LANDING LOADS AND DYNAMICS OF THE X-15 AIRPLANE

i ?_il i::i ! ¸¸i¸71¸¸ O_ kO I

<

<

Z

TECHNICAL MEMORANDUM

X-659 .....

LANDING LOADS AND DYNAMICS OF THE X-15 AIRPLA_._ By James M. McKay and E!don E. Kordes Flight Research Center Edwards, _ai±f.

AERONAUTICS AND SPACE ADMINISTRATION

March 1962

CONFIDENTIAL

CONFIDENTIAL NATIONAL AERONAUTICS AND SPACE ADMINISTRATION TEC_ICAL MEMORANDUM X-639 LANDING LOADS AND DYNThMICS OF THE X-15 AIRPLANE* ** By James M. McKay and Eldon E. Kordes SUMMARY This paper presents a discussion of the loads, accelerations, and displacements of the X-I_ airplane and landing-gear system measured during landing impact. The measured quantities are related to the initial touchdown conditions and are compared with data from a theoretical analysis. The applicability of the analysis to the X-15 landing gear has been investigated for the gear in the absence of drag loads. Studies have also been made to determine the effects of variations in such parameters as elevator positions, skid coefficient of friction, main- gear location, and initial touchdown conditions beyond the range of the experimental data.

INTRODUCTION One of the major problems that must be considered in the design of glide reentry vehicles is the provision for a safe landing on return.

Landing-gear systems for these vehicles must meet all the usual require- ments and, in addition, must be able to withstand the temperatures .resulting from reentry. Also if adequate ground steering is not pro- vided, the landing-gear system must give good stability during the runout.

The X-15 marks the beginning of a class of reentry vehicles with a landing gear that is designed to meet these requirements. The X-15 landing-gear system consists of a main gear with steel skids placed well back on the fuselage_ along with a conventional, nonsteerable nose gear placed well forward.

Because the landing-gear configuration represents a marked departure from previously used configurations, this paper reports on the landing loads experience of the X-15. A further purpose of the paper is to *This document is based on a paper presented at the Conference on the Progress of the X-15 Project, Edwards Air Force Base, Calif., November 20-21, 1961. An appendix has been added to show the analyt- ical relations used in an analog study of X-15 landing response.

**Title_ Unclassified.

CONFIDENTIAL

2 CONFIDENTIAL

review the dynamics of landing and to present results of a recent

analytical study of the effects of various parameters on the landing

loads. The landing flare maneuverand someof the slideout character-

istics are covered in reference i.

SYMBOLS

meanaerodynamic chord, ft

Lift

CL 0

basic lift coefficient_ qS

CL_ lift-curve slope, per radian

Cmo basic pitching-moment coefficient, Pitching moment

qS Cm5 h horizontal-tail-effectiveness parameter, per deg SC m e_ vertical ground reaction due to pneumatic force in main-gear _a shock strut, ib vertical ground reaction due to bending of main-landing-gear strut, ib vertical ground reaction due to hydraulic force in main-gear Fm h shock strut, ib pneumatic forces in nose-gear shock strut_ ib rn a hydraulic force in nose-gear shock strut, ib Fn h vertical force, applied to nose-gear tire at ground, ib

Fn t

horizontal-tail aerodynamic load, ib F t vertical ground reaction, ib F v acceleration due to gravity, ft/sec 2 g moment of inertia about Y-axis, slug-ft 2 Iy CONFIDENTIAL

CONFIDENTIAL

L

lifting force, ib vertical distance from airplane center of gravity to ground, ft

Lcg

horizontal distance from airplane center of gravity to point of contact of landing gear with ground, ft vertical distance from fuselage reference to point of landing- gear contact With ground at landing-gear location_ ft m m_ss, W/g q dynamic pressure, ib/sq ft S wing area, sq ft V free-stream velocity, ft/sec airplane sinking speed at initial touchdown, ft/sec

Vv o

W airplane landing weight, ib weight of nose-gear lower mass below shock strut, Ib

Wn

angle of attack, deg initial angle of attack at touchdown, deg

%

angle between pitch-attitude angle and angle of attack, deg vertical displacement of upper mass from position at initial contact, ft horizontal-tail deflection, deg 5h vertical displacement of nose-gear lower mass from position at 5 t initial contact, ft pitch attitude, deg pitching velocity, radians/sec coefficient of friction Sub script s: m main gear n nose gear CONFIDENTIAL

4 CONFIDENTIAL

A dot over a symbol indicates the derivative of the quantity with

respect to time.

A double dot over a symbol indicates a second derivative of the

quantity with respect to time.

GENERAL DESCRIPTION

Because of the airplane configuration, the landing characteristics

of the X-15 are somewhatunusual. A typical landing sequenceis illus-

trated in figure !. The sketch at the top of the figure showsthat a

nose-high attitude is established just prior to main-gear touchdown.

The airplane weight, wing lift, and tail loads are indicated by the

arrows in each sketch, and the springs represent both the main and nose

landing gear. During main-gear contact, the airplame rotates and impacts

on the nose gear, as shownin the second sketch. During nose-gear

compression, a second reaction occurs on the main gear, as indicated in

the third sketch. It is significant to mention that this second reaction

is far greater than the first, as will be shown subsequently. The air-

plane then rests on both gears for the slideout, as shownin the bottom

sketch.

Thus far, 45 landings have been madewith the X-15. The first four

pointed out certain deficiencies in gear design. The principal defi-

ciency can be brought out by reference to figure 2, which showsone of

the main gears of the X-15 and also serves to indicate the unusual nature

of the gear operation. The gear consists of a steel skid and an Inconel X

strut which is attached to the fuselage by trunnion fittings and through

bell-crank arms to shock struts inside the fuselage. The skids are free

in pitch and roll, but are fixed for parallel alinement. Drag braces are

attached to the fuselage ahead of the trunnion fitting and to the skid at

the strut-attachment pin. The bungee springs are used to keep a nose-up

position of the skids just before landing. During flight the skids and

landing-gear struts are folded forward against the outside of the fuselage.

After release, they are extended simply by gravity and air loads.

The main changes that were madein this main-gear arrangement were

simply to replace the shock struts by struts having greater energy-

absorbing characteristics and to '_eef up" the gear back-up structure

somewhat. These changeswere brought about mainly because the gross

weight of the airplane had increased and also because the down-load on

the elevator during landing was found to be greater than that taken into

account in design.

In connection with this discussion, the fourth X-15 landing, which

was an emergencylanding madeafter an engine explosion, should be

CONFIDENTIAL

CONFIDENTIAL 5

mentioned. It is significant to note that the failure of the fuselage

during this landing was not attributed to a design error; rather, it

occurred because the airplane landed in an overweight condition, since

all the fuel could not be jettisoned, and because of a high nose-gear

load, caused by foaming of the gas and oil mixture in the shock strut.

A permanent solution to the foaming problem was achieved by using a free-

floating piston inside the strut to separate the gas and oil. With

these main changes_the last 41 landings have been without major incident.

EXPERIMENTAL RESULTS

Main-Gear Response

During the X-15 program, the airplane has been instrumented to

measure gear loads_ gear travel, and accelerations. Figure 3 showsthe

main-gear shock-strut force and travel measuredon a typical landing.

The upper curve is the strut travel and the lower curve is the strut

force, measuredfrom time after main-gear touchdown. At touchdown, the

angle of attack s0 was 8 °, the sinking speed Vvo was 3 feet per

second, and the landing weight W was 14,500 pounds. The sketches at

the top of this figure are used to aid in identifying the landing

sequence. It is important to note that both the shock-strut force and

travel are appreciably higher during the second reaction on the main

gear following the nose-gear touchdown than during the initial portion

of the landing. These high values are due to several factors_ primarily

to the main-gear location well back of the airplane center of gravity

and to the pronounced aerodynamic load on the tail 3 the negative wing

lift during this portion of the landing_ and the airplane inertia loads.

The increasing air load on the tail is brought about by two sizable

increases in angle of attack_ namely_ the rotation of the airplane onto

the nose gear, and a change in the wind-flow direction to nearly hori-

zontal due to arresting the vertical descent. Experience with the X-15

has shownthat the horizontal-tail angle, and hence the tail loads are

also increased by the stability augmentation system as the airplane

pitches down. The time history of only one gear is shown since all

landings have been nearly symmetrical and both skids have been solidly

on the lakebed before nose-gear touchdownoccurred.

The influence of airplane sinking speed on main-gear response for

many landings with the modified gear system is shownin figure 4. Air-

plane vertical travel at the main gear_ and shock-strut force for the

first- and second-peak values are presented in terms of airplane sinking

speed at initial touchdown. Values measured at the first peak are shown

by circles and at the second peak by squares. These data are for angles

of attack between 4 ° and ii°_ and ground speeds at touchdownbetween

145 and 238 knots. Note that there is good correlation between sinking

C 0NF IDENT IAL

6 CONFIDENTIAL

Speedand the measuredquantities at the first peak. It is important to

point out that the values at the second peak are independent of sinking

speed. It should be noted also that as the sinking speed increases, the

values at the first peak approach those of the second. No definite

correlation for the first peak has been found between vertical travel or

shock-strut force and angle of attack or forward speed at touchdown.

Nose-GearResponse

The influence of airplane sinking speed on nose-gear response is

shown in figure _. Nose-gear contact velocity, shock-strut travel, and

vertical reaction are presented for various airplane sinking speeds.

The results indicate that there is little change in the measured

quantities with airplane sinking speed. The large magnitudes of the

quantities are, of course, due to the rapid rotation of the airplane

after the initial touchdown. The loads resulting from the high nose-

gear contact velocities cause high but acceptable accelerations on the

pilot during this phase of the landing. However, the lack of any indi-

cated trend with initial sinking speed is probably due to the absorption

by the main gear of a larger portion of the total energy during the

first peak at the higher airplane sinking speeds.

Analytical Resuit s

Experience during the program has shownthat the pilots tend to

land the X-15 in a similar way on each flight. Therefore, the effect

of many of the variables cannot be determined from the experimental

data. In order to study someof the conditions that affect the gear

response, an analytical study has been conducted (see appendix). Results

from the calculations are comparedwith X-15 data in figure 6, where the

time history of the main-gear-skid vertical reaction is shownfor a

typical landing. The initial conditions are angle of attack _0 of 8 ° , airplane sinking speed Vvo of 3 feet per second, and airplane landing weight W of 14,500 pounds. The method used for obtaining the skid reaction from data of an actual landing necessarily resulted in faired values, as indicated by the solid line. The dashed curve is used to show the calculated values. Although there is a slight time difference at the second peak, the magnitudes of the maximum first and second reactions agree extremely well. The good agreement between calculated and measured results gives confidence in the ability of the analysis to determine the X-15 landing response.

CONFIDENTIAL

CONFIDENT IAL 7

Horizontal-Tail Load

The downward-acting horizontal-tail load during landing is large and has a marked influence on the vertical reaction on the main-gear skid.

The results of an analysis which calculated its effects are shown in figure 7_ in which skid vertical reaction is given as a function of air- plane sinking speed for an initial angle of attack of 8 °. The results, along with some experimental data, are shown for both the maximum first reaction and the maximum second reaction per skid. The dashed curves apply to the condition where the elevator position is held constant at -4 ° during the landing. The solid curves are for the condition brought about by the stability augmentation system, where the elevator position varies from the angle of trim of -4 ° at touchdown to -19 ° at nose-gear contact. The latter condition is one that usually exists for actual landings of the X-15 airplanes. The differences between the solid and dashed curves are due to the increased tail loads associated with the difference in elevator position. Note the large decrease in the magnitude of the second reaction obtained by keeping the elevator angle small. In fact_ a greater reduction in load would be expected with the horizontal tail rotated to a positive angle, leading-edge up_ at the instant of main-gear contact. These results show the desirability of including an automatic system to control the elevator positions after touchdown; and_ hence, to reduce the second reaction on the main-gear system.

Skid Coefficient of Friction Several different types of skids have been proposed for reentry- type vehicles, including wire-brush skids. One of the main differences in the skids is in the value of the skid coefficient of friction. The influence of the skid coefficient of friction on the landing response has been calculated and the results are shown in figure 8. The skid and the nose-gear vertical reaction are presented as a function of air- plane sinking speed. The solid curve shows the results for a skid- friction coefficient _ of 0.33; which is representative of the skid on the X-15 airplane. The dashed curve is for a friction coefficient of 0.7, which is typical of the values for a wire-brush skid. The results indicate that increasing the coefficient of friction tends to reduce the vertical skid reaction slightly and, as might be expected, to increase the nose-gear vertical reaction. Even though the vertical reactions are not appreciably affected by increasing the coefficient of friction, the drag loads would be affected to a larger degree.

Main-Gear Location Another factor that would be expected to affect the gear loads is the location of the main gear with respect to the airplane center of CONFIDENTIAL

8 CONFIDENT IAL

gravity. The next results are intended only to showthe effect of moving

the main-gear location and should not be interpreted to imply any change

to the X-iS. This effect has been calculated by using X-15 parameters.

The results of the calculations are shownin figure 9 for two positions

of the main gear. The skid and nose-gear vertical reactions are shown

again as a function of airplane sinking speed. The solid curve is for a

gear distance Lhm of 15.9 feet aft of the center of gravity_ which is

the value for the X-15; the dashed curve represents the results obtained

by moving the gear to a position one-third of the distance to the center

I %

of gravity (Lhm = 11.3 ft). The results indicate that the second

% reaction on the main gear is not affected to a great extent; however, the effect of moving the gear forward increases the first reaction in such a way that, at the higher sinking speeds_ the values of the first and second reaction approach each other. The results do show that moving the main gear forward reduces the nose-gear vertical reaction. It can be seen that a change in the gear position to a little over ii feet does not have as much effect as might be expected. However, other results not shown here indicate that if the gear is moved still closer to the center of gravity, there is an appreciable reduction in the second main reaction; thus, a configuration representing that of a present-day fighter aircraft is approached_ wherein the first reaction is the one that is critical.

The analytical program is being continued to study the effects of other parameters on the landing-gear requirements for reentry vehicles.

CONCLUSIONS Landings with the X-15 airplane have shown that the main-gear loads, measured during the second reaction after nose-gear contact, are several times larger than the loads experienced during the initial phase of the landing. The large loads during the second main-gear reaction are attrib- uted to the main-gear location as well as to the large tail loads, the negative wing lift, and the airplane inertial loads after nose-gear touchdown. The high nose-gear contact velocities due to the airplane pitching down result in high nose-gear ioads_ and, consequently_ in high accelerations on the pilot during this phase of the landing. Calculated results are used to show that the main-gear reaction can be reduced by proper control of the elevator angle during touchdown. These results also show that increasing the skid coefficient of friction reduces the main-gear reaction slightly, but increases the nose-gear reaction. The calculated resul_s also show that moving the main gear forward increases the first main-gear reaction but reduces the nose-gear reaction.

Flight Research Center National Aeronautics and Space Administration Edwards, Calif., November 20, 1961 CONFIDENTIAL

CONFIDENT IAL 9

APPENDIX

Analytical Relations Used in Analog Study of X-15 Landing Response

An analytical study was madeto determine the effect of the X-15

landing response to such quantities as horizontal-tail loads_ skid

friction coefficients, gear location, and initial touchdown conditions.

This analysis was conducted on an electric analog with four degrees of

freedom; main-gear motion, nose-gear motion, airplane pitch, and

vertical translation.

The program madeuse of the following relations to describe the

motion of the airplane upper mass, which was considered to be rigid.

Airplane pitch:

Iy8 = qS[(Cmo+ CmSh$ h + _V Cm8 _)

+ FvnIS(Lvn + _nLhn) + (Lhn - _nLvn)1

Vertical translation: 7mV = qS(CLo + CL_)+ (Fvn + 2Fv m - W)cos 7 For the main-gear-skid vertical reaction, the following relations were used: Before the upper-mass displacement began Fv m =Fm b After the upper-mass displacement began Fvm = Fma + Fmh and _m = -Lcg + 8Lh m CONFIDENTIAL

I0 CONFIDENTIAL

Relations used for the nose-gear vertical reaction were as

follows:

Before the beginning of shock-strut deflection

Fvn = Fnt

The equation of motion used for the nose-gear lower mass was Wn + (Fvn - Fnt) - W-_n_'t = Og After the beginning of shock-strut deflection Fvn = Fn a + Fnh and _n = -Lcg - eI_n December 21p 1961 CONFIDENTIAL

CONFIDENTIAL ii

REFERENCES

i. White, Robert M., Robinson, Glenn H., and Matranga, GeneJ.: Resume

of Handling Qualities of the X-15 Airplane. NASA TMX-715, 1962.

CONFIDENTIAL

"--: .-_ ._. "'_._. ";;. "L'.,:"i ...i ...: "" _ !..: CONFIDENTIAL

X-15 TOUCHDOWN SEQUENCE

WIND_ RELATIVE ____,_ MAIN-GEAR TOUCHDOWN ROTATION NOSE-GEAR TOUCHDOWN SECOND MAIN-GEAR REACTION SLIDEOUT Figure I

X-15 MAIN LANDING GEAR

I I I I ! I I f _"'-CENTER OF ROTATION Figure 2 CONFIDENTIAL CONFIDENTIAL MAIN-GEAR SHOCK-STRUT FORCE AND TRAVEL aO=8°, %=3 FT/SEC, W=14,500 LB MAIN-GEAR NOSE-GEAR SECOND TOUCHDOWN TOUCHDOWN REACTION (T RELATIVE/"! "_'_W_ w- Ft WIND z/////////z//z//////1///,, _//z_//, ---T//_H_//, SHOCK- STRUT TRAVEL, IN.

60xlO _ SECOND PEAK SHOCK-STRUT 30 l FIRST _ _ FORCE, LB / PEAK _._'!

'' I I I II I I I 0 .2 4 6 8 1.0 12 14 TIME AFTER MAIN-GEAR TOUCHDOWN, SEC Figure 3 INFLUENCE OF AIRPLANE SINK SPEED ON MAIN-GEAR RESPONSE 50xlO_ uQ BQB 40/u B _t u SECOND 1.6[ SECOND PEAK I_ _ DPEAK 3O o SHOCK-STRUT AIRPLANE FORCE, LB 20 o VERTICAL .8 o TRAVEL, FT I0 o .4 t_f FIRST O_j_ FIRST PEAK I I 4 8 0 4 8 AIRPLANE SINK SPEED, FT/SEC Figure 4 CONFIDENTIAL CONFIDENTIAL !4 INFLUENCE OF AIRPLANE SINK SPEED ON NOSE-GEAR RESPONSE CONTACT VELOCITY, 15 o 8 o O t_ o FT/SEC _o_ o o

301 °

O STRUT TRAVEL, 20Ioo@ o_%oO_otJ B o o IN. 0 L 40 X I03 VERTICAL REACTION, 20 o =° o o LB 0 o

t oo

i I I i I 0 2 4 6 8 I0 AIRPLANE SINK SPEED, FT/SEC Figure 5 MAIN-GEAR-SKID VERTICAL REACTION a 0 ,, 8 o Vvo=3 FT/SEC W = 14,500 LB 3 MAXIMUM SECOND _0 I0 NOSE-GEAR /1--- _^t, TlnN TOUCHDOWNS.. _" ....

/ MAIN- GEAR-SKID VERTICAL REACTION 4 I'A " PER SKID, LB MAXIMUM FIRST / ,,_' .....

_l " I i I ' *_ " I I J 0 .4 .8 1.2 1.6 TIME AFTER MAIN-GEAR TOUCHDOWN,SEC Figure 6 CONFIDENTIAL CONFIDENTIAL INFLUENCE OF TAIL LOAD ON MAIN-GEAR-SKID VERTICAL REACTION CALCULATEDRESULTS aO= 8" W, 14,500 LB EXPERIMENTAL I0 x 103 o MAXIMUM FIRST REACTION _..__..._..-I:T- ---_ r-, MAXIMUM SECOND REACTION " MA_MUM SECOND REACTION MAIN-GEAR-SKID 6 VERTICAL REACTION -- INCREASE TO Bh = -15" PER SKID, LB --- CONSTANT 8 h TO TRIM (-4")

2 ///

I I i , I 1 0 2 4 6 8 I0 AIRPLANE SINK SPEED, FT/SEC Figure 7 INFLUENCE OF SKID-FRICTION COEFFICIENT ON MAIN- AND NOSE-GEAR VERTICAL RE/WCTION CALCULATED RESULTS ao=B'_ , W=14,500 LB I0 I0 _ 50 MAXIMUM SECOND REACTION l / NOSE-GEAR 50 MAIN-GEAR- 6 VERTICAL SKID VERTICAL REACTION REACTION, LB 20 PER SKID, LB 4 _F=.33 I0 MAXIMUM .... _=.70 FIRST REACTION ..J I0 0 5 I0 0 AIRPLANE SINK SPEED, FT/SEC Q Figure 8 CONFIDENTIAL CONFIDENTIAL INFLUENCE OF MAIN-GEAR LOCATION ON MAIN- AND NOSE-GEAR VERTICAL REACTION CALCULATED RESULTS aO=8 % W--14,500 LB iOxlO 3 50xlO 3 MAXIMUM SECOND REACTION 4O i / /// , NOSE-GEAR 30 MAIN-GEAR- 6 ,." VERTICAL SKID VERTICAL REACTION ,.." REACTION, PER SKID, LB 4 LB 20 6 _ILM i0 MAXIMUM --Lhm= 15.9 FT FIRST REACTION I I .... Lhm=ll.3 FT 5 I0 5 I0 0 AIRPLANE SINK SPEED, FT/SEC Figure 9 NASA-L,-g,,y, ,962 H-245 CONFIDENTIAL