Document
DRAG A N D WEAR CHARACTERISTICS
OF VARIOUS SKID MATERIALS O N
DISSIMILAR LAKEBED SURFACES D U R I N G
OF THE X-15 AIRPLANE
THE SLIDEOUT
by Ronald J. WiZson
FZight Research Center
Edwards, CLZ Z $
, . ;+
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 1966 TECH LIBRARY KAFB, NM
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O L 3 O O L O NASA T N D-3331 DRAG AND WEAR CHARACTERISTICS O F VARIOUS SKID MATERIALS ON DISSIMILAR LAKEBED SURFACES DURING THE SLIDEOUT O F THE X-15 AIRPLANE By Ronald J. Wilson Flight R e s e a r c h Center Edwards, Calif.
.
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 - Price $0.40 DRAG AND WEAR CHARACTERISTICS O F VARIOUS SKID MATERIALS
ON DISSIMILAR W~EBED SURFACES DURING THE SLIDEOUT OF THE x-15 AIRPLANE
By Ronald J. Wilson F l i g h t Research Center SUMMARY An investigation w a s made t o determine t h e c o e f f i c i e n t s of f r i c t i o n and t h e wear c h a r a c t e r i s t i c s f o r X-15 landing-gear skids of various materials.
Data a r e presented f o r skids made of 4130 s t e e l , with and without cermet coating, and InconelX f o r several lakebed-surface conditions. The mean coef- f i c i e n t of f r i c t i o n on a dry-hard surface was found t o be 0.30 f o r 4130 s t e e l 0.36 f o r 4130 s t e e l skids with cermet coating, and 0.33 f o r Inconel X skids, skids. The mean coefficient of f r i c t i o n f o r t h e cermet-coated skids on a s o f t surface w a s 0.46; f o r Inconel X skids on a damp surface t h e mean value was 0.25. Flight data are compared with experimental ground-tow t e s t data on n a t u r a l and simulated lakebed surfaces. Also included i s t h e v a r i a t i o n of skid wear with slideout distance.
INTRODUCTION Landing gear t h a t use skids instead of wheels have been used on a number of f l i g h t vehicles and have been proposed f o r some f u t u r e vehicles. Some of t h e advantages of t h i s type of gear are i t s simplicity, r e l i a b i l i t y , t h e a b i l i t y t o withstand aerodynamic heating, minimum space and weight require-
The X-15 research a i r -
ments, and t h e a b i l i t y t o s u s t a i n high landing speeds.
plane u t i l i z e s a landing gear of t h i s type. Some aspects of operating with skid gear on t h e X-15 a r e reported i n references 1 t o 6.
Coefficients of f r i c t i o n and wear c h a r a c t e r i s t i c s f o r skids of various materials have been studied by several investigators. The r e s u l t s of experi- at v e l o c i t i e s up t o mental ground-tow t e s t s on simulated and n a t u r a l lakebeds
82.5 knots a r e reported i n references 7 t o 9. Flight t e s t s of operational
skids, fabricated from s t r u c t u r a l s t e e l s , on hard lakebed surfaces were made i n t h e investigations of references 3 and 10. O p t i m u m f r i c t i o n coefficients f o r landing skids w e discussed i n reference ILL. Additional information i s required, however, on c o e f f i c i e n t s of f r i c t i o n and wear at high speeds on hard, s o f t , and damp lakebed surfaces. I n order t o obtain information of this
type during high-speed slideout, an X-15 airplane was instrumented by t h e NASA
F l i g h t Research Center, Edwards, C a l i f ., t o measure p e r t i n e n t q u a n t i t i e s during landing. Twelve tests were conducted with skids of various materials
I II I l l I l l I
This paper presents t h e and f o r several d i f f e r e n t lakebed surface conditions.
results of t h e investigation.
TEST VEHICLZ AND EQUIPMENT The X-15 airplane ( f i g . 1) used as t h e t e s t vehicle i n t h i s investigation i s a rocket-powered a i r c r a f t capable of a t t a i n i n g a Mach number of 6 and alti- tudes i n excess of 3OO,OOO f e e t . The airplane i s described f u l l y i n r e f e r - ences 1 2 and 13.
Gear system.- The X-13 landing-gear system, dis- cussed i n d e t a i l i n r e f e r - ence 3, consists of a non- s t eerable, full- c a s t e r ing ( 3 6 0 " ) nose gear located well forward of the airplane center of g r a v i t y and a skid-type main gear located under t h e E-7905 t a i l , well a f t of the center Figure 1.- X-15 airplane.
o f gravity.
The main-gear l e g s are Inconel X s t r u t s attached t o t h e fuselage by trunnion f i t t i n g s ( f i g . 2 ) and through bellcrank a r m s t o high-pressure shock s t r u t s i n s i d e t h e fuselage. The t w o skids are universally mounted. t o t h e f o r s t r u t s t o allow f o r pitching and r o l l i n g motion, but are restrained i n yaw p a r a l l e l alinement. The drag braces are attached t o t h e fuselage by semi- universal f i t t i n g s and are s i m i l a r l y connected t o t h e skids ahead of t h e -Instrumented for Instrumented for shock-strut force/ shock-strut force-
Instrumented for f a
drag-brace tension Skids - 3 feet long 6 inches wide Figure 2.- X-15 main landing gear and instrumentation.
strut-attachment pin. Bungees connect t h e leading edge of the skid t o t h e main-landing-gear l e g t o insure a nose-up a t t i t u d e of t h e skids before touch- down.
-- Landing-gear skids.- Three skid materials were used i n t h i s investiga- t i o n . The first s e t of skids, which i s standard equipment on t h e X - 1 5 a i r - w a s fabricated from 4130 s t e e l . The second s e t was of 4130 s t e e l with plane, a cermet coating t h a t was o r i g i n a l l y developed f o r t h e X-20 nose-gear skid.
X.
The t h i r d s e t was fabricated from Inconel The skids were 6 inches wide and 3 f e e t long, with contact surfaces ap- proximately 4.8 inches wide and 30.6 inches long. The wearing surfaces were approximately 0.15O-inch t h i c k f o r t h e 4130 s t e e l skids, 0.40-inch t h i c k €or t h e cermet-coated skids, and 0.15O-inch t h i c k f o r t h e Inconel X skids.
The cermet-coating process ( f i g s . 3(a) and 3(b) consisted o f : (1) copper-brazing inch screened
-27-
tungsten carbide chips t o t h e skid surface a f t e r precoating t h e surface with flux ( t h e chips were applied t o t h e surface by hand); ( 2 ) flame- spraying a 0.020-inch- t o I 0 ~ 0 4 0 - i ~ c h - t h i c k matrix of tungsten E-10822 (35 percent), chrome, nickel, and (a) Bottom view.
boron (65 percent) on top of t h e copper-brazed carbide chips, then fusing and g r i t blasting; and ( 3 ) flame-spraying with a copper-nickel matrix. The cermet coating w a s then ground t o a nominal thickness of 4 0.20 inch.
I
TEST LANDING CONDITIONS E- 10820 (b) End view of both skids.
Figure 3.- Cermet-coated skid.
The landing conditions f o r t h e t e s t s discussed herein are summa- r i z e d i n t a b l e I. A s shown, the landing weights f o r the X-15 ranged from The t r u e ground speed varied f r o m 164 knots 14,500 pounds t o 15,855 pounds.
t o 221 knots. The maximum slideout distance was 8968 f e e t , and the minimum distance was 3520 f e e t . The maximum wind velocity across the lakebed was ap- proximately 10 knots.
A l l t h e landings of t h i s study except one were made on t h e hard, smooth lakebed of Rogers D r y Lake, at Edwards, C a l i f . ; one landing was made on t h e s o f t surface of Cuddeback D r y Lake, near Edwards.
Lakebed-surface conditions were variable and presented a range of hard- ness values dependent on weather conditions, location, and s o i l c h a r a c t e r i s t i c s .
Surface-hardness values were not obtained d i r e c t l y f o r all of t h e surface con- ditions. Hardness values, expressed i n terms of California Bearing Ratio (CBR)1, were obtained from reference 9 f o r dry-hard surface conditions.
The values f o r t e s t s 1 t o 9 and ll averaged 76, with a high of 94 and a low of 5 9 .
A CBR of 30 or greater i s considered s a t i s f a c t o r y f o r operations of heavy j e t - transport a i r c r a f t .
TAEZE I SUMMARY OF TEST CONDITIONS I I Distance from Velocity a t Velocity a t Time of Lakebed Landing main-gear t o Slideout Wind touchdown touchdown nose-gear Test Type Of surface weight, nose-gear distance, velocity, (indicated ( t r u e ground impact , condition lb touchdown, f t knots airspeed), speed), sec f t knots knots Dry-hard 14,700 8 0.7 7920 2 07 _ _ _ _ Dry-hard .a C a l m 14,500 304 1% - _ _ _ Dry-hard 14,600 218 10 .54 1 % Dry-hard 8170 Calm 14,950 294 .74 204 4130 Dry-hard .60 4488 2 164 15,150 2 05 Cermet Dry-hard Calm 14,920 252 * 72 5702 Cermet Dry-hard 15, loo .61 4805 208 253 3 Cermet Dry-hard 15,100 5204 C a l m 310 .83 Cermet Dry-hard 5808 C a l m 14,750 320 .89 Dry-soft 271.6 14,920 181 76 3520 5 Dry-hard 287.8 C a l m 15,798 . 7 u 60% Damp-hard C a l m 15,855 365 *715 8968 221 Surface CBR values were not determined f o r t h e s o f t surface of Cuddeback 10, Lake, used i n t e s t because of the remote location of t h e lake. A CBR of 16 t o 23 w a s estimated on t h e b a s i s of ground t e s t s t h a t provided t h e same coefficient of f r i c t i o n f o r t h e same skid material. N o estimates of surface hardxess were made f o r t h e damp surface condition of t e s t 12.
B a l l t e s t s were conducted t o compare surface-hardness values of t h e natu- ral lakebed surfaces with those of the simulated lakebed surface of r e f e r - ence 7. A 17.9-pound s t e e l b a l l , 5 inches i n diameter, w a s dropped onto t h e lakebed from a height of 6 f e e t . The diameters of t h e indentations l e f t by t h e b a l l on t h e hard lakebed surface ranged from 2.25 inches t o 2.50 inches. O n t h e s o f t lakebed surface, indentation diameters were from 4.0 inches t o 4.50 inches. The b a l l t e s t s on t h e simulated lakebed of reference 7 resulted i n diameters of 3.0 inches.
AND DATA REDUCTION INSTRUMENTATION t Airspeed, shock- s t r u t force, and drag-brace tension load were measured during t h e approach, touchdown, and slideout phases of t h e landings. Airspeed 1 data were obtained f r o m t h e X-15 flow-direction sensor i n t h e nose of t h e - _ _ k a l i f o r n i a Bearing Ratio i s defined as t h e r a t i o o f t h e bearing strength of t h e s o i l surface i n question t o a standard high-quality compacted crushed- stone surface.
This standard has a bearing strength of 1000 lb/sq i n . at a depth of 0.10 inch.
a i r c r a f t and s t a t i c pressure pickups on t h e a i r c r a f t fuselage. S t r a i n gages mounted on each of t h e main-gear bellcrank arms and drag braces ( f i g . 2 ) were arranged t o measure t h e normal skid and drag loads.
synchro- The measured q u a n t i t i e s were recorded on standard oscillographs, nized a t 0.1-second i n t e r v a l s t o a comon timer. The n a t u r a l frequency and damping r a t i o of the recording galvanometers were 20 cps and 0.64, respec- t i v e l y . Recordings were accurate within 3 2 percent of f u l l - s c a l e readings.
The t r u e ground speed at touchdown w a s calculated by dividing t h e measured distance between t h e main-gear and nose-gear touchdown points on t h e lakebed by t h e time i n t e r v a l between main-gear and nose-gear touchdown (as obtained from oscillograph records). The t r u e ground speed during slideout w a s determined by correcting t h e t r u e airspeed, obtained from onboard recording, f o r t h e d i f f e r - The data a r e e n t i a l between t h e airspeed and t h e ground speed at touchdown.
accurate t o within +1 knot.
Landing-gear loads, which a r e normal skid and drag loads, were determined The s t r a i n from data recorded f o r each main-gear skid during t h e slideout.
gages on t h e l e f t and the r i g h t main drag braces were calibrated t o give t h e drag-brace tension loads r e s u l t i n g from t h e drag loads on t h e skids. From t h e the drag-brace tension loads were used t o geometry of t h e main-gear system, calculate t h e drag reaction between t h e skids and t h e ground. No i n t e r a c t i o n e x i s t s between t h e drag-brace load and the main-gear shock s t r u t , since pivot points a t t h e fuselage f o r t h e drag brace and t h e landing-gear l e g f a l l on a l i n e t h a t i s e s s e n t i a l l y p a r a l l e l t o the longitudinal centerline of t h e fuse- lage.
The s t r a i n gages on t h e main-gear bellcrank arm were calibrated t o give t h e a x i a l load on t h e shock-strut cylinders. Since only s l i g h t pitching, ver- t i c a l , and r o l l i n g motions were experienced during t h e slideout, t h e main-gear shock-strut reaction t o t h e v e r t i c a l load of t h e skid w a s regarded as being equal t o t h e s t r u t airspring force; t h a t is, t h e airplane was e s s e n t i a l l y riding on t h e airspring force of t h e shock s t r u t s . A c a l i b r a t i o n on t h e main- gear system correlated t h e e f f e c t of the v e r t i c a l force of t h e skid on t h e shock-strut-cylinder reaction and shock-strut displacement.
TEST RESULTS Twelve t e s t s were conducted with skids of various materials and f o r several d i f f e r e n t lakebed-surface conditions ( t a b l e I).
Tests 1 t o 5 were made with t h e 4130-steel skids on a dry-hard surface, The which w a s smooth except f o r a few indications of previous a i r c r a f t use.
t y p i c a l v a r i a t i o n of skid loads with time and of coefficients of f r i c t i o n with forward speed during slideout on these t e s t s i s shown i n t h e d a t a of f i g - ures 4(a) and 4(b). During landing impact, t h e i n i t i a l values of the coeffi- c i e n t s of f r i c t i o n were somewhat similar i n magnitude t o t h e values obtained during wheel spin-up on conventional landing gear. These high values were not 8 X l O3 I Normal skid load Normal skid drag load, Ib 0 4 8 12 16 2 0 2 4 2 8 3 2 3 6 4 0 44 Time after initial touchdown, sec 0 00 0 0 0 00 Coefficient O o c 0 m o o 0 O 0 @ @ c 0 O m o @ O of friction I I I I I I I I I I I 0 2 2 0 2 0 0 180 160 140 120 100 8 0 60 4 0 2 0 Velocity, knots (a) Left main gear.
Figure 4.- Drag characteristics of main gear on test 1. 4130 steel skids; dry-hard lakebed.
10x103
I j
Normal skid 6 Normal skid load drag load, I b 4 0 a 12 16 2 0 2 4 2 8 32 3 6 4 0 4 4 4 Time after initial touchdown, sec
.6 *'I
Coefficient of friction ' 4 1 2 2 0 2 0 0 180 160 140 120 100 8 0 6 0 4 0 2 0 0 Velocity, knots (b) Right main gear.
Figure 4.- Concluded.
analyzed because of t h e t r a n s i e n t conditions of impact, rebound, and m a x i m u m loading. For t h i s paper, t h e c o e f f i c i e n t s of f r i c t i o n were evaluated during t h e s t a b i l i z e d portion of t h e slideout.1 During t h i s period, t h e c o e f f i c i e n t of f r i c t i o n decreased t o a r e l a t i v e l y constant value. A t approximately 50 knots, it began t o increase t o i t s maximum value (impending s t a t i c f r i c t i o n ) at t h e end of s l i d e o u t .
Tests 6 t o 9 were a l s o conducted on a dry-hard lakebed, but with cermet-
coated skids. Landing-loads da%a were not obtained on t e s t 7 because of a loss of instrumentation during f l i g h t . Typical d a t a from these t e s t s a r e presented i n figures 5(a) and ? ( b ) .
Test 10 w a s conducted with t h e cermet-coated skids on a s o f t lakebed sur- face t h a t w a s r e l a t i v e l y rough and wavy. The v a r i a t i o n of landing loads with time and c o e f f i c i e n t of f r i c t i o n with forward speed during slideout i s shown i n f i g u r e s 6(a) and 6(b). The drag loads could not be obtained u n t i l 2 sec- onds a f t e r touchdown because of indistinguishable oscillograph t r a c e s . A second gap i n t h e data occurred a t 13.4 seconds a f t e r touchdown as a r e s u l t of contact with a graded road. Because of surface i r r e g u l a r i t i e s , t h e measured loads fluctuated considerably more than during t h e previous t e s t s .
Tests 1 1 and 12 were made on dry-hard and damp-hard lakebeds, respec- t i v e l y , using Inconel X skids. Data from the t e s t s a r e presented i n f i g - ures 7(a) and 7 ( b ) and 8 ( a ) and 8 ( b ) .
Table I1 summarizes pertinent q u a n t i t i e s r e s u l t i n g from each slideout.
TABLE I1 SUMMARY O F TEST CONDITIONS AND MEASURED DATA Mean absolute Range of Skid mean Mean Type of Estimated d e v i a t i o n of c o e f f i c i e n t bearing Test surface c o e f f i c i e n t skid CBR c o e f f i c i e n t of f r i c t i o n pressure, condition of f r i c t i o n
- -
of f r i c t i o n lb/sq i n .
Low High ~~ ~ .~ ~~ - ~ -~ ~
I
1 4130 Dry-hard 59 t o 94 0.31 0.01 0.25 0.36 26.5 2 4130 Dry-hard .02 59 t o 94 -29 -25 * 33 30.7 4130 Dry-hard .28 .02 .22 3 59 to 94 31.2 .34 4 4130 Dry-hard 59 t o 94 .02 .18 -32 .38 25.7 4130 Dry-hard .02 28.3 5 59 to 94 -32 .19 -37 6 Cermet Dry-hard .02 59 to 94 -37 -33 -47 25 *5
---- ---- ---- ---- ----
Cermet Dry-hard 59 t o 94 Cermet Dry-hard 59 t o 94 .03 -29 27.8 . 3 6 -45 Cermet Dry-hard .02 9 59 to 94 27.4 .38 .34 .45 10 Cermet Dry-soft 1 6 t o 23 .46 .04 .64 -35 25.7 11 Inconel X Dry-hard .18 .46 59 to 94 -35 -03 3 0 . 6 --------
12 Inconel X Damp - hard . c 6
- 2 5 26.4
.07 .47 - - 'Stabilized slideout w a s considered t o be t h e period between 2 seconds a f t e r impact and 6 seconds before termination of slideout, at which time i m - pending s t a t i c f r i c t i o n s t a r t e d t o increase t h e c o e f f i c i e n t of f r i c t i o n .
a
8x103 Normal skid Normal skid load I
v I I I I I 1 1 1 1
0 4 8 12 16 20 24 28 32 36 4 0 Time after initial touchdown, sec 0 I I I I I 1 1 1 1 1 200 180 160 140 120 100 80 6 0 4 0 20 0 Velocity, knots (a) Left main gear.
Figure 5.- Drag characteristics of main gear on test 9. Cermet-coated skids; dry-hard lakebed.
P 10 03 Normal skid 6 load and Normal skid load drag load,
-
Ib 4 r I ' I I I I I I I I I I 4 8 12 16 2 0 2 4 28 32 3 6 4 0 Time after initial touchdown, see .8 . 6 A Coefficient .4 of friction .2 I I I I I I I I I I 2 0 0 180 160 140 120 100 8 0 60 4 0 2 0 0 Velocity, knots (b) Right main gear.
Figure 5.- Concluded.
8X1O3
I
6 - Normal skid Normal skid load load and drag load, Ib
Drag load ,--- /'
\Jlr\zy~gcJ+-+VV---- \,- w\P".L,PzwL%--..
I I I I 1 I I I I I I I I 0 2 4 6 8 10 12 14 16 18 2 0 2 2 24 2 6 Time after initial touchdown, sec .8 .6 Coefficient .4 of friction .2 01 I I I I I I I I I I 2 0 0 180 160 140 120 100 80 60 4 0 2 0 0 Velocity, knots (a) Left main gear.
Figure 6.- Drag characteristics of main gear on test 10. Cermet-coated skids; dry-soft lakebed.
I - ' P 8 -
-
Normal skid 6
-
.2 I I I I I I I I I I (b) Right main gear.
Figure 6.- Concluded.
1 2 x 1 0 ~ Normal skid load and drag load, Normal skid load Ib
w w
/ r D r a g load 0 4 8 12 16 2 0 2 4 2 8 3 2 36 40 Time after initial touchdown, sec .8 .6 0 Coefficient .4 o o o o o 0 0 0 0 of friction oo 0 O 0 0000 000 0 0 o o o o o o 0 .2
c8
I I I I I I I I I I 2 2 0 2 0 0 180 160 140 120 100 8 0 60 40 20 0 Velocity, knots (a) Left main gear.
Figure 7.- Drag characteristics of main gear on test 11. Inconel X skids; dry-hard l&-&d.
12 x i 0 9 Normal skid load and drag load, Ib Normal skid load 4 ' / Drag load //
2 1 ----------------. --I/----_, , /-,,A
L' J I I I I I I I I I I 0 4 8 12 16 2 0 2 4 2 8 3 2 3 6 4 0 Time after initial touchdown, sec .6 *8[ 0 Coefficient 0 0 0 0
-
of friction .4 O O 0 0 0 0 0 0 ~ 0 0 0 o o o o o o oooo 0 0 0
oo 0
.2 - O0
0 I I I I I I I I I I I 2 2 0 2 0 0 180 160 140 120 100 8 0 6 0 4 0 2 0 0 Velocity, knots (b) Right main gear.
Figure 7.- Concluded.
8 - 6 - Normal skid load and drag load, Ib Time after initial touchdown, sec
*'C .4 OOB
Coefficient of friction 0 0 0 0 0 .2 0 O 0 o o o o o o o 0 % 0% o@ooo $o&@o @ 8 O 0 1 I I I I I I I I I I 2 4 0 2 2 0 2 0 0 180 160 140 120 100 80 6 0 4 0 2 0 0 Velocity, knots (a) Left main gear.
Figure 8.- Drag characteristics of main gear on test 12. Inconel X skids; damp-hard lakebed.
1 0 x 1 0 ~
I
8 -
-
Normal skid 6 load and drag load, Ib Normal skid load f Drag load /'
- . - , q $ - # /---- 0--0------4-- ,----- --c@---cI .- I/- --c- ---c- --+'
I I I I I I I 0 4 8 12 16 2 0 2 4 2 8 32 3 6 4 4 4 8 5 2 5 6 60 40 Time after initial touchdown, sec
I o
2 4 0 2 2 0 2 0 0 180 160 140 120 100 8 0 6 0 4 0 2 0 0 Velocity, knots (b) Right main gear.
Figure 8.- Concluded.
Included are the mean c o e f f i c i e n t of f r i c t i o n and t h e mean absolute devi- a t i o n ( r e f . 1 4 ) of t h e f r i c t i o n coefficient, e q r e s s e d by where (pi - c ) = absolute deviation from t h e mean c o e f f i c i e n t of f r i c t i o n of a d a t a value p i fi = t h e number of occurrences of t h e associated value pi N = t o t a l number of data points considered DISCUSSION OF RFSULTS Coefficient of F r i c t i o n The process of a skid s l i d i n g on a r e l a t i v e l y s o f t surface such as a lake- bed involves shearing and ploughing, two of t h e p r i n c i p a l f a c t o r s which pro- duce t h e r e s i s t a n c e t h a t determines t h e c o e f f i c i e n t of f r i c t i o n . The shearing term i s e s s e n t i a l l y independent of skid pressure, whereas t h e ploughing term i s a function of skid penetration, which, i n turn, i s a function of skid shape, load d i s t r i b u t i o n , and pressure ( r e f . 8). For t h i s analysis, t h e values of drag force and normal load on each skid were determined from d a t a recorded f o r each main-gear skid during landing impact and slideout.
E f f e c t of surface hardness.- The e f f e c t of surface hardness on t h e . . ...
shearing and ploughing terms of t h e s l i d i n g c o e f f i c i e n t of f r i c t i o n i s shown i n f i g u r e s 9 t o 11.
.8 -- X-15, tests 1 to 5 , average CBR 7 6 of t h e c o e f f i c i e n t s of f r i c t i o n
--__ Experimental data, for t e s t s 1 t o 5 with t h e experi-
CBR 60 mental ground-tow t e s t s of r e f e r - *' ----Experimental data, CBR 16 to 23 ence 8 a t low v e l o c i t i e s . The f i v e recorded s l i d e o u t s were made Coefficient
-
on a surface with an average CBR of friction .4 of 76. The experimental d a t a of reference 8, using t h e same type
- of skid, apply t o a surface w i t h a
.2 CBR of 60 and 16 t o 23. The coef- f i c i e n t s of f r i c t i o n from f l i g h t I I I I 1 t e s t s ( 0 . 3 0 ) a r e considerably Figure 10 shows, photographically, t h a t skid penetration i n t h e area o f landing impact and slideout, with a CBR of 59 t o 94, w a s s l i g h t , with l i t t l e breakthrough of t h e surface ( m a x i - mum v e r t i c a l load occurred during _ _ _ _ _ - * - . - .. ....--rc= -= 52- ~ - . 5 5 = : - . - - nose-gear impact).
-
. - Figure ll compares t h e average c o e f f i c i e n t s of f r i c t i o n f o r t h e cermet-coated skids during t e s t s 6, 8, and 9 on a hard surface ( e s t i - . . . . .
- , . ., .
mated CBR of 39 t o 94) and during t e s t 10 on a s o f t surface ( e s t i - . - mated CBR of 16 t o 2 3 ) . This com- /,,,.TI . .
parison i l l u s t r a t e s t h e e f f e c t of I t 2 % .
surface hardness and skid ploughing on t h e measured coefficients of
-
. I f r i c t i o n . The increase i n t h e values of t h e mean coefficients of f r i c t i o n (0.36 to 0.46) can be at- c , . , . 'L ' , t r i b u t e d t o t h e ploughing of t h e I . . .
, , . skid i n t o t h e s o f t e r surface, in- E-5233 asmuch as t h e skid bearing pres- Figure 10.- Landing impact and slideout area. Rogers sures are approximately equal.
Dry Lake.
(Additional t e s t s may indicate t h a t of skids i s t h e ploughing f a c t o r p a r t i a l l y dependent on bearing pressure, i.e., a s k i d o f t h e same size, shape, and load distribution, on t h e same lakebed would produce d i f f e r e n t values of f r i c t i o n coefficients with varying bearing pressure or skid penetration.)
- 8 r L,- // -- .
Coefficient .4 of friction
.2 - Tests 6, 8, and 9 (dry-hard lakebed) '1
I I I 1 - - 1 I 1 1 I J As i n t e s t s 1 t o 3 , t h e skid penetration of t h e cermet-coated skids i n and 9 w a s s l i g h t , with l i t t l e breakthrough of t h e surface; how- tests 6, 8, ever, a s e r i e s of grooves l e f t by t h e exposed tungsten carbide chips could be O n t h e dry-soft surface of t e s t 10, t h e penetration of t h e skids observed.
at t h e time of nose-gear impact was approximately 2.5 inches t o 2.75 inches 12), as compared t o t h e i n s i g n i f i c a n t penetration o f t e s t s 1 t o 6, 8, ( f i g .
and 9. During t h e s t a b i l i z e d portion of t h e slideout on t h e dry-soft surface, t h e penetration w a s approximately 0.10 inch t o 0.15 inch, except a t impact with surface i r r e g u l a x i t i e s . The skids broke through t h e surface, leaving a
1 8
compacted-powder residue along t h e skid t r a c k t h a t could be removed t o a depth of approximately 0.20 inch before a f i r m surface was reached.
Figure 13 compares the recorded f l i g h t data from t e s t s 6 and 8 t o 10 with the experimental d a t a of r e f e r - ence 8, i n which the same type of s k i d was used but with a thinner cermet coating. The recorded f l i g h t data agree closely with t h e experi- mental r e s u l t s above 40 knots, as- suming t h a t t h e experimental data remain constant at high speeds.
The i r r e g u l a r i t y of t h e curve ( s e e f i g . ll a l s o ) f o r t e s t 10 i s due primarily t o t h e roughness of t h e d i f f e r e n t lakebed surface. Although t h e lakebed surface f o r t h e ground- tow t e s t s of reference 8 w a s not i d e n t i c a l t o t h e lakebed surface f o r f l i g h t t e s t s 6 and 8 t o 10, t h e hard and s o f t surfaces do y i e l d similar r e s u l t s . These r e s u l t s show t h a t t h e ploughing e f f e c t r e s u l t s i n t h e c o e f f i c i e n t of f r i c t i o n being par- t i a l l y dependent on t h e skid bearing pressure.
E- 11308 12.- Landing impact and slideout area on Figure Effect of material.- The e f f e c t Cuddeback Dry Lake.
of various s k i d materials on t h e c o e f f i c i e n t of f r i c t i o n during s l i d e o u t on n a t u r a l and simulated
p 8 r
lakebed surfaces ( r e f . 7) i s shown i n f i g u r e 14. This f i g u r e compares t h e average c o e f f i c i e n t s of f r i c t i o n as a function of forward v e l o c i t y f o r skids of 4130 s t e e l , 4130 s t e e l X with cermet coating, and Inconel on t h e same landing s i t e , and f o r a X-15, tests 6, 8, and 9, CBR 7 6 1020 s t e e l skid, with a contact area ---- X-15, test 10, CBR 16 to 2 3
of 4 inches by 24 inches, on a sirnu-
Experimental data, CBR 60 l a t e d lakebed. Coefficients of !ZZZ?ZlZ Experimental data, CBR 16 to 2 3 f r i c t i o n f o r t h e 4130 s t e e l s k i d and 100 80 60 40 2 0 0 f o r t h e 1020 s t e e l skid on a simu- Velocity, knots l a t e d lakebed agree well a t veloci- t i e s above 70 knots. The skid Figure 13.- Variation of coefficient of friction with true bearing pressure was 26.8 lb/sq i n .
ground speed for recorded X-15 flight data and experi-
f o r t h e 4130 s t e e l s k i d and
mental data. Cermet-coated skids.
22.4 lb/sq i n . f o r t h e 1020 s t e e l skid.
l l l l l l l l I
Tests 1 to 5, 4130 steel Tests 6, 8, a n d 9, cermet coating --- Test 11, Inconel X Simulated lakebed, 1020 steel (ref. 7) 0 Coefficient of friction *4 J 2 2 0 200 180 160 140 120 100 8 0 6 0 4 0 2 0 0 Velocity, knots Figure 14.- Effect of various skid materials on the coefficient of friction.
Dry-hard runways.
Figure 14 a l s o illustrates t h e rise i n t h e mean c o e f f i c i e n t of f r i c t i o n f o r t h e cermet-coated skids (0.36) over t h a t f o r 4130 steel skids (0.30).
This increase i s a t t r i b u t e d t o t h e s o f t copper-nickel matrix and t h e exposed tungsten carbide chips, which r e s u l t e d i n an increased ploughing component.
Figure 13 shows a bottom view of t h e cermet-coated skid, revealing t h e exposed tungsten carbide chips and the sheared copper-nickel matrix.
I-- _ _ E- 10825 Figure 15.- Bottom view of cermet-coated skid after first slideout.
The r e s u l t s of t e s t 1 1 i n which an InconelX skid w a s used are a l s o shown The reason f o r t h e increase i n t h e mean coefficient o f f r i c t i o n i n f i g u r e 14.
f o r t h e Inconel X skid over t h a t f o r t h e 4130 s t e e l skid has not been estab- l i s h e d .
Effect of moisture.- The e f f e c t of moisture on t h e coefficient of f r i c - t i o n i s shown i n t h e r e s u l t s from t e s t s 1 1 and 12. The same InconelX skids and t h e same landing location were used i n t h e two t e s t s ; however, on t e s t 12 the lakebed surface w a s damp, because of a recent r a i n f a l l .
Figure 16 presents t h e average values f o r t h e two landings. The drop i n t h e mean value (0.35 f o r t e s t 1 1 and 0.25 for t e s t 12) between t h e two f l i g h t s i s a t t r i b u t e d t o t h e dampness o f t h e lakebed surface. Skid penetration f o r t e s t 1 1 was s l i g h t , with some surface breakthrough during t h e i n i t i a l phase of t h e slideout. Skid penetration f o r t e s t 12 w a s a l s o s l i g h t ; however, the skid tracks were com- pacted and l e f t a r e l a t i v e l y s l i c k surface because of t h e moisture content o f t h e lakebed material.
c / Test 11 (dry-hard lakebed)
i
Coefficient of friction '4 I I I I I I I O J ~ 1 1 I I I 2 2 0 2 0 0 180 160 140 120 100 8 0 6 0 40 2 0 0 Velocity, knots Figure 16.- Effect of a damp surface on coefficient of friction. Inconel X skids.
The e f f e c t of a damp surface on a skid-type landing i s shown a l s o i n t h e slideout distance recorded on t e s t 12. Although there i s some correlation between t h e X-15 slideout distance and touchdown velocity, t h e longest slide- out of 8968 f e e t f o r a touchdown v e l o c i t y of 221 knots was experienced on t h i s t e s t . A slideout of 7228 f e e t w a s recorded for t h e highest touchdown velocity of 238 knots ( r e f . 5 ) .
The r e s u l t s of t e s t =--the lower coefficient of f r i c t i o n and t h e re- s u l t i n g slideout distance--indicate t h e e f f e c t of a damp surface on a skid- type landing.
Skid Wear Skid wear i s caused by t h e shearing of surface i r r e g u l a r i t i e s from t h e skid surface and t h e separation of t h e i r r e g u l a r i t i e s from t h e skid material by t h e surface over which it i s s l i d i n g . The amount of skid wear depends on t h e speed of sliding, t h e strength and hardness of t h e skid material, t h e strength of t h e surface material, and t h e sliding distance.
The thickness of t h e X - Y j skids was measured a f t e r each f l i g h t t o deter- mine t h e usefulness of t h e skid f o r t h e succeeding f l i g h t . Skids were re- jected when t h e minimum thickness (generally near the point of attachment of t h e main s t r u t ) w a s 0.06 inch or l e s s . Because of deep grooves i n t h e skids, t h e thickness w a s taken as t h e average of several measurements. The weight of t h e material removed during each slideout w a s determined by multiplying t h e volume removed by t h e s p e c i f i c weight of t h e material. The skids were not weighed a f t e r each f l i g h t because of the d i f f i c u l t y of removal and r e i n s t a l - l a t ion.
Wear data for t h e 4130 s t e e l skids were obtained from previous X-15
f l i g h t s , r a t h e r than from t h e f l i g h t s of t h i s investigation, because it w a s not possible t o phase a new s e t of skids i n t o t h e program at t h e t h e t h e drag- brace instrumentation w a s i n s t a l l e d . Data from f i v e f l i g h t s , numbers 1-9-17
I
Skid l i f e for t h i s type o f skid i s to 1-13-25 ( s e e ref. 5 ) , a r e presented.
about f i v e landings. The sketches i n figures 1 7 ( a ) and 17(b) show t h e approxi- mate positions a t which thickness w a s measured on t h e 4130 s t e e l skids a f t e r each landing. Below t h e sketches axe p l o t s of t h e average t o t a l skid thick- ness as a function of longitudinal position.
+ + + + +
+ + + + + E;]- 1
+ + + + +
-___ Skid measurement locations Flight 0 1-9-17 .4 .
0 1-10-19 0 1-11-21 A 1-12-23 - Total .3 V 1-13-25 skid thickness,
In. .2 -
I 1 1 1 I ~ 1 -.I- l a
0 4 8 12 16 2 0 2 4 2 8 3 2 3 6 Skid length, in.
( a ) Left main gear.
c E r - [-::r::
+ + + + +
Skid measurement locations . 5 r Flight 0 1-9-17 0 1-10-19 0 1-11-21 Total .3 A 1-12-23 skid V 1-13-25 thickness, in. .2 @ @ - = -
I I I I I I I I I J
0 4 8 12 16 2 0 2 4 2 8 3 2 3 6 Skid length, in.
(b) Right main gear.
Figure 17.- Variation of total skid thickness of the main gear for five X- 15 flights. 4130 steel skids.
with skid length Figures 18( a) and 18(b) present similar information on t h e cermet-coated s k i d s .
Considerable wear occurred during t h e first landing because of t h e s o f t outer layer of copper-nickel. Less wear occurred during l a t e r landings as t h e Some amount of material flow, or trans- tungsten carbide chips were uncovered.
f e r , w a s noted, which made measurements d i f f i c u l t t o i n t e r p r e t . The heat Skid measurement locations Total skid
thickness, .21 Test
in.
0 Before 6 0 6 0 7 .1 b 8 A 9 v 1 0 I I I I I I 1 I.
0 4 8 12 16 2 0 2 4 2 8 32 36 Skid length, in.
(a) Left main gear.
Skid measurement locations Total skid thickness, Test .- 111.
0 Before 6 0 6 0 7 b 8 A 9 - v 1 0 I I I I I I 1
I
0 4 8 12 16 2 0 2 4 2 8 32 3 6 Skid length, in.
(b) Right main gear.
Figure 18.- Variation of total skid thickness of the main gear with skid length for tests 6 to 10. Cermet-coated skids.
generated by t h e slideout and t h e physical c h a r a c t e r i s t i c s of t h e copper- nickel matrix are believed t o be responsible f o r t h i s t r a n s f e r of material.
The weaz of coated and uncoated skids on a n a t u r a l lakebed surface i s compared i n f i g u r e 19 i n terms of pounds of material removed as a function of s l i d i n g distance. The data f o r t h e 4130 s t e e l skids showed an increasing amount of skid wear as t h e s l i d i n g distance increased beyond 6400 f e e t .
The data f o r t h e cermet skids revealed a considerable amount of w e a r f o r t h e first
slideout of 5702 f e e t , which w a s expected because of t h e s o f t copper-nickel
matrix. The remaining landings showed a reduced amount of wear, similar t o t h e experience with t h e 4130 s t e e l skid. This reduction i n wear i s a r e s u l t of the increasing proportion of contact with t h e hard carbide chips and work- hardening of t h e copper-nickel matrix.
2.4 0 4130 steel 0 Cermet coating 2.0 1.6 Skid wear, 1.2 Ib 0 0 .8
- O O
.4 0 ono 0 C D 000
4 I I
Slideout distance, ft Figure 19.- Variation of wear for 4130 s t e e l and cermet-coated skids on a lakebed surface for various slideout distances.
Wear c h a r a c t e r i s t i c s of the Inconel X skids were not determined because of t h e d i f f i c u l t y o f measuring t h e chemically milled areas inside t h e skid.
However, preliminary d a t a indicate wear resistance superior t o t h a t of 4130 s t e e l , with o r without a cermet coating.
CONCLUSIONS Results of an investigation t o determine t h e c o e f f i c i e n t s of f r i c t i o n and the wear c h a r a c t e r i s t i c s f o r X-15 landing-gear skids of various materials showed t h a t : 1. The mean c o e f f i c i e n t of f r i c t i o n determined from f l i g h t data f o r t h e
4130 s t e e l skids was 0.30. Comparison of f l i g h t data and ground-tow t e s t s
on the same lakebed r e s u l t e d i n lower values of f r i c t i o n coefficients f o r t h e X-15. The r e s u l t s a l s o tend t o agree with ground-tow t e s t s on a simu- l a t e d lakebed f o r v e l o c i t i e s exceeding 70 knots.
2 . The mean c o e f f i c i e n t of f r i c t i o n f o r t h e cermet-coated skids on a dry- i hard surface was 0.36. The ploughing action of skids and i t s e f f e c t on t h e measured coefficient of f r i c t i o n i s i l l u s t r a t e d by t h e increase of measured to 0.46 on a dry-soft surface.
values 3. The moisture content of t h e lakebed surface has a marked e f f e c t on t h e coefficient of f r i c t i o n . The mean coefficient of f r i c t i o n of t h e Inconel X skids was 0.35 on a dry lakebed surface and 0.25 on a damp lakebed surface.
4. The wear experienced by t h e 4130 s t e e l skids tended t o be constant up to a slideout distance of 6400 f e e t , with increased wear during greater slide- out distances.
Flight Research Center, National Aeronautics and Space Administration, Edwards, C a l i f . , December 3, 1965.
REFERENCES 1. Houbolt, John C . ; and Batterson, Sidney A . : Some Landing Studies Pertinent t o Glider-Reentry Vehicles. NASA TN D-448, 1960.
2. McKay, James M.; and Kordes, Eldon E . : Landing Loads and Dynamics of t h e X - 1 5 Airplane. NASA TM X-639, 1962.
3. McKay, James M . ; and Scott, Betty J.: Landing-Gear Behavior During Touch-
down and Runout f o r 17 Landings of t h e X-15 Research Airplane. NASA
TM x-518, 1961.
Measurements Obtained During t h e F i r s t Landing of t h e 4. McKay, James M . : North American X - 1 5 Research Airplane. NASA TM X-207, 1959.
5. Noll, Richard B.; Jarvis, Calvin R . ; Pembo, Chris; Lock, Wilton P.; and Scott, Betty J.: Aerodynamic and Control-System Contributions t o t h e X-15 Airplane Landing-Gear Loads. NASA TN D-2090, 1963.
6. Noll, Richard B.; and Halasey, Robert L.: Theoretical Investigation o f t h e Slideout Dynamics of a Vehicle Equipped With a Tricycle Skid-Type NASA TN D-1828, 1963.
Landing-Gear System.
7. Dreher, Robert C . ; and Batterson, Sidney A.: Coefficients of F r i c t i o n and Wear Characteristics f o r Skids Made of Various Metals on Concrete, Asphalt, and Lakebed Surfaces. NASA TN D-999, 1962.
8. Tebben, Gerald D . : An Investigation of t h e Coefficients of F r i c t i o n and Wear Properties of Cermet Material Proposed f o r t h e X-20A Nose Gear Skid.
FTC-TDR-63-20, U. S. Air Force, Dee. 1963.
9. Tebben, Gerald D.: An Investigation of the Coefficients of Friction and
Wear Property of Wire Brush Skids Constructed With Ren6 41 Bristles.
FTC-TDR-64-=, U.S. Air Force, Aug. 1954.
1 0 . Walker, H.; Deutschman, J.; and van Summern, J. : Pitch Landing Condition.
Nose Gear Loads. Rep. No. 52-941-010, Bell Aircraft Corp., Apr. 1953
(rev. July 1 9 5 4 ) .
U. Blanchard, Ulysse J.: Landing Characteristics of a Winged Reentry Vehicle With All-Skid Landing Gear Having Yielding-Metal Shock Absorbers.
NASA TN D-1496, 1962.
12. Tremant, Robert A.: Operational Experiences and Characteristics of the X-15 Flight Control System. NASA TN D-1402, 1962.
13. Yancey, Roxanah B.: Flight Measurements of Stability and Control Deriva-
tives of the X-15 Research Airplane to a Mach Number of 6.02 and an
Angle of Attack of 23".
NASA TN D-2532, 1964.
14. Kenney, J. F.; and Keeping, E.S.: Mathematics of Statistics. Part One.
Third ed., D. Van Nostrand Co., Inc., 1954, p. 76.
2 6 NASA-Langley, 1966 H-392
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