Document
N95- 33024
-,/5 .-_
NASA Dryden Flight Research Center
"Dynamic Ground Effects Flight Test of the NASA F-15 Airp
Table of Contents
Navigating Around the Workshop:
[Workshop Home] [Session Agenda] [Submit Response / Read Responses]
HELP is Available
CONTENTS:
• Abstract and Introduction (p. 1)
• Ground Effects Background (p. 2)
• Approach Speed, Flap setting and Sink Rate Effects (p. 3)
• Previous Ground Effect Comparison (p. 4)
• Improvement in F-15 Ground Effects Flight Simulator Model (p. 5)
Author: Stephen Corda
Affiliation: PRC, Inc.
Phone: 805-258-2103
Fax: 805-258-3744
Address: Bldg. 4839, P. O. Box 273, Edwards, CA 93523-0273
e-mail: Stephen_Corda@QMGATE.DFRF.NASA.GO V
Author." Mark T. Stephenson
Affiliation: NASA Dryden Flight Research Center Address: M. S. D2033, P. O. Box 273, Edwards, CA 93523-0273 Author: Frank W. Burcham Jr.
Affiliation: NASA Dryden Flight Research Center Phone: 805-258-3126 Fax: 805-258-3744 Address: M. S. D2033, P. O. Box 273, Edwards, CA 93523-0273 e-mail: Bill Burcham@QMGATE.DFRF.NASA.GOV
Dynamic Ground
Effects Flight Test
of an F-15 Aircraft
Stephen Corda PRC, Inc.
Edwards, Catifomia Mark T. Stephenson and Frank W. Burcham NASA Dryden Flight Research Center Edwards, Califom{a Abstract Aerodynamic characteristics of an aircraft may significantly differ when flying close to the ground rather than when flying up and away. Recent research has also determined that dynamic effects (i.e., sink rate) influence ground effects (GE). A ground effects flight test program of the F-15 aircraft was conducted to support the propulsion controlled aircraft (PCA) program at the NASA Dryden Flight Research Center.
Flight data was collected for 24 landings on 7 test flights. Dynamic ground effects data were obtained for low- and high-sink rates, between 0.8 and 6.5 ft/sec at two approach speed and flap combinations. These combinations consisted of 150 kt with the flaps down (30 ° deflection) and 170 kt with the flaps up (0 ° deflection), both with the inlet ramps in the fuU-up position. The aerodynamic coefficients caused by ground effects were estimated from the flight data. These ground effects data were correlated with the aircraft eed, flap setting, and sink rate. Results are compared to previous ht test and wind-tunnel ground effects data for various wings and for complete aircraft.
Radar altimeter -_ 8.74 It 12.04 ft 5.66 ft 4.17 ft (3.67 m) (1.73 m) (1.27 m) 04O339 F-15 at touchdown attitude o Without ground, free flow [] In proximity of the ground 1.2 -- .2 .8 1.0- _ Induced Lift drag .6 _;_ O Without ground, coefficient coefficient .1 free flow .4 [] In proximity of
F
.2 the ground 10 20 30 0 .2 .4 .6 .8 1.0 1.2 i I I I Angle of attack, deg CL2 O4OO31 iML0332 Ground Effects Background Ground effects may be explained by the interaction of the aircraft wing_p vortices with the ground. This interaction reduces the strength of these vortices. The weakened wingtip vortices reduce the downwash which increases the lift and decreases the induced drag or the drag due to lift. These figures show this change for a 40 ° sweptback'.K wing.winIn addition, the reduced downwash at the wing trailing edge increases the angle-of-attack of the relative wind at the elevator, resulting m a nose-clown pitching moment.
Ground effects data can be obtained in the wind tunnel or in flight. In conventional wind-tunnel ground effects testing, measurements are taken for a stationary aircraft modeI at various fixed ground heights. The results are called static ground effects data. Unfortunately, this static data simulates the aircraft flying' neanear the ground at a constant altitude rather than simulating the transient or dynamic effects of the aircraft descendingthrough a given altitude, termed "dynamic" ground effects data.
Note that static conditions, whether in the wind tunnel or in flight, produce significantly different ground effects on an atrcraft than those produced by dynamic conditions.
F-15 flight data, gear down • 170 kt, flaps up Approach Speed, Flap Setting, and • 150 kt, flaps down Sink Rate Effects m ACL,G E = 0.2/(1 + I_) + 0.02 .20 These figures show the F-15 ground .15 effects flight data plotted versus as a function approach speed, flap setting, ACL,GE .10 and sink rate. These figures show the .05 _ _ -_ changes due to ground effect of the I I I I I I I lift, drag, and pitching moment 0 1 2 3 4 5 6 7 coefficients as a function of sink rate.
Sink rate, Wsec _o_ Changes in the aerodynamic coefficients were calculated at touchdown. Sink rates ranged from a low of 0.7 ft/sec (42 ft/min) to a high of 6.5 ft/sec (390 ft/min). For F-15 flight data, gear down reference, the F-15 landing gear has a 170 kt, flaps up 150 kt, flaps down maximum sink rate capability of ACD,GE = 0.035 - 0.005 I_ about 10 ft/sec (600 ft/min).
.15 .10_ In general, these figures show that ground effect becomes more significant _CD,GE as sink rate decreases. The changes in the lift coefficient and the nose-down .050 t__ pitching moment increase with -.05 0 1 2 3 4 5 6 7 decreasing sink rate. The changes Sink rate, Wsec _o367 because of ground effect decrease and approach zero as the sink rate increases.
These trends are not as clear for the drag coefficient.
F-15 flight data, gear down The approaches at 150 kts with the flaps • 170 kt, flaps up down s-how more significant ground • 150 kt, flaps down effects. This difference is most apparent ACM,GE = - 0.035/(1 + I_) -- _CM,GE = - 0.06/(1 + I_) for the pitching moment. This increase may be caused by a camber effect due to -.01 the flaps being down.
_,02 ACM'GE -.03 These figures show simple correlation curves that have been fit through the m.04 I gthround effects data. These curves give I I I I I I I I -.05 e change in lift, drag, and pitching 0 1 2 3 4 5 6 7 moment coefficients because of ground Sink rate, _s_ _0_8 effect as a function of sink rate.
All data for hJb = 0.3 except as noted: ,, Delta wings: AStatic, • Dynamic 50 _ Dynamic data for various aircraft: • \_, Wing sweep, A --- Wing: % AC. ,._ = (0.2/AR + 0.04) x 100 _ 7_;° _-,_.- _', "A- _ Alrcmft: % ACL,GE = (0.2/AR) x 100 40 _,, o O F-15 DATCOM static prediction V,, 70 Percent 30 \', A 65.6 ° (XB-70 wing) increase _, A in lift \ _, F-104A coefficient 20 _-"'.. 60 ° • _...&. Zl 27.3 ° (F-104A wing) FSD-1 with ogee wing'_ ...... F_()_" .... . F-15 (IVb = 0) 10 - )( B.7_X:29 "(-h_-__- -6)- 6 F5D-1 _F-15 (h/b : 03_ I I I I I I I I 0 .5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Aspect ratio Previous Ground Effect Data Comparison The F-15 ground effects lift data resulting from this investigation were compared to other wind tunnel and flight data for various wings and for complete aircraft.
Tl'Lis fit_ure shows the percent increase in the lift coefficient caused by ground effect as a function of the aspect ratio for the various wings and aircraft. The percent increase in the lift coefh_cient is defined as the O.ifference between the lift coefficients in and out of ground effect divided by the out of ground effect lift coefficient. Static and dynamic data are shown. These data are for a height above the ground divided by wing span, h/b, of 0.3. The F-15 data are for the 170 kts with the flaps up configuration.
Correlation curves for the wing and for the aircraft are shown. In general, the F-15 fh_ght data correlate well with the available aircraft dynamic ground effect data.
These data show a decrease inthe percent change in the lift coefficient as the aspect ratio increases. The changes in lift appear to approach nearly constant values for aspect ratios greater than about 3.
'i 40t I ........ New High Sink Rate M°del (150 and 170 kts) I "_ 30.-1P, I "'" New Low Sink Rate Model (150 and170 kts)l I\ I -- Old Simulator Model I _ i i ............................................. ' I 0.00 0.05 0.10 0.15 0.20 Change in Lift Coefficient Due to Ground Effect, ACL,GE A 40-.
¢= v ........ New High Sink Rate Model !
30- z_ .................................................... -- - New Low Sink Rata Model I ¢3 \ ---- Old Simulator Mo._.._/ J 20- i ...............................................................................................
> < 10- o) "t- O, " ' ' ' I " " " I ' ' ' ' i .... I .... I .... I 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Change in Drag Coefficient Due to Ground Effect, ACD.GE .--. 40 I ........ New 170 kte High Sink Rate Model I '1o r- I "'" New 170 kts Low Sink Rate Model I -_ 30 .................................. I .... New 150 kts Model I (9 I -- Old Simulator Model I ¢ 20 > _.t , < 10 ...... "_ _ .................................................................................................................
e-
o
I .... |,,P,, | .... | .... | .... | .... | , • • • !
0.00 -0.01 -0.02 -0.03 -0.04 -0.05 -0.06 -0.07 Change In Moment Coefficient Due to Ground Effect, _CM,GE Improvement in F-15 Ground Effects Flight Simulator Model This figure shows the improvements made to the NASA Dryden F-15 flight simulator modeling of ground effects based on the ground effects flight test data. The changes in the aerodynamic coefficients are shown as a function of height above the ground. The new ground effects model is a function of approach velocity and sink rate. The new model more closely duplicates actual flight data as seen in _e results presented in the flight test paper by Burcham and Maine ( Flight Test of a Propulsion Controlled Aircraft System on the NASA F-15 Airplane ).