Experimental investigation of active loads control for aircraft landing gear

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A C T I V E CONTROL ................................................................. 6 2

T R A N S I T I O N ..................................................................... 6 2

ROLL-OUT PHASE ................................................................. 6 2

iv SUMMARY A i r c r a f t dynamic loads and v i b r a t i o n s r e s u l t i n g from landing impact and from runway and taxiway unevenness are recognized a s s i g n i f i c a n t f a c t o r s i n causing f a t i g u e damage, dynamic stress on t h e airframe, crew and passenger discomfort, and reduction of t h e p i l o t ' s a b i l i t y t o c o n t r o l the a i r c r a f t during ground operations.

One p o t e n t i a l method f o r improving operational c h a r a c t e r i s t i c s of a i r c r a f t on t h e is t h e a p p l i c a t i o n of active-control technology t o t h e landing gears t o reduce ground ground loads applied t o t h e airframe.

An experimental i n v e s t i g a t i o n w a s conducted which simulated t h e landing dynamics of a l i g h t a i r p l a n e t o determine t h e f e a s i b i l i t y and p o t e n t i a l of a 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 main landing gear. The experiments involved a p a s s i v e gear and an a c t i v e - c o n t r o l gear. Results of t h i s i n v e s t i g a t i o n show t h a t a s e r i e s - h y d r a u l i c a l l y c o n t r o l l e d gear i s f e a s i b l e and t h a t such a gear i s very e f f e c t i v e i n reducing t h e loads t r a n s m i t t e d by t h e gear t o t h e airframe during ground operations.

INTRODUCTION A i r c r a f t dynamic loads and v i b r a t i o n s r e s u l t i n g from landing impact and from runway and taxiway unevenness are recognized a s s i g n i f i c a n t f a c t o r s i n causing f a t i g u e damage, dynamic stress on t h e airframe, c r e w and passenger discomfort, and reduction of t h e p i l o t ' s a b i l i t y t o c o n t r o l the a i r c r a f t during ground operations.

These ground-induced dynamic loads and v i b r a t i o n s have been encountered with some conventional subsonic t r a n s p o r t a i r c r a f t ( r e f s . 1 and 2 ) . They are magnified f o r supersonic-cruise a i r c r a f t because of t h e increased 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 t i n these slender-body, thin-wing designs. These o p e r a t i o n a l problems with supersonic-cruise a i r c r a f t have occurred a t high take-off and landing speeds on s o m e runways which provide only marginal performance f o r some conventional a i r c r a f t . A p o t e n t i a l method f o r improving o p e r a t i o n a l c h a r a c t e r i s t i c s of such aircraft on t h e ground i s t h e a p p l i c a t i o n of active-control technology t o t h e landing gears t o reduce t h e ground loads a p p l i e d t o t h e airframe.

I n r e f e r e n c e 3, a model of a series-hydraulic a c t i v e landing gear f o r control- l i n g t h e loads during impact and r o l l - o u t was developed and programmed f o r d i g i t a l - computer operation. The c o n t r o l o p e r a t e s t o l i m i t t h e a i r p l a n e mass-center f o r c e t o a minimum (command l i m i t f o r c e ) compatible with t h e a v a i l a b l e shock-strut s t r o k e and t h e a i r p l a n e k i n e t i c energy. AS long a s the mass-center f o r c e i s greater than t h e l i m i t force, the c o n t r o l system removes f l u i d from t h e strut a t a rate which v a r i e s w i t h t h e magnitude of t h e f o r c e difference. A s t h e mass-center f o r c e decreases toward t h e l i m i t f o r c e , the c o n t r o l system reduces t h e rate a t which f l u i d i s removed f r o m t h e s t r u t . When t h e mass-center f o r c e becomes less than t h e l i m i t force, t h e c o n t r o l system adds f l u i d t o t h e strut. I n reference 3, a n a l y t i c a l r e s u l t s , u s i n g t h e developed c o n t r o l l a w s , i n d i c a t e d t h a t the a c t i v e gear s u b s t a n t i a l l y reduced forces t r a n s m i t t e d t o t h e airframe. Consequently, an e l e c t r o n i c c o n t r o l w a s designed, f a b r i c a t e d , and t e s t e d and the results are presented i n r e f e r e n c e 4. The d r o p - t e s t ( z e r o ground speed) r e s u l t s of reference 4, which used a modified l a n d i n g gear from a g e n e r a l a v i a t i o n a i r p l a n e , indicated active-gear f o r c e reductions, rela- t i v e t o t h e forces obtained with the passive gear, from 9 t o 31 p e r c e n t depending on t h e a i r c r a f t sink rate and t h e gear charging pressure.

The purpose of t h i s paper i s t o p r e s e n t t h e r e s u l t s of an experimental i n v e s t i - gation of a series-hydraulic a c t i v e - c o n t r o l gear t o demonstrate t h e f e a s i b i l i t y and t h e p o t e n t i a l of t h i s type of a c t i v e - c o n t r o l concept. For t h i s i n v e s t i g a t i o n , a s t r u t from a single main gear of a 3000-kg-class (200-slug) a i r p l a n e w a s modified t o accommodate a series-hydraulic a c t i v e - c o n t r o l system. Landing-simulation tests w e r e conducted a t ground speeds t o 80 knots and v e r t i c a l - d r o p tests a t zero ground speed.

The forward-speed tests involved landing impacts and r o l l - o u t s over discrete bumps as w e l l as r o l l - o u t s over n a t u r a l surface unevenness.

APPARATUS LANDING-GEAR MODIFICATION The series-hydraulic c o n t r o l concept r e q u i r e s t h e hydraulic f l u i d i n t h e landing-gear p i s t o n t o be removed or added t o c o n t r o l t h e shock-strut hydraulic force. To accomplish t h i s f l u i d exchange, t h e gear w a s modified (see f i g . 1 ) t o Com- provide a conduit between t h e f l u i d i n t h e p i s t o n and a c o n t r o l servovalve.

ponents of the l i g h t - a i r c r a f t main landing gear used i n t h i s i n v e s t i g a t i o n are shown i n f i g u r e l ( a ) . The modification t o t h e gear c o n s i s t e d of adding a smaller diameter tube i n s i d e the e x i s t i n g single-wall orifice support tube t o provide an annular pas- sage through the c y l i n d e r i n t o t h e p i s t o n . The e x i s t i n g o r i f i c e plate, but not t h e o r i f i c e , was reduced i n diameter and i n s t a l l e d i n t h e smaller diameter tube as shown i n f i g u r e l ( b ) . To provide f o r f l o w between the annular passage and t h e servovalve, a c y l i n d e r head a d a p t e r w a s mounted on top of the c y l i n d e r , w a s mated with t h e annu- l a r passage i n t h e o r i f i c e support tube, and w a s connected t o the servovalve by a f l e x i b l e hose. D e t a i l s of the modified strut assembly are shown i n f i g u r e 1 ( c ) .

The p i s t o n w a s modified by mounting a p r e s s u r e transducer i n t h e base of t h e p i s t o n ( f i g . l ( a ) ) t o provide a s i g n a l t o the e l e c t r o n i c c o n t r o l l e r f o r b i a s i n g t h e servovalve power spool t o maintain t h e f u l l y extended charging p r e s s u r e i n t h e s t r u t prior to touchdown on the runway surface. A s l i d e w i r e mounted on t h e c y l i n d e r w i t h I t h e wiper s h a f t a t t a c h e d t o t h e half fork a t t h e base of t h e p i s t o n provided a strut s t r o k e s i g n a l f o r use by t h e e l e c t r o n i c c o n t r o l l e r i n applying the c o n t r o l l a w s .

I HYDRAULIC POWER UNIT

The hydraulic power u n i t used i n t h i s i n v e s t i g a t i o n is shown i n f i g u r e 2 a t t a c h e d t o the test c a r r i a g e a t the Langley Landing Loads Track. T h i s u n i t s u p p l i e s high-pressure (20.7 M P a (3000 p s i g ) ) f l u i d t o t h e c o n t r o l servovalve, p r o v i d e s a l o w - p r e s s u r e (101 kPa (14.7 p s i g ) ) r e s e r v o i r f o r s t o r i n g and r e c y c l i n g f l u i d removed f r o m the gear by the c o n t r o l servovalve, provides p r e s s u r e r e l i e f t o avoid damage t o t h e gear i n t h e event of a servovalve f a i l u r e i n the high-pressure mode of operation, and permits operation of the modified gear i n e i t h e r t h e passive or a c t i v e modes. To keep the u n i t compact f o r mounting t o t h e c a r r i a g e , several 90° elbows w e r e r e q u i r e d i n t h e p i p i n g between the servovalve and the gear; t h e r e f o r e , t h i s i n s t a l l a t i o n d i d n o t provide f o r minimum f l o w losses. The v a r i o u s components of the hydraulic power u n i t are shown i n f i g u r e 2 and component s p e c i f i c a t i o n s are l i s t e d i n table I. Some of t h e m o r e p e r t i n e n t components are d i s m s s e d i n t h e following sections.

The three-stage servovalve used i n t h i s i n v e s t i g a t i o n had a maximum fluid-f l o w I 3 c a p a b i l i t y of 0.757 m /min (200 g p m ) for a p r e s s u r e drop across the valve Of 6.9 M P a I (1000 psig). For low-pressure operation ( t h a t is, f l o w from t h e gear i n t o the l o w - p r e s s u r e r e s e r v o i r ) and a static gear p r e s s u r e i n t h e range of 3.4 MPa (500 p s i g ) , t h e maximum flow rate through t h e servovalve would be approximately 0.534 m3/min g p m ) . For high-pressure o p e r a t i o n s ( t h a t is, flow from t h e high-pressure (141 accumulator i n t o t h e gear) and a static gear p r e s s u r e of 3.4 M P a (500 p s i g ) , a maximum flow rate through t h e servovalve of approximately 1.20 m /min (316 g p m ) could The servovalve w a s mounted on the hydraulic power u n i t with t h e power- be expected.

spool a x i s o r i e n t e d normal t o t h e d i r e c t i o n of c a r r i a g e a c c e l e r a t i o n t o reduce i n e r t i a l e f f e c t s during c a r r i a g e launch.

Two p r e s s u r e - r e l i e f v a l v e s used t o p r o t e c t t h e system were set, based upon tests, t o operate f o r p r e s s u r e s g r e a t e r than 5.2 M P a r e s u l t s of vertical-drop The i n t a k e ports of (750 p s i g ) . t h e valves were i n s t a l l e d between t h e servovalve and t h e i s o l a t i o n valve, and t h e exhaust ports were connected t o t h e low-pressure r e s e r v o i r .

The i s o l a t i o n valve (a manually operated gate valve) w a s mounted between t h e c o n t r o l servovalve and a f l e x i b l e hose which w a s a t t a c h e d t o t h e landing-gear c y l i n d e r head adapter. The valve permitted tests of t h e modified gear i n t h e a c t i v e (valve open) o r p a s s i v e (valve closed) modes. When f u l l y open, t h e valve provided a flow area e q u i v a l e n t t o a 3.8-cnrdiameter (1.5-in. ) tube. Since t h e f l e x i b l e hose had a flow area equivalent t o a 3.18-cm (1.25-in.) diameter, t h e i s o l a t i o n valve accommodated a f l u i d flow rate g r e a t e r than t h a t of t h e f l e x i b l e hose. The 3.05-m (IO-ft) l e n g t h of f l e x i b l e hose provided a conduit f o r f l u i d flow between t h e hydraulic power u n i t and t h e landing gear.

The bias-pressure pickup supplied a feedback s i g n a l t o t h e e l e c t r o n i c c o n t r o l l e r f o r r e g u l a t i n g t h e charging p r e s s u r e of t h e f u l l y extended gear p r i o r t o touchdown during t h e active-gear tests. The surge suppressor helped t o a l l e v i a t e p r e s s u r e spikes a p p l i e d t o t h e bias-pressure gage during servovalve operation i n t h e high- p r e s s u r e mode. The c o n t r o l panel contained a switch and f u s e s for o p e r a t i o n of t h e electric motor.

ELECTRONIC EQUIPMENT The c o n t r o l , signal-conditioning, and diagnostic e l e c t r o n i c equipment are shown i n a bench s e t u p i n f i g u r e 3. The function of t h e c o n t r o l equipment, namely t h e e l e c t r o n i c c o n t r o l l e r and the servocontroller, is t o apply t h e c o n t r o l l a w s , operate t h e servovalve, and thus, c o n t r o l t h e gear f o r c e a p p l i e d t o t h e airframe. The signal-conditioning equipment c o n s i s t s of a regulated power supply, a c o n t r o l box, amplifiers, and analog f i l t e r s f o r preparing t h e s i g n a l s from t h e v a r i o u s d a t a trans- ducers f o r recording on frequency-modulated ( F M ) tape recorders. The d i a g n o s t i c equipment shown (oscilloscope, oscillograph, and t e l e t y p e w r i t e r ) w a s employed during t h e i n v e s t i g a t i o n t o isolate problems encountered with t h e e l e c t r o n i c c o n t r o l l e r and t o v a l i d a t e hardware and software modifications t o t h e c o n t r o l system.

I TEST FIXTURE The test f i x t u r e and equipment are shown i n f i g u r e 4 mounted on t h e test c a r r i a g e of t h e Langley Landing Loads Track. The test f i x t u r e c o n s i s t s b a s i c a l l y of a standoff s t r u c t u r e , a vertical-drop frame, and a p i t c h i n g beam. The standoff s t r u c t u r e (which is a t t a c h e d t o the framework of the c a r r i a g e ) supported a set of r a i l s t o restrict t h e drop frame t o v e r t i c a l motion. The drop frame, which i s a t t a c h e d t o t h e standoff s t r u c t u r e by an e l e c t r i c a l l y operated quick-release mecha- nism, moved on rollers along t h e s e rails. I n a test, t h e drop frame w a s r e l e a s e d , allowing t h e landing-gear t i r e t o c o n t a c t t h e runway surface. A t touchdown, t h e t i r e produced a p i t c h i n g moment which caused the p i t c h i n g beam t o rotate t o a h o r i z o n t a l p o s i t i o n . Also a t touchdown, t h e drop frame a c t i v a t e d a microswitch t o enable t h e c o n t r o l l e r .

L i f t , elevator, and nose gear f o r c e s i m u l a t o r s were incorporated i n t o t h e drop frame. The l i f t f o r c e simulator c o n s i s t e d of a double-acting a i r c y l i n d e r and p i s t o n mounted v e r t i c a l l y on t h e drop frame and charged t o a p r e s s u r e of 207 kea (30 p s i g ) .

A steel p l a t e was attached t o t h e end of the p i s t o n rod, which passed through a h o l e i n a f i x e d p l a t e a t t a c h e d t o t h e standoff s t r u c t u r e . The p i s t o n w a s p o s i t i o n e d so t h a t the p l a t e on the end of t h e p i s t o n rod w a s a t a d i s t a n c e above t h e f i x e d p l a t e equal t o t h e vertical-drop height. These two p l a t e s made c o n t a c t a t touchdown, thereby developing a f o r c e equal to b u t opposing the g r a v i t a t i o n a l f o r c e a p p l i e d t o t h e drop mass. The l i f t f o r c e w a s dumped s h o r t l y a f t e r touchdown by an e l e c t r o n i - c a l l y operated solenoid valve mounted i n the a i r c y l i n d e r l i n e . The e l e v a t o r f o r c e simulator, a constant-f orce, bending-wire, energy-dissipating mechanism mounted between the drop frame and t h e p i t c h i n g beam, opposed the p i t c h i n g moment developed by t h e landing gear, thereby simulating a nose-up e l e v a t o r force. The nose gear f o r c e simulator c o n s i s t e d of a shaped-aluminum honeycomb block mounted on a m e m b e r of t h e drop frame so t h a t c o n t a c t with the p i t c h i n g beam a t an angle of 2 O simulated nose gear touchdown. A s t h e beam continued t o rotate, t h e honeycomb w a s crushed p r o v i d i n g a constant f o r c e of 8.9 kN ( 2 0 0 0 l b f ) t o remove t h e r o t a t i o n a l energy dur- i n g simulated n o s e gear impact. The bending w i r e and honeycomb w e r e replaced a f t e r each test.

The p i t c h i n g beam was a welded, open, r e c t a n g u l a r s t r u c t u r e of s t e e l I-beams 457 cm (180 i n . ) long and 61 cm (24 i n . ) wide. A c r o s s beam a f t of t h e forward end of t h e beam served a s t h e crushing p l a t e €or t h e nose gear f o r c e simulator and attachment point f o r t h e e l e v a t o r f o r c e simulator. The p i t c h i n g beam w a s a t t a c h e d t o t h e drop frame with a 5.08-cwdiameter (2-in.) steel pin, t h u s e s t a b l i s h i n g t h e c e n t e r of r o t a t i o n of the beam. The modified main gear w a s a t t a c h e d t o t h e p i t c h i n g beam 68 c m (26.8 in. 1 a f t of t h e beam c e n t e r of r o t a t i o n , which is t h e same as t h e fore-and-aft distance between t h e gear attachment t o t h e wing and t h e c e n t e r of g r a v i t y of t h e l i g h t a i r p l a n e .

S i n c e only one landing gear w a s t e s t e d , t h e t o t a l mass of t h e drop f i x t u r e w a s l i m i t e d t o 1518 kg (104 s l u g s ) , which is approximately one-half t h e mass of t h e l i g h t a i r c r a f t . Within t h i s mass r e s t r i c t i o n , l e a d weights w e r e a t t a c h e d a t t h e rear of t h e p i t c h i n g beam t o balance the beam about t h e c e n t e r of r o t a t i o n . The p i t c h i n g m a s s moment of i n e r t i a of t h e balanced beam w a s determined t o be 997.9 kg-m (736 s l u g - f t 1 which i s only 40 p e r c e n t ,of t h a t r e q u i r e d t o simulate one-half t h e p i t c h i n g mass moment of i n e r t i a of t h e l i g h t a i r c r a f t .

TEST FACILITY The i n v e s t i g a t i o n was performed on t h e 29 500-kg (65 000-lbm) t e s t c a r r i a g e (shown i n f i g . 4 ) a t the Langley A i r c r a f t Landing Dynamics F a c i l i t y ' described i n r e f e r e n c e 5.

A l l t e s t s w e r e conducted on a dry runway surface.

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'Called the Langley a i r c r a f t landing l o a d s and t r a c t i o n f a c i l i t y i n r e f e r e n c e 5.

TEST BUMPS To determine t h e performance and evaluate t h e e f f e c t i v e n e s s of the a c t i v e - c o n t r o l gear during t r a v e r s e of abrupt elevation changes i n t h e runway surface, two step bumps w e r e i n s t a l l e d ( f i r s t bump a t s t a t i o n 4 0 0 ) on t h e test surface so t h a t t h e gear would encounter t h e bumps during t h e roll-out phase of t h e simulated landings.

Photographs and t h e geometry of the step bumps are shown i n f i g u r e 5. The dimensions of t h e bumps a r e given i n f i g u r e 5( a ) . The p r o f i l e of t h e bumps c o n s i s t e d of a 1.9-cm (0.75-in. 1 step a t t h e leading edge which increased t o 3.2 c m (1.25 i n . 1 a t 0.3 m (1.0 ft), remained c o n s t a n t a t t h a t height f o r 1.5 m (5.0 f t ) , and then decreased t o a 1.9-cm (0.75-in.) s t e p t o t h e runway surface i n 0.3 m (1.0 f t ) .

To determine t h e e f f e c t on t h e gear of t h e frequency of bump encounters, t h e bumps were spaced a t d i s t a n c e s determined by t h e forward speed of t h e c a r r i a g e t o produce frequencies of 2 and 4 Hz. The spacing d required t o produce t h e s e f r e - quencies a t v a r i o u s forward speeds i s shown i n t h e t a b l e i n f i g u r e 5 ( b ) .

I n a r e c e n t l y r e p a i r e d s e c t i o n of t h e runway surface, long wavelength changes i n surface e l e v a t i o n of approximately the same magnitude a s t h a t of t h e s t e p bumps w e r e measured. These changes, which a r e designated a s n a t u r a l bumps i n t h i s paper, are i l l u s t r a t e d and defined i n f i g u r e 6. The photograph of f i g u r e 6 w a s taken with t h e s u r f a c e flooded t o a depth of approximately 1.3 c m ( 0 . 5 in. ) t o i n d i c a t e t h e high p o i n t s on t h e surface. The s t a t i o n s i d e n t i f i e d i n t h e photograph c o r r e l a t e t h e peaks and troughs of t h e s u r f a c e with t h e measured s u r f a c e p r o f i l e shown a t t h e top of t h e f i g u r e . The most s i g n i f i c a n t d i f f e r e n c e between t h e s t e p bumps and t h e n a t u r a l bumps i s t h e wavelength, 2.1 m (7 f t ) f o r t h e step bumps and approximately 18.0 m (59 f t ) f o r the n a t u r a l bumps.

INSTRUMENTATION Twenty-seven v a r i a b l e s measured and recorded during t h i s i n v e s t i g a t i o n are l i s t e d and defined i n t a b l e 11. The types of measuring instruments used included: servo- type accelerometers t o determine drop-f r a m e forces; strain-gage- type trans- ducers f o r measuring pneumatic and hydraulic pressures; slide-wire potentiometers f o r measuring displacements of the drop-f rame and t h e landing-gear shock strut; a t i m e - code generator f o r synchronizing timing of events; and an e l e c t r o n i c t i m e r f o r o b t a i n i n g forward speed of the c a r r i a g e . A l l outputs from t h e instruments were transmi t t e d through signal-conditioning equipment, with t h e exception of those instruments supplying s i g n a l s t o t h e e l e c t r o n i c c o n t r o l l e r . (See t a b l e 11.) The o u t p u t s f r o m t h e signal-conditioning equipment and t h e e l e c t r o n i c c o n t r o l l e r w e r e recorded on frequency-modulated tape recorders.

TEST PROCEDURE The s t e p s i n t h e t e s t i n g technique f o r t h e a c t i v e gear were: ( 1 1 set t h e p i t c h - i n g beam t o t h e d e s i r e d p i t c h angle; ( 2 ) r a i s e t h e drop frame t o t h e a p p r o p r i a t e h e i g h t above t h e runway surface t o provide t h e d e s i r e d sink r a t e ; (3) charge t h e l i f t f o r c e simulator t o produce t h e required l i f t f o r c e ; ( 4 ) open t h e i s o l a t i o n valve and charge t h e f u l l y extended landing-gear strut t o t h e d e s i r e d hydraulic pressure; (5) propel1 t h e c a r r i a g e t o t h e d e s i r e d speed; ( 6 ) a c t u a t e t h e quick-release mechanism a t a p r e s e l e c t e d p o s i t i o n along the runway t o allow t h e drop frame and t h e landing gear t o f a l l t o t h e surface; ( 7 ) apply t h e l i f t f o r c e a t touchdown, and s h o r t l y t h e r e a f t e r , a c t i v a t e a microswitch to enable t h e c o n t r o l l e r and a l s o allow t h e lift f o r c e t o be removed i n approximately 1 sec. The only d i f f e r e n c e i n technique f o r the passive-gear tests w a s t o leave t h e i s o l a t i o n valve closed t o p l a c e t h e modified gear i n a normal configuration. During each test, t h e drop frame, p i t c h i n g beam, and landing gear w e r e subjected t o touchdown impact, rebound, p i t c h - over, and roll-out. A t touchdown, t h e p i t c h i n g beam r o t a t e d downward opposed by t h e e l e v a t o r f o r c e simulator, contacted and crushed t h e nose gear simulator, and remained i n a h o r i z o n t a l p o s i t i o n during t h e roll-out.

For active- and passive-gear modes, tests w e r e made with i n i t i a l pitching-beam a t t i t u d e s over a range of O o t o 13O and a range of sink rates from 0.9 t o 1.7 m/sec ( 3 t o 5.5 f w s e c ) . For t h e c a r r i a g e , nominal forward speeds ranged f r o m 8 t o 8 0 knots. The 8-knot tests w e r e made by towing t h e test c a r r i a g e with a ground to f orward-speed tests, s t a t i o n a r y v e r t i c a l - d r o p tests were vehicle. I n addition made over a similar range of sink r a t e s with t h e p i t c h i n g beam locked i n a h o r i z o n t a l p o s i t i o n .

DATA REDUCTION A l l d a t a were filtered t o 1000 H z and recorded on analog magnetic tape.

Following each test, the data channels w e r e f i l t e r e d to 100 Hz and reproduced on a n o s c i l l o g r a p h to permit e v a l u a t i o n of t h e q u a l i t y of t h e test. A f t e r completion of t h e test program, the analog-tape data w e r e processed through a low-pass f i l t e r ( c u t - o f f frequency of 400 Hz) , d i g i t i z e d a t 400 samples per second, and used t o generate time-history p l o t s f o r data a n a l y s i s . The d i g i t i z e d data from t h e c o n t r o l accelerom- eter, t h e wing/gear (mass c e n t e r ) accelerometer, and t h e l i f t - c y l i n d e r p r e s s u r e transducer were converted to f o r c e s ( s i n c e t h e mass of t h e drop f i x t u r e and t h e pis- ton a r e a of the l i f t c y l i n d e r w e r e constant) p r i o r t o generating t h e computer p l o t s .

ReSULTS AND DISCUSSION D a t a and t e s t conditions from the simulated landing t e s t s of t h e a c t i v e and p a s s i v e modified landing gear are presented i n t a b l e 111. The c o n t r o l nomenclature and philosophy employed i n t h i s i n v e s t i g a t i o n are presented i n t h e appendix. Repre- s e n t a t i v e d a t a from tests over t h e range of touchdown parameters i n v e s t i g a t e d are presented i n f i g u r e s 7 t o 13. The data i l l u s t r a t e t h e c o n t r o l operation and show t h e e f f e c t i v e n e s s of t h e a c t i v e gear i n reducing forces a p p l i e d t o t h e mass c e n t e r w i t h i n the a v a i l a b e shock-strut stroke.

ILLUSTRATION O F CONTROL OPERATION To i l l u s t r a t e t h e operation of t h e e l e c t r o n i c c o n t r o l l e r and t h e r e s u l t i n g effects on t h e p e r t i n e n t landing-gear v a r i a b l e s , o s c i l l o g r a p h traces of c o n t r o l l e r outputs and landing-gear v a r i a b l e s f o r a t y p i c a l landing test a r e shown i n f i g - 7 . Figure 7 ( a ) shows d a t a obtained during landing impact and f i g u r e 7 ( b ) shows u r e d a t a obtained during t r a v e r s e of t h e s t e p bumps. The d a t a t r a c e s i n f i g u r e 7 ( a ) o r i g i n a t e a t an a r b i t r a r i l y s e l e c t e d time ( 0 sec) before release of t h e drop f i x t u r e , continue through drop-f i x t u r e a c c e l e r a t i o n t o touchdown, touchdown impact, and r o l l - o u t t o s t a t i c conditions a t approximately 2 . 5 sec. The t r a c e s i n f i g u r e 7 ( b ) are f o r t r a v e r s e of the s t e p bumps f o r t h e t i m e period of 15.5 t o 17.5 sec. A d e t a i l e d explanation of t h e c o n t r o l l e r f u n c t i o n s and e f f e c t s are p r e s e n t e d i n t h e following discussion.

Free-Fall The data of f i g u r e 7 ( a ) show t h a t , a t approximately 0 . 2 sec, t h e drop f i x t u r e w a s r e l e a s e d and a c c e l e r a t e d under t h e influence of g r a v i t y , as i n d i c a t e d by t h e c o n t r o l and mass-center a c c e l e r a t i o n t r a c e s , t o touchdown a t approximately 0.4 sec.

The mass c e n t e r a l s o displaced toward the runway surface during t h i s t i m e p e r i o d .

w a s t h e The only o t h e r c o n t r o l l e r - o u t p u t q u a n t i t y t h a t v a r i e d during t h i s p e r i o d force error. T h i s q u a n t i t y is s i m p l y a scaled value of the c o n t r o l a c c e l e r a t i o n , s i n c e t h e computation of t h e force error i s not performed prior t o c o n t r o l l e r enable.

Touchdown The c o n t r o l l e r i s enabled a t touchdown (0.4 sec), and simultaneously, t h e l i f t f o r c e i s a p p l i e d as i n d i c a t e d by t h e increase i n t h e l i f t - c y l i n d e r pressure. The c o n t r o l and mass-center a c c e l e r a t i o n s make the t r a n s i t i o n f r o m a c c e l e r a t i o n through z e r o t o deceleration. The f o r c e e r r o r i n d i c a t e s t h e summation of t h e c o n t r o l f o r c e (scaled value of the c o n t r o l a c c e l e r a t i o n ) and t h e l i m i t f o r c e command, which i s z e r o a t t h e t i m e of c o n t r o l l e r enable. The hydraulic and pneumatic p r e s s u r e s i n t h e strut have n o t changed since t h e strut has n o t stroked (strut p o s i t i o n equal z e r o ) . The strut p o s i t i o n e r r o r , which i s t h e summation of t h e i n p u t value of t h e d e s i r e d s t a t i c stroke (12.7 c m (5.0 i n . ) f o r t h i s i n v e s t i g a t i o n ) and t h e instantaneous value of t h e strut p o s i t i o n , i s set equal t o t h e i n p u t value of t h e static s t r o k e , s i n c e t h e s t r u t p o s i t i o n i s zero. The c o n t r o l l e r i n i t i a t e s energy c a l c u l a t i o n s , and when t h e tire/ground i n t e r f a c e f o r c e exceeds t h e charging force of t h e strut, t h e strut begins t o stroke, then the s t r u t hydraulic pressure, pneumatic pressure, and d e c e l e r a t i o n force increase. The s t r u t p o s i t i o n e r r o r decreases as t h e strut p o s i t i o n approaches t h e i n p u t value of t h e static stroke. When the p o t e n t i a l energy of t h e s t r u t equaled or exceeded t h e k i n e t i c energy of the mass center a t approximately 0 . 4 3 sec, t h e c o n t r o l l e r s t o r e d t h e instantaneous scaled value of the c o n t r o l a c c e l e r a t i o n as t h e l i m i t f o r c e command and i n i t i a t e d c o n t r o l by enabling t h e servo loop.

Impact The gear i s now under a c t i v e control. The c o n t r o l d e c e l e r a t i o n i n c r e a s e s , t h e l i m i t force command i s constant, and hence, the f o r c e e r r o r increases. However, t h e r e i s a s l i g h t delay b e f o r e t h e servo spool d i s p l a c e s t o i n i t i a t e removal of f l u i d f r o m t h e strut, and hence, t h e strut hydraulic p r e s s u r e i n c r e a s e s . A t a t i m e of approximately 0.45 sec, t h e servo spool is a g a i n s t the mechanical stop and i s remov- i n g f l u i d f r o m t h e s t r u t , as shown i n f i g u r e 7 ( a ) by t h e decrease i n t h e hydraulic pressure, even while t h e shock strut i s compressing a t near t h e maximum rate. The h y d r a u l i c p r e s s u r e decreases t o a value below t h e charging p r e s s u r e of t h e f u l l y extended strut, and the c o n t r o l d e c e l e r a t i o n reaches a maximum value and starts t o decrease. As t h e c o n t r o l d e c e l e r a t i o n decreases and t h e l i m i t f o r c e command remains c o n s t a n t a t t h e impact value, t h e force e r r o r changes sign as it p a s s e s through zero, r e q u i r i n g t h a t f l u i d be added t o t h e s t r u t t o maintain t h e c o n t r o l d e c e l e r a t i o n f o r c e a t t h e value of the l i m i t f o r c e command u n t i l t h e wing/gear i n t e r f a c e v e l o c i t y (mass- c e n t e r v e l o c i t y f o r t h i s i n v e s t i g a t i o n ) has decreased t o t h e t r a n s i t i o n v e l o c i t y .

Note t h a t again t h e r e i s a s l i g h t delay between t h e f o r c e e r r o r s i g n a l , which changes s i g n a t approximately 0.57 sec, and t h e response of t h e servo-spool displacement, which starts t r a n s i t i o n from removal of f l u i d t o t h e a d d i t i o n of f l u i d a t approxi- mately 0.60 sec.

T r a n s i t i o n The mass-center v e l o c i t y (touchdown sink rate minus decrement i n wing/gear i n t e r f a c e v e l o c i t y ) decreases during t h e 0.57- t o 0.60-sec t i m e p e r i o d t o the value of t h e t r a n s i t i o n v e l o c i t y , and t h e c o n t r o l l e r starts t r a n s i t i o n of t h e impact l i m i t f o r c e command t o the r o l l - o u t l i m i t f o r c e command of zero. The c o n t r o l continues t o add f l u i d t o t h e strut i n a n attempt t o maintain t h e c o n t r o l d e c e l e r a t i o n f o r c e equal t o t h e instantaneous t r a n s i t i o n value of t h e l i m i t f o r c e command. The mass-center displacement, and hence t h e strut p o s i t i o n , reach a maximum value and start t o decrease as the mass rebounds. The c o n t r o l continues t o add f l u i d t o t h e strut and i n i t i a t e s t h e t r a n s i t i o n of t h e l i m i t f o r c e command. When t h e shock strut becomes f u l l y extended (strut p o s i t i o n equal to zero) a t approximately 0.88 sec, t h e c o n t r o l l e r terminates the t r a n s i t i o n phase, sets t h e l i m i t f o r c e command t o zero, and d e a c t i v a t e s control, as i n d i c a t e d by t h e f o r c e e r r o r r e t u r n i n g t o zero a t approximately 0 . 8 8 sec. Again, a delay i n t h e servo-spool response t o t h e force- error s i g n a l r e s u l t s i n f l u i d being added to t h e f u l l y extended strut with a dramatic i n c r e a s e i n s t r u t hydraulic and pneumatic pressures. H o w e v e r , s i n c e t h e t i r e h a s rebounded above the runway surface, as i n d i c a t e d by t h e strut p o s i t i o n , t h e i n c r e a s e d strut p r e s s u r e s do n o t a f f e c t t h e c o n t r o l o r mass-center f o r c e s . A f t e r t h e delay, t h e servo spool d i s p l a c e s through the bias p o s i t i o n and removes f l u i d from the strut, t h u s reducing the strut pressures.

Ro 1 1-Ou t S i n c e t h e t r a n s i t i o n phase of the c o n t r o l l o g i c w a s terminated by t h e c o n t r o l l e r during mass-center rebound, subsequent c o n t r o l of t h e gear i s accomplished u s i n g t h e l o g i c of t h e roll-out phase. The t i r e r e t u r n s t o t h e s u r f a c e and t h e strut starts t o s t r o k e a t approximately 1.06 sec. F l u i d i s being removed from the strut as i n d i c a t e d by t h e servo-spool displacement and t h e decreasing strut p r e s s u r e s . A s t h e strut continues to compress and t h e servo spool approaches t h e bias p o s i t i o n , t h e pressures s t a b i l i z e , and t h e c o n t r o l and mass-center a c c e l e r a t i o n s decrease. A t approximately 1.49 sec, t h e control d e c e l e r a t i o n f o r c e exceeds t h e 2.2-kN (500-lbf) deadband l i m i t .

The c o n t r o l l e r sets t h e l i m i t f o r c e command t o 2.2 k N ( 5 0 0 l b f ) , t h e f o r c e e r r o r commands t h e removal of f l u i d from t h e s t r u t , and a f t e r a s l i g h t delay, t h e servo- spool displacement i n d i c a t e s t h a t f l u i d i s b e i n g removed. The strut p r e s s u r e s , which have been increasing, decrease and c o n t r o l and mass-center d e c e l e r a t i o n s diminish.

A t approximately 1.59 sec, t h e c o n t r o l d e c e l e r a t i o n f o r c e d e c r e a s e s below t h e 2.2-kN (500-lbf) deadband l i m i t and t h e c o n t r o l l e r sets t h e l i m i t f o r c e command t o zero.

The strut p o s i t i o n and mass-center displacement reach t h e i r maximum value a t approxi- mately 1.7 sec, and s i n c e t h e strut p o s i t i o n i s g r e a t e r than t h e d e s i r e d s t a t i c s t r o k e , t h e c o n t r o l l e r adds f l u i d t o t h e strut a s i n d i c a t e d by t h e servo-spool d i s - placement trace. A t approximately 2.05 sec, t h e strut p o s i t i o n i s a t t h e d e s i r e d s t a t i c s t r o k e , a s i n d i c a t e d by t h e z e r o value of t h e strut p o s i t i o n error, and t h e servo spool r e t u r n s t o the b i a s p o s i t i o n .

S t e p Bumps The roll-out continues with no c o n t r o l r e q u i r e d u n t i l t h e f i r s t s t e p bump i s encountered a t approximately 15.54 sec ( f i g . 7 ( b ) ) . The gear compresses, t h e strut p r e s s u r e s increase, and t h e c o n t r o l d e c e l e r a t i o n f o r c e exceeds t h e 2.2-kN (500-lbf) deadband l i m i t a t approximately 15.59 set. The c o n t r o l l e r sets t h e l i m i t f o r c e com- mand t o 2.2 kN (500 l b f ) , and t h e f o r c e error commands t h e removal of f l u i d f r o m t h e strut. F l u i d is removed as i n d i c a t e d by the servo-spool displacement, and the strut p r e s s u r e s and t h e c o n t r o l d e c e l e r a t i o n decrease. A t approximately 15.64 sec, t h e c o n t r o l d e c e l e r a t i o n f o r c e becomes less than t h e 2.2-kN (500-lbf) deadband l i m i t and t h e c o n t r o l l e r sets the l i m i t f o r c e command t o zero and t h e servo-spool displacement r e t u r n s t o t h e r o l l - o u t b i a s p o s i t i o n . A t approximately 15.68 sec: t h e c o n t r o l a c c e l e r a t i o n f o r c e exceeds t h e 2.2-kN (500-lbf) deadband l i m i t ; t h e c o n t r o l l e r sets t h e l i m i t f o r c e command t o 2.2 kN (500 l b f ) , r e v e r s e s t h e sign of t h e analog l i m i t f o r c e command s i g n a l , and solves for the force e r r o r s i g n a l which i n d i c a t e s t h a t f l u i d should be added t o t h e strut. F l u i d i s added as i n d i c a t e d by the servo-spool displacement, and the s t r u t p r e s s u r e s increase and the c o n t r o l a c c e l e r a t i o n decreases. The c o n t r o l l e r continues t o function i n t h e manner described through t h e second bump encounter, which occurs a t approximately 16.16 sec.

The preceding discussion i l l u s t r a t e s the response of the landing gear t o t h e a c t i v e - c o n t r o l system which is d i r e c t e d by the e l e c t r o n i c c o n t r o l l e r through the a p p l i c a t i o n of the c o n t r o l l o g i c and s i g n a l s from t h e feedback transducers.

EFFECTIVENESS OF ACTIVE GEAR The e f f e c t i v e n e s s of the active-control gear r e l a t i v e t o t h e passive gear i n reducing ground loads applied t o t h e simulated a i r p l a n e during touchdown impact and landing r o l l - o u t over an uneven surface i s i l l u s t r a t e d by comparisons of mass-center f o r c e s and shock-strut s t r o k e s f o r s i m i l a r touchdown parameters. The v a r i a b l e s of mass-center f o r c e and shock-strut stroke w e r e selected, s i n c e the purpose of t h e a c t i v e gear i s t o reduce t h e f o r c e s applied t o the a i r c r a f t during ground o p e r a t i o n s w i t h i n t h e s t r o k i n g c a p a b i l i t y of the landing-gear shock s t r u t . Data a r e p r e s e n t e d and discussed for two c a t e g o r i e s of tests, vertical-drop tests and landing-simulation tests. R e s u l t s of t h e vertical-drop t e s t s ( z e r o ground speed) i l l u s t r a t e t h e e f f e c - t i v e n e s s of t h e a c t i v e gear as a f u n c t i o n of touchdown sink rate when the effects of strut binding f r i c t i o n due t o p i t c h a t t i t u d e and h o r i z o n t a l v e l o c i t y (ground speed), are minimal. The data obtained from t h e landing-simulation tests, hmever, are more r e p r e s e n t a t i v e of t h e e f f e c t i v e n e s s t h a t may be obtained during a i r c r a f t landings w i t h t h e a c t i v e gears.

The f o r c e plots, used f o r analyzing the e f f e c t i v e n e s s of t h e a c t i v e - c o n t r o l gear, w e r e f a i r e d through the mean values of t h e high-frequency o s c i l l a t i o n s t o o b t a i n t h e b a s i c f o r c i n g function. Typical t i m e h i s t o r i e s of t h e mass-center f o r c e d a t a and t h e f a i r i n g technique applied t o these data are presented i n f i g u r e 8 f o r a n active-gear test. The f o r c e data i n f i g u r e 8 ( a ) w e r e obtained during t h e impact phase of an active-gear test. The accelerometer w a s mounted a t t h e mass c e n t e r beside t h e c o n t r o l accelerometer which supplied t h e s i g n a l used by t h e e l e c t r o n i c c o n t r o l l e r t o apply t h e c o n t r o l l a w s . The f o r c e data shown i n f i g u r e 8 ( b ) w e r e o b t a i n e d f r o m t h e mass-center c o n t r o l accelerometer during t r a v e r s e of t h e step bumps.

Vertical-Drop T e s t s The mass-center f o r c e and strut stroke d a t a from v e r t i c a l - d r o p tests of a c t i v e and p a s s i v e g e a r s are presented i n figure 9 f o r a p i t c h a t t i t u d e of O o a t zero ground speed. The strut charging p r e s s u r e v a r i e d from 1248 t o 1834 kPa (181 t o 266 p s i g ) f o r t h e s e tests. The data are p l o t t e d from t h e t i m e of release of t h e drop f i x t u r e (0 sec), through impact, rebound of t h e f i x t u r e , and touchdown for secondary impact.

I n f i g u r e 9, p o s i t i v e mass-center f o r c e s represent g r a v i t a t i o n a l a c c e l e r a t i o n during f r e e - f a l l t o o b t a i n t h e d e s i r e d touchdown sink rate; whereas, t h e negative mass- c e n t e r forces represent t h e d e c e l e r a t i o n of t h e f i x t u r e from ground f o r c e s a p p l i e d I through t h e landing-gear shock strut.

Touchdown Impact For t h e data i n f i g u r e 9 ( a ) , t h e touchdown sink rate w a s 0.9 m/sec (3.0 ft/sec) and t h e charging p r e s s u r e for t h e f u l l y extended strut was 1248 kPa (181 p s i g ) .

During i n i t i a l impact, t h e a c t i v e gear reduced t h e d e c e l e r a t i o n f o r c e by 8 p e r c e n t r e l a t i v e t o the passive gear. However, t h e shock-strut stroke required by t h e a c t i v e gear t o achieve t h i s f o r c e reduction w a s 18 p e r c e n t g r e a t e r during t h e i n i t i a l impact The strokes required by t h e a c t i v e and p a s s i v e than t h e stroke of t h e p a s s i v e gear.

s m a l l percentage of t h e a v a i l a b l e g e a r s during i n i t i a l impact r e p r e s e n t only a s t r o k e , as shown i n f i g u r e 9. The major s t r o k i n g of t h e gear occurs during secondary impact, as i l l u s t r a t e d i n f i g u r e 9 ( d ) .

The data i n f i g u r e 9 ( b ) a r e for a touchdown sink r a t e of 1.2 m / s e c (4.0 f t / s e c ) and a charging p r e s s u r e of 1351 kPa (196 p s i g ) f o r t h e f u l l y extended strut. For t h e s e conditions, t h e a c t i v e gear reduced t h e d e c e l e r a t i n g f o r c e experienced by t h e I p a s s i v e gear by 19 percent and required a s t r o k e 38 p e r c e n t g r e a t e r than t h a t of t h e p a s s i v e gear.

The d a t a €or a touchdown sink rate of 1.5 m/sec (5.0 ft/sec) and a charging p r e s s u r e of 1351 kPa (196 p s i g ) are presented i n f i g u r e 9 ( c ) . The a c t i v e gear reduced t h e d e c e l e r a t i n g f o r c e r e l a t i v e t o t h a t of t h e p a s s i v e gear by 20 percent and required a 34-percent g r e a t e r stroke.

D a t a a r e presented i n f i g u r e 9 l d ) for a touchdown sink r a t e of 1.7 m / s e c (5.5 f t / s e c ) and a s t r u t charging p r e s s u r e of 1834 kPa (266 p s i g ) . The data a r e t i m e of touchdown ( 0 sec) through i n i t i a l impact, drop-fixture p l o t t e d from the rebound, and secondary impact t o an e s s e n t i a l l y s t a t i c p o s i t i o n ( g e a r supporting drop-fixture mass). For these touchdown parameters, t h e a c t i v e gear reduced t h e i n i t i a l impact d e c e l e r a t i n g f o r c e r e l a t i v e t o t h a t of t h e p a s s i v e gear by 32 p e r c e n t w i t h a corresponding 43-percent i n c r e a s e i n shock-strut stroke.

Secondary Impact During secondary impact, t h e a c t i v e gear reduced t h e d e c e l e r a t i n g f o r c e by I 57 percent, but a t the expense of an apparently l a r g e i n c r e a s e i n strut s t r o k e when compared with t h e recorded passive-gear stroke. (See f i g . 9 ( d ) . ) However, upon observing t h e small s t r o k e of the gear a f t e r t h e passive-gear drop test, t h e drop f i x t u r e was manually v i b r a t e d and t h e f i x t u r e s e t t l e d onto t h e gear u n t i l t h e gear stroked t o approximately 12.1 cm (4.75 i n . ) . Therefore, t h e s m a l l value of t h e recorded s t r o k e f o r t h e p a s s i v e gear w a s a t t r i b u t e d t o f r i c t i o n a l e f f e c t s between t h e drop f i x t u r e and t h e standoff s t r u c t u r e . consequently, t h e active-gear s t r o k e r e q u i r e d w a s only 52 percent g r e a t e r than t h a t of t h e p a s s i v e gear when t h e f r i c t i o n force was relieved.

Such e f f e c t s would be a l l e v i a t e d during t h e active-gear t e s t because t h e c o n t r o l would be adding or removing f l u i d from t h e gear, t h u s applying o s c i l l a t i n g forces to t h e test f i x t u r e .

Response t o Control The l i m i t f o r c e command (an output s i g n a l generated by t h e e l e c t r o n i c c o n t r o l l e r ) i s superposed on t h e mass-center f o r c e p l o t i n f i g u r e 9 ( d ) t o i l l u s t r a t e t h e v a r i a t i o n of mass-center f o r c e i n response t o t h e c o n t r o l system. During i n i t i a l impact, t h e c o n t r o l l e r set t h e impact-limit f o r c e command t o 9.2 kN (2080 l b f ) and t h e mass-center force f o r t h e a c t i v e gear peaked a t 20.5 kN (4600 l b f ) , which i s considerably lower than the 30 kN (6740 l b f ) obtained during t h e passive-gear test.

A s long as t h e mass-center f o r c e i s g r e a t e r than t h e l i m i t f o r c e command, t h e c o n t r o l system removes f l u i d from t h e strut a t a r a t e which v a r i e s as t h e magnitude of t h e f o r c e d i f f e r e n c e . As the mass-center force decreases toward t h e l i m i t f o r c e command, t h e c o n t r o l system reduces t h e r a t e a t which f l u i d i s removed from t h e strut, and when t h e mass-center f o r c e becomes less than t h e l i m i t f o r c e command, adds f l u i d t o t h e s t r u t . T h i s operation i s i l l u s t r a t e d between 0.2 and 0.4 sec, by t h e e s s e n t i a l l y c o n s t a n t mass-center force.

When t h e s t r u t becomes f u l l y extended a t about 0.41 sec, t h e c o n t r o l becomes i n a c t i v e , as i n d i c a t e d by t h e zero output of t h e l i m i t f o r c e command. Control i s i n i t i a t e d a g a i n when t h e gear starts t o stroke during t h e secondary impact a t approximately 0.61 sec. A t t h i s t i m e t h e mass-center f o r c e i n d i c a t e s t h a t t h e drop f i x t u r e i s a c c e l e r a t i n g toward t h e s u r f a c e with a f o r c e a p p l i e d t o t h e drop f i x t u r e of approximately 4.9 kN (1100 l b f ) . The c o n t r o l l e r sets t h e l i m i t f o r c e command t o 2.7 kN (600 l b f ) and pumps f l u i d i n t o t h e gear t o decrease t h e a c c e l e r a t i n g f o r c e t o a value w i t h i n t h e f2.7-kN (f600-lbf) deadband; and, when t h i s is accomplished, sets t h e l i m i t f o r c e command t o zero. When t h e mass-center d e c e l e r a t i n g f o r c e exceeds 2.7 kN (600 l b f ) , t h e c o n t r o l l e r sets the l i m i t force command t o 2.7 kN (600 l b f ) and removes f l u i d f r o m t h e strut t o l i m i t t h e d e c e l e r a t i n g force. I n t h e c o n t r o l philos- as f 2 . 2 kN (f500 l b f ) , ophy s e c t i o n ( t h e appendix), t h e f o r c e deadband was defined based on r e s u l t s from a n a l y t i c a l simulations. However, the f o r c e deadband during c o n t r o l operation w a s output as f2.7 kN (f600 l b f ) , which i s a t t r i b u t e d t o a l t e r a t i o n of t h e s i g n a l i n t h e e l e c t r o n i c c i r c u i t r y . T h i s phenomenon occurred c o n s i s t e n t l y throughout t h e test program. The c o n t r o l system was more e f f e c t i v e during t h e secon- dary impact than during t h e i n i t i a l impact, s i n c e t h e mass-center f o r c e exceeded the l i m i t force command by only 23 p e r c e n t during secondary impact; whereas, the mass- c e n t e r f o r c e during i n i t i a l impact peaked a t a value 120 p e r c e n t g r e a t e r than t h e l i m i t f o r c e . This increased e f f e c t i v e n e s s is a t t r i b u t e d t o t h e lower strut compres- s i o n v e l o c i t y during secondary impact which permits t h e c o n t r o l system, even w i t h t h e delayed response previously noted, t o be more e f f e c t i v e .

I n summary, t h e v e r t i c a l - d r o p test r e s u l t s i n d i c a t e d t h a t t h e e f f e c t i v e n e s s of t h e a c t i v e gear increased with touchdown sink rate. During i n i t i a l impact, decel- e r a t i n g f o r c e reductions of 8 p e r c e n t a t 0.9 m / s e c ( 3 f v s e c ) t o 32 p e r c e n t a t 1.7 m / s e c (5.5 f t / s e c ) w e r e obtained. A s shown i n f i g u r e 9 ( d ) , d e c e l e r a t i n g force r e d u c t i o n s as g r e a t as 57 percent may be obtained during secondary impact. Although it is n o t shown on a l l of t h e time-history plots, t h e maximum s t r o k e occurs during secondary impact and t h e maximum s t r o k e required f o r any of t h e s e tests, when t h e c o n t r o l system w a s o p e r a t i n g properly, was less than 85 p e r c e n t of t h e a v a i l a b l e stroke.

Landing-Simulation T e s t s Landing-simulation tests provide a more r e a l i s t i c r e p r e s e n t a t i o n of the loads and motions imposed on t h e landing gear during an a c t u a l a i r c r a f t landing than those obtained during v e r t i c a l - d r o p tests, which a r e normally employed t o v e r i f y landing- gear designs. The p r i n c i p a l d i f f e r e n c e between t h e s e types of tests i s t h e s t r u t binding-friction f o r c e . S t r u t binding f r i c t i o n r e s u l t s f r o m moments developed on t h e gear by t h e fore-and-aft f o r c e s a p p l i e d a t t h e a x l e during wheel spin-up a t impact or by encounters w i t h e l e v a t i o n unevenness of t h e landing surface during r o l l - o u t . The nominal touchdown ground speed f o r t h e a i r c r a f t simulated i n t h i s i n v e s t i g a t i o n w a s 8 0 knots a t a p i t c h a t t i t u d e of approximately 13O. However, tests were made a t l o w e r ground speeds t o obtain data during t r a v e r s e of t h e step and n a t u r a l bumps a t varying r o l l - o u t speeds. Consequently, touchdown impact data were obtained a t a l l ground speeds i n v e s t i g a t e d and are employed i n t h e d i s c u s s i o n of t h e e f f e c t i v e n e s s of t h e a c t i v e gear.

D a t a f r o m t h e landing-simulation tests of t h e a c t i v e and p a s s i v e g e a r s are pre- sented i n f i g u r e s 10 t o 13 f o r t h e following touchdown and r o l l - o u t parameters: h o r i z o n t a l v e l o c i t i e s (ground speeds) f r o m 8 t o 80 knots; p i t c h a t t i t u d e s from 2 O t o 13O; sink r a t e s from 0.9 t o 1.7 m/sec (3.0 t o 5.5 f t / s e c ) ; gear charging p r e s s u r e s a t touchdown from 662 t o 2317 kPa (96 t o 336 p s i g ) , and step bump encounters a t fre- quencies of 2 and 4 Hz. These data r e p r e s e n t t h e t h r e e s i g n i f i c a n t phases of t h e simulated landing t e s t s ; touchdown impact, t r a v e r s e of t h e s t e p bumps, and t r a v e r s e of t h e n a t u r a l bumps.

Touchdown Impact Phase Data f o r the mass-center f o r c e , l i m i t force command, and shock-strut s t r o k e obtained during t h e touchdown impact phase of t h e landing-simulation tests a r e pre- sented i n f i g u r e s 1 0 ( a ) t o 1 0 ( g ) f o r t h e various touchdown parameters. The data are p l o t t e d from touchdawn ( z e r o t i m e ) through i n i t i a l impact, mass (drop f i x t u r e ) rebound, and secondary impact t o an e s s e n t i a l l y s t a t i c condition.

S t r u t charging pressure.- P r i o r t o each test, t h e i n i t i a l s t r u t charging pres- sure w a s set t o t h e same value €or both t h e a c t i v e and p a s s i v e gears. However, t h e s t r u t p r e s s u r e s of the a c t i v e gear a t touchdown f o r 40 and 8 0 knots a r e lower than those of t h e passive gear, a s noted i n f i g u r e 10. A n examination of t h e test d a t a d u r i n g c a t a p u l t and p r i o r t o touchdown showed t h a t , f o r t h e a c t i v e gear, t h e i n i t i a l charging pressure decreased. This decrease is a t t r i b u t e d t o a launch a c c e l e r a t i o n force of approximately 39 f o r t h e test c a r r i a g e , which apparently a f f e c t s t h e operation of the servovalve t h a t c o n t r o l s the strut pressure. As previously noted, t h e servovalve w a s mounted on t h e hydraulic power u n i t with t h e a x i s of motion of t h e power spool oriented 90° t o t h e d i r e c t i o n of c a r r i a g e launch a c c e l e r a t i o n i n a n attempt t o minimize i n e r t i a l motion of t h e power spool during launch. T h i s launch impulse i s unique t o the test f a c i l i t y and, hence, i s n o t encountered i n a c t u a l a i r p l a n e landings.

F u r t h e r i n d i c a t i o n t h a t launch a c c e l e r a t i o n a f f e c t s t h e strut charging p r e s s u r e of the a c t i v e gear i s shown by t h e v e r t i c a l - d r o p t e s t s ( f i g . 9) and t h e 8-knot towed test ( f i g . l O ( a ) ) . With no launch a c c e l e r a t i o n , t h e v a l u e s of s t r u t charging pres- s u r e s for both the active- and passive-gear tests w e r e approximately t h e same. An attempt was a l s o made t o o b t a i n t h e same strut charging p r e s s u r e f o r t h e a c t i v e and p a s s i v e gears a t impact by charging t h e a c t i v e gear t o a higher p r e s s u r e than t h e p a s s i v e gear p r i o r t o launch. T h i s technique w a s p a r t i a l l y s u c c e s s f u l as shown by comparing t h e active- and passive-gear p r e s s u r e s i n f i g u r e s 1 0 ( b ) and l O ( c ) . How- ever, t h e technique was n o t a s e f f e c t i v e f o r t h e higher gear p r e s s u r e s ( f i g s . 1 0 ( f ) and 1O(g) 1, possibly because of t h e higher servovalve f l o w rates which r e s u l t when t h e p r e s s u r e drop a c r o s s t h e servovalve i s g r e a t e r .

E f f e c t of ground speed.- The e f f e c t i v e n e s s of t h e a c t i v e gear f o r reducing t h e f o r c e s a p p l i e d t o t h e airframe during the touchdown impact phase a r e i l l u s t r a t e d by comparisons of active- and passive-gear forces during i n i t i a l impact, rebound from i n i t i a l impact, secondary impact, and i n i t i a t i o n of r o l l - o u t . The e f f e c t of ground speed on t h e performance of t h e a c t i v e gear i s shown i n f i g u r e s 1 0 ( a ) t o lO(c) f o r a nominal sink rate of 1.7 m/sec (5.5 f t/sec) and ground speeds of 8 , 40, and 8 0 k n o t s , r e s p e c t i v e l y . Data f o r a ground speed of 8 knots, f i g u r e 1 0 ( a ) , show t h a t during i n i t i a l impact t h e a c t i v e gear reduced the d e c e l e r a t i n g f o r c e r e l a t i v e t o t h a t of t h e p a s s i v e gear by 31 p e r c e n t with a 57-percent i n c r e a s e i n strut stroke. During rebound of t h e m a s s , t h e f o r c e w a s reduced by 55 percent. This reduction i s a r e s i d - u a l e f f e c t of c o n t r o l during t h e i n i t i a l impact, s i n c e t h e c o n t r o l i s n o t a c t i v e during t h e rebound period once the shock strut becomes f u l l y extended. Deactivation of t h e c o n t r o l is i n d i c a t e d by t h e z e r o value of t h e l i m i t f o r c e command. During secondary i m p a c t , t h e l i m i t f o r c e command shows t h a t t h e c o n t r o l was r e a c t i v a t e d when t h e d e c e l e r a t i n g f o r c e exceeded t h e -2.7-kN (-600-lbf) deadband l i m i t force. The d e c e l e r a t i n g f o r c e w a s reduced by 30 percent with an 87-percent i n c r e a s e i n strut stroke.

D a t a f o r a ground speed of 40 knots ( f i g . 1 0 ( b ) ) show t h a t , during i n i t i a l impact, t h e a c t i v e gear reduced t h e d e c e l e r a t i n g f o r c e by 9 p e r c e n t with a 40-percent i n c r e a s e i n strut stroke. During rebound, the active-gear a c c e l e r a t i n g f o r c e w a s 37 p e r c e n t g r e a t e r than t h a t which occurred with t h e p a s s i v e gear. The mass-center d e c e l e r a t i n g f o r c e during secondary impact exceeded t h e -2.7-kN (-600-lbf) deadband ( r o l l - o u t ) l i m i t f o r c e a t a t i m e of approximately 1 sec b u t w a s l i m i t e d by t h e c o n t r o l t o t h i s value. The o s c i l l a t o r y motion of t h e l i m i t f o r c e command, occurring a t about 1 sec, r e s u l t s from t h e c o n t r o l l e r responding t o the mass-center f o r c e o s c i l l a t i o n s above and below -2.7 kN (-600 l b f ) as shown a t a t i m e of approximately 1.2 sec i n f i g u r e 8 ( a ) . The strut stroke required by the a c t i v e gear during secondary impact was 41 p e r c e n t g r e a t e r than t h a t required by t h e p a s s i v e gear.

The d a t a f o r a ground speed of 80 knots ( f i g . l O ( c ) ) show t h a t , during i n i t i a l impact, t h e a c t i v e gear reduced t h e d e c e l e r a t i n g f o r c e by 11 p e r c e n t with an i n c r e a s e i n strut s t r o k e of 168 percent. During rebound, t h e active gear was e f f e c t i v e i n reducing t h e a c c e l e r a t i n g f o r c e by 36 percent. The active-gear d e c e l e r a t i n g f o r c e during secondary impact w a s l i m i t e d t o the d e c e l e r a t i n g deadband l i m i t f o r c e of -2.7 kN (-600 l b f ) as shown a t a t i m e of 1.1 sec and required an increase i n strut s t r o k e of 258 p e r c e n t r e l a t i v e t o t h a t occurring with the p a s s i v e gear.

Binding-f r i c t i o n e f f e c t s . - As previously s t a t e d , t h e series-hydraulic c o n t r o l concept reduces t h e ground loads applied t o t h e airframe by c o q t r o l l i n g t h e shock- strut h y d r a u l i c force. Consequently, the e f f e c t i v e n e s s of t h e c o n t r o l i s g r e a t l y reduced when s i g n i f i c a n t f r i c t i o n f o r c e i s p r e s e n t i n t h e shock strut. The presence of s i g n i f i c a n t b i n d i n g - f r i c t i o n f o r c e s i n the shock strut during t h e touchdown impact phase for t h e tests conducted a t 40- and 80-knot ground speeds is discussed i n t h e f ollowing section.

Under dynamic conditions t h e presence of l a r g e binding-f r i c t i o n forces i n a landing-gear shock strut cause a " s t i c k - s l i p " phenomenon t o occur a s t h e strut s t r o k e s . (See r e f . 6. ) Vertical-drop tests of landing gears have m i n i m a l b i n d i n g - f r i c t i o n e f f e c t s where t h e shock-strut l o n g i t u d i n a l a x i s i s o r i e n t e d normal t o t h e s u r f a c e and t h e ground speed i s zero. For example, snooth s t r o k i n g t o t h e maximum touchdown impact value occurred f o r t h e a c t i v e gear during t h e touchdown impact phase a t ground speeds of 0 and 8 knots ( f i g s . 9 ( d ) and l O ( a ) , r e s p e c t i v e l y ) .

I n c o n t r a s t , t h e active-gear s t r u t - s t r o k e data presented i n f i g u r e s 10 ( b ) and 10 ( c ) f o r t h e 40- and 80-knot ground speeds show t h a t t h e strut stops s t r o k i n g during t h e p e r i o d of maximum s t r o k i n g v e l o c i t y and resumes s t r o k i n g toward t h e maximum touchdown impact s t r o k e , a f t e r which t h e " s t i c k - s l i p " phenomenon i s a l s o observed as the strut s t r o k e decreases. Binding f r i c t i o n would a l s o be m o r e l i k e l y t o occur during t h e touchdown impact phase, s i n c e t h e spacing between t h e shock-strut b e a r i n g s u r f a c e s i s a minimum when the strut i s f u l l y extended. These f a c t o r s support t h e o b s e r v a t i o n t h a t considerable binding f r i c t i o n w a s p r e s e n t during t h e touchdown impact phase of t h e tests conducted a t 40- and 80-knot ground speeds and i s a t t r i b u t e d t o t h e g r e a t l y increased "spin-up" drag l o a d s developed a t t h e s e higher speeds.

basis of t h e d a t a presented i n f i g u r e s 1 0 ( a ) t o 1 O ( c ) , t h e a c t i v e g e a r On t h e w a s e f f e c t i v e i n reducing the ground l o a d s a p p l i e d t o t h e mass c e n t e r during touch- down impact a t the ground speeds i n v e s t i g a t e d . However, t h e presence of binding f r i c t i o n i n t h e shock strut w a s m o s t pronounced a t t h e higher ground speeds and dras- t i c a l l y reduced t h e e f f e c t i v e n e s s of t h e active gear. The e f f e c t i v e n e s s of t h e a c t i v e gear during rebound i s a f u n c t i o n of t h e o p e r a t i o n of t h e c o n t r o l during i n i - t i a l impact, and i n two of t h e t h r e e tests presented, w a s very e f f e c t i v e . During secondary impact, t h e mass-center d e c e l e r a t i n g f o r c e w a s l i m i t e d by t h e c o n t r o l t o t h e designed r o l l - o u t l i m i t - f o r c e deadband of f2.7 kN (f600 lbf 1 .

E f f e c t of st'rut charging pressure.- The active-gear d a t a i n f i g u r e s lO(d) and 1 0 ( e ) are f o r landing-simulation tests i n which t h e strut p r e s s u r e a t touchdown w a s t h e only d i f f e r e n c e i n t h e touchdown parameters. The p r e s s u r e i n t h e a c t i v e gear w a s 662 kPa (96 p s i g ) f o r t h e d a t a shown i n f i g u r e lO(d) and 993 kPa (144 p s i g ) f o r t h e d a t a shown i n f i g u r e 1 0 ( e ) . During touchdown impact, t h e e f f e c t i v e n e s s of t h e a c t i v e g e a r i s a function of t h e responsiveness of t h e c o n t r o l l e r and t h e rate of flow of t h e h y d r a u l i c f l u i d through t h e servovalve. For t h e s e tests, t h e response of t h e c o n t r o l l e r is the same; hence, t h e e f f e c t i v e n e s s of t h e a c t i v e gear would be a fun- t i o n of t h e servovalve flaw which i s dependent upon t h e p r e s s u r e drop across t h e servovalve. The d a t a i n f i g u r e lO(d) f o r t h e lower strut p r e s s u r e show a decelerating-force reduction of 13 p e r c e n t during i n i t i a l impact. The decelerating- f o r c e r e d u c t i o n during impact f o r t h e higher strut p r e s s u r e ( f i g . 1 0 ( e ) ) w a s 20 per- cent. The s t r o k i n g c h a r a c t e r i s t i c s of t h e strut during i n i t i a l impact are s i m i l a r f o r b o t h tests, i n d i c a t i n g t h a t t h e b i n d i n g - f r i c t i o n f o r c e s are s i m i l a r . Therefore, s i n c e t h e p r i n c i p a l d i f f e r e n c e between t h e s e t w o active-gear tests w a s t h e strut p r e s s u r e , t h e i n c r e a s e i n t h e e f f e c t i v e n e s s of t h e a c t i v e gear may be a t t r i b u t e d t o t h e i n c r e a s e d flaw c a p a b i l i t y of t h e servovalve t o remove P l u i d from t h e strut because of t h e higher p r e s s u r e drop across t h e valve.

R e p e a t a b i l i t y of active-gear data.- The d a t a shown i n f i g u r e s 1O(f) and 1O(g) i l l u s t r a t e t h e degree of r e p e a t a b i l i t y of active-gear d a t a during i n i t i a l impact f o r tests conducted a t approximately t h e same touchdown parameters. The d e c e l e r a t i n g forces f o r t h e a c t i v e g e a r s w e r e about t h e same d u r i n g i n i t i a l impact and r e s u l t e d i n a nominal 8-percent reduction r e l a t i v e t o t h a t generated with t h e p a s s i v e gear. The s t r u t s t r o k e s during i n i t i a l impact f o r t h e t w o tests have d i f f e r e n t s i g n a t u r e s .

T h i s d i f f e r e n c e i s probably caused by strut binding. Consequently, t h e c o n t r o l l e r performed d i f f e r e n t l y during t h e rebound phase. I n both tests, t h e c o n t r o l l e r set t h e impact-limit f o r c e command t o 3 . 6 kN ( 8 0 0 l b f ) , b u t t r a n s i t i o n w a s i n i t i a t e d a t about 0.1 sec f o r t h e d a t a presented i n f i g u r e 1 0 ( f ) and a t about 0.2 sec f o r t h e d a t a presented i n f i g u r e 1O(g). A s a consequence, t h e a c c e l e r a t i n g f o r c e s f o r t h e t w o active-gear tests were d i f f e r e n t d u r i n g rebound, and hence, t h e d e t a i l e d opera- t i o n of t h e c o n t r o l l e r d i f f e r e d during secondary impact. I n s p i t e of t h e s e d i f - f e r e n c e s , t h e control w a s e f f e c t i v e i n l i m i t i n g t h e a c c e l e r a t i n g and d e c e l e r a t i n g f o r c e s t o t h e r o l l - o u t l i m i t command f o r c e s of f2.7 k~ (f600 l b f ) during secondary impact and i n i t i a t i o n of r o l l - o u t .

Data f o r t h e touchdown impact phase of the landing-simulation tests show t h a t t h e a c t i v e gear w a s e f f e c t i v e i n reducing the mass-center f o r c e s r e l a t i v e t o those generated by t h e p a s s i v e gear. During i n i t i a l impact, mass-center f o r c e r e d u c t i o n s of 31 p e r c e n t a t a ground speed of 8 knots t o 8 p e r c e n t a t a speed of 80 knots were obtained. The reduction i n t h e e f f e c t i v e n e s s of t h e a c t i v e gear a t t h e higher ground speeds may be a t t r i b u t e d t o t h e l a r g e r binding-friction f o r c e s generated i n t h e strut which t h e hydraulic c o n t r o l system cannot control. As a r e s u l t of c o n t r o l during i n i t i a l impact, t h e a c t i v e gear w a s generally very e f f e c t i v e i n c o n t r o l l i n g t h e mass- c e n t e r a c c e l e r a t i n g f o r c e s during rebound of t h e drop-f i x t u r e mass.

During secondary impact, t h e mass-center f o r c e s w e r e l i m i t e d by t h e c o n t r o l t o t h e designed r o l l - o u t l i m i t f o r c e s of f2.7 kN (f600 l b f ) . The maximum shock-strut s t r o k e required by t h e a c t i v e gear, which occurs during t h e secondary impact of t h e touchdown impact phase, d i d n o t exceed 85 p e r c e n t of t h e a v a i l a b l e stroke.

Traverse of Step Bumps Mass-center f o r c e and strut s t r o k e data from t h e landing-simulation tests of t h e a c t i v e and p a s s i v e gears during t r a v e r s e of the s t e p bumps are presented i n figure 11 f o r t h e range of ground speeds and bump-encounter frequencies. T h e data are p l o t t e d from an a r b i t r a r y t i m e before encounter with t h e f i r s t bump ( z e r o t i m e i n t h e f i g u r e ) through t r a v e r s e of t h e second bump.

Encounter frequency of 2 Hz.- Data for the a c t i v e and p a s s i v e g e a r s during tra- v e r s e of t h e s t e p bumps a t ground speeds of 8, 40, and 80 knots and bump spacings f o r a n encounter frequency of 2 Hz are presented i n f i g u r e s l l ( a ) t o l l ( c ) t o permit e v a l u a t i o n of t h e e f f e c t i v e n e s s of t h e a c t i v e gear as a f u n c t i o n of ground speed.

The t i m e h i s t o r i e s of t h e mass-center f o r c e f o r t h e a c t i v e and p a s s i v e g e a r s d i f f e r s i n c e t h e c o n t r o l l e r r e g u l a t e s t h e gear force i n response t o t h e dynamic loading r e s u l t i n g f r o m previous c o n t r o l action. Therefore, t h e e f f e c t i v e n e s s of t h e a c t i v e gear must g e n e r a l l y be defined i n terms of t h e peak f o r c e s generated by each of t h e g e a r s during t r a v e r s e of t h e s t e p bumps. When compared on t h i s b a s i s , t h e d a t a i n f i g u r e 11 ( a ) i n d i c a t e t h a t t h e a c t i v e gear was very e f f e c t i v e i n reducing t h e mass- c e n t e r d e c e l e r a t i n g f o r c e s during t r a v e r s e of t h e s t e p bumps a t a ground speed of 8 knots; 28 p e r c e n t reduction during i n i t i a l c o n t a c t w i t h f i r s t bump, and 60 p e r c e n t r e d u c t i o n during i n i t i a l c o n t a c t with t h e second bump. The a c t i v e gear w a s a l s o e f f e c t i v e i n reducing the mass-center decelerating f o r c e s during t r a v e r s e of t h e bumps a t 40 and 80 knots, as shown i n f i g u r e s l l ( b ) and l l ( c ) , r e s p e c t i v e l y . A t a ground speed of 40 knots, t h e mass-center decelerating f o r c e reductions w e r e 31 per- c e n t during i n i t i a l c o n t a c t w i t h t h e f i r s t bump and 55 p e r c e n t during i n i t i a l c o n t a c t with t h e second bump. For the 80-knot ground speed, the reductions were 28 and 15 percent, r e s p e c t i v e l y .

A s shown, t h e e f f e c t i v e n e s s of t h e a c t i v e gear generally decreased during i n i - t i a l encounter with t h e second bump with increasing ground speed. This decrease i n e f f e c t i v e n e s s may be a t t r i b u t e d t o two f a c t o r s ; l a r g e binding-f r i c t i o n f o r c e s developed i n t h e strut during encounter with t h e s t e p bumps and/or inadequate response of t h e c o n t r o l system. Previously, it has been shown t h a t , during touchdown impact, l a r g e binding-f r i c t i o n f o r c e s reduce t h e e f f e c t i v e n e s s of t h e a c t i v e gear.

The magnitude of t h e s e f o r c e s during encounter with t h e step bumps i s unknown.

However, t h e strut has been stroked t o near the s t a t i c p o s i t i o n prior t o encounter w i t h t h e bumps; hence, t h e d i s t a n c e between the s t r u t bearings i s g r e a t e r and t h e binding-f r i c t i o n f o r c e s should be smaller than those occurring a t touchdown. With r e g a r d t o t h e c o n t r o l system response a s the ground speed i n c r e a s e s , t h e s t r o k i n g rate of t h e shock-strut i n c r e a s e s a s t h e gear t r a v e r s e s the bumps; hence, t h e rate of change of t h e strut hydraulic p r e s s u r e and t h e shock-strut f o r c e increase. Thus, a n i n c r e a s e i n the response of t h e c o n t r o l system would be required a t t h e higher ground speeds f o r t h e a c t i v e gear t o maintain t h e e f f e c t i v e n e s s demonstrated a t t h e 8-knot ground speed.

E f f e c t of ground speed.- The following d i s c u s s i o n i l l u s t r a t e s t h e d e t e r i o r a t i o n of t h e response of t h e c o n t r o l system with i n c r e a s e i n ground speed during t r a v e r s e of t h e 2-Hz bumps. The c o n t r o l l i m i t e d t h e mass-center d e c e l e r a t i n g f o r c e s t o v a l u e s w i t h i n t h e range of t h e r o l l - o u t l i m i t f o r c e s programmed i n t o t h e computer (f2.7 k N (f600 l b f ) during encounter with t h e bumps a t t h e 8-knot ground speed ( f i g . l l ( a ) 1 .

During encounter with t h e bumps a t the 40-knot ground speed ( f i g . 1 1 ( b ) ) , t h e mass- c e n t e r d e c e l e r a t i n g f o r c e s f o r t h e a c t i v e gear during i n i t i a l c o n t a c t w i t h each of the bumps w e r e 33 p e r c e n t g r e a t e r than the r o l l - o u t l i m i t forces. The data f o r a ground speed of 80 knots ( f i g . l l ( c ) ) show t h a t t h e mass-center d e c e l e r a t i n g force f o r c o n t a c t w i t h t h e f i r s t bump was 83 p e r c e n t g r e a t e r than t h e r o l l - o u t l i m i t f o r c e and was even g r e a t e r during i n i t i a l c o n t a c t with the second bump. The f a c t t h a t t h e c o n t r o l system could n o t l i m i t t h e mass-center d e c e l e r a t i n g f o r c e s t o t h e value of t h e programmed r o l l - o u t l i m i t f o r c e s i n d i c a t e s t h a t t h e response of t h e c o n t r o l sys- t e m was marginal during encounter with t h e step bumps a t speeds of 40 and 8 0 knots.

The marginal performance of t h e c o n t r o l system during t r a v e r s e of t h e s t e p bumps i s f u r t h e r i l l u s t r a t e d by data i n t a b l e I11 which show t h a t , during t r a v e r s e of t h e s t e p bumps, only one test of t h e e i g h t conducted a t a ground speed of 40 knots and t w o of t h e seven conducted a t a speed of 80 knots i n d i c a t e d e f f e c t i v e operation of t h e a c t i v e gear. T h i s marginal performance of t h e c o n t r o l system a t t h e s e speeds may be a t t r i b u t e d t o the following: t h e e l e c t r o n i c c o n t r o l l e r was designed and tuned pri- marily f o r t h e touchdown impact phase and not t h e r o l l - o u t phase (see r e f . 4) , and t h e design of the hydraulic power u n i t was n o t optimized f o r providing minimum flow l o s s e s .

I n s p i t e of t h e s e f a c t o r s , t h e a c t i v e gear w a s s u r p r i s i n g l y e f f e c t i v e i n reduc- i n g t h e mass-center f o r c e s r e l a t i v e t o those obtained with t h e p a s s i v e gear. T h i s was t r u e f o r those tests during which t h e c o n t r o l system w a s a b l e t o respond t o t h e ground f o r c i n g function during t r a v e r s e of t h e step bumps spaced f o r a n encounter frequency of 2 Hz.

Encounter frequency of 4 Hz.- Data f o r t h e a c t i v e and p a s s i v e g e a r s during tra- v e r s e of s t e p bumps spaced f o r encounter frequencies of 2 and 4 Hz a t a ground speed of 40 knots a r e presented i n f i g u r e s l l ( b ) and l l ( d ) , r e s p e c t i v e l y . During i n i t i a l c o n t a c t with the first bump i n t h e test conducted a t t h e 4-Hz spacing, t h e a c t i v e gear reduced the mass-center d e c e l e r a t i n g f o r c e by only 6 percent, compared w i t h t h e 31-percent reduction achieved during the test conducted f o r an encounter frequency of 2 Hz- This d i f f e r e n c e may be a t t r i b u t e d t o t h e low-frequency response of t h e c o n t r o l system, a s previously discussed, during bump encounters a t t h e 40-knot ground speed.

As a r e s u l t of the c o n t r o l system operation f o l l a w i n g t r a v e r s e of t h e f i r s t bump and the s h o r t e r time i n t e r v a l between bump encounters, the c o n t r o l system d i d n o t respond during t r a v e r s e of t h e second bump, as i n d i c a t e d by t h e 54-percent i n c r e a s e i n mass- c e n t e r d e c e l e r a t i n g f o r c e f o r the 4-Hz t e s t compared with the 55-percent reduction obtained during t h e 2-Hz test.

Servovalve/relief-valve i n t e r a c t i o n . - Another factor which influenced t h e e f f e c t i v e n e s s of the a c t i v e gear during t r a v e r s e of the step bumps w a s an unexpected i n t e r a c t i v e e f f e c t between t h e p r e s s u r e - r e l i e f v a l v e s and t h e servovalve. The e f f e c t of i n a d v e r t e n t operation of these valves on t h e e f f e c t i v e n e s s of t h e a c t i v e gear i s i l l u s t r a t e d i n f i g u r e 12 f o r t r a v e r s e of t h e s t e p bumps a t a ground speed of 8 0 knots and a bump encounter frequency of 2 Hz. A t i n i t i a l encounter w i t h t h e first step bump, t h e strut hydraulic p r e s s u r e ( f i g . 1 2 ( b ) ) very quickly exceeds t h e opening p r e s s u r e f o r t h e relief valves, thus i n i t i a t i n g f l o w from t h e gear t o the l o w - p r e s s u r e r e s e r v o i r . Maximum s t r u t p r e s s u r e occurring when t h e servo-spool displace- ment i s zero i n d i c a t e s t h a t flow w a s through the p r e s s u r e - r e l i e f valves and not t h e c o n t r o l servovalve. The subsequent phase d i f f e r e n c e s between t h e o p e r a t i o n of t h e servovalve and t h e p r e s s u r e - r e l i e f valves r e s u l t e d i n an undesirable i n t e r a c t i v e e f f e c t c h a r a c t e r i z e d by l a r g e mass-center force o s c i l l a t i o n s a t a frequency of about 8 Hz. As shown i n f i g u r e 1 2 ( a ) , t h e i n t e r a c t i v e phenomena continue u n t i l t h e gear h a s completely t r a v e r s e d t h e s t e p bumps. As a r e s u l t of t h i s i n t e r a c t i o n t h e a c t i v e gear was n o t e f f e c t i v e i n reducing the mass-center f o r c e s r e l a t i v e t o those obtained w i t h t h e p a s s i v e gear. The i n t e r a c t i v e operation i s t h e r e s u l t of an overly con- s e r v a t i v e s e t t i n g of t h e opening p r e s s u r e f o r t h e r e l i e f valves and n o t a design f a u l t i n t h e e l e c t r o n i c c o n t r o l l e r .

Summarizing t h e r e s u l t s obtained during t r a v e r s e of t h e s t e p bumps, t h e a c t i v e gear w a s s u r p r i s i n g l y e f f e c t i v e i n reducing t h e mass-center f o r c e s a t a l l ground speeds i n v e s t i g a t e d when t h e e f f e c t s of p r i o r c o n t r o l a c t i o n , strut binding f r i c t i o n , c o n t r o l response, and/or servovalve/relief-valve i n t e r a c t i o n w e r e n o t dominant f a c t o r s .

Traverse of Natural R u m p s Presented i n f i g u r e 13 are t y p i c a l data f o r mass-center forces and shock-strut strokes from a c t i v e - and passive-gear landing- simulation tests during t r a v e r s e of t h e n a t u r a l bumps a t ground speeds of 40 and 8 0 knots.

Data w e r e not obtained f o r a ground speed of 8 knots. The data are presented t o e v a l u a t e t h e e f f e c t i v e n e s s of t h e a c t i v e gear during t r a v e r s e of surface unevenness having approximately t h e same amplitude a s t h a t of t h e step bumps, b u t with a more gradual onset. T h e data are p l o t t e d f r o m a t i m e ( z e r o t i m e ) corresponding t o a t r a c k s t a t i o n of approximately 980 (see f i g . 6 ) through track s t a t i o n 1180.

E f f e c t of f o r t h e a c t i v e and ground speed.- Data are presented i n f i g u r e 1 3 ( a ) p a s s i v e g e a r s during t r a v e r s e of t h e n a t u r a l bumps a t a ground speed of 40 knots.

The c o n t r o l deadband of f 2 . 7 kN (f600 l b f ) i s shown i n t h e f i g u r e t o i l l u s t r a t e t h a t n e i t h e r t h e a c t i v e nor t h e passive gears produced mass-center f o r c e s g r e a t e r than t h e r o l l - o u t (deadband) l i m i t f o r c e s during t r a v e r s e of t h e n a t u r a l bumps a t 40 knots.

Within t h e r o l l - o u t l i m i t command f o r c e s , the a c t i v e and p a s s i v e g e a r s w e r e a b l e t o accommodate t h e strut s t r o k i n g r a t e s and t h e maximum s t r o k e s imposed on t h e gear by t h e f o r c i n g f u n c t i o n a p p l i e d during traverse of t h e bumps a t a speed of 40 knots.

Consequently, t h e c o n t r o l system f o r t h e a c t i v e gear was i n a c t i v e and t h e f o r c e s produced by the a c t i v e gear w e r e about t h e same as those of t h e p a s s i v e gear.

D a t a obtained from tests of t h e a c t i v e and p a s s i v e gears during t r a v e r s e of t h e n a t u r a l bumps a t a ground speed of 8 0 knots are presented i n f i g u r e 1 3 ( b ) .

The num- bers assigned t o t h e mass-center decelerating-force peaks correspond t o those shown on t h e t r a c k s u r f a c e p r o f i l e ( f i g . 6 ) and a r e used t o c o r r e l a t e mass-center f o r c e s profiles. To i l l u s t r a t e t h e e f f e c t s of bump amplitudes w i t h t h e changes i n s u r f a c e and t h e r a t e of change of these amplitudes on gear f o r c e s , hence mass-center f o r c e s , t h e data obtained during t r a v e r s e of bumps 1 and 2 a r e discussed. The t r a c k s u r f a c e profile ( f i g . 6 ) shows t h a t bump 1 has an amplitude almost twice t h a t of bump 2, b u t had a shallower slope than bump 2.

The mass-center f o r c e s f o r t h e p a s s i v e gear ( f i g . 1 3 ( b ) ) during t r a v e r s e of bump 2 a r e approximately twice those developed during t r a v e r s e of bump 1. Thus, t h e rate of change of surface e l e v a t i o n s had a g r e a t e r i n f l u e n c e on t h e f o r c e developed by t h e gear than t h e amplitude of t h e e l e v a t i o n change. This agrees with oleo-pneumatic shock-strut theory t h a t d e f i n e s t h e (which accounts f o r about 90 p e r c e n t of t h e dynamic f o r c e developed hydraulic force by t h e strut) a s a function of t h e shock-strut v e l o c i t y . Therefore, t h e series- hydraulic c o n t r o l system, assuming adequate c o n t r o l system response and s i m i l a r binding-f r i c t i o n e f f e c t s , should be more e f f e c t i v e during t r a v e r s e of the bumps having s t e e p e r slopes. A s shown i n f i g u r e 1 3 ( b ) , t h e a c t i v e gear w a s e f f e c t i v e i n reducing t h e mass-center d e c e l e r a t i n g f o r c e , r e l a t i v e t o t h a t of t h e p a s s i v e gear, by 62 p e r c e n t during t r a v e r s e of bump 2 . During t r a v e r s e of bump 1, t h e reduction i n mass-center d e c e l e r a t i n g f o r c e w a s only 13 percent. The a c t i v e gear w a s also e f f e c t i v e i n reducing t h e mass-center d e c e l e r a t i n g f o r c e during t r a v e r s e of bump 3 by 24 percent. The c o n t r o l system response was adequate for c o n t r o l l i n g the gear during t r a v e r s e of the n a t u r a l bumps a t t h e 80-knot ground speed, s i n c e a l l active-gear mass-center forces, d e c e l e r a t i n g and a c c e l e r a t i n g , w e r e l i m i t e d t o t h e r o l l - o u t l i m i t f o r c e command of f2.7 kN (f600 l b f ) .

Data f o r s t r u t hydraulic p r e s s u r e and servo-spool displacement during t r a v e r s e of the n a t u r a l bumps a t a ground speed of 80 knots a r e shown i n f i g u r e 1 3 ( c ) . These data show t h a t , when t h e s t r u t hydraulic p r e s s u r e does n o t exceed t h e opening pres- sure of t h e r e l i e f valves, t h e i n t e r a c t i v e e f f e c t s between the c o n t r o l system and t h e r e l i e f v a l v e s a r e avoided and t h e c o n t r o l system performs e f f e c t i v e l y . (See f i g . 1 3 ( b ) . ) Servovalve/relief-valve i n t e r a c t i o n . - Additional d a t a i n figures 1 3 ( d ) and 1 3 ( e ) f o r t r a v e r s e of t h e n a t u r a l bumps a t a ground speed of 80 knots i l l u s t r a t e t h e detri- mental e f f e c t of i n t e r a c t i o n between t h e control system and t h e p r e s s u r e - r e l i e f valves. The a c t i v e gear was e f f e c t i v e i n reducing t h e mass-center f o r c e s during t r a v e r s e of the f i r s t and second bumps ( s e e f i g . 1 3 ( d ) ) when t h e s t r u t hydraulic p r e s s u r e was less than the opening p r e s s u r e f o r the r e l i e f valves. (See f i g . 1 3 ( e ) . ) T h e mass-center d e c e l e r a t i n g and a c c e l e r a t i n g f o r c e s w e r e reduced by 51 and 72 per- cent, respectively. However, t h e s t r u t hydraulic p r e s s u r e exceeded t h e opening pres- s u r e of t h e r e l i e f valves during encounter w i t h the t h i r d bump, and t h e i n t e r a c t i v e effect r e s u l t e d i n overcontrol of t h e servovalve, as shown by t h e l a r g e t r a n s i e n t s i n servo-spool displacement and t h e c h a r a c t e r i s t i c 8-Hz o s c i l l a t i o n s of t h e mass-center f o r c e . Consequently, t h e a c t i v e gear w a s not e f f e c t i v e during t r a v e r s e of t h e t h i r d bump.

I n summary, t h e r e s u l t s f o r t h e a c t i v e and p a s s i v e g e a r s during t r a v e r s e of t h e n a t u r a l bumps show t h a t t h e a c t i v e gear is very e f f e c t i v e i n reducing t h e f o r c e a p p l i e d by t h e gear t o t h e mass center. The d e t e r i o r a t i o n i n t h e e f f e c t i v e n e s s of t h e a c t i v e gear r e s u l t i n g from i n t e r a c t i o n between t h e c o n t r o l system and t h e p r e s s u r e - r e l i e f valves i s due t o improper s e t t i n g of t h e r e l i e f p r e s s u r e and n o t t o f a u l t y c o n t r o l system design.

CONCLUDING REMARKS An experimental i n v e s t i g a t i o n w a s conducted on a s e r i e s - h y d r a u l i c a l l y Controlled main landing gear f o r a l i g h t a i r p l a n e t o determine t h e f e a s i b i l i t y and t h e p o t e n t i a l of such a gear i n reducing ground loads a p p l i e d through t h e gear t o t h e airframe.

The i n v e s t i g a t i o n included v e r t i c a l - d r o p ( z e r o ground speed) and landing-simulation tests.

The p o t e n t i a l of t h e a c t i v e gear w a s evaluated for performance during touch- down impact and during t r a v e r s e of two types of surface unevenness: a b r u p t discon- t i n u i t i e s ( s t e p bumps), r e p r e s e n t a t i v e of uneven s e t t l e m e n t of runway s e c t i o n s ; and longer wavelength v a r i a t i o n s i n runway e l e v a t i o n ( n a t u r a l bumps).

R e s u l t s show t h a t , during the impact phase of a landing, t h e a c t i v e gear was t o e f f e c t i v e i n reducing ground loads applied t o t h e simulated a i r p l a n e r e l a t i v e t h o s e generated by t h e p a s s i v e gear. Data from the v e r t i c a l - d r o p tests show t h a t t h e e f f e c t i v e n e s s of t h e a c t i v e gear i n c r e a s e s with i n c r e a s e s i n touchdown sink rate.

For example, r e s u l t s showed an 8-percent reduction for a sink rate of 0.9 m/sec ( 3 ft/sec) and a 32-percent reduction f o r a sink rate of 1.7 m/sec (5.5 f t / s e c ) .

However, d a t a f o r t h e touchdawn impact phase of t h e landing-simulation tests show t h a t , f o r a c o n s t a n t touchdown sink r a t e , the e f f e c t i v e n e s s of t h e a c t i v e gear i s reduced as t h e touchdown ground speed is increased; 31 p e r c e n t reduction a t 8 knots, 9 p e r c e n t reduction a t 40 knots, and 11 percent reduction a t 80 knots. These r e d u c t i o n s i n e f f e c t i v e n e s s may be a t t r i b u t e d t o i n c r e a s e s i n shock-strut binding- f r i c t i o n f o r c e s r e s u l t i n g from increased wheel "spin-up" drag a t t h e higher ground speeds.

Data obtained during t r a v e r s e of t h e step bumps was n o t a s c o n s i s t e n t as t h a t obtained during t h e touchdown impact phase. The e l e c t r o n i c c o n t r o l l e r w a s designed and tuned f o r t h e touchdown impact phase of a landing and n o t f o r t h e r o l l - o u t phase.

I n a d d i t i o n , an unexpected i n t e r a c t i o n between p r e s s u r e - r e l i e f valves i n t h e hydraulic power u n i t and t h e c o n t r o l servovalve r e s u l t e d i n adverse performance of the c o n t r o l system during t r a v e r s e of the step bumps f o r some of t h e tests. Because of t h e s e f a c t o r s , only a l i m i t e d number of tests i n d i c a t e d e f f e c t i v e operation of t h e a c t i v e gear during t r a v e r s e of t h e s t e p bumps. For those tests during which t h e c o n t r o l system w a s o p e r a t i n g properly, t h e a c t i v e gear w a s very e f f e c t i v e i n reducing t h e mass-center d e c e l e r a t i n g forces; maximum reductions of 60 p e r c e n t a t a ground speed of 8 knots, 55 p e r c e n t a t a speed of 40 knots, and 2 8 p e r c e n t a t a speed of 8 0 knots.

The r e s u l t s from the tests during t r a v e r s e of t h e n a t u r a l bumps show t h a t , f o r a ground speed of 40 knots, t h e r a t e of change of s u r f a c e e l e v a t i o n d i d n o t r e s u l t i n developing mass-center f o r c e s g r e a t e r than t h e r o l l - o u t l i m i t f o r c e s programmed i n t o t h e c o n t r o l system. Consequently, t h e control system w a s i n a c t i v e and the mass- c e n t e r f o r c e s produced by t h e a c t i v e gear were about t h e same a s those of t h e p a s s i v e gear. I n t e r a c t i o n between t h e servovalve and t h e r e l i e f valves a l s o occurred during t r a v e r s e of t h e n a t u r a l bumps a t a ground speed of 80 knots, b u t t h e occurrence w a s not encountered as f r e q u e n t l y as it was during t r a v e r s e of t h e s t e p bumps. During t r a v e r s e of t h e n a t u r a l bumps a t a ground speed of 8 0 knots, t h e a c t i v e gear was e f f e c t i v e i n reducing t h e mass-center forces; maximum reductions i n d e c e l e r a t i n g f o r c e of 62 p e r c e n t and a c c e l e r a t i n g f o r c e of 72 p e r c e n t w e r e obtained.

The o v e r a l l r e s u l t s of t h i s i n v e s t i g a t i o n show t h a t a s e r i e s - h y d r a u l i c a l l y c o n t r o l l e d landing gear i s f e a s i b l e and t h a t such a gear i s very e f f e c t i v e i n reducing t h e loads, r e l a t i v e t o those generated by t h e p a s s i v e gear, t r a n s m i t t e d by t h e gear t o t h e airframe during ground operations.

Langley Research Center N a t i o n a l Aeronautics and Space Administration Hampton, VA 23665 June 29, 1982

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2 1 TABLE 111.- S U M M A R Y OF TEST CONDITIONS AND TEST DATA ( a ) Touchdown S t r u t V” ‘ h pressure Test Gear A t t i tude, lumber type de9 m/sec f t/sec m/sec knots 1.35 1 Passive 0 1 . 5 5.0 0 0 196 2 Active 0 1.5 5 . 0 0 0 1.35 196 0 1.30 188 3 Passive 0 .91 3 . 0 0 4 Active 0 .91 3 . 0 0 0 1.30 188 5 Passive 0 1.2 4.0 0 0 1.35 196 6 Active 0 1.2 4.0 0 0 1.35 196 40 1.41 204 14 Passive 8 1.5 5.0 21 1 6 Active 8 1.5 5.0 21 40 1.32 192 23 Active 8 .9 3.0 21 40 .67 24 Passive 8 . 9 3 . 0 21 40 1.43 208 8 1.63 236 27 Active 8 .9 3.0 4.1 28 Active 8 .9 3.0 4 . 1 1.54 224 30 Active 8 .91 3.0 21 40 .99 144 31 Active 8 .91 3.0 21 40 1.38 200 80 1.71 248 32 Active 8 .91 3.0 41 34 Passive 8 .91 3.0 41 80 2.32 3 36 35 Active 8 .91 3.0 41 80 1.65 240 36 Active 8 1.7 5.5 41 80 1.13 164 Passive 8 1.7 5.5 41 80 1.74 252 80 1.16 168

* ; : Active 8 1.7 5.5 41

1.71 248 40 Passive 2 1.7 5.5 41 80 42 Active 2 1.7 5.5 41 80 1.35 196 40 1.74 252 43 Active 2 1.7 5.5 21 44 Passive 2 1.7 5.5 21 40 2.12 308 45 2 1.7 5.5 21 40 1.65 240 Active 47 Passive 2 1.7 5.5 4.1 8 1.65 240 48 Active 2 1.7 5.5 4.1 8 1.65 240 49 Passive 0 1.7 0 1.82 264 5.5 0 51 0 0 268 Active 1.7 5.5 0 1.85 Active 13 1.7 5.5 21 40 .83 120 53 Active 13 1 . 7 80 1.16 168 5.5 41 - - *Roll-out limit-force input a t fl.11 kN (f250 lbf).

TABLE 111.- Continued (b) Touchdown impact phase I n i t i a l impact Rebound Secondary impact Test Gear Mass-center Maximum L i d t Yass-center L i d t Mass-center L i m i t Maximum force force stroke lumber f orce l i f t force force force type

- -

in.

lbf kN lbf kN lbf kN lbf k N lbf lbf lbf kN c m

- - -

--- _---

4.3 -730 10.9 1 Passive -6060 -3210 12.90 2900 -3.25 -26.96 -14.28 600 5 . 0 2.67 -4850 -3150 7.92 1780 -2.58 -580 12.7 -21.57 -14.01 2 AC t i v e

--- ----

4.8 3 Pa 8 s i v e -3200 -2920 7.38 1660 -4.00 -900 12.2 -14.23 -12.99 600 5.5 2.67 4 AC t i v e -2940 -3040 5.60 1260 -2.85 -640 14.0 -13.08 -13.52

--- ----

4.6 -3160 -3.60 -810 11.7 -14.06 5 Passive -5100 9.70 2180 -22.69 600 5.2 2.67 6 Active -4150 -33 10 5.69 1280 -2.67 -600 13.2 -18.46 -14.72

--- -___

4.2 14 Passive -5320 -3420 6.67 1500 -2.76 -620 10.7 -23.66 -15.21 400 6.0 1.78 -4880 -3320 -2.49 -560 15.2 -21.71 -14.77 1 6 Active 9.16 2060 640 6.7 2.85 23 Active -2760 -2940 6.32 1420 -2.40 -540 17.0 -12.28 -13.08

--- ----

3 . 2 24 Passive -3160 -2900 7.12 1600 -1.42 -320 8 . 1 -14.06 -12.90 600 6 .O 2.67 27 Active -2860 -2960 5.25 1180 -2.67 -600 15.2 -12.72 -13.17 540 6.0 2.40 28 -3020 -2.49 -560 15.2 Active -2760 4.45 1000 -12.28 -13.43 600 7 . O 2.67 30 -2520 -3020 3.91 880 -2.49 -560 17.8 -1 1.21 -13.43 Active 7 . 0 2.85 -2.67 -600 17.8 640 31 Active -2700 -3160 4.45 1000 -12.01 -14.06 600 6.2 2.67 32 -4600 -3260 2.94 660 -3.56 -800 15.7 -14.50 Active -20.46

--- ----

1 . 0 -4.18 -940 2.5 34 Passive -5020 -3040 6.14 1380 -22.33 -13.52 18.8 600 7.4 2.67 35 Active -4680 -3600 4.36 980 -3.83 -860 -20.82 -16.01 600 7 . 1 2.67 36 6.85 1540 -3.29 -740 18.0 Active -6160 -3500 -27.40 -15.57

--- --__

-2.67 -600 2 . 1 Passive -7040 -3520 7.14 1740 5.3 -31.32 -15.66

300 8.8 1.33 *:: AC t i v e -3520 10.85 2440 -9.43 -2120 22.4 -15.66

-6600 -29.36

--- ----

4.3 40 12.01 2700 -4.18 -940 10.9 Passive -6960 -3560 -30 -96 -15.84 600 5 . 1 2.67 42 4.89 1100 -2.67 -600 13.0 Active -6300 -3520 -28.02 -15.66 600 7.2 2.67 43 Active -3680 8.90 2000 -3.47 -780 18.3 -26.78 -16.31 -6020 ----

---

2 . 3 44 -3800 15.12 3400 -7.30 -1640 5.8 Passive -6900 -30.69 -16.90 600 7.1 2.67 45 Active -5320 -3780 9.88 2200 -4.00 -900 18.0 -23.66 -16.81

--- ----

4.8 -4.72 -1060 12.2 47 Passive -6560 -3680 12.46 2800 -29.18 -16.37 -720 600 7.7 2.67 48 Active -4500 -3780 5.69 1280 -3.20 1 9 . 6 -20.02 -16.81

--- ----

2.5 49 Passive -6740 14 .SO 3260 -5.87 -1320 6.4 -15.48 -3480 -29.98 600 7.3 2.61 51 AC t i v e -4600 -3640 14.50 3260 -3.29 -740 18.5 -20.46 -16.19 600 8.3 2.67 -3.47 -780 21.1 52 Active -5200 -3700 6.94 1560 -23.13 -16.46 600 7.4 2.67 53 Active -5400 -3600 6.94 1560 -3.56 -800 18.8 -24.02 -16.01 -

- - - -

Decelerating force t Accelerating force

* Roll-out limit-force input a t fl.11 l d i (k250 l b f ) '

---_ Control system inactive

TABLE 111.- Continued ( c ) Traverse of step bumps [Blank space indicates no data obtained] F i r s t bump Second bump L i d t Maximum Test Gear Mass-center' L i d t Maximum Mass-center stroke number force stroke force force type force ( * ) (

- - - - - -

- - - - -

CUI kN lbf kN lbf cm in. kN lbf lbf i n

- - - - - - - - - -

1 Passive 2 Active 3 Passive AC t i v e 5 Passive 6 Active

---- --- ---

-1730 15.5 6 . 1 14 Passive -5.34 -1200 14.2 5.6 -7.70 -830 10.7 0 . 2 -770 600 11.4 4.5 16 Active -3.69 2.67 600 -3.43 23 Active -5.43 -1220 2.67 6 0 0 45.7 6.2 -6.94 -1560 6 0 0 17.5 6.9

---- --- ---

-830 11.7 4.6 24 Passive -6.14 -1380 11.7 4.6 -3.69 27 AC t i v e -3.20 -720 2.67 6 0 0 13.2 5 . 2 -4.18 -940 600 13.2 5 . 2 $28 -6.23 -1400 2.40 540 14.0 5 . 5 -3.56 -800 540 17.5 6.9 Active Active -7.47 -1680 2.67 600 18.3 7.2 -5.34 -1200 6 0 0 15.5 6 . 1 $30 -1540 17.3 6.8 -6.49 -1460 60 0 16.8 6.6 531 Active -6.85 2.67 600 4.5 §32 Active -6.94 -1560 2.67 600 13.5 5 . 3 -5.07 -1 140 6 0 0 11.4 --- ---- --- -6.09 -1370 5 . 1 2 . 0 -7.70 -1730 5.6 2.2 34 Passive -1650 18.8 7.4 535 Active -7.65 -1720 2.67 600 14.7 5.8 -7.34 600 $36 -7.38 -1660 2.67 600 17.0 6.7 -5.96 -1340 600 17.0 6.7 Active

---- --- ---

9 . 1 3 . 6 Passive -6.63 -1490 8 . 1 3 . 2 -9.21 -2070 8 . e

* : ; Active -6.41 -1440 1 . 3 3 300 13.7 5.4 -23.6 -5300 300 22.4

---

---- --- 6.2

40 Passive -4.54 -1020 14.2 5.6 -6.41 -1440 15.7 $42 -4.80 -1080 600 16.8 6.6 -7.70 -1730 600 18.0 7 . 1 Active 2.67 15.7 6.2 43 Active -5.60 -1260 2.67 600 15.2 6.0 -5.78 -1300 600 ---

---- ---

44 8.4 3 . 3 -7.74 -1740 9 . 4 3.7 Passive -6.23 -1400 45 14.7 5.8 -5.07 -1 140 600 13.2 5 . 2 Active -5.78 -1300 2.67 600

---

---- ---

47 Passive -570 13.7 5.4 -3.87 -870 1 4 . C 5.5 -2.54 48 14.2 5.6 -1.65 -370 600 1 4 . 7 5.8 Active -1.87 -420 2.67 600 49 Passive Active 52 Active -5.78 -1300 2.67 600 18.8 7 . 4 -5.65 -1270 600 1 8 . C 7 . 1 553 Active -4.89 -1100 2.67 600 16.3 6.4 - 1 1 . 1 -2500 600 1 6 . E 6 -6

- - -

Decelerating force

* Roll-out limit-force input a t fl.11 kN (f250 l b f )

Relief-valve/servovalve interaction I

-_-- Control system inactive

TABLE 111.- Concluded (d) Traverse of natural bumps [Blank space indicates no data1 R 1 1 bumps Third bump Second bump F i r s t bump Maximum lass-center L i d t Maximum Mass-center Maximum T e s t Gear lass-center stroke force stroke force lumber stroke force force t m e ( * I ( * ) ( * ) -

-

- in. in. cm kN lbf cm in. lbf cm kN lbf

-

- - -

- - - - -

1 Passive 2 R c tive 3 Passive 4 Active 5 Passive Active 5.6 5 . 4 -280 13.1 -300 -1.33 14.2 5.5 -220 14.0 -1.25 Passive -0.98 3.7 3.7 9.4 -300 -1.33 9.4 3.6 -240 9.1 -1.51 -340 -1.07 16 Rctive 4 . 4 -290 -1.29 11.2 4.3 4.3 -330 11.0 -1.29 -290 10.9 23 R c tive -1.47 4 . 4 -490 -2.18 11.2 4.3 4.2 11.0 -1.78 -400 10.1 -350 24 Passive -1.56 27 nc tive $28 Active 5.5 -680 -3.02 14.0 4.8 4.8 12.2 -1.69 -380 12.2 -240 $30 Active -1.07 4 .e 4.8 -340 12.2 -520 -2.31 12.2 4.7 -260 11.9 -1.51 -1.16 §31 Active

--- ----

4 . 7 -650 1 1 . 9 -1660 -7.38 4 . 7 1 1 . 9 -2.89 -6ia $32 Active -2.71

1.7 - 1080 -4.80 2.9

1.8 -1310 4.6 1.5 -670 3.8 -5.83 34 Passive -2.98

--- ----

4.4 -480 1 1 . 2 -187a -8.32 5.6 14.2 -2.14 -600 535 Active -2.67 5.4 4 . 5 ( I ) 13.7 -1.60 -360 1 1 . 4 ( $ 1 ( § ) (P) -500 $36 Active -2.22 2 . 9 3.1 -1330 7.G -84C -3.74 7.4 2.8 -69C 7.0 -5.92 Passive -3.07 ( I) ( 1 ) ( I) ( 1 ) ( 1) ( 1 )

( I) ( 1 ) ( I) ( I) ( I) *:: Active

( 1 ) 5 . 8 -26a -1.16 14.7

5.5 14.5 - .98 -220 14.C

5.7 -44c 40 Passive -1.96 5.3 4 . 3 -510 10.1 -631 -2.80 13.5 5.2 -6OC 13.3 -2.27 $42 -2.67 Active 4.8 4.6 -620 12.; -260 -1.16 11.7 4.5 11.4 -2.76 -58C 43 Active -2.58 3 . 1 3.1 -220 7.5 -22c -.98 7 . 9 2.8 - 3 O C 7.1 -.98 44 -1.33 Passive 4 . 2 -36C -1.60 10.7 4 . 2 4.4 10.7 -1.42 -320 1 1 . ; -34c 45 Active -1.51 47 Passive 48 Active 49 Passive 51 Active 5 . 2 5 . 2 -300 13.: -58( -2.58 13.2 5.1 13.0 -1.33 -.98 -22( 52 Active 5.1 -44c -1.96 13.0 4.9 5.6 12.4 -2.94 -660 14.: -60( 953 Active -2.67 -

- - -

- - -

Decelerating force

* Roll-out limit-force input a t t 1 . 1 1 kN (t250 l b f )

Relief-valve/servovalve interaction § I Controller malfunctioned

---- Control system inactive

L-80-7327.1 ( a ) Components.

Figure 1.- Modified main l a n d i n g gear.

(b) End view of modified o r i f i c e s u p p o r t tube.

Figure 1 .- Continued.

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Figure 1 .- Concluded.

ii pnien t

~ ~~ L-82- 160

Figure 3 .- Bench setup of Control, signal-conditioning, and

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I L-81-10,134.1 - Photograph and track surface p r o f i l e of natural bumps.

Figure Control til o n acceleration Mass-cen t e r 3 t i i on acceleration Li ft-cy1 i nder pressure L i m i t force comnand Force error i S t r u t hydraulic pressure c S t r u t pneuma t i c pressure S t r u t position S t r u t p o s i t i o n error Mass-center displacement B i Servo-spool d i spl acement WIG velocity decrement Servo-1 oop enable Time, sec ( a ) Landing impact.

Figure 7.- Typical o s c i l l o g r a p h t r a c e s of active-gear data. V h = 8 knots; vv = 1.7 m/sec (5.5 f t / s e c ) ; t e s t 4 8 .

Control ti on acceleration Mass-center ti on accel erati on Lift-cylinder pressure L i m i t force command Force error S t r u t hydraul i c pres sure S t r u t pneumatic pressure S t r u t position S t r u t position error Mass-center displacement t B i Servo- spool di spl acernent W/G velocity decrement Servo-loop enable Time, sec (b) Traverse of step bumps.

Figure 7 .- Concluded.

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APPENDIX CONTROL PHILOSOPHY The c o n t r o l philosophy of the o r i g i n a l v e r s i o n of the e l e c t r o n i c c o n t r o l l e r i s shown i n t h e software flow c h a r t of reference 4 . I n t h i s i n v e s t i g a t i o n it became a p p a r e n t t h a t t h e o r i g i n a l c o n t r o l philosophy w a s n o t adequate t o c o n t r o l t h e gear during more realistic landing simulations. The c o n t r o l l a w s , t h e r e f o r e , w e r e changed through hardware and software modifications of t h e e l e c t r o n i c c o n t r o l l e r .

START The software flow c h a r t f o r t h e e l e c t r o n i c c o n t r o l l e r used i n t h i s i n v e s t i g a t i o n i s presented i n f i g u r e A l . The following discussion i l l u s t r a t e s the o p e r a t i o n of t h e e l e c t r o n i c c o n t r o l l e r as t h e c o n t r o l laws were a p p l i e d t o operate t h e c o n t r o l servo- valve and, hence, t h e gear. P r i o r t o a test, the c o n t r o l l e r w a s "on" i n the d i s a b l e d (reset) mode (START on t h e flow c h a r t ) . With a p p l i c a t i o n of power t o t h e c o n t r o l l e r , the c e n t r a l processing u n i t (CPU) disabled the i n t e r r u p t s , i n i t i a l i z e d t h e arithmetic board, set t h e e l e c t r o n i c switches, and set t h e front-panel l i g h t s t o i n d i c a t e t h e s e conditions. The CPU i n i t i a t e d a query loop for determining the d i s a b l e d (reset) o r enabled s t a t u s of t h e c o n t r o l l e r . For t h e experiments of t h i s study, a c t i v a t i o n of a microswitch enabled the c o n t r o l l e r when the t i r e contacted the track surface. A t t i m e of a c t i v a t i o n , t h e CPU enabled the c o n t r o l l e r and set a l i g h t on the f r o n t p a n e l of t h e c o n t r o l l e r t o i n d i c a t e t h i s condition.

MODE DETERMINATION The landing mode of o p e r a t i o n of the c o n t r o l l e r r e q u i r e s d i f f e r e n t c o n t r o l l a w s f o r t h e touchdown impact phase and t h e landing r o l l - o u t phase. For t h e take-off mode of operation, t h e c o n t r o l l a w s are e s s e n t i a l l y t h e s a m e as those employed i3uring t h e l a n d i n g r o l l - o u t phase. Because of t h i s difference, t h e c o n t r o l l e r must determine of operation. A s t h e a i r c r a f t approaches t h e runway i n t h e landing mode, the mode t h e gear shock s t r u t s are f u l l y extended; whereas, t h e a i r c r a f t i s supported by t h e gear shock s t r u t s a t e s s e n t i a l l y the designed s t a t i c gear d e f l e c t i o n i n t h e take-of f mode. Therefore, a f t e r t h e c o n t r o l l e r i s enabled, t h e CPU acquires t h e strut p o s i t i o n and compares t h e value t o a n i n p u t take-off o r landing threshold (TLTHmSH) value of t h e s t r u t stroke. The TLTHReSH value for t h i s i n v e s t i g a t i o n w a s i n p u t as 2 . 5 4 c m ( 1 -0 i n . ) s i n c e t h e static s t r o k e for the gear w a s 11.4 cm ( 4 . 5 i n . 1 . For a strut p o s i t i o n g r e a t e r than TLTHRESH, the CPU t r a n s f e r s t o the take-off l o g i c s e c t i o n of the program.

LANDING MODE T h i s i n v e s t i g a t i o n w a s conducted f o r the landing mode only; hence, the gear w a s above t h e t r a c k s u r f a c e ( g e a r f u l l y extended and s t r u t p o s i t i o n equal to zero) prior t o a test. There€ore, the CPU selects t h e landing mode, enables t h e i n t e g r a t o r cir- c u i t , and sets l i g h t s on t h e f r o n t p a n e l to i n d i c a t e t h e s e conditions. The i n t e g r a - tor c i r c u i t i n t e g r a t e s the wing/gear i n t e r f a c e a c c e l e r a t i o n (W/G ACCEL) t o o b t a i n t h e change i n wing/gear i n t e r f a c e v e l o c i t y (wing/gear i n t e r f a c e v e l o c i t y decrement, W/G VEL DEC). The touchdown value of the wing/gear i n t e r f a c e v e l o c i t y is e q u i v a l e n t t o t h e sink rate. The C P U a c q u i r e s t h e sink rate, t h e instantaneous value of t h e W/G 6 1

APPENDIX VEL DEC, and c a l c u l a t e s t h e instantaneous Value of t h e wing/gear i n t e r f a c e v e l o c i t y (w/G VEL) through an a l g e b r a i c summing of t h e sink rate and t h e W/G VEL DEC. The CPU t h e n a c q u i r e s 'the instantaneous v a l u e s of t h e wing/gear i n t e r f a c e i n e r t i a f o r c e (FWG), t h e W/G VEL, and the strut p o s i t i o n (strut s t r o k e ) . With t h e s e v a l u e s t h e CPU calculates t h e k i n e t i c energy a t t h e wing/gear i n t e r f a c e ( K E ) and t h e remaining work c a p a b i l i t y of the shock strut (PE). The CPU compares t h e s e e n e r g i e s and, i f t h e PE i s less than the KE, loops back i n t h e program to o b t a i n t h e sink rate, updates t h e v a l u e s of F W G and W/G VEL DEC, and r e c a l c u l a t e s t h e energies. The CPU remains i n t h i s loop u n t i l t h e PE equals or exceeds t h e K F , . When PE > KE, t h e CPU stores t h e i n s t a n t a n e o u s value of t h e F W S as t h e l i m i t f o r c e command (LFC) . T o i n s u r e t h a t t h e i n i t i a l c o n t r o l e f f e c t on the gear is t h e removal of f l u i d , t h e CPU a c q u i r e s t h e FWG and compares it with t h e LFC. I f t h e F W G i s less than t h e LFC, t h e CPU continues t o sample updated values of the F W S and compare t h e v a l u e s with t h e LFC.

ACTIVE CONTROL When t h e F W 2 becomes g r e a t e r than t h e LFC, the CPU i n i t i a t e s a c t i v e c o n t r o l of t h e gear by d i s a b l i n g t h e bias-pressure loop (which maintains charging p r e s s u r e i n the gear) and enabling t h e servo loop. The servo loop then i n i t i a t e s t h e computation of t h e f o r c e e r r o r (Fa-LFC) and t r a n s m i t s a s i g n a l p r o p o r t i o n a l t o t h e f o r c e error to t h e servovalve. The CPU continues t o c o n t r o l t h e servovalve and, a d d i t i o n a l l y , c a l c u l a t e s t h e t r a n s i t i o n v e l o c i t y (TRANS VEL), calculates an updated value of t h e W/G VEL, and compares the t w o values. If t h e W/G VEL is g r e a t e r than t h e TRANS VEL, t h e CPU loops back i n t h e program, c a l c u l a t e s an updated value of t h e W/G VEL, and continues i n t h i s loop u n t i l t h e W/G VEL becomes less than t h e TRANS VEL. When t h e W/G VEL becomes less than TRANS VEL, t h e CPU i n i t i a t e s the t r a n s i t i o n phase by a c q u i r i n g t h e value of t h e LFC and decreases LFC a t t h e i n p u t design value of t h e t r a n s i t i o n rate.

TRANS I T I ON During t h e t r a n s i t i o n phase, t h e CeU continues the comparison of t h e updated values of t h e LFC and t h e F W S and c o n t r o l s the gear. The t r a n s i t i o n phase continues u n t i l t h e LFC becomes less than zero or t h e shock strut becomes f u l l y extended.

Control of t h e gear i s e s t a b l i s h e d on t h e basis t h a t t h e gear i s i n c o n t a c t with t h e landing surface and t h e shock strut is compressed. Therefore, t h e CPU a c q u i r e s t h e S t r u t p o s i t i o n and compares it with a strut threshold value of 1.0 c m ( 0 . 4 i n . 1 f o r t h i s i n v e s t i g a t i o n . If t h e strut p o s i t i o n is g r e a t e r than t h e t h r e s h o l d value, t h e CPU a c q u i r e s the LFC and compares it w i t h zero. I f t h e LFC is g r e a t e r than zero, t h e CPU loops back and continues c o n t r o l of t h e gear and t r a n s i t i o n of t h e LFC.

ROLL-OUT PHASE If t h e a i r c r a f t rebounds and t h e shock strut becomes f u l l y extended or t h e LFC becomes less than zero, t h e CPU i n i t i a t e s t h e roll-out phase; t h a t is, t h e LFC is Set i d e n t i c a l l y equal t o zero and t h e f o r c e feedback switch i s opened ( c o n t r o l d e a c t i - v a t e d ) . To i n s u r e t h a t t h e gear i s n o t active as long as it i s f u l l y extended and t o accommodate c o n t r o l during secondary impact, the mu g e t s the strut p o s i t i o n and compares it w i t h t h e threshold value.

If t h e strut p o s i t i o n i s n o t g r e a t e r than t h e t h r e s h o l d value, the CPU loops back, o b t a i n s an updated value of t h e strut p o s i t i o n and compares it with t h e t h r e s h o l d value.

m e n t h e strut p o s i t i o n becomes g r e a t e r than the threshold value, t h e CPU sets the t r i p p o i n t (a storage a d d r e s s ) t o the I

APPENDIX designed value of the r o l l - o u t LFC and compares it with t h e FW2. I f t h e F W G is not l e s s than the t r i p p o i n t , the CPU s e t s t h e LFC t o the r o l l - o u t value, c l o s e s t h e f o r c e feedback switch ( a c t i v a t e s c o n t r o l of t h e g e a r ) , s e t s t h e t r i p p o i n t t o t h e cutoff value of t h e LFC, Computes the force e r r o r , and c o n t r o l s the gear. The CPU continues t o c o n t r o l t h e gear i n t h i s mode u n t i l t h e FW2 becomes less than t h e LFC cutoff value of t h e t r i p point. The CPU then r e s e t s the t r i p p o i n t t o the LFC r o l l - o u t value, checks t o a s c e r t a i n if t h e FWG is negative, and i f so, compares t h e abso- l u t e value of the F W G with the t r i p point. If t h e a b s o l u t e value of the F W G is n o t l e s s than t h e t r i p p o i n t , the CPU sets t h e LFC t o t h e r o l l - o u t value, c l o s e s t h e f o r c e feedback switch, r e v e r s e s the sign of t h e LFC analog s i g n a l , s e t s t h e t r i p t h e LFC, computes t h e f o r c e e r r o r , and c o n t r o l s the p o i n t t o t h e cutoff value of gear. I f t h e absolute value of the FLG is less than t h e t r i p p o i n t , t h e CPU loops back t o t h e start of t h e r o l l - o u t phase. The CPU continues t o operate i n t h i s mode u n t i l the c o n t r o l l e r i s reset.

I

I TAKE-OFF MODE

If t h e s t r u t p o s i t i o n had been g r e a t e r than the TLTHRESH value following con- CPU would have t r a n s f e r r e d t o t h e take-off mode i n t h e computer t r o l l e r enable, t h e program. The CPU s e t s t h e l i g h t s and switches f o r the take-off mode and c o n t r o l s t h e gear i n t h e same manner a s t h e previously discussed landing r o l l - o u t phase. However, an a d d i t i o n a l strut p o s i t i o n t e s t is included following t h e negative FhG test, and i f t h e s t r u t p o s i t i o n i s g r e a t e r than t h e threshold value, t h e CPU continues t o operate as it does i n the landing r o l l - o u t phase. When t h e gear has become f u l l y extended a s it would a t l i f t - o f f , t h e s t r u t p o s i t i o n becomes l e s s than t h e threshold value and causes the CPU t o d i s a b l e the servo loop, enable t h e bias-pressure loop, and r e t u r n t o t h e start of t h e program.

APPENDIX 1 1 PE AND KE INTERRUPTS N 0 L I M I T FORCE AND SWITCHES

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shown are for gear employed i n t h i s investigation.

APPENDIX SET LFC = 2244N (500 LBF) CLOSE FORCE FEEDBACK

‘1 REVERSE SIGN OF LFC ANALOG

T K I P POINT= 1779N (400 LBF) I

c $ 2 d < T R I P POINT? < T R I P POINT?

-

TAKE-OFF REVERSE S I t i N SET T R I P POINT= Figure A1 .- Concluded.

6 5 REFERENCES 1. DC-10 Landing G e a r Modified. Aviat. Week Space Technol., vol. 98, no. 12, Mar. 19, 1973, p. 181.

2 . Ropelewski, Robert R. : Airbus T e s t Tempo Quickening. A v i a t . Week & Space Technol., vol. 98, no. 10, Mar. 5, 1973, pp. 32-35.

3. McGehee, John R . ; and Carden, Huey D. : A Mathematical Model of a n Active C o n t r o l Landing Gear f o r Load Control During Impact and Roll-Out. N A S A TN D-8080, 1976.

4. Ross, I r v i n g ; and Edson, Ralph: An E l e c t r o n i c C o n t r o l f o r a n E l e c t r o h y d r a u l i c A c t i v e C o n t r o l A i r c r a f t Lznding C-zsr. XASA CR-3113, 13?9.

5. Dreher, Robert C.; and yager, Thomas J.: F r i c t i o n C h a r a c t e r i s t i c s o f 2 0 x 4.4, Type VII, A i r c r a f t T i r e s Constructed With D i f f e r e n t Tread Rubber Compounds.

N A S A TN D-8252, 1976.

6 . F a s a n e l l a , Edwin L.; McGehee, John R.; and Pappas, M . Susan: Experimental and A n a l y t i c a l Determination of C h a r a c t e r i s t i c s A f f e c t i n g L i g h t A i r c r a f t Landing- G e a r Dynamics. N A S A T M X-3561, 1977.

7 . Standard f o r Metric P r a c t i c e . E 380-79, American SOC. T e s t i n g hi Mater., c. 1980.

SYMBOLS The u n i t s used f o r the p h y s i c a l q u a n t i t i e s d e f i n e d i n t h i s paper are given f i r s t i n t h e I n t e r n a t i o n a l System of U n i t s ( S I ) and p a r e n t h e t i c a l l y i n t h e U . S . Customary Units. Measurements and c a l c u l a t i o n s w e r e made i n U.S. Customary Units. F a c t o r s r e l a t i n g t h e two systems are given i n reference 7.

d d i s t a n c e between step bumps, m ( f t ) f frequency, Hz P strut h y d r a u l i c pressure, kPa ( p s i g ) V v e l o c i t y , m / s e c ( k n o t s ) W/G wing/gear i n t e r f a c e 0 p i t c h angle, deg Subscripts: a a c t i v e gear h h o r i z o n t a l P p a s s i v e gear V v e r t i c a l 2. Government Accession No.

1. Report No. 3. Recipient's Catalog No.

NASA TP-2042 I I

I 5. Report Date 4. Title and Subtitle

A u g u s t 1982 EXPERIMENTAL INVESTIGATION O F ACTIVE LOADS CONTROL FOR AIRCRAFT LANDING GEAR 6. Performing Organization Code

I I 505-44-33-0 1

8. Performing Organization Report No

I

John R. M c G e h e e and R o b e r t C. D r e h e r L- 15224 10. Work Unit No.

, 9. Performing Organization Name and Address 1 1 . Contract or Grant No.

NASA Langley Research C e n t e r H a m p t o n , VA 23665 13. Type of Report and Period Covered 12. Sponsoring Agency Name and Address Technical Paper N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n 14. Sponsoring Agency Code Washington, DC 20546 17. Key Words (Suggested by Author(s)) 18. Distribution Statement A i r c r a f t landing gear

U n c l a s s i f i e d - U n l i m i t e d

A c t i v e controls Landing loads Subject C a t e g o r y 05

1 22. Rice 19. Security Classif. (of this report1 20. Security Classif. (of this page)

1 21. NO. of pages

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