 7 8 Time, sec (a) Airframe-gear forces: uphill runway.

Figure A1.- Airframe-gear force and shock-strut-stroke time histories for landing simulations of airplane with passive and active main gears. Small mass configuration: runway B; sink rate, 1.5 m/sec (5.0 ft/sec) .

"I 22 .55 - Active gear Maximum stroke 20 Maximum stroke - - - --Passive gear , 5 0 I- - 1 8 .45 i- - 16 .40 I - - 14 .35 - S t r u t Strut stroke, stroke, in.

rn X Time, sec (b) Shock-strut strokes; uphill runway.

Figure A1 .- Continued.

- Active gear ""_ -50 X lo3 Passive gear -40 -30 -l20l- I Airframe-

4 -20

gearforce, Airframe- kN gear force, I bf

i H

X

1 Nosegear 1 Nosegear

Touchdown Touchdown t contact t contact t t

80 r 80 r

I I I I I 9 20 9 20 0 I 2 3 4 5 6 0 I 2 3 4 7 8 6 7 8 Time, sec (c) Airframe-gear forces; downhill runway.

Figure A1 .- Continued.

- 22

.55 -

Active gear - - "_ Passive gear ' Maximum stroke Maximum stroke 2o .MI-

I

- 1 8

45 - - 16 40 -

- 1 4

.35 -

.30- P - '2 Strut

Strut : ", stroke, stroke, in.

G

-8 E

X - 6 - 4 - 2 Time, sec (dl Shock-strut strokes; downhill runway.

Figure A1 .- Concluded.

h, W -240 -Active gear -"" -2*[ Passive gear

1 - 4 0

"I - ; -30

-120 1

I Airframe-

- 1 -20

gear force, -80- Airframe- kN gearforce, I bf - 1 -10 Time. sec (a) Airframe-gear forces; uphill runway; sink rate, 0.3 m/sec (1 . O ft/sec).

Figure A2.- Airframe-gear-force and shock-strut-stroke time histories for landing simulations o f airplane with passive and active main gears. Small mass configuration; runway A.

- 22

.55 - Active gear _"" Passive gear Maximum stroke 20 Maximum stroke 50 - - I 8 . 4 5 ~ - I6 - 4 0 - - 1 4 35 -

.30 -

-I2 Strut S t r u t stroke, .25- . 2 0 - -6 .E- - 4 / I I /

'

"2 Nosegear 4 contact I I

0 I 2 3 4 5 6 7 8 go

Time, sec (b) Shock-strut strokes; u p h i l l runway; s i n k rate, 0.3 m/sec ( 1 .O ft/sec) .

F i g u r e A2.- Continued.

-240 r

-%X lo3

-Active gear

""_

-200 !

Passive gear

- 1 6 0 t

-"

4 -30

-1201-

I

" -20 Airframe- gear force,

I bf %

- -10

H i

X - I O

' Nosegear

[Touchdown contact I I 0 I 2 3 4 5 6 7 8 920 Time, sec (c) Airframe-gear forces;uphill runway; s i n k rate, 1.5 m/sec (5.0 f t / s e c ) .

Figure A2.- Continued.

- 22

.55 ,-

Active gear Maximum stroke Maximum stroke 2o ""- Passive gear "

.50 17

- 1 8 . 4 5 c

- 1 6

.a"

- 14

.35 -

S t r u t S t r u t stroke, stroke, in.

m Time, sec (d) Shock-strut strokes; uphill runway; sink rate, 1.5 m/sec (5.0 ft/sec) .

Figure A2.- Continued.

-240 r

-50x10~ Active gear

""_

Passive gear

-200 t

- 1 6 0 1

1 "

4 -30

-120 c

Airframe- 41-20

gearforce, -80 -

~ Airframe- kN ~ gear force, I bf - -10

-40 -

5.

I , I 40 - -! I O

8o +[Touchdown ; Nosegear contact

I I I 0 I 2 3 4 5 6 7 8 920 Time, sec (e) Airframe-gear forces; downhill runway; sink rate, 0.3 m/sec (1.0 ft/sec).

Figure A2.- Continued.

- 22 .55 " Active gear _"" Passive gear Maxi m u m stroke Maximum stroke 2o

.x-

- 1 8

.45 t

- 16 .40-

- 14

. 3 5 - .3G- Strut S t r u t stroke, stroke, In.

m . 2 5 - . 2 0 - - 8 . 15 - - 6 .IO- - 4 I Time, sec (f) Shock-strut strokes; downhill runway; s i n k rate, 0 . 3 m/sec (1 .0 ft/sec).

F i g u r e A2.- Continued.

h) W

-%X t o3

-Active gear

_"" Passive gear

-I-

- -30 Airframe- - -20 ; -80 Airframe- gearforce, kN gear I bf - -10 -40 X 0 0 - I O

k T o u c h d o w n * Nosegear

contact 1 1 0 I 2 3 4 5 6 7 8 920 Time,set (4) Airframe-gear forces; downhill runway; s i n k rate, 1 . 5 m/sec (5.0 f t / s e c ) .

Figure A2 .- Continued.

1 22 .55 - , Active gear ""- Passive gear Maximum stroke Maximum stroke 2o

.50 I F

S t r u t S t r u t stroke, stroke, in.

m x T i m e , sec ( h ) Shock-strut strokes; downhill runway; s i n k rate, 1 . 5 m/sec ( 5 . 0 f t/sec) .

F i g u r e A2 .- Concluded.

W h) -50x10~ -Active gear _ " " Passive gear

-200 "r

I

4-4 - 1 6 0 t 1 - 3 0 Airframe- - 1 -20 gearforce, - 8 0 1 Airframe- kN gear I bf - -10 $ 1 I ' 40 t I"' -' I O I

8o +Touchdown 1 Nose-gear contact

I ?O 0 I 2 3 4 5 6 7 8 9 ' Time, sec (a) Airframe-gear forces; uphill runway; sink rate, 0.3 m/sec (1 . O ft/sec).

Figure A3.- Airframe-gear-force and shock-strut-stroke time histories for landing simulations of airplane with passive and active main gears. Medium mass configuration; runway A.

- 22 .55 - Active gear

""_

M a x i m u m stroke Passive gear Maximum stroke 20 - ""

.50 -

- 1 8 .45 .- - I6 .40 - - 1 4

.35 -

Strut stroke, m Time, sec (b) Shock-strutstrokes: u p h i l l runway: s i n k rate, 0 . 3 m/sec ( 1 . O ft/sec).

Figure A3.- Continued.

W W W Ip -240 .U)XIO~ Active gear ””_ Passive gear -200 -40 - 1 6 0 -30 I- . -120 I t I 1 Airframe- 1 1 ; I I -20 gearforce, -80 ~h, Airframe- kN gearforce, I bf -10 I O

7 Nose-gear

[Touchdown t contact I I 0 I 2 3 4 5 6 7 8 Time, sec (c) Airframe-gear forces; uphill runway; sink rate, 1.5 m/sec (5.0 ft/sec).

Figure A3.- Continued.

- 22

.55 -

-Active gear "_" Passive gear Maximum stroke Maximum stroke .50 1 - S t r u t Strut stroke, stroke, in.

m Ti me, sec (a) Shock-strut strokes; u p h i l l runway; s i n k rate, 1.5 m/sec (5.0 ft/sec).

F i g u r e A3.- Continued.

W QI

, -50x10~

-Active gear _"" Passive gear

t

Airframe- " -20

gearforce, -80 - Airframe-

gearforce, kN I bf 40 I - - I O

7 Nose-gear

L-Touchdown A contact go \/ I I ! I I I I 0 I 2 3 4 5 6 7 8 Time, sec ( e ) Airframe-gear forces; downhill runway; s i n kr a t e , 0 . 3 m/sec ( 1 . 0 ft/sec).

F i g u r e A3 .- Continued.

1 22

.55 -

Active gear _"" I Passive gear ' Maximum stroke Maximum stroke

- 1 8

.45 k " - 16 .40 I - 14 .35 r -

.30 -

S t r u t stroke, .25- I I

20 -

0 ) ' f" -8

I -6

.I5 -

- 4

t Nosegear

contact 0 1 2 3 4 I 5 L " L 2 A 2 6 7 8 9O T i m e , s e t ( f )S h o c k - s t r u t strokes; downhill runway; s i n k rate, 0 . 3 m/sec (1.0 ft/sec).

F i g u r e A3.- Continued.

-240 -50x10~ -Active gear

""_

Passive gear -200 -40 - 1 6 0 -30 -120 Airframe- -20 Airframe- gearforce,

I bf %

-10

ii

E

X I O I A

1 Nose-gear

,-Touchdown f contact 80 j- I I I I I 0 I 2 3 4 5 6 1 8 Time, sec (9) Airframe-gear forces; downhill runway: sink rate, 1.5 m/sec (5.0 ft/sec).

Figure A3 .- Continued.

- 22 .55 - Active gear " _ " Passive gear Maximum stroke Maximum stroke . 5 0 - - 1 8 .45 -

- 16

-40 8 - - 1 4 S t r u t S t r u t stroke, stroke, in.

m \ \

\

- 6 - 1 4

~) 1 Nosegear

I/ ,-Touchdown ', ' \,r contact

$ 1 I I

Y

0 I 2 3 4 5 6 7 8 Time,sec (h)Shock-strut strokes; downhill runway; s i n k rate, 1 . 5 m/sec (5.0 ft/sec), F i g u r e A3.- Concluded.

W ID Ip -240 .%X lo3 -Active gear _"" Passive gear -200 -40 - 1 6 0 -30 -12c Airframe- -20

gearforce, -80 -

Airframe- kN gearforce, I bf -10

-40 -

I I

40 -

l r

1 Nose-gear

,-Touchdown t contact on (a) Airframe-gear forces; uphill runway; sink rate, 0.3 m/sec (1.0 ft/sec).

Figure A4.- Airframe-gear-force and shock-strut-stroke time histories for landingsimulations of airplane with passive and active main gears. Large mass configuration; runway A .

1 22 .55 , - -Active gear " _ " Passive gear Maximum stroke Maximum stroke ' 20 . 5 0 - 1 1 8 .45 - - 16 .40 - S t r u t S t r u t stroke, stroke, in.

m I , - 4 I I I I I - 2

, 1 Nose-gear

/ + contact I I 7 8 9O 0 I 2 3 4 5 6 Time, sec (b) Shock-strut strokes; uphill runway; sink rate, 0 . 3 m/sec (1.0 ft/sec).

Figure A4.- Continued.

-240 -5OXlO3 - Active gear -200 " _ " Passive gear -40 - 1 6 0 -30 -120 Airframe- gear force, -80 Airframe- kN gearforce,

I bf %

A -40 - I \ -10 I \

H i

X I I - IO Time, sec (c) Airframe-gear forces; uphill runway; sink rate, 1.5 m/sec (5.0 ft/SeC).

Figure A4.- Continued.

1 22 .55 - Active gear - - _ " Passive gear Maximum stroke Maximum stroke 2o .50 - - 1 8

- 16

S t r u t stroke, m - 8 H x - 6 1 4 Time, sec (a) Shock-strutstrokes; uphill runway; sink rate, 1 . 5 m/sec (5.0 ft/sec).

Figure A4.- Continued.

I P W -50x10~ Active gear _"" Passive gear

-2401 -200

I - r n

7 2 0 Airframe- gear force, I bf L Nosegear ,-Touchdown contact ; r I I I

-1-

0 I 2 3 4 5 6 7 8 T i me, sec (e) Airframe-gear forces; downhill runway; sink rate, 0.3 m/sec (1.0 ft/sec).

Figure A4 .- Continued.

- 22

.55 -

Active gear

I Maximum stroke - - - --Passive gear

Maximum stroke .50 r-

- 1 8

.45 -

I 1 I6 40 -

" - 14

stroke, stroke, in.

m

- I O

.25 ,r

- -8

.20 - - 6 . 1 5 1 4 " 2

\ /.. I'

0 I 2 3 4 5 6 7 8 9 Time, sec ( f )S h o c k - s t r u t strokes; downhillrunway;sink rate, 0.3 m/sec ( 1 . O ft/sec) .

F i g u r e A4.- Continued.

-50x10~ Active gear

-240r

" _ " Passive gear

t

I .

-30 Airframe- gearforce, Airframe- kN gearforce, I bf

i H

X Time. sec (9) Airframe-gear forces; downhill runway; sink rate, 1.5 m/sec (5.0 ft/sec).

Figure A4.- Continued.

- 22

.55 I - -Active gear ""_ Passivegear Maximum stroke Maximum stroke , 2o

.x,-

- 1 8

.45 -

- 16

.40 - - 1 2 S t r u t S t r u t stroke, stroke, in.

m H - 8 X -6 - 4 - 2 Time, sec (h)Shock-strut strokes; downhillrunway;sinkrate, 1 . 5 m/sec (5.0 ft/SeC) F i g u r e A4 .- Concluded.

REFERENCES 1 . DC-10 Landing Gear M o d i f i e d .' A v i a t . Week & Space Technol.,vol.98,no. 1 2, Mar. 19, 1973, p. 181.

2. R o p e l e w s k i , Robert R.: A i r b u s T e s t Tempo Q u i c k e n i n g .A v i a t . Week & Space Technol., vol. 98, no. 10, Mar. 5, 1973, pp. 32-35.

3. Wignot, Jack E.; DUrup, P a u l C.; and Gamon, Max A.: D e s i g nF o r m u l a t i o na n d A n a l y s i s of anActiveLanding Gear. Volume I. A n a l y s i s . AFFDL-TR-71-80, V o l . 1 , U . S . A i r F o r c e , Aug. 1971. ( A v a i l a b l e f r o m DDC as AD 887127L.)

4 . Bender, E. K.; Berkman, E . F.; and Bieber, M.: A F e a s i b i l i t yS t u d y of A c t i v eL a n d i n g Gear. AFFDL-TR-70-126, U . S . A i r F o r c e ,J u l y1 9 7 1 .

( A v a i l a b l e from DDC as AD 887451L.)

5. McGehee, J o h n R . ; andCarden, Huey D . : A MathematicalModel of a n A c t i v e ControlLanding Gear f o r Load C o n t r o lD u r i n g Impact andRoll-Out. NASA TN D-8080, 1976.

6 . Ross, Irving;andEdson,Ralph: An E l e c t r o n i cC o n t r o l for a nE l e c t r o - h y d r a u l i cA c t i v eC o n t r o l Aircraft Landing Gear. NASA CR-3113, 1979.

7 .W e s t f a l l ,J o h n R.; M i l w i t z k y ,B e n j a m i n ;S i l s b y , Norman S . ; andDreher, R o b e r t C . : A Summary of Ground-Loads S t a t i s t i c s . NASA TN 4008, 1957.

8 . Carden, Huey D.; and McGehee, John R.: V a l i d a t i o no f a F l e x i b l e Aircraft Take-OffandLandingAnalysis (FATOLA) . NASA TP-1025, 1977.

4 8 TABLE I.- INTEGRATION ERROR TOLERANCES KIR DEPENDENT VARIABLES Upper bound of local Dependent variable r e l a t i v e t r u n c a t i o n error . ~~ .. " ~- T r a n s l a t i o n a l velocities, m/sec (f t/sec) 0.0003 (1.0 x R o t a t i o n a l velocities, rad/sec 0.01 0.0003 (1.0 x T r a n s l a t i o n a ld i s p l a c e m e n t s , m ( f t ) R o t a t i o n a ld i s p l a c e m e n t s , rad 0.01 48.3(1.01) S h o c k - s t r u th y d r a u l i cp r e s s u r e ,P a ( l b f / f t 2 ) Servo-valve spool a c c e l e r a t i o n , cm/sec2 (in/sec2) 2.54(1 .OO) Servo-valve spool velocity, cm/sec ( i n / s e c ) 0.0254 (1 . O O x 0.000254(1 . O O x 1 0-4) Servo-valve spool d i s p l a c e m e n t , cm ( i n . ) 1.64 X 10-9 (2.60 X 10-5) H y d r a u l i cf l u i d flow rates, m3/sec (gal/min) 2.83 x l o e 5 (5.98 x F l u i dv o l u m et r a n s f e r r e d , m3 ( p i n t s ) cn TABLE 11.- RESULTS OF PARAMETER STUDY FOR LANDING SIMULATIONS ON RUNWAY A WITH ACTIVE LOAD-CONTROL GEAR Maximum flow Maximum fluid Maximum flow Maximum strut Maximum fluid Allowable added TouchdownPercentforcereduction, rate from strut removeda rate into strut stroke added fluidb sink rate loo* (1 - Fa/FD)

Main-gear response m3 1 pints

I to nose-gear impact ~~ ~~ ~ Small mass configuration, uphill slope 0.3 1.0 22.9 0.0108 11.0 0.279 0.2 0.0001 254 0.016 5.0 0.0024 151 0.010 5 1 7 24.1 .0114 11.3 .287 .4 .0002 199 .013 8.4 .0040 166 .010 57 15 3.0 .9 28.2 .0133 12.8 .325 .4 .0002 242 .015 12.5 .0059 213 .013 48 24 5.0 1.5 Small mass configuration, downhill slope 8 -1 3 0.009 25.0 0.0118 10.4 0.264 0.8 0.0004 172 0.011 1.5 0.0007 142 48 28.0 .0132 10.9 .277 . 6 .0003 229 .014 6.0 .0028 157 .010 28.2 .0133 11.9 .3 . 9 .0004 302 .019 10.4 .0049 197 .012 Medium mass configuration, uphill slope 0.3 1.0 18.5 0.0088 11.9 0.302 5.2 0.0025 163 0.010 10.3 0.0049 156 0.010 67 15 22.0 .0104 13.0 .330 6.8 .0032 205 .013 12.7 .0060 173 .011 48 24 3.0 . 9 26.6 .0126 17.5 .445 14.0 .0066 290 .018 17.2 .0081 224 .014 55 48 5.0 1 .5 Medium mass configuration, downhill slope Large mass configuration, uphill slope 0.011 13.0 0.0062 151 0.010 43 14 0.3 1.0 20.0 0.0095 13.8 0.351 5.4 0.0026 170 23.5 .0111 15.8 .401 9.0 .0043 209 .013 16.3 .0077 170 .011 84 47 3.0 .9 240 .015 26.8 .0127 18.9 .480 15.1 .0071 284 .017 16.8 .0079 89 5 1 5 . 0 1 .5 Large mass configuration, downhill slope 0.0018 141 0.009 16 0.3 25.4 0.0120 13.6 0.345 5.7 0.0027 189 0.012 3.8 70 1.0 24.5 ,0116 12.4 .315 10.7 .0051 187 .012 11.0 .0052 163 .010 78 2 1 3.0 .9 .014 27.9 .0132 14.7 .373 15.0 .0071 253 .016 12.2 .0058 222 87 40 5.0 1.5 aShock-strut piston contains 0.0103 m3 (21.80 pints) of hydraulic fluid for fully extended strut.

bAllowable added fluid to shock strut at time of maximum fluid added.

TABLE 111.- POTENTIAL FOR REDUCTION OF FATIGUE DAMAGE WITH ACTIVE LOAD-CONTROL MAIN LANDING GEAR Peak rms average forces , Sinkrate Active gear Passive gear Fa FP I DJDp = ( F , / F p ) Positive Positive Negative Negative lbf kN lbf kN lbf kN lbf kN kN (a) lbf m/sec ft/sec Small mass configuration, uphill slope 0.3 1.05 6394 28.44 6 326 28.14 3317 14.75 3078 13.69 2018 8.98 4 307 19.16 1.0 .07 2788 12.40 4 681 20.82 1006 4.48 1781 7.92 2221 9.08 2 640 11.74 3.0 .9 .27 4729 21.04 6 131 27.27 1881 8.37 2849 12.67 2531 11.26 3 599 16.01 5.0 1.5 Small mass configuration, downhill slope 0.11 4 530 1 2 2 4 0.3 2894 12.87 20.15 5.45 1670 7.43 2156 9.59 2 374 10.56 1.0 .17 2999 13.34 4 292 19.09 1332 5.93 1667 7.42 1996 8.88 2 295 10.21 3.0 .9 .51 4574 20.35 5 225 23.24 2083 2490 11.08 9.27 1909 8.49 3 316 14.75 5.0 , 1.5 Medium mass configuration, uphill slope Medium mass configuration, downhill slope 0.3 1.0 15.78 3 548 13.70 3079 7.00 1573 5.06 1 1 3 8 29.48 6 627 12.06 2711 bo. 01 14.65 3 294 11.91 2677 8.81 1 9 8 0 6.99 1 5 7 2 26.56 5 971 15.80 3552 .07 .9 3.0 2140 5787 .07 1.5 5.0 15.47 3 477 10.27 2308 9.52 5.69 1279 25.74 15.21 3419 aCalculated for Dp = 1.0.

bO.O1 established as lower limit.

:el er'ometer

,- Cy1 i nder

\Orifice with snubber valve (Ao) Pressure -rc I transducer (a) Passive.

(b) Active.

Figure 1 . - Cross-sectional schematics of passive and active shock struts.

Y ( i n t o paper) Aerodynanli c X b * (into paper)

0- center of pressure

Elevator

*- hinge a x i s

2.150 m (7.06 f t )

f

m h \

0.345 m rl (1.13 f t ) l "

- perturbations

z " 'z Figure 2.- Schematic of rigid-airframe configuration used for study. (Dimensions arenot to scale.)

cn W -1.8 Perturbations "" General slope -1.5 /RunwayA -1.2 I

- -3

Runway elevation, Runway m elevation.

ft ..

/Touchdown I I I I I I I I I 30 120 240 360 480 600 720 840 960 1080 1200 Runway distance. m I I I I I I I I I I I I I 0 3 6 9 12 15 18 21 24 27 30 33 36 3!9 x lo2 Runway distance, ft Figure 3 . - Elevation profiles of two operational runwaysused for landing simulations.

- I I. a - Elevator deflection (FATOLA) "" ELCON (ACOLAG)

'I-

1 1 . 6

- 1.4 r-----l

- 1.2

I I I i I

I I

- 1.0

-5,- I I ELCON I Elevator deflection, deg - 4 1 -.a

\ J

I I " - . 6 -1.4 - - . 2 -1 Nose-gearf Touchdown contact I

r I I I I I

I I I 2 3 5 6 7 a go T i m e . sec (a) Elevator-control inputs.

Figure 4 . - Comparison of data time histories obtained from FATOLA and ACOLAG for a symmetrical landing with a passive gear on a flat smooth runway. Sink rate, 1.5 m/sec (5.0 ft/sec).

FATOLA ACOLAG - Pitch rate - - - "" Pitch attitude - " I O Pitch attitude, deg contact

-2 - -126 /Touchdown, Y ; I Nose-gear I I I I

I 2 3 4 5 6 7 8 Time, sec (b) Pitch rates and pitch attitudes.

Figure 4 .- Continued.

- FATOLA "" ACOLAG " I I " - -30 - -60.

Strut Strut -46 force, "16 force, I bf kN i -26 I * - /"" I -., ."C . c -1% - \ J \ J

-180- L -180

'/ Nose-gear

0 0 Time, sec Time, sec (c) Main-gear shock-strut forces.

Figure 4.- Continued.

x10-l - FATOLA - - - - ACOLAG - I . 05 3 . 2 3 L /.

I I \

2.8 -

-. 90 " I . ,e-., .". /". -.

. / X " / / L"

'.- *'

.*'

-. 75

Strut -. 60 stroke, ft

-. 45

I. 2 I -. 30

-. 1 5

. 4

f Nose-gear

Touchdown contact

.(I L I I I I I I I I

0 I 2 3 4 5 6 7 8 9 O Time, sec (.a) Main-gear shock-strutstrokes.

Figure 4.- Concluded.

- -4 x 10 -16 - 0 Experimental

- Analytical

" " _ Shifted experimental

- -3

-12 -

- -2 A i r f r a m e y e a r Airframe-gear force, kN

- -' force, Ibf

4r - 1

Time, sec (a) Airframe-gear forces.

Figure 5.- Comparison of experimental data with data computed from ACOLAG for drop test of active control main gear from light aircraft. S i n k rate, 1.5 m/sec (5.0 ft/sec).

m

-e- Experimental

.24 Analytical .21 .18 .15 S t r u t S t r u t stroke, stroke, m .12 in.

.06 2 :

- 1

. 0 3 Touchdown ~ .1 . 2 . 3 . 4 Time, sec (b) Shock-strut strokes.

F i g u r e 5. - Concluded.

-1% - - -28~1d -120:- - Active gear Initial , I I ""_ I I Passive gear impact 1

- and -Rotation to nose-gear c o n t a c t - ! Nose I

rebound /

- "21

-90 - Initial impact and rebound L

- -14

-60 -iT Airframe- y, gear force, kN I Airframe- - -1 gear force, -30 !- \, I bf L Nose-gear I contact I I I 1 0 I 2 3 5 6 7 8 9 l4 Time, sec (a) Airframe-gear forces; small mass configuration; uphill runway.

Figure 6 .- Airframe-gear-force and shock-strut-stroke time historiesfor typical landing simulations of airplane with passiveand active main gears. Runway A; sink rate, 0.9 m/sec (3 f t/sec) .

- Active gear " "_ Maximumstroke Passive gear Maximum stroke 5 . 0

"'r 4.0

3. 5 -

i l4

3.0 -

S t r u t Time, sec (b) Shock-strut strokes; small mass configuration; uphill runway.

6.- Continued.

Figure - 1 5 0 - " 2 8 X l d -120- - Active gear ""_ Passive gear - -21 -90 - I

-@lly - - I 4

Airframe- Airframe- - -7 gear force, I bf contact I 0 I 2 3 4 5 6 7 a Time, sec (c) Airframe-gear forces; small mass configuration; downhill runway.

Figure 6 .- Continued.

5.5k

- Activegear

1 Maximum stroke

""_ 5.0 - Passive gear Maximum stroke 4.5 I - 4. 0 -

i

3. 5 -

S t r u t

stroke, 3.0 -

m ~ stroke, I..

2.0 -.

Time , sec (d) Shock-strut strokes: small mass configuration; downhill runway.

Figure 6 .- Continued.

-1%- - -28x13 -120 - - Active gear Passive gear

- -21

-90- - -14 -611 Airframe- I gear force, kN

\ : Airframe-

-30 : :

! Nose-gear

? contact i -/ Touchdown I I ffi 1 1 I I I 1

0 i

I 2 3 4 5 6 7 8 Time, sec (e) Airframe-gear forces; medium mass configuration; uphill runway.

Figure 6 . - Continued.

a a XIO" 6 . 0 r

5.5c

__ Active gear - - ---Passive gear Maximumstroke

Maximum stroke Azo

5. 0 - 1 8 4. 5 - - 16 4.0 -

- 1 4

3. 5 - S t r u t S t r u t stroke, m stroke, v in.

/ - I O 2.5 1 -

I

\ - 8 \ \ \ \ I - 6 ', \ I \ I \ - 4 I \ !

/

\ I \ I - 2 \ I Nose-gear \ , I $. contact /-Touchdown 1 " 1 , ,I' I I I 0 I 2 3 4 5 6 7 8 9 O Time, sec (f) Shock-strut strokes; medium mass configuration; uphill runway.

Figure 6.- Continued.

-1501- - -28x10 -120 - - Active gear ""_ Passive gear

- -21

-90 -

- -14

-60 - Airframe- I d Airframe- 1 1 gear force, I bf 30 - "7

1 Nose-gear

t + contact

I 6o p T o u c h d o w n I I 1 1 0 I 2 3 6 7 8 Time. sec (9) Airframe-gear forces; medium mass configuration; downhill runway.

Figure 6 .- Continued.

6. 0 , XlO" 5. 5 - 22

- Active gear

Maximum stroke ""_ " Passive gear Maximum stroke 2o - 5. 0 - 4. 5 4. 0

- i 1 8 16

- 3.5 1 4 #- \ S t r u t I \ . . I I-\

- ," h. ' /--, ""\\ ,-., ' I-\ \

stroke, 3.0 \ - 2 \

' '\ ,L 12

/ '1 \ " e '.-4' - -2 L 0 ' '. '

S t r u t r * L' 8 ' m stroke, 4 -' - / ft 2. 5 / - 8 - 6 - 4 - 2 Time, sec (h) Shock-strut strokes; medium mass configuration; downhill runway.

Figure 6.- Continued.

-150 - - -28x10 -120 - -Active gear "". Passive gear Airframe- gear force, I bf (i) Airframe-gear forces; large mass configuration; uphill runway.

Figure 6.- Continued.

Activegear 5' 5.0 5[Maximum stroke " _ " Passive gear

4 . 5 1 -

S t r u t

stroke, 3.0 -

Strut m stroke, In.

- I O

-8 - 6 I 1. 0 \ I - 4 \ I \ i I I " ' 1 I I . 5 - 2

\ I t Nose-gear

I , 9 contact

/Touchdown I -. I

\ I I I 0' I 2 3 4 5 6 7

8 9O

T i m e , sec (j) Shock-strut strokes; large mass configuration; uphill runway.

Figure 6.- Continued.

- -28x12

-Active gear ""_ Passive gear

- -21

Airframe- gear force, I bf (k) Airframe-gear forces; large mass configuration; downhill runway.

Figure 6.- Continued.

i -Active gear

""_

Maximumstroke Passivegear Maximumstroke

I l8

-! I 6 S t r u t / stroke, , - 1 2 I strut 3.01- / m stroke, I I n .

"i IO " 8 i 6 - " 4 , / \ I I . 5 I ,

, I 1 Nose-gear " 2

/ Touchdown I, I t contact I \ I 1 I 0 I 2

3 4 5 6 7 8 d o

Time , sec (1) Shock-strut strokes: large mass configuration; downhill runway.

Figure 6.- Concluded.

XI0 -66 1 -

- -14x10

- Activegear

"" Passive gear 4 -12 -+to Airframe- gear force, kN (a) Airframe-gear forces.

Figure 7.- Airframe-gear-force and shock-strut-stroke time historiesfor landing simulations with passive and active gears at above design touchdown sink rate. Large mass configuration; uphill slope of runway A; sink rate, 2.4 m/sec (8.0 ft/sec).

- Activegear

"" Passive gear Maximum stroke

54L Maximum stroke

S t r u t stroke, Strut rn stroke, in.

- 6 - 4 " 2

T Nose-gear

f contact I I

I I I

3 4 7 8 5 6 Time, sec (b) Shock-strutstrokes.

Figure 7 .- Concluded.

x 10 - -48x 10 Passive gear "" Active oear: constant accumulator pressure -16 - - -32 -14 - - -24 - -16 Airframegear force, Ibf - 16 Time, sec (a) Airframe-gear forces.

Figure 8.- Effect of high-pressure accumulator design on airframe-gearforces and shock-strutstrokes for landingsimulations w i t h activegear. Large mass configuration; runway A w i t h uphill slope; s i n k rate, 1.5 m/sec ( 5 . 0 ft/sec) .

" Passive gear . - - - - - - Activegear;constantaccumulatorpressure = O.ooo6 m 3 / sec ( 9 gall min) "- Activegear;Vac,t = 0.0379 m 3 (10.0gal);Vac,i = 0.0189 m 3 (5.0 gal); 9 3 3 Pump

- 0.0032 m 3 / sec (50 gall minl

"" Active gear; V = 0.0189 m (5.0 gal); Vac,i = 0.0076 m (2.0 gal); Q ac,t Pump

1 " Maximum stroke

Strut stroke, in.

(b) Shock-strutstrokes.

Figure 8.- Concluded.

--28XIO - Active gear

-120 1 -

""_ Passive gear

- -21

. .

(a) Airframe-gear forces.

Figure 9.- Airfrarne-gear-force and shock-strut-stroke time historiesforlandingsimulations w i t h antiskid braking. Large mass configuration; runway A w i t h uphillslope: s i n k rate, 0.9 m/sec ( 3 . 0 ft/sec) .

5. 5 " x10-l Activegear

1 Maximum stroke

""_

Passive gear Maximum stroke 1 8

4. 5.01 5

Strut S t r u t stroke, stroke, in.

m , l '- , ./Touchdown ' I 0 I 2 (b) Shock-strutstrokes.

Figure 9 .- Concluded.

-42.0

-

Activegear - - " Passive gear -3.6 - Time, sec (a) Sink rate, 0.3 m/sec (1 .O ft/sec) .

Figure 10.- Gravity Z-axis displacement time histories of fuselage mass center. Medium mass configuration; runway A with uphill slope.

-12.0 - Activegear "" Passive gear Displacement, ft I Nosegear contact - 1 - 9 . 6 -2*9/Touchdown I 1 I I I 0 I 2 3 4 7 5 6 Time, set (b) S i n k r a t e , 0.9 m/sec ( 3 . 0 f t / s e c ) .

F i g u r e 10.- C o n t i n u e d .

-3.7 , -

- -12.0 - Active gear "" passive gear I -3.6 - - 4 1 . 6 Displacement, f t (c) Sink rate, 1.5 m/sec (5.0 ft/sec) .

Figure 10.- Concluded.

-150 t r - -120 - ~ Active gear

l-28x'o

- - --Passive gear

I

gearforce, I b f Figure 11.- Airframe-gear-forcetime histories w i t h rms bounded forcesforindicatingpotential for reduced fatigue damage w i t h theactivegear. Large mass configuration; runway A w i t h uphill slope; s i n k rate, 0.9 m/sec ( 3 . 0 ft/sec).(Solidhorizontallines show active-gear force bounds and dashed horizontallines show passive-gearforce bounds.)

-~ . . "" . .. . -.

1. Report No. I 2. Government Accession No.

T=i,ipieny-cata~og .. .. . NO. .

5. Report Date

1 December 1 9 7 9

ANALYTICAL INVESTIGATION OF THE: LANDING DYNAMICS OF A LARGE AIRPLANE WITH A LOAD-CONTROL SYSTEM I N THE 6. PerformingOrganization Code MAIN LANDING GEAR " . . . . . - -. . . -.

I - - . ". - -

7. Author(s) I 8. Performing Organization Report No.

John R . McGehee and Huey D. Carden L-13250 - ___" _ _ _ ~ ~ 10. Work Unit No.

9. PerformingOrganizationNameand Address NASA LangleyResearchCenter Hampton, VA 23665 ." " ". ~~ _ _ _ - ~ " TechnicalPaper 2. Sponsoring Agency Name and Address NationalAeronauticsandSpaceAdministration . .

Washington, DC 20546 14. Sponsoring Agency Code

+--

" 1 ~~~ " " .___ . . " _ ~.

5. Supplementary Notes - _ _ _ _ _ ~ ~ ~~ " _ ~ ~~ = ..-. . - .

6. Abstract T h i sp a p e rp r e s e n t st h e results o fa ne v a l u a t i o n of a na c t i v el o a d - c o n t r o ll a n d i n g g e a r computer program (ACOLAG) for p r e d i c t i n gt h el a n d i n gd y n a m i c s of a i r p l a n e s w i t hp a s s i v ea n da c t i v em a i ng e a r s . ACOLAG was u s e di na na n a l y t i c a li n v e s t i g a t i o n a l a r g ea i r p l a n ew i t hb o t hp a s s i v ea n da c t i v em a i ng e a r s .

ofthelandingdynamicsof The results of t h ee v a l u a t i o n of ACOLAG i n d i c a t et h a tt h ep r o g r a m is v a l i d for pre- d i c t i n gt h el a n d i n gd y n a m i c so fa i r p l a n e sw i t hb o t hp a s s i v ea n da c t i v em a i ng e a r s .

The results o ft h ea n a l y t i c a li n v e s t i g a t i o no ft h el a n d i n gd y n a m i c so ft h el a r g e a i r p l a n e show t h a tt h ea c t i v eg e a rr e d u c e sa i r f r a m e - g e a rf o r c e sa n da i r p l a n em o t i o n s f o l l o w i n gi n i t i a li m p a c ta n dh a st h ep o t e n t i a l for s i g n i f i c a n tr e d u c t i o n si ns t r u c - t u r a l f a t i g u e damage r e l a t i v e to t h a t w h i c ho c c u r sw i t ht h ep a s s i v eg e a r .

~ -E " . - . . . " ~ " ~ " " ~.

. ~ '. Key Words (Suggested by Author(s)) 18. Distribution Statement A i r c r a f tl a n d i n gg e a r U n c l a s s i f i e d - Unlimited A c t i v ec o n t r o l s L a n d i n g l o a d s S u b j e c tC a t e g o r y 05 . .-__- L. ." - "~ - - . . -. . ..

I. Security Classif. (of this report] 20. Security Classif. (of this p a g e ) U n c l a s s i f i e d U n c l a s s i f i e d - -- _ _ $6.00 -~ -~ " * For sale by the Natlonal Technical Information Service, SprlnRfleld. Vlrnlnla 22161

- -

NASA-Langley, 1979 THIRD-CLASS BULK R A T E National Aeronautics and Postage and Fees Paid National Aeronautics and Space Administration Space Administration NASA451 Washington, D.C.

20546 Official Business Penalty for Private Use, $300 12 1 l U , A , 121179 S00903DS DEPT O F T H E A I 3 FORCE AP 'REAPOIJS LABORATORY ATTN: TECHNICAL LIERlARf (SUL) K I R T L A N D A F B BPI 87117 POSTMASTER: If Undeliverable (Section 1 5 8 Postal Manual) Do Not Return