An experimental simulation study of four crosswind landing gear concepts

NASA T ECHNICAL NOTE

NASA TN D-7864

A N EXPERIMENTAL SIMULATION STUDY

OF FOUR CROSSWIND LANDING-GEAR CONCEPTS

Yundy M . Stubbs, Thomas A. Byrdsong,

and Robert K. Sleeper

Langley Research Center

Hamptolt, Vu. 23665

N A T I O N A L A E R O N A U T I C S A N D SPACE A D M I N I S T R A T I O N W A S H I N G T O N , D. C. M A R C H 1975 CROSSWIND LANDING-GEAR CONCEPTS NASA Langley Research Center Hampton, Va. 23665 National Aeronautics and Space Admi~listration An experinlental investigation w a s conducted i n o r d e r to evaluate s e v e r a l crosswind landing-gear concepts which have a potential application to tricycle-gear-configured, s h o r t take-off and landing (STOL) a i r c r a f t lariding a t c r a b o r heading angles up to 30°. In t h i s investigation, the landing g e a r s w e r e installed on a dynamic model which had a scaled Illass distribution and g e a r spacing but no aerody~lamic silllilarities when compared with a typical STOL a i r c r a f t . The lllodel w a s operated a s a f r e e body with radio-control steering and w a s launched onto a runway sloped laterally i n o r d e r to provide a simulated crosswind s i d e force.

During the landing rollout, the g e a r f o r c e s and the model trajectory w e r e measured and the various concepts w e r e colnpared with each other. Within the t e s t limitations, the landing- g e a r system, in which the g e a r s w e r e alined by the pilot and locked i n the direction of motion p r i o r to touchdown, gave the s n ~ o o t h e s t runout behavior with the vehicle maintaining i t s c r a b angle throughout the landing runout.

Aircraft landing Unclassified - Unlimited Crosswind landing gear Experimental model For sale bv the National Technical Information Service, Springfield, Virginia 22151 AN EXPERIMENTAL SIMULATION STUDY OF FOUR CROSWIND L m D I N G - G E A R CONCEPTS Sandy M. Stubbs, Thomas A. Byrdsong, and Robert K. Sleeper Langley Research Center SUMMARY An experimental investigation was conducted in o r d e r to evaluate several crosswind landing-gear concepts which have a potential application to tricycle-gear-configured, s h o r t take-off and landing (STOL) aircraft landing a t crab o r heading angles up to 30°.

In this investigation, the landing g e a r s were installed on a dynamic model which had a scaled m a s s distribution and gear spacing but no aerodynamic similarities when compared with a typical STOL aircraft. The model was operated a s a f r e e body with radio-control steering and was launched onto a runway sloped laterally in order to provide a simulated crosswind side force. During the landing rollout, the gear forces and the model trajec- tory w e r e measured and the various concepts were compared with each other. Within the t e s t limitations, the landing-gear system, in which the gears were alined by the pilot and locked in the direction of motion prior to touchdown, gave the snloothest runout behavior with the vehicle maintaining its c r a b angle throughout the landing runout.

INTRODUCTION Airports constructed f o r short take-off and landing (STOL) aircraft will provide fewer choices f o r runway headings than conventional airports do, and thus will have the potential of exposing tne aircraft to crosswinds which could impede landing and, possibly, take-off operations. In addition, STOL aircraft have typically low landing and take-off speeds which further contribute to their vulnerability to crosswinds. It is conceivable that under sonie conditions the velocity of the crosswind could be a s high a s 50 percent of the touchdown speed of such aircraft. T h e r e a r e several techniques employed by pilots to land an airplane equipped with conventional gear under the influence of a crosswind.

T h e most preferred technique is to crab, o r head the airplane into the wind during the approach, and to perform a transition maneuver (decrabbing o r slipping the aircraft) immediately prior to touchdown. This transition maneuver and the subsequent rollout could pose problems to STOL aircraft where c r a b angles up to 30' a r e encountered, These problems include: excessive gear loading and passenger discomfort associated with an imperfect decrab maneuver; an increased worlrload required of the pilot in regu- lating the powered lift, monitoring airspeed, decrabbing the airplane, and so forth; and controlling the a i r c r a f t once on the ground to keep itwithin the confines of the runway.

Some of these problems w e r e emphasized in a recent simulator study conducted on a STOL transport; they a r e discussed in reference 1. In that study, the pilots concluded that during landing, a continuous wings-level crabbed touchdown and crabbed rollout was preferred to conventional techniques. However, to provide a n airplane with a crabbed touchdown and rollout capability would necessitate an unconventional o r crosswind landing- gear system.

Several landing-gear concepts, proposed in the late 1940's and early 19501s, would permit an aircraft touchdown in a crabbed attitude. These concepts, described in refer- ences 2 to 6, were originally developed f o r tail-wheel aircraft and some flight experience was obtained with various concepts on several aircraft. One of the concepts is currently employed on the B-52 and C-5A aircraft, but with a 20' crab-angle limitation. Compara- tive t e s t s to establish whether this concept is the best approach f o r tricycle-geared STOL aircraft, operating at c r a b angles up to 30°, have not been conducted.

The purpose of this paper is to present the results of an experimental investigation conducted in o r d e r to evaluate various crosswind landing-gear concepts which have appli- cation to tricycle-gear-configured STOL aircraft landing a t crab angles up t o 30'. In this investigation, four different crosswind gear concepts utilizing a free-body, radio- controlled, dynamic model on a runway sloped laterally to simulate a crosswind side force were tested. Different steering techniques were used for the gear concepts. The model track and heading of the four concepts were compared with one another to show the behavioral characteristic of each gear concept during the landing runout. The basis f o r the evaluation of the various gear concepts was minimum vehicle lateral excursions and pilot effort.

APPARATUS AND PROCEDURE The crosswind landing study was conducted in an enclosed facility using a simple radio-controlled dynamic model having a tricycle landing-gear arrangement. A side wind was simulated by using a laterally sloping runway; otherwise, no attempt w a s made to simulate aircraft aerodynamic effects on the model.

The study was conducted in two phases. The f i r s t phase employed a noninstrumented vehicle to obtain qualitative data for the various crosswind landing-gear concepts; prelim- inary results from that phase were discussed in reference 7. F o r the second phase, the vehicle was instrumented to obtain rneasuieemerrts of gear forces, gear steering angle, and wheel speed.

Description of Model The model used in the investigation Was patterlied a f t e r a L . S T O E - ~ ~ ~ ~ aircraft, ~h~ nlodel was not scaled aerodynamic all^; however, m a s s p r o p e r ties and gear spacing were simulated.

Basic vehicle.- A sketch of t h e model with the p e r t i n e n t dinlensions is given in fig-.

ure 1 and photographs of both the noninstrumented and i n s t r u m e n t e d versions a r e pre- sented in figure 2. Longitudinal and l a t e r a l aluminum a n g l e s were attached to a solid- model body i n order to provide a m e a n s f o r mounting the ballast weights f o r obtaining the desired inertia properties. The pertinent m a s s ~ a r a l n e t e r s of the instrumented model The t r i c y c l e landing-gear a r r a n g e m e n t used on the llnodel was a r e presented in table I.

Detailed photographs of the landing-gear components composed of three identical gears.

a r e shown in figures 3 to 8 (and f u r t h e r discussion of the d e s i g n is given in the appendix).

Each gear was capable of being s t e e r e d by radio-controlled Servomechanisms, locked in any position, o r f r e e swiveling within the limits provided by mechanical stops. In addition, each gear possessed a simple d r a g brake which could b e energized by a radio-control link.

sizes of landing-gear forks w e r e used t o provide v a r i o u s amounts of trail o r caster (offset distance of the t i r e behind the swivel axis). B e n c h t e s t s were made to determine the best t r a i l location f o r each landing-gear concept. P n e u m a t i c , model airplme-type tires, 1 1 . 4 cm ( 4 . 5 in.) in diameter, were used on e a c h gear.

Crosswind gear concepts.- Four crosswind landing- gear concepts were examined in this study. Figure 9 presents a schematic illustration of each concept together wi& a brief explanation of its operating technique both prior to and subsequent to touchdown.

Concept A utilizes free-swiveling gears prior to touchdown in order to achieve an alinement with the direction of the motion on contact.

After alinement, the gears a r e either locked o r steered. The steering Can be accomplished by using only the nose gear, both the nose and the main g e a r s together, o r the nose and the main gears independently, A trail is needed on this configuration in order to a i d i n the rapid alinement of the gear when the t i r e contacts the runway.

all gears a r e f r e e to swivel p r i o r t o touchdown, but mechanical stops, In concept s e t on the main gear, prevent outward swiveling.

T h e purpose of the stops is to facilitate the steering by developing side forces on the upwind main-gear wheel without actively having to lock the main gear at touchdown a s f o r concept A.

For Concept B, the downwind wheel alines with the direction of motion but the upwind wheel is held against the stop until Should the vehicle decrab beyond 0°, a downwind side force is the vehicle decrabs to 0°, develop&. a s the downwind wheel is then held against its stop. The steering is accorn- piished only through the nose gear.

concept l 2 alego allows dl gears to swivel f r e e l y prior to totzchdown, but differs from concepts A and B in t h a h armsbaa= li&age c o n ~ e ~ t s Elie forks on the nlain gear so they The geometry of the crossbar is such that a vehicle crab attitude will act together, induces a main wheel toe-out which varies proportionally wit11 the crab angle, For this concept, toe-out i s defined a s occurring when the front of the main-gear t i r e s a r e farther apart than the r e a r of the tires. It is theorized in concept C that the more heavily loaded dowr~wind wheel will aline itself with the direction of motion and the more lightly loaded upwind wheel will toe-out, and thereby produce a small upwind side f o r c e to facilitate the steering. For the tests reported in this paper, the crossbar linkage was s e t to provide a 3 O toe-out at a c r a b angle of 30°. Mechanical stops were added to this concept to r e s t r i c t the main-gear swivel angle to -+30°.

In concept D the pilot, prior to touchdown, p r e s e t s all gears in the direction of motion, Since the g e a r s require no self-alining mechanism a t touchdown, no trail is needed f o r this concept. As with concept A, directional control during rollout can be accomplished by steering the nose gear only o r by steering all gears.

Steering mechanism.- The model was steered remotely using radio-control equip- ment. Each gear was equipped with a single s e r v o n ~ o t o r to engage a clutch which con- verted the landing gear f r o m a free-swiveling to a steerable mode. Dual servomecha- nisms w e r e used on each in order to provide the necessary steering torque. Each gear was steered by a separate transmitter-control stick and a mechanical linkage was inserted between the sticks when it was desirable to s t e e r all gears together o r to s t e e r the nose and main gears independently. (Steering the nose gear only o r steering all g e a r s in the s a m e direction concurrently required only one hand; steering independently required two The radio-control system was hands, one for the nose gear and one f o r the main gears.)

proportional; that is, the servomechanisms displaced proportionally to the control-stick displacement.

Runway and Launch Apparatus The runway and launch apparatus a r e shown in figure 10. The runway, 61 m (200 ft) long and 4.1 m (13.6 ft) wide, was covered with plywood in an attempt to achieve a smooth surf ace. The runway was inclined laterally 4.5O to simulate a crosswind side f o r c e on the model (see the following sketch) estimated to be equivalent to that which would occur in a 90° crosswind of one-half of the aircraft-landing velocity: 15.6 N (3.51 lbf) Side force

' Horizontal

I i

698 N (44.62 lbf) 199 N (44-76 lbf) Weight Normal force The launch apparatus a s shown in figure 10 consisted of a model-supporting carriage nzounted on a m o n o r ~ l , A c o n t i n u o ~ ~ s electrically powered, winch-driven cable was attached to the carriage and was used to accelerate the model and carriage to the desired horizontal velocity. Near the end of the monorail, the drive cable was separated f r o m the c a r r i a g e and the carriage was arrested, allowing the model to slip f r e e and continue down the runway.

Instrumentation and Measured P a r a m e t e r s P a r a m e t e r s measured during the course of each test consisted of the vehicle track and heading, both acquired f r o m motion-picture coverage, and the touchdown velocity a s determined f r o m the speed of the launch carriage immediately prior to model release.

The instrumented model was equipped to measure the time histories of the forces, the steering torque, the steering angle, and the wheel angular velocity of each gear, together with the vehicle normal acceleration.

Normal, longitudinal and lateral forces, and steering torque were measured on each landing gear using a six-component force balance specially developed f o r this model. The balance rotated with the gear assembly and thus the measured f o r c e s a r e oriented with the gear. To permit the resolution of the gear f o r c e s along the vehicle body axis, the s t e e r - ing angle of each gear with respect to the body was measured using a variable potentiom- e t e r geared to the shaft a s shown in figure 4. A continuous wheel angular-velocity sig- nal was derived from the frequency of pulses generated by an optical device which sensed each twelfth of a wheel revolution. (See fig. 8.) The normal acceleration of the vehicle w a s measured at the vehicle center of gravity by a piezoresistive, strain-gage accelerom- eter. Signals f r o m these data-acquisition devices were multiplexed onboard the model and transmitted through four small coaxial cables to two frequency-modulated tape record- e r s . All the taped data except the wheel angular velocity were filtered in o r d e r to atten- uate frequencies above 100 Hz. The wheel angular-velocity signal was attenuated above 1000 Hz and, in the data processing, all data were attenuated above 110 Hz and digitized at a sample rate of 500 samples/sec.

The force and torque loading applied to the balance and defined a s positive a r e shown i n figure 11. A positive steering angle is also shown. ' The data were corrected f o r balance interactions but not corrected f o r gear orientation. For instance, if the model is not level, normal force affects the longitudinal and lateral forces. The sign convention f o r the lateral and longitudinal model position and the heading angle a r e shown in figure 12 and an upward acceleration was considered positive.

Motion- picture coverage was obtained f r o m six overhead c a m e r a s positioned along the runway in order to determine the vehicle runout trajectory and the model-to-runway heading angle. Two additional movie c a m e r a s recorded the entire runout and a video recorder was used to provide an immediate review of the test conditions and landing behavior, A time-code signal was recorded on the instrumentation tapes and along the edge of the rnovie film in order to facilitate data reduction f r o m the tapes and to provide synchronization of the tape and the film data.

The model was positioned on the carriage close to the runway in o r d e r to minimize the vertical velocity at touchdown, The landing speed was determined electronically f r o m the speed of the launch carriage during a coast phase which existed immediately p r i o r to release of the model f r o m the carriage.

Testing Technique The testing technique involved launching the model as a f r e e body in a crabbed atti- tude onto a laterally sloping runway. The behavior of the model to various steering inputs a s it freely rolled to a stop was evaluated.

Before each run, the t i r e pressure, the t r a i l position, and the initial model heading o r c r a b angle were s e t and the gears were visually alined with the runway. After launch, the model became a f r e e body steered to a complete stop through radio control by the operator a t a position adjacent to the launch point.

The brakes on the model were applied by the operator for only high-speed runs.

A number of r u n s w e r e conducted using each of the landing-gear concepts with vari- ations made in the initial c r a b angle, the landing speed, the initial gear alinement, the t i r e inflation pressure, and the steering technique. Most of the runs were made with the model preset on the carriage at a c r a b angle of 30°, which simulated an aircraft landing i n a 90° crosswind equal to one-half of the landing speed. Similarly, most r u n s w e r e initiated with model landing speeds of approximately 6.1 m/s (20 ft/s). These landing speeds simulated a full-scale velocity of 19 m/s (63 ft/s), and seemed to provide the most authentic simulation of the last two-thirds of a landing runout since aerodynamics would be l e s s effective in this period. The lower speed also permitted better control and, hence, better differentiation between the various concepts that otherwise might not be controllable f o r this simulation a t higher speeds. Another advantage of the lower speed was that brak.- ing, which could contribute complicating effects into the steering behavior, was not required. However, a few landings were made a t model landing speeds of 11.4 m/s (37.4 ft/s) and brakes were applied.

RESULTS AND DISCUSSION Limitations to Simulation Early in the investigation several problems emerged that were inherent in relating the model results to those of a full-scale aircraft. One of the most apparent was related to the need for abnormally quick pilot response, For example, dimensional ec~uivalence requires that the pilot's response f o r a l/l0-scale model be over three times that of a pilot of a full-scale aircraft. The response time lags of the radio-control system also aggravated this condition. These problems were compounded by the fact that the pilot was not in the vehicle where he could sense motion cues but, instead, was positioned near the touchdown point of the model where his visual cues diminished with model runout.

Another limitation in the simulation is that there was no thrust and aerodynamic control on the model to keep the horizontal forces in balance a t touchdown and until s t e e r - ing of the landing g e a r s could begin. F o r a STOL aircraft touching down in a crabbed attitude with a crosswind landing gear, it is assumed that the horizontal forces will be in equilibrium. The model, however, touching down on a laterally sloping runway, h a s an unbalance in f o r c e s that immediately starts a downwind drift which continues until the steering of the landing gears can be initiated. Because of the pilot response and the lags i n the model radio-control mechanisms, when steering is finally attempted, the momentum of a downwind-drift velocity and any yaw angular velocity that h a s been initiated must be overcome. In o r d e r to minimize initial drift and yaw changes of the f r e e swiveling con- cepts (concepts A, B, and C), some tests were made with the g e a r s alined with the direc- tion of motion and the steering engaged prior to touchdown, and thus the free-swiveling feature at touchdown was eliminated.

An additional limitation of the simulation was that the side force developed by the laterally sloping runway acts at the vehicle center of gravity instead of at the aerodynamic center of pressure. Therefore, there is no weathervaning moment such as might occur on an actual aircraft. This runway slope was maintained constant over the entire length of the runway and thus produced a constant side force perpendicular to the runway center line. In an actual aircraft landing, however, the side force, resulting from aerodynamics, v a r i e s during rollout because of changes in the windspeed and changes in the aircraft ground speed and heading with respect to the resultant a i r s t r e a m direction.

Although these problems with model testing place limitations on the direct applica- tion of the model test results to full-scale aircraft, these shortcomings apply to all four concepts investigated and a comparison of the relative m e r i t s of the various configurations appears to be justifiable.

T e s t Criterion The basic criterion used in comparing the various landing-gear configurations was that the vehicle experience a minimum lateral displacement during rollout on the runway.

Another requirement was that the vehicle have a minimum, o r a t least a slow yaw attitude change during runout; that i s , a vehicle touching down a t a 30' crab angle would run out at a 30" crab angle o r would decrab slowly.

Noninstrumented Model T e s t s Results in this section, obtained from over 60 runs, a r e qualitative in nature, They a r e presented in t e r m s of the experience with the various gear concepts.

Concept A.- In the initial t e s t s with concept A, the landing gears were f r e e to swivel and, upon ground contact, to aline themselves with the direction of motion.

In o r d e r to obtain alinement, some amount of trail was needed but it was found that there w a s a range of trail values that produced shimmy problems at a given tire-inflation pressure. After several tests, a trail of 1.3 times the t i r e radius and a tire-inflation p r e s s u r e of 60 k P a (9 psi) were selected in order to eliminate the shimmy and to provide an adequate aline- ment capability. On the lighter loaded nose gear, a t r a i l equal to one t i r e radius and an inflation p r e s s u r e of 55 k P a (8 psi) were found to minimize the shimmy. In both cases, lower inflation p r e s s u r e s with o r without shorter trail resulted in a moderate to a severe shimmy. Unfortunately, the need f o r long trail to reduce the shimmy imposed severe demands upon the available steering torque. Initial t e s t s with concept 4 and other con- cepts that were f r e e to swivel prior to landing, gave poor landing behavior. When landings were made with all the gears prealined with the direction of motion and the steering clutch engaged prior to touchdown, very good crabbed runouts w e r e obtained by utilizing only the nose-gear steering. When the steering is engaged prior to touchdown, concept A is simi- l a r to concept D except f o r the trail and the flexibility of the steering mechanism. The engagement of the steering clutch eliminates the free-swiveling feature a t touchdown.

Figure 13(a) shows sequence photographs of a typical run using concept A and only nose- gear steering. The runout was good, the vehicle maintaining a track very near the runway center line.

Figure 13(b) is a sequence of photographs showing a landing with steering control attempted by both nose and main g e a r s turning equally and simultaneously, using one- steering input (one-hand control). When such steering w a s attempted, the r e s u l t s were not satisfactory because of either a slight preset nlisalinement of the g e a r s with respect to each other, o r a misalinement of the gears caused by uneven loading. The slight mis- alinement produced a slow continuing yaw change in the vehicle and, although the vehicle could be displaced laterally on the runway with steering, it would continue yawing until its gears hit mechanical stops, whereupon the vehicle diverged from the runway.

Steering the nose and main gears independently with two controls was also unsatis- factory even though some good runs were obtained. Figure 13(c) shows sequence photo- graphs utilizing this independent steering. When differential inputs were made, such a s steering nose gear windward and main gear leeward, the yawing motions w e r e very rapid and confusing to the pilot, with an occasional loss of control a s illustrated i n the photo- graphs, No attempt was made to s t e e r the nose and main gears in opposite directions with a single steering input by the pilot, With reduced sensitivity and ad&tioraal refinements, steering the nose and main gears independently might prove feasible; however, it was 2 . 2011. corlsidered an unnecessary eomplic 'c* Concept Be- The main-landing gear of concept B was f r e e to swivel only inward to aline with the direction of motion on ground contact. T o facilitate the gear alinement, trail again was used - the s a m e amount of t r a i l a s f o r concept A. When the vehicle landed crabbed o r yawed into the wind, the nose gear and downwind main gear alined with the direction of motion. However, the upwind main gear was forced against a stop which kept it alined with the longitudinal axis of the model and produced a side force aft of the Only nosewheel steering was used with concept B, and center of gravity into the wind.

f o r the test shown by the sequence photographs in figure 14(a) it was actuated after touch- down. F o r the sequence shown, a large windward side force was developed by the upwind t i r e alined at a 30° yaw angle with respect to the direction of motion. The large side f o r c e acting behind the vehicle center of gravity produced a large decrabbing o r counter- clockwise torque and a violent decrabbing motion. The angular nlomentum generated by the rapid decrabbing rendered the model uncontrollable. It was felt that the violent decrabbing motion caused by the upwind wheel could be reduced, but not altogether elim- inated, on an actual aircraft by directional stability and rudder control.

Additional runs were made with concept B, wherein the nose gear and downwind main gear w e r e prealined with the direction of motion and the nose steering clutch w a s engaged prior to touchdown. Photographic results of this test a r e presented in fig- u r e 14(b). The model decrabbed rapidly a s shown between the f i r s t and second photo- graphs and full right-steering input (300) was needed throughout the remaining runout in order to maintain control. This type of steering imput was marginal and was considered to be unsatisfactory.

Concept C.- As with concepts A and B, the landing gear of concept C was f r e e to swivel prior to contact and, like that of concept B, only nosewheel steering was available.

The t r a i l used to achieve alinement was the s a m e a s that for concepts A and B. Since both main g e a r s were tied together by a crossbar linkage, it would be expected that the downwind gear, which was more heavily loaded, would aline itself with the direction of motion. The more lightly loaded upwind gear would toe-out (3O for 30° c r a b angles) and produce a small force in the windward direction to facilitate steering.

No satisfactory runs were made when the steering clutch was engaged after contact.

When the gear was alined with the direction of motion and the steering clutch engaged p r i o r to contact, good runs were obtained a t a 30° c r a b angle. (See fig. 15(a).) However, i t was necessary in those tests to s e t mechanical stops on the main gear at 30°; otherwise, during runout the tail would continue to swing downwind. To support these findings several landings were made a t 0" yaw (fig. 15(b)), An undesirable yaw motion was observed for all landings until the maill gear hit tile 3O0 stops, as is shown to occur during the run in f r a m e 4 and again in f r a m e 12 of figure 15(b). Throughout this crabbing maneu- ver, i t was found that the nose gear must be steered or the model would be uncontrollable, The toe-out of this concept did not produce enough side force to facilitate steering and there was no directional control unless the main gear was against a mechanical stop.

Concept D.- F o r a landing using concept D, it is assumed that a mechanism would be provided to permit the pilot to aline all three landing gears with the direction of motion and to lock them in position prior to touchdown. Since the self-alining feature was not needed, no trail was used f o r this concept. With no trail on the landing gears, there was no shimmy problem, the torques required to s t e e r the model were considerably reduced, and the steering was quite responsive. Thus, in effect, concept D is essentially the s a m e a s concept A when the gear of A is alined and locked in the direction of motion; however, concept D lacks the shimmy tendency and the severe steering-torque demands that occur f o r the long trail required f o r concept A. Good runs were obtained immediately by using concept D with nose-gear steering. Figure 16 shows a typical run where the model touched down in a 300 c r a b and a straight, uneventful runout followed.

An interesting observation with this concept was that, even though the model was crabbed 30° and the gear lined up with the direction of motion, the model weathervaned o r crabbed even f a r t h e r because of the uneven loading of the main gears. With no steering inputs, the vehicle on touchdown moved leeward slightly, then weathervaned, and started a slow windward drift. Small leeward steering inputs a r e needed for control, and control i s relatively easy. Several runs were made with a 5O preset e r r o r in the gear alinement with the direction of motion in order to simulate pilot e r r o r . The vehicle was controll- able but not without some initial weaving down the runway. When lo0 e r r o r s i n alinement were tried, the vehicle was still controllable but initial lateral motions tended to be exces- sive and some t i r e squeal was noted. With aerodynamic controls, however, i t was f e l t that landings with even larger alinement e r r o r s could be satisfactorily made.

As was observed with concept A, steering all g e a r s of concept D together was unsat- isfactory because a slow ground loop resulted. Steering both nose gear and main g e a r s independently was tried and a satisfactory run was obtained. However, independent steer- ing increased the sensitivity of an already adequately sensitive steering system and added an unnecessary complication.

Instrumented Model Tests Eight representative c a s e s were chosen f r o m over 70 instrumented t e s t s f o r quan- titative discussion. The c a s e s a r e summarized in table 11 and detailed time histories of these c a s e s have been included in figures 17 to 24, 4 1 tests were made with the initial c r a b angle p r e s e t to 300, the gears alined with the runway, and the steering clutch engaged F o r all but the last case of table PI, the lower (not f r e e swiveling) prior to touchdown, landing speed, which simulated the last two-thirds of a landing rollout, was used and no braking was applied. These c a s e s include effects due to two t i r e p r e s s u r e s and two steer- ing techniques. For gear concepts A, B, and C, the wheels were positioned in their forks t o have 0.052 m (2.04 in.) t r a i l on the nose gear acd 0.071 m (2.79 in.) t r a i l on the main gears; smaller forks without trail were used in concept D. To aid in interpreting the r e s u l t s from the instrumented tests, the following paragraphs discuss in detail (1) a typi- c a l time history and (2) the maximum value model data for the c a s e s presented.

Time histories.- The time histories derived f r o m case 1 and presented in figure 17 have been arbitrarily selected f o r discussion. These histories describe the rollout characteristics of concept A utilizing only nose-gear steering. Force data a r e presented i n figure 17(a) and acceleration, steering angle (gear position), wheel angular velocity, vehicle displacements, and heading data a r e presented in figure 17(b). The run starting time f o r the time-history data was determined by the first indication of force on one of the t h r e e landing gears. Dashed-line fairings of the maximum-force data (fig. 17(a)) a r e an attempt to eliminate structural oscillations caused by model and gear elasticity. The maximum values obtained f r o m such fairings a r e presented in table 11. Because the steering inputs a r e small in this run, the longitudinal and lateral f o r c e s during the run- out w e r e relatively constant.

The normal acceleration shown in figure 17(b) has oscillations a t approximately the s a m e frequency a s the normal-force t r a c e s of the main gears shown in figure 17(a). The oscillations in the steering angle at the s t a r t indicate a brief shimmy of the nose gear before the steering becomes steady. The right and left main-gear steering angles remained constant throughout the run because they w e r e locked. Except f o r initial trans- ients in the wheel angular-velocity traces, a smooth velocity decay is shown f o r each wheel. The good controllability of this run is reflected in the smoothness of the lateral displacement and the heading-angles traces. The greatest lateral displacement f r o m the runway center line was approximately 0.5 m (20 in.) and heading angle changes were small and gradual indicating a smooth runout.

Maximum value data.- A summary of the maximum values obtained f r o m the eight time-history r e c o r d s (figs. 17 to 24) is given in tabular form in table E[.

Figure 25 is a b a r chart of runout distance and excursions in the vehicle lateral displacement and the heading angle f o r the four concepts. Windward lateral and yawing excursions w e r e con- sidered to be positive. In general, concepts A and D have smoother runouts than do con- cepts B and C. In the trajectory of concept B the vehicle decrabbed to 0°, which required the steering=control stick to be held hard over against a stop ir? order to maintain control, CONCLUDING REMARW An experimental investigation was conducted to evaluate various crosswind landing- gear concepts which have potential application to tricycle-gear-configured, short take-off and landing (STOL) aircraft landing a t c r a b angles up to 30°, Four crosswind gear con- cepts were tested by utilizing a free-body, radio-controlled model having a scaled m a s s distribution and gear spacing but no aerodynamic similarities. The model was landed on a runway sloped laterally to simulate a crosswind side force, Relative comparisons of the concepts were made but certain limitations were found that w e r e inherent in relating the model results to those of a full-scale aircraft. The more significant of these were the lack of motion cues to the pilot, the requirement that the pilot respond three times a s fast a s is necessary for the full-scale aircraft, and the lack of aerodynamic control at touchdown.

Of the concepts examined, concept D, which required the pilot to aline all three gears in the direction of motion prior to touchdown and to s t e e r with the nose gear about the pre- s e t landing-gear position, gave the best performance. Satisfactory runs w e r e consistently made with this system even when the g e a r s were misalined up to 10' a t touchdown.

Langley Research Center, National Aeronautics and Space Administration, Hampton, Va., January 20, 1975.

CROSSWIND LMDTNG-GEAR DESIGN Each of the three landing gears of the short take-off and landing crosswind model w a s identical in design and construction, A photograph of a gear is presented in figure 3 where the principal subassemblies a r e identified. The upper section constitutes the steering-control system, the middle section defines that portion of the gear which is attached to the body of the model, and the lower section comprises the force balance and the tire, wheel, and brake assembly. A closeup photograph of the upper and midsections with callouts of the visible components is presented in figure 4 and all component p a r t s f o r these sections a r e pictured in figures 5 and 6. The lower section is shown in figure 7, and i t s components in figure 8. The three sections a r e connected by a steering shaft, shown in figure 6, which transmits any applied steering torque to the tire. To convert the gear from a f r e e swiveling to a steerable mode, the steering-actuator servomechanism identified in figure 4 was used to engage a spring-loaded clutch. Also shown in figure 4 is the gear position potentiometer for landing-gear yaw-position measurements. F o r those r u n s using the restrained mode for the main gear, a mechanical-stop plate and pin (fig. 4) were used to fix the gear.

The force balance (figs. 7 and 8) was a lightweight, six-component balance but, because of the other instrumentation limitations, only four components were recorded.

The wheel fork (fig. 8) was fabricated to provide three possible t r a i l positions and a sim- ple spring-loaded friction brake was used to provide a fixed brake force f o r the high- speed runs. The angular velocity pickup, also shown in figure 8, was mounted on the wheel fork in order to monitor wheel angular velocity.

REFERENCES 1. Grantham, William D,; Nguyen, Luat T.; Patton, J a ~ n e s M., Jr,; Deal, P e r r y E.; Champine, Robert A.; and Carter, C. Robert: Fixed-Base Simulator Study of an Externally Blown Flap STOL Transport Airplane During Approach and Landing.

NASA TN D-6898, 1972.

2. Anon.: Landing Gear Load Measurements on DC-3 Airplane Equipped With Cross-Wind Landing Gear. Tech. Memo. Rep. WCNS 52-25, Wright Air Develop. Center, U.S. Air Force, June 30, 1952.

T T 34-61 (PTR AE-633011), 3. Anon.: Evaluation of Geisse Cross-Wind Landing Gear.

U.S. Naval Air T e s t Center, Aug. 15, 1952.

Service T e s t of T-6 Goodyear Cross-Wind Landing Gear. Mem. Rep.

4. Anon.: No. MCRFT-2325, Air Materiel Command, U.S. Air Force, Dec. 18, 1950.

5. Anderson, Seth B.; Gadeberg, Burnett L.; and McAvoy, William H.: Effect of a 90° Cross-Wind on the Take-Off Distance of a Light Airplane Equipped With a Cross- Wind Landing Gear. NACA TN 1898, 1949.

6. Breuhaus, W. 0.; and James, C. W.: Taxi, Take-Off, and Landing T e s t s of an Ercoupe Airplane Equipped With a Cross-Wind Landing Gear. Rep. No. BC-717-F-1, Cornell Aeronaut. Lab., Inc., Aug. 28, 1951.

7. Stubbs, Sandy M.; and Byrdsong, Thomas A.: Model Studies of Crosswind Landing- Gear Configurations f o r STOL Aircraft. 8th Aerospace Mechanisms Symposium, NASA TM X-2934, 1973, pp. 145-154.

TABLE I.- MASS PARAMETERS OF INSTRUMENTED CROSSWIND MODEL

SI / U.S. Customary

Complete vehicle (nominal) Units Units 1.39 slugs Mass . . . . . . . . . . . . . . . . . . . .

1.39 slug-ft2 Roll m a s s moment of inertia . . . . . .

1.51 slug-ft2 Pitch m a s s moment of inertia . . . . . .

2.64 slug-ft2 Yaw m a s s moment of inertia . . . . . .

Components: 0.04961 slug Gear m a s s , long fork . . . . . . . . . .

0.04488 slug Gear m a s s , short fork . . . . . . . . . .

0.01513 slug T i r e m a s s . . . . . . . . . . . . . . . .

TABLE 11. - SUMMARY OF TEST CONDITIONS AND RESULTS - Landing-gear t r a i l T i r e p r e s s u r e Touchdown Initial Aircraft/runway Crosswind velocity, c r a b heading angle Case gear Nose, Right main, Nose, Right main, Left main, technique Left main, m / s angle, concept m m kPa m k P a (ft/s) deg (lbf/in2) (lbf/in2) (in.) (in.) (in.) (1&%2) 0.052 0.071 0.071 (2.04) (2.79) (2.79) '0.071 0.052 0.071 (2.04) (2.79) (2.79) 0.052 0.071 0.071 (2.04) (2.79) (2.79) C Nose gear 0.052 0.071 0.071 (2.04) (2.79) (2.79) Nose gear 0 0 0 (0) (0) (0) Nose gear 0 0 0 (0) (0) (0) Nose gear 0 0 0 (0) (0) (0) Nose gear 0 0 0

(37.4) 1 (10)

(0) (0) (0) Steering both nose and main g e a r s independently.

TABLE II.- SUMMARY O F TEST CONDITIONS AND RESULTS - Continued Maximum nose gear forces Maximum right main gear forces aximum left main gear forces Crosswind Normal, Normal, Longitudinal, Lateral, S $ , " g T ; ' , " : gear N N N N concept N -m (lbf) (lbf) (Ibf) (in/lbf) (lbf) ( - 14) (2.8) (-7.4)

TABLE 11.- SUMMARY OF TEST CONDITIONS AND m S U L T S - Concluded

Crosswind Remarks Notes Case gear concept 1 A Good runout. Simulated a free-swiveling g e a r p r i o r to touchdown.

Large lateral displacements and heading Steering both nose and main gear changes. independently.

Abrupt motions a t touchdown, runout barely Main g e a r s swivel limited by stops.

controllable.

I ~ I I

Crossbar linkage between main gears.

D / Good runout. 1 All g e a r s alined with direction of motion. /

1 5 /

1 6 1 D ( Abrupt motions at touchdown, but controllable. / All g e a r s misalined leeward lo0. !

I Abrupt motions a t touchdown, but controllable.

All g e a r s misalined windward lo0.

1 7 1 D

1 8 1 D 1 Model pilot response inadequate for initial I High velocity run with brakes.

i

speed, but runout was satisfactory.

Figure 1. - Crosswind model configuration. Dimensions a r e given in meters and parenthetically in inches.

L- 72-4848.1 (a) Noninstrumented model.

L-73-5475.1 (b) Instrumented nlodel.

Figure 2. - Crosswind landing-gear test models.

-111-face (simulated) Figure 3,- Model landing gear assembly.

L-74-290.1 Figure 4.- Upper and midsections of landing-gear assembly.

Figure 5.- Components of upper section of landing-gear assembly.

ism Figure 7.- Lower section of landing-gear assembly.

C;, r- c; Figure 11.- Positive direction of gear forces.

Figure 12.- Crab- and heading-angle definitions.

L-75-129 (a) Nose-gear steering only.

Figure 13.- Sequence photographs of landing runout f o r model with concept A.

1 3 14 15 L-75-130 (b) Steering all g e a r s together.

Figure 13. - Continued.

14 15 L-75-131 .ent steering of nose and main gears.

(c) Independ Figure 13. - Concluded.

1.6 17 18 L-75-132 (a) Steering actuated after touchdown.

Figure 14.- Sequence photographs of landing runout for model with concept B.

L-75-133 touchdown.

(b) St€ :ering actuated prior to

Figure 14. - Conclude

13 14 15 L-75-134 (a) Model landed at 30' crab.

Figure 15.- Sequence photographs of the landing runout for model with concept C.

(b) Model landed a t 0 ' c r a b F i g u r e 1 5 . - Concluded.

13 14 Figure 16.- Sequence photographs of typical landing rul nout for model with concept D.

'sxa3aurex-ed d x o p a [ - e q - a p ~ y a pu-e 'spaads TaayM 'sal2u-e 2u;xaa~s xzaf) (q) U ' l A l 'Ni -~ - -- - - ..

'IN3U3 UldSlO --..- -- = - 1 I N U3387 SIO

-- 1nn1lu -~~ - - -- i n n 3 l d i I Oh 8 LBF >-5 EOSE -uo GERR 5 I O E

DRFG FORE. . . 0 , , , , , - ~ ~ I F F

, , , . + . . . u A .

LEFT PlA!N GERR -90- q G r (a) Gear forces.

Figure 18.- Time histories of case 2 (concept A).

SIGHT W I N M I E L UNGLKRR VELOCIIY 100 lul. RRDISEC L E F i n R i N KHiEL RIiGjtRR VELOC I iY 1 0 0 1 ,dl. RRDISEC LRIERRL LR'ERRL 0 DISPLRT.EREN1.

IN.

01SPLRCEfii:17 0 I Y I . ll - -'1u - 1 (b) Gear steering angles, wheel speeds, and vehicle-trajectory parameters.

Figure 18.- Concluded, ORFG

FOIJSE. 0 - 1 : F S b ! . LBF

-5 N i S E -'lo Y O - GERR S LO€ -------^1-----~ SLOE FOSE. 0 0 FOSE. LBF -5 I 0 IOrnIJE.

TOWUE. 1 tl'll 'irvx~-----/ ,- IN'LBF - I 0 IORn* NlRtlRL FOSE. -100 - 2 0 FORCE. LBF 0 0 mffi ORR; FORE. 0 0 FUSE.

LBF -i RIGHT M R I N GERR 5 SIDE 0 F U S E . LBF -5 a Q f G mffi -- 0 FOSE.

-------- LBF L E F T -5 N R l N GERR -YO- S ICE 5 ID€ FO%E. 0 0 FOSE.

LBF -5 1 - 1 0 - ~ o L ~ ' ~ ~ ; ' : ~ ~ ' L ' ~ ' A ' ~ ' ~ o ' 1IIIE.SEC (a) Gear forces.

Figure 19.- Time histories of case 3 (concept B).

LC.

- LRTLRRL LR'ESRL 0 DISPLRCEfiENI nISPi9CEfiE"J+ I Y I . n IN.

(b) Gear steering angles, wheel speeds, and vehicle-trajectory parameters.

Figure 19.- Concluded.

OR% F O K E .

M S E GERR S I D E F U Z E .

OR% FORCE.

RTGHT i 1 R I I4 GERR >iDE FORCE.

- 2 3 I , j L , , , \ , , . , , ; - ., , . - -25. ffiK.E.

LPF O R R , MR; FGf$E. O F O K E .

L E F LEFT - 5 PIR 1 N GERR 5 l i l E i LEE F G K E .

. , .~-.---- - .., - -%- .--- G f f i K E .

..*, L P F - 5 (a) G e a r forces.

Figure 20.- Time histories of case 4 (concept C).

1.0SE F i g 7 - - STEERLI!: R'.(ILE 161. 'IEG - - - - W-- -- - - - ~ - - - ~ - -- --- LEFT h 4 1 h GERR STEERING R h K E -.0b i b l . OEG LEFT (iRili **EEL RNGULRR VELOCITY IOU - - - _ _ I w I , RRO/SEC .. - . -. " - - \ - l-I.Y,->.--.~ -..-.I,L, .\ -LI.. " "

t

O RUNWT - - _ - - .

RUYUUT .

1W OISTRHCE.

_ _ ___+- --- 0 1 ST9!iCE 1 x 1 . 1 -- - ~- -YO 1 - LRIERRL 01k?:&??il'~NT 0- 0 DISPLRCEHENI.

IN.

I Y i . n --YO 1 - 80 - RlRrRRFl/RUUiiRY HE~o!Ns RNGLE YO- 1 0 1 . OEG

0 6 - - - + ; 4 ; . - + - - + i + -* - . - - - 3

.."c . IL.SCC (b) Gear steering angles, wheel speeds, and vehicle-trajectory parameters.

Figure 20.- Concluded.

NOSF G t U k S T F t R I N G UNGLF 101. OFT, R I G t i - n R ! I G f H H 5 l t E H I N G RNriLF ( 6 1 . ll<G L E F l n R l N GERR STEERING ANGLE 1 6 1 . D F G NO E UliEEL R N G ~ L ~ ~ R V E L ~ C I i Y I ui . RRO/ CC RIGHT nR!N UHEEL n t t c u L n R VELQCI rr i up. RRO/LEC L E F T n R l N WHE L R N G ~ L D R ~ E L O C 15" rul. RRU/SEC RlRCRRFllRUNWRY HERD!NG RNGLE 101. OEG (b) Gear steering angles, wheel speeds, and vehicle-trajectory parameters.

Figure 21.- Concluded.

NCCE GEG" - a s ; r E

-

0 i O S C E , ! SF - -5 INORARL \ r , 2 0 FORCE.

- L i O r ORRG , , < . . ",wlliL . , i - r - +<' .---- *,. . . ~ ' \ , I , . , I < , . . .

F O V E . 0 LEFT L* -<''lr .''.I '- a- -5 N R I N GERS - Y o Y O 7 (a) Gear forces.

Figure 22,- Time histories of case 6 (concept D).

-60 z - ST&%'~I:$%G~LE -30 1 6 1 . OEG 1 - 6 O j - g o - - - - - - - - - R I G H T STEERING n R I N UNGLE GLRR 1 6 1 . OEG I RNGg% %:%IT" - - ~. ~ . - .. ~ - .- --.---..--.-..-X- 1-1. RROISEC . . . Y r , . - u ; r , u , ~ h , . ~ ~ ~ ~ w - I_____ - - ~. - - - _. .

gEg$;ELK5$\ - - -- - - .. . _ _ C ~ _ _ _ _ I w l . RnOISEC

_ ___CrX"____~* -*_^__ _ ,__

80 - RIRCRRFTIRUNNRY qo 4 HEROlNG RllGLE 1 8 1 . OEG I , I , l +-*-$--"--i ' ' I ' i ' i b I t 12 1 0 0 i ' i ' TinE.SEC (b) Gear steering angles, wheel speeds, and vehicle-trajectory parameters.

Figure 22.- Concluded.

. .,,7 NORtlRL - - -20 IOPCE.

d F ~ ~ ' 0 ORfiG ~ ~ .~ ~ -- .- 0 FORCE.

LBF - 5 NOSE G E R R SIDE

-

C FORCE.

V 1 a i -5 I --. ~ . . - , - A * + - - FORCE.

- < O I R 1 GHT - r o L l l M R l N GERR L.. 5 SIDE 0 FORCE.

L BF -5 I U F- - - TORWE. IN'LBF --+ \.n--:7-.#= .- -- ---- - - -- - -10 DRRG -.- .--, ---..--..- - - . . -.-____ 1: F @ .

- L . - ~ . ~ * ' T ' :i ;P\-W ,.,qiJiJr C ~ o , - x . . , k . r , LEFT -5 t l R l N GERR -80 FORCE.

.--Y : ' , . J (a) Gear forces.

Figure 23.- Time histories of case 7 (concept D).

IIOSE ni>=t!

R:IGULRR V t i O C I - Y I w l . 'IROISEC .- -'in LRTERRL - - - - - -- - LRTERAL 0 DISPLRCERENI.

O I S P L R C L r E t I T I Y I n - - Y O (b) Gear steering angles, wheel speeds, and vehicle-trajectory parameters.

Figure 23 .- Concluded.

- ORRG NOSE GERR

-

2 L SIGHT nRlN GERR rlOHliRL 2 0 FORCE.

LEFT n R I N GERR (a) Gear forces.

Figure 24.- Time histories of case 8 (concept D).

trunk " ~ E E .

. ; c " L m I w l , a f n / s i c vcLoci17 c i r w

I 1

O L p . - - - -- I !

(b) Gear steering angles, wheel speeds, and vehicle-trajectory parameters.

Figure 24.- Concluded.

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