Wear, Friction, and Temperature Characteristics of an Aircraft Tire Undergoing Braking and Cornering

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NASA Technical Paper 1569

Wear, Friction, and Temperature Characteristics of an Aircraft Tire Undergoing Brakingand Cornering

John L. McCarty, Thomas J. Yager, and S. R.Riccitiello

DECEMBER 19 79 . ~~

TECH LIBRARY KAFB, NM

OL3477b NASA Technical Paper 1569

Wear, Friction, andTemperature Characteristics of an Aircraft Tire Undergoing Brakingand Cornering

John L. McCarty and Thomas J. Yager Langley ResearchCellter HamptotZ, Virgitlia S. R.Riccitiello Ames ResearchCelzter MoffettField, Califoruia

National Aeronautics and Space Administration

Scientific and Technical Information Branch

1979 SUMMARY

An experimental investigation was conducted to evaluate the wear, friction, and temperature characteristics of aircraft tire treads fabricated from differ- entelastomers. Braking and corneringtests were performed on size 22 X 5.5, type VI1 aircraft tires retreaded with currently employedand experimental elastomers. The braking testsconsisted of gearingthe tire to a driving wheel of a ground vehicle to provideoperations at fixed slip ratios on dry surfaces ofsmooth and coarseasphalt and concrete. The corneringtests involved freely rolling the tire at fixed yaw angles of O0 to 24O on thedry smooth asphalt surface. The results show thatthe cumulative tread wear varieslinearly with distancetraveled at all slip ratios and yaw angles. Thewear rateincreases with increasing slip ratio duringbraking and increasing yaw angleduring cornering. The extent ofwear in eitheroperational mode is influenced by the character of the runway surface. Of thefour tread elastomers investigated, 100-percentnatural rubber was shown to be theleast wear resistant and the state-of-the-artelastomer, comprised of a 75/25 polyblend of cis-polyisoprene and cis-polybutadiene, proved most resistantto wear. The resultsalso show that the tread surface temperature and the friction coefficient developed during braking and cornering is independent of thetread elastomer. A comparison of tire-tread data obtained during the cornering tests with those from thebraking tests, on thebasis of equivalentslip velocities, suggests that the amountof tread wear is comparable but friction and surfacetemperatures are greater duringbraking operations. The difference is attributedto the tire being softer in the lateral direction whichwould tend to reduce the relative slippage between the tire and the pavementand thereforeprovide a lower effective slip ratio in cornering.

INTRODUCTION

Tire replacement is of majoreconomic concern tothe aviation industry. The reasonsfor tire replacementinclude cutting, which is generallyattributed to characteristics of the runway surface and to the presence of foreign objects; tearing and chunking, where strips or chunks of rubber areseparated from the tire; and tread wear, which results from braking, yawed rolling maneuvers, and wheel spin-up at touchdown. The primaryreason is tread wear, particularly that due tothe braking andyawed rollingrequired during the landing roll-out and taxi phases of the normal ground operations of an airplane. In viewof the economicand inherent safety considerations, NASA undertook a program in the early 1970's to examine theeffects of tire tread wear attributed to the various ground operations of an airplane. Reference 1 presentsthe results from the initial study which exploredthe wearand related characteristics of friction and treadsurface temperatures for an aircraft tire duringbraking. The purpose of theinvestigation reported in this paper is to extend that initial study(ref. 1) toinclude (1) differenttread materials with known elastomeric composition, (2) different runway surfaces, (3) more complete temperature coverageboth onand beneaththe tread surface, (4) higher slip ratios in the

I" braking mode, and (5) operations in the yawed rolling mode.The tiresfor this study were size 22 x 5.5, type VI1 aircraft tires retreaded with both currently employedand experimentalelastomers. The experimentalelastomers are part of a program beingconducted by the Chemical Research Projects Office at the NASA Ames ResearchCenter to seek new elastomeric materials whichwould provide improved tiretread wear, traction, and blowout resistance. This programand someof the early test results are discussed in reference 2.

SYMBOLS

Values are given in both SI and U.S. Customary Units. The measurements and calculations were made in U.S. Customary Units.

Nb number of revolutions of braked wheel over a measured distance

NO numberof revolutions of free-rolling(unbraked) myawed wheel Over a measured distance

RS ratioslip % test speed of ground vehicle

Vt circumferentialtire velocity vr,bresultant slip velocity of a braked, unyawed tire

Vr 19 resultant slip velocity of a free-rolling yawed tire 9 yaw angle

APPARATUS AND TEST PROCEDURE:

Tires

The tires of this investigation were size 22 x 5.5, 12-ply rating, type VII, aircraft tires whichwere retreaded with stocks which used the fourdifferent elastomers defined in thefollowing table:

Elas t- Canposition .. A- A- [ 100%natural rubber

B 75%rubber (85% natural, 15% synthetic) 25%cis-polybutadiene

C 75%natural rubber 25% vinylpolybutadiene

D 75%natural rubber 25%trans polypentenemer -

. Elastomer A was selectedfor testing because natural rubber has been considered the elastomer that would best satisfy the tire requirements for supersonic transport-type aircraft. State-of-the-art treads for jet transports are typi- callycomprised of a 75/25polyblend of cis-polyisoprene, either as natural rubber or "synthetic"natural rubber, and cis-polybutadiene. One such tread stock, inan optimized formulation developed to satisfy various airline carrier requirements, is identifiedin the table as elastomer B. Elastomers C and D were experimentaland the formulation of the tread stock fromthose materials was notoptimized. All retreads were curedin the same mold: thus, all tires hadan identical tread pattern of three circumferentialgrooves. During the retreading process, half of the tires were equippedwith two thermocouples each,which were installed on the same shoulderof the carcass beneaththe tread. The purposeof these thermocouples was to measurethe hysteretic heat- ingwithin the carcass of the tire at a locationwhere tire flexing is great. For all tests, the tires were verticallyloaded to 17.8 kN (4000 lb) which was the approximate maximum loadingavailable with the test fixture andsomewhat below therated loading of 31 .6 kN (71 00 lb) for this tire size. The tire inflationpressure for all butone test was reduced from the rated 1620 kPa (235 psi) to 738 kPa (107 psi),the pressure necessary to produce a 30-percent tire deflection under the test loading. Wear, friction, andtemperature data were obtained for eachof the fixed-slip-ratioconditions, generally throughout the life of each tire tread,that is, until most of thetread rubber was removed. Inseveral of these tests, one tire was used to providedata for two test conditionsbecause of the limited number of available tires.

Test Surf aces

The fixed-slip-ratio tests were conductedon one concrete and two asphalt surfaces at the NASA WallopsFlight Center. Close-up photographs of thesurfaces are presentedin figure I. The surfaceidentified as coarseasphalt had anaverage texture depth of 0.39 mm (0.0152in.) as measured by the grease techniquedescribed in reference 3. The texturedepth of the smooth asphalt averaged0.27 mm (0.01 04 in.) . The concrete had the lowest averagetexture depth (0.24 mm (0.009in.)) but had a veryabrasive sandpaperlike surface. All tire yaw tests were conductedonly on the smooth asphalt. Both the fixed-slip ratio andthe yaw tests were performed with thesurfaces dry.

Ground TestVehicle and Instrumentation

Figure 2 is a photographof the powered ground test vehicle employed in thisinvestigation and figure 3 shows the wheel test fixturein the fixed- slip-ratio mode. Figure 4 is a close-up of theinstrumented wheel test fixture in the yaw test operational mode. Also identifiedin figures 3 and 4 are the keycomponents to each test operation. Vertical load was applied to the tire bymeans of two pneumaticcylinders and this load, together with the drag and side loads on the tire, was measuredby straingage beams inthe wheel test fixture. To provide operations at fixed slip ratios, the test tire was driven through a universalcoupling by interchangeablegears, which in turn were chain- driven by a drivingwheel on the vehicle. Changing the slip ratio entailed merelyreplacing and positioning the gear at thedriving end of the universal

3 coupling. This techniquefor changing slip ratio proved to be farsuperior to that employed in reference 1 forsimilar tests. For the yaw tests,the universal coupling was disconnected and, as shown in figure 4, the entire fixture rotated andclamped at the preselected yaw angle. . During the fixed-slip-ratio tests on tires with elastomers C and Dl and during all yaw tests, an optical pyrometer (shown in fig. 4 but removed in fig.3) wasmounted on thefixture in a positionto monitor treadtemperature after the tire had rotatedapproxi- mately 3/8of a revolutionout of thefootprint. This location of the pyrometer was necessary to avoidcontaminating the sensor element. The output from the pyrometer and those from the two tire-carcass theromocouples, ascollected through slip rings, were recorded onan oscillograph mounted in the vehicle driving compartment. The instrumented trailing wheel, seen in all photographs of thevehicle and identified in figures 3 and 4, provided an accurate measure- mentof vehicle speed and distance, and a cam-operated microswitch transmitted a signalfor each test wheel revolution to the oscillograph as well as to a visualcounter.

Test Technique

The testing techniqueinvolved driving the ground vehicle at a speed of 32 km/hr (20 mph) over a known distance with the test tire driven at a fixed slip ratio or freely rolling at a fixed yaw angle, depending upon the test operational mode, while the number of test wheel revolutions, the tread wear, thevarious tire loadings, and tire tread and carcasstemperatures weremoni- tored. Tread wearwas obtained by weighing the tire and wheel assembly prior totesting and atfrequent intervals during the tire test life. Since the vehicle had to be stopped and the tire and wheel assembly removed forweighing, a number of passes over a known distance weremade on each surface between weighings. In general,the pass lengths were set at 305 m (1000 ft) , but at the high slip ratios and thelarge yaw angles, where the tire wear rates were high,these lengths were necessarily reduced to, in some cases,as low as 152 m (500 ft) and the tire was weighed following each pass. At otherconditions where thetread wearwas slight, numerous passes weremade between weighings.Several free- rolling tests were also conducted onunyawed tires at distances up to 5 km (3.1 miles)to better understand thetemperature buildup in thetire carcass.

During a typical test run, either at one slip ratio or at one yaw angle, the tire was first lawered tothe surface and loaded to 17.8 kN (4000 lb) while thevehicle remained stationary. The vehicle was thendriven at 32 km/hr (20 mph) over thefixed distance, with theacceleration and decelerationphases asbrief as possible. During thepass an oscillograph recorded thevehicle speed and distance, the test wheel revolutions , the outputs from the load beams , and, when available,the response of the two thermocouples and theoptical pyrometer.Figure 5 is a reproduction of a typicaloscillograph record taken during a test at a fixed slip ratio and figure 6 is typical of thosetaken during yaw tests of a freelyrolling tire. At theconclusion of thepass, the vertical load on the tire was removed, the tire was raised from thesurface, and thevehicle was realinedfor another pass. If a tire-weardata point was desiredthe tire andwheel assembly wasremoved andweighed at this time and then reinstalledfor additional passes. Testing of a tire was concluded when mostof thetread rubber had beenremoved. Because of thelimited number of

4 available tires, each tire in the yaw tests was used to obtain data at two angles,and in the fixed-slip-ratio tests a tire was run at two slip ratios wheneverpossible.

Beforeand after each discrete fixed-slip-ratio test, thedriving gear was disengagedand the test tire, whilefully loaded, was drivenover the pass length in the free-rolling or unbraked state andthe number ofwheel revolutions recorded. A knowledgeof the braked and unbraked wheel revolutions was necessary to definethe test slip ratio ofthe braked tire. Similarly, to obtainan equivalentbraking slip ratio duringthe yaw tests, the tire was freely rolled at zero yaw angleprior to andfollowing each yaw test.

RESULTS AND DISCUSSION

General

Data fromthe fixed slip-ratio tests included a revolutioncount of the freely rolling andgear-driven test wheelover the pass distance, tire weights at variousdistance intervals, and time historiesof the vertical anddrag tire loadingsplus the output from the various temperature sensors, when available.These data definedthe test slip ratio, the tire-tread wear rate, the drag-forcefriction coefficient, the approximate tread surface temperature, andthe temperature of the tire carcass beneaththe tread in the shoulder area. The test slip ratio was computedfrom the relationship

No - Nb Slip ratio = NO where No and Nb are the number ofunbraked (free-rolling) and braked test wheelrevolutions, respectively, over the same distanceon the test surface. The tire tread wear was determined from thecumulative loss in tire weight as a functionof distance. The drag-forcefriction coefficient was defined as the ratio ofthe measured drag load to theapplied vertical load.

Similardata, together with the side loadingon the tire, were collected duringthe yaw tests. Thisside loading (perpendicular to thewheel plane) was converted to a side-force friction coefficient by dividing it by theapplied verticalload and, subsequently, when combinedwith the drag-force friction coefficientmeasured parallel to thewheel plane, was trigonometricallytransformed intocornering anddrag friction coefficients, perpendicular and parallel to thevehicle direction of motion,respectively. The tire loadings,revolutioncount, and, when available,temperatures were monitoredduring every pass togetherwith the test vehiclespeed and distance. Figure 5 is a typical oscillographrecord of a pass with the tire operating in the fixed-slip-ratio mode andfigure 6 is typicalof the yawedmode. The oscillationsnoted in the force- gageresponses of these figures are attributed to surfaceunevenness, to motions inthe vehicle andwheel test fixture, and to the tire elastic behavior.These oscillations occurred about faired levels which were observed to reachessen-

5 tiallyconstant values throughout each pass. The frictioncoefficients for a pass werecomputed by dividing the faired drag and/or sideload by the faired vertical load.Figures 5 and 6 also illustrate how, duringthe course of a pass,the tire-tread surface temperature generally increased to somemaximum level whichwas essentially maintainedfor the remainder of theconstant-speed portion of thepass. This level is referredto as the maximum treadtemperature in thediscussions to follow. It is readilyapparent from thetime histories thatthe tire-carcass temperature continues to increase with distancetraveled. Since no equilibrium or quasi-static carcass temperature is reachedduring a pass, the actual measured temperatures from start to completion of each pass arepresented. Temperature data from just one thermocouple arepresented since only slight differences were evernoted between thetemperature levels of the two transducers.

Fixed-Slip-Ratio Tests

Data which typify the results from tires operating at fixed slip ratios are presented in figure 7 for three slip ratios, where the tire wear, steady-state drag-forcefriction coefficient, maximum treadtemperature, and carcass tempera- tureare plotted as a function of distancetraveled. These particulardata are for tires with elastomer C in their tread and the testing was performed on a smooth asphaltsurface. Tire wearwas obtained fram weighings atdistance intervals commensurate with the wear rate; hence, the number of passes between weighings and theactual pass lengths decreased with increasingslip ratio. The frictioncoefficient as well as tread and carcasstemperatures were obtained from theoscillograph records during every pass, and values of the friction coefficient and the maximum tire-treadtemperatures are plotted at distances which representthe midpoint of each pass (fig. 7). The actualcarcass temperatures are plotted as measured duringeach pass since no equilibriumvalue was reached. The data of figure 7 illustratethe linear relationship between tire wearand thetraversed distance, and also show that over a givendistance the wear increases with increasingslip ratio. The nonlinearity noted at a slip ratio of 0.396 can be explained by tread chunking which was observed with that particular combination of tire,surface, and slip ratio.

Figure 7 also shows that,for the slip ratios considered, the drag-force frictioncoefficient increases with increasingslip ratio. However, it is shown later that the friction level at a slip ratio of 0.396 is slightly less thanthe maximum which was developed at a somewhat lower slip ratio. Also, the friction coefficient remains essentially constant with distanceduring operations at a given slip ratio, although it appears toincrease slightly, particularly at thehigher slipratios. Both the magnitude of the maximum tread temperature and therate of temperature rise in thetire carcass increase with increasing slip ratio (fig. 7) .

The followingparagraphs examine in some detail the effects of slip ratio on tread wear, drag-forcefriction coefficient, and maximum tire-treadsurface temperaturefor the four tire-tread elastamers operating on the three test surfaces(figs. 8 to 11). Also included is a discussion of the results from separatestudies to examine thetemperature buildup in the tire carcass during fixed-slip-ratio and free-rollingoperations (figs. 12 and 13).

6 Tread wear .- Wear data from tires equipped with treads made from the four elastamersoperating on the three test surfaces are summarized in figure8. This figurerelates the tire-wear rate to slip ratio, where therate is defined by theslope of thewear/distance curves such asthose presented on the bottom of figure 7. The data from allthree surfaces clearly indicate that the 100-percent natural rubber tread(elastomer A) is theleast wear resistant of the fourelastomers and thestate-of-the-art tread (elastomer B) is the most wear resistant. Treads fabricated from the two experimentalelastomers had similar wear characteristics that approached those of the state-of-the-art tread. It is apparent from figure 8 thatthe tire-wear rate is influenced by thenature, or texture, of thesurface. The lower plot of figure 10 better illustrates this surfaceeffect. The data of figure 10 arefor treads with elastomer A and are typical of all treads examined in terms of the relative difference between the wear rates on thedifferent surfaces. Tire wearon the smooth asphalt is significantly less than that on thecoarse asphalt or concrete surfaces,particularly at the high slipratios. Wearon thelatter two highly texturedsurfaces is quitesimilar. The wide difference in the wear rates associated with the two asphalttest surfaces, as noted in thedata of figures 8 and 10, servesto validate the observation made in reference 1 thatsurface texture is a major factor in tire-tread wear.

Friction.- The variation of drag-force friction coefficient with slip ratio forthe different combinations of tire-treadelastomers and surfaces is presented in figure 9. Each plottedvalue of frictioncoefficient represents the average of that measured over the life of thetread at each slip ratio. The figure shows thatthe developed friction is essentially independent of theelastomer since one curveappears to fair all the data gathered from each surface over theslip-ratio test range. The effect of testsurface on friction can be obtained by comparing thethree sets of data in figure 9 or by examining the upper plot of figure 10 which presents all the friction data collected from tires equipped with elastomer A duringfixed-slip-ratio operations. As has been noted in otherreferences (e.g., refs. 1 and 3), thefriction coefficient is generallyinsensitive to the character of the runway surface when dry.

The magnitude of the maximum friction coefficient developed under the conditions of thesetests is approximately 0.8, which is in good agreement with thevalue of 0.81, predicted from theempirical expression developed in reference 4 at very low ground speeds. However,when compared with thefric- tiondata of reference 1, which reports on similar tests on the same size of aircraft tire, there is a difference in the slip ratio at which the maximum frictioncoefficient is developed. Maximum friction in reference 1 occurred at a slip ratio of approximately 0.18, whereas in the current tests the maximum occurred atslip ratios in the 0.30 to 0.40 range. The explanationfor this difference could be the lower inflationpressure (738versus 827kPa (107 psi versus 120 psi) ) and higherply rating (12 versus 8) of the tires in these tests.

Temperatures.- Difficulties early in the program with a variety of optical pyrometers limited measurements of the tire-tread temperatures to only tires equipped with elastomers C and D, the experimentalelastomers, in thefixed- slip-ratio phase of this program. Themaximum approximate treadsurface temperatures reached by these tires on thethree test surfaces are presented in fig-

7 ure11. These temperaturesare the average of thoserecorded during each pass over the life of thetread at a fixed slip ratio. Figure 11 shows thatthe treadtemperature is insensitiveto the elastomer and thatthe temperatures generatedduring operations on concreteare less thanthose generated on asphaltat the same slipratio. Similar trends were observed in reference 1 and explained on thebasis of thermalconductivity. The conductivity of asphalt is less than that of concrete;therefore, more heat is dissipatedto the concrete which results in acooler tiretread. It is interestingto note that the temperatures of figure 11 are slightly higherthan the temperatures recorded at the same slip ratio in reference 1. The difference in theinflation pressures and plyratings of the two sets of tires is againoffered as an explanation.

It is readilyapparent from the sample oscillographrecord of figure 5 and thetypical results of figure 7 thatthe temperature in the tire carcass, beneath thetread in theshoulder area, does notreach equilibrium during the course of apass. Thus, it is meaningless toassign a carcass temperature value to each pass.Figure 7 does suggest however, thatthe rate of temperature buildup in thecarcass varies with slip ratio and, as noted in thedata at a slip ratio of 0.187, the rate of thatbuildup appears to increase with subse- quentpasses. Figure 12 was prepared tobetter illustrate these points. In this figure,the measured temperatureincrease per unit distancetraveled is plotted as a function of slip ratio for tires equipped with elastomer C operating on the smooth asphaltsurface. Each datapoint is identified with a number thatcorresponds tothe pass number at each slip ratio. The dataare bounded by two curves. The lower curve fairsthe first-pass values and is labeled "new tread," and the upper curve fairsthe last-pass values, wheremost or all of the tread rubber had beenremoved, and is labeled "worn tread." Very little tread wasremoved as a result of eight passes at the lowest slip ratio identified in this figure.Figure 12 shows thatthe rate of temperaturebuildup in thecar- cassincreases with increasingslip ratio, and further,that the rate increases with decreasingtread depth. These increasesare attributed to greater carcass flexing which can be expected to occur at higher slip ratios and to the transfer of heatinto the carcass from the much hottertread surface. Transfer of heat is perhaps an explanationfor the increased rates during subsequent passes at one slip ratio since the distance from the embedded thermocouple to the hot tread surface decreases with eachpass.

In view of the limited available experimental data which describe the temperaturesgenerated beneath the tread of an aircraft tire, several additional tests were conducted specifically to give a better understanding of tire-carcass temperatures. The results from thesetests, all performed with afree-rolling unyawed tire,are presented in figure 13. Figure13(a) shows theeffect of test speed on carcasstemperature where tires equipped with thermocouples were loaded to 17.8 kN (4000 lb) and exposed tothree passes approximately 2.5 km (8200 ft) long down an active runway atthree test speeds. The vehicle speed during one test was increased on eachsubsequent pass with a relatively longpause for runway availability between passes. The speed forthe other test was 84 km/hr (52 mph) forthe first pass, 32 km/hr (20 mph) forthe second, and 64 km/hr (40 mph) forthe third with somewhat shorter pauses betweenthem. It appears thatat the conclusion of both tests the tires were approaching an equilibrium temperature slightlygreater than 80° C. Figure13(a) shows thatthe rate of carcasstemperature rise appears to be afunction of test speed,tending to

8 increase with speed, and the temperature differential with respectto the equilibriumtemperature. More precisely,the colder the tire at the onset of the test, the greaterthe rate sf temperature rise. In an attempt tovalidate the existence of a possibleequilibrium temperature under theloading conditions of these tests, oneof the tires was subjectedto two passes over a 5-km (3.1 miles) highway routeat a constant speed of 32 km/hr (20 mph) . The carcasstemperature recorded during those passes is plotted as a function of distance in figure13(b) and, as in theearlier tests, it appearsas though the temperatureapproaches 83O C as an asymptote which suggeststhat value as an equilibriumtemperature under thosetest conditions.

Figure13(c) shows the effect of underinflation on carcasstemperature. Data arepresented for similar passes with the tire at its normal test inflation pressure of738 kPa (107 psi) and at a pressure of 641 kPa (93 psi). As would be expected, because of theincreased carcass flexing under the same vertical load,the underinflated tire experienced higher temperatures and in time would no doubt reach a somewhat higherequilibrium temperature.

The findings from thesetemperature studies are in agreement with those from similarstudies conducted on automotive tires. (See refs. 5 to 8 for examples .)

Yaw Tests

Figure 14 presentstypical results from the yaw tests for tires retreaded with elastomer C and operated on smooth asphalt.Tire wear, side-forcefriction coefficient, maximum treadtemperature, and carcasstemperature are plotted as a function of distancetraveled at yaw angles $ of8O, 12O, and 20°. As in the fixed-slip-ratio tests, tire wearwas obtained from weighings at distance intervals commensurate with the wear rate: hence the numberof passes betweenweigh- ings and the actual pass lengthsdecreased with increasing yaw angle where the wearwas more pronounced. For thedata of figure 14, weighings occurredafter everypass, but thepass length decreased from305 m (1000 ft), at J, = 8O and 12O, to 152 m (500 ft) at J, = 20°. The friction and temperaturedata were taken from oscillographrecords during every pass and values of theside-force friction coefficient and maximum tire-treadtemperature are plotted in the figureat distances which representthe midpoint of each pass. Again, no equi- libriumcarcass temperature was reached: hence theactual temperatures are plotted in thefigure. The data of thefigure exemplifythe linearvariation of tire wear with distancetraveled andshow that over a given distance the wear increases with increasing yaw angle. The side-forcefriction coefficient, which is based upon theforce developed perpendicularto the wheel plane, increases,as expected, with increasing yaw angle and appearsto be rela- -tivelyconstant throughout the test distance, although a slight increase can be observed at thehighest yaw anglepresented. Figure 14 also shows that the maximum tire-treadtemperature increases with yaw angle but that the rate of temperaturebuildup in thecarcass (at least, at the site of theinstalled thermocouples)appears todecrease with yaw.

9 The detailed effects of yaw angle on tire wear, cornering, and drag friction coefficients and thevarious tire temperaturesare discussed in thefollowing paragraphs .

Tread wear.- Thewear data from the treads of the freely rolling tires in the yaw tests are summarized in figure 15 where the wear rate is plotted as a function of the yaw angle. The different symbols identifythe four tread elastomers examined. As was thecase in thefixed-slip-ratio tests, thecurrent state-of-the-arttread,.elastomer B, is the most resistant to wearand the natural rubber tread,elastomer A, is the least wear resistant. Thewear rate of the two experimentalelastomers, although better than natural rubber, did notdemonstrate the significant improvement noted in thefixed-slip-ratio tests(fig. 8). The data of figure15 depict clearly the increase in tire-wear rate with increasing yaw angle,despite a certain amountof scatter.

Friction.- The side-force friction coefficient of figure 14 is the ratio of theside load on thetire, perpendicular to the wheel plane,to the vertical load and, asobserved in thefigure, can reachlevels at least as high as 0.75. However, thedrag load parallelto the wheel plane is observed to be quitesmall in the time history of figure 6 and, on thebasis of these and othertests, is assumed equal in magnitude tothe tire rolling resistance regardless of yaw angle within the range of thetests reported herein. These tire loadings were trigonometricallytransformed to the tire cornering force, perpendicular to the vehicle motion, and drag force,parallel to the vehicle motion. Both arepresented in coefficient form in figure 16. As was observed in thefixed-slipratio tests, the friction coefficients appear to be independent of thetread elastomersince each corneringfriction coefficient suggests a peak value of approximately 0.65 near a yaw angle of 25O. The drag-forcefriction coefficient,as expected, shows a gradualincrease with yaw angle. By definition, the drag-force friction coefficient will reach a maximum value at a yaw angle of 90° where the cornering-force friction coefficient would reduce to zero.

Temperatures.-Figure 17 presents the maximum approximate tread surface temperatures measured by the optical pyrometer as tires with elastomers B, C, and D were subjectedto the range of test yaw angles. The datasuggest that thetread temperature is independent of theelastomer as was observed in the fixed-slip-ratiotests. The temperature is shown toincrease with increasing yaw angle whichwould be expectedsince the tire experiences a greaterscrub- bing action at the higher yaw angles.

An examination of thecarcass temperatures presented in figure 14 suggests that the rate of temperaturebuildup in thecarcass is reduced when the yaw angle is increased. This reduction is attributedto the physical location of the embedded thermocouples. In thetest described by figure 14, theshoulder area housing the thermocouples was not rolledinto the footprint when the tire wasyawed; thus the rate of temperaturebuildup tended to decreaseas the yaw angle was increased. One series of tests was conducted where, during yaw, the instrumentedshoulder of the tire was rolledinto the footprint. For theselat- ter tests, the rate of temperature riseincreased rapidly with increasing yaw angle. In eitherset of tests, no attempt wasmade toestablish an equilibrium temperature at any yaw angle.

10 Braking and Cornering Relationships

The purpose of this section is to attempt to relate tread wear, friction, and temperaturedata betweenan unbraked, yawed tire and a braked, unyawed tire. To effect this relationship, the assumption is made that these tire characteristics are the direct result of the relative slip velocity between the tire and the pavement. Thus, it is assumed that if theresultant slip velocity is the same, whether stemmingfrom brakingor yawing a tire, the magnitude of these tire characteristics would be the same.

For a braked, unyawed tire the lateral slip velocity is zero and hence theresultant slip velocity Vr,b equalsthe slip velocity in the fore-and- aftdirection. The simplerelatlonship

vr, b = VgRs

applies where V9 is thevehicle ground speed and R, is thebraking slip ratio.

For an unbraked, yawed tirethe lateral slip velocity is Vt sin $, where Vt is thetire circumferential velocity and Q is the yaw angle. The slip velocity in thefore-and-aft direction for the yawed tire is thedifference V - Vt cos Q . For small yaw angles, Vt can be approximated by V9; thus, tzelateral slip velocity becomes Vg sin Q and thefore-and-aft sllp velocity becomes V (1 - cos $). Data collectedduring the course of the yaw tests to aid in vali%atingthe fore-and-aft portion of this approximation are repro- duced in figure 18. This figurepresents, as a function ofyaw angle,the equivalentfore-and-aft slip ratio asdetermined experimentally from the relationship

No - NQ Equivalentfore-and-aft R, = NO

where No is the numberof revolutions of the unbraked andunyawed test wheel over a specifieddistance and NQ is the numberof revolutions of the unbraked, yawed tire over the same distance. These faireddata are compared with the curvedescribed by (1 - cos $) which, forsmall angles, corresponds to the equivalentfore-and-aft slip ratio for the yaw tests. The figure suggeststhat the small-angle assumption is a good one. Thus, theresultant slip velocity Vr,$, which is thevectorial sum of the lateral and fore-and-aft slip velocities, simply becomes

11 A comparison of equations (1) and (3) suggeststhat the tread characteristics of tires operating in the twomodes can becompared on thebasis of slip ratio where, in the yawed case, an equivalent slip ratio is computed for each yaw angle by theexpression

Equivalentresultant R, = {2( 1 - cos 9)

Figures 19, 20, and 21 compare thetire wear rate,friction coefficient, and treadsurface temperature, respectively, of thepure braking operational mode with those of the pure yawing operational mode.The comparison is made on thebasis of resultantslip ratio. For thebraking case in figure 19, the fairedtire-wear-rate curves of figure 8 for smooth asphaltare replotted. For the yawing case,the faired data of figure 15 arerepeated with the yaw angleconverted tothe equivalent slip ratio as defined by equation (4). The figure shows that at the same equivalent slip ratio, or at the same resultant slipvelocity, the tread wear in braking and corneringare similar with the exception of elastomer D.

The friction coefficients of figure 20 forthe braking mode arerepre- sented by thefairing of the smooth asphaltdata of figure 9. Those forthe unbraked yawing mode arethe vector sum of thecornering and drag coefficients of figure 16 plottedas a function of theequivalent slip ratio. Themaximum friction defined by the yawing mode at the highest test yaw angle, which corresponds to an equivalent slip ratio of 0.42, is shown in thefigure to approach the maximum frictionlevel defined by thebraking mode.The differenceat the low slip ratios is due perhapsto differences between the lateral and fore-and- aftelastic behavior of thetire. Aircraft tires are stiffer in the fore-and- aft (braking)direction than in thelateral direction. Unpublished data on tires of similarsize show fore-and-aftspring constants which typically double thelateral spring constants for the same loadingconditions. The softertire carcass would tend to reduce therelative slippage between the tire and the pavement, thus resulting in a lower frictionlevel. This corresponding lower effective slip ratio in yawingcompared with that in braking may also explain the lower wear rates of figure 19 and the lower tire-treadsurface temperatures noted in figure 21.

CONCLUSIONS

12 1. Cumulative tire wear (tread rubber removed) varieslinearly with distance traveled at all slip ratios and yaw angles, and the wear rate (mass removed per unitdistance) increases with increasing slip ratio during braking and increasing yaw angleduring cornering. Of thefour tread elastomers investigated, 100-percent natural rubber was the least wear resistant and the state- of-the-artelastomer, comprised of a 75/25 polyblend of cis-polyisoprene and cis-polybutadiene, proved most resistantto wear. As observed in earlier tests, tread wear is highlyinfluenced by thecharacter of the runway surface.

2. The magnitude of the friction coefficient developed duringbraking andyawing is independent of thetread elastomer. The drag-forcefriction coefficients measuredfrom the fixed-slip-ratio tests are in good agreement with thoseobserved in otherinvestigations in that they are insensitive to thecharacter of the runway surface when dry.

3. Tread surfacetemperature increases with increasing slip ratio and increasing yaw angle and is independent of theelastomer. The temperatures generatedduring braking operations on concreteare less than those generated on asphalt.

4. Tirecarcass temperature beneath the tread in theshoulder area never reached an equilibriumvalue over theshort test lengths during either thebraking or corneringoperations. The rate of temperaturebuildup did increase with increasing slip ratio in thebraking tests and is attributed to tireflexing whichbecomes more pronounced asslip ratios are increased. Dur- ingfree-rolling unyawed tests this temperatureappears to reach an equilibrium value at a rate dependent upon the test speed and thetemperature differential. An underinflatedtire generates higher carcass temperatures due tothe increased tire flexure.

5. A comparison of tire tread data obtained during the cornering tests with those from thebraking tests on the basis of equivalent slip velocities suggeststhat the amountof tread wear is comparable but friction and surface temperaturesare greater during braking operations. The difference is attributed to the tire being sozter in the lateral direction whichwould tend to reduce the relative slippage between the tire and the pavementand therefore provide a lower effective slip ratio in cornering.

Langley Research Center NationalAeronautics and Space Administration Hampton, VA 23665 November 9, 1979

13 REFERENCES

1.McCarty, John Locke: Wear andRelated Characteristics of an Aircraft TireDuring Braking. NASA TN D-6963, 1972.

2. Yager, Thomas J.; McCarty,John L.; Riccitiello, S. R.; andGolub, M. A.: Developments in New Aircraft TireTread Materials. AircraftSafety andOperating Problems, NASA SP-416, 1976, pp. 247-256.

3. Leland,Trafford J. W.; Yager, Thomas J.; andJoyner, Upshur T.: Effects of Pavement Textureon Wet-Runway BrakingPerformance. NASA TN D-4323, 1968.

4. Smiley,Robert F.; andHorne, Walter B.: MechanicalProperties of Pneumatic Tires WithSpecial Reference to Modern Aircraft Tires. NASATR R-64, 1960.

5.Conant, F. S.: Tire Temperatures.Rubber Chem. Technol.,vol. 44, no.2, 1 971 , pp.397-439.

6.Nybakken, G. H.; Collart, D. Y.; Staples, R. J.; Lackey, J. I.; Clark, S. K.; andDodge, R. N.: PreliminaryMeasurements on Heat Balancein Pneumatic Tires. NASA CR-121239, 1973.

7. Clark, S. K.: TemperatureRise-Times in Pneumatic Tires. DOT/TST-75-144, U.S. Dep. Trans., Dee. 1975.(Available from NTIS as PB 266 652/7.)

8. Clark, S. K.; and Loo, M.: Temperature Effects onRolling Resistance of PneumaticTires. DOT-TST-76-93, U.S. Dep. Trans.,Apr. 1976. (Available from NTIS as PB 263622/3.)

14 (a) Coarse asphalt.

(c) Concrete. 679-31 6 Figure 1.- Close-upphotographs of test surfaces.Scale in inches (1 in. = 2.54 cm). 1 5 L-79-317 Figure 2.- Ground test vehicle. Loadingcylinder -a

I arns e=%

ting bracket

I.. ;1 ip ring

.. .

Figure 3.- Wheel test fixture in fixed-slip-ratio mode showing major components.

I L-78-2220.1 Figure 4.- Close-up of wheel test fixture in yaw operational mode showing major components. Treadsurface temperature 1 t 114T C 63' C .

Vertical loa- /

/4 . .

Testqround speed

Test distance

Dragload Test wheel counter (1 perrevolution)

Figure 5.- Reproduction of typical oscillograph record showing instrumentation response during fixed-slipratiooperations. Slip ratio = 0.137; concrete runway surface. hl 0 Treadsurface temperature

J t Jt

Testground speed Sideload

Test wheel counter (1 perrevolution) .. - -- .-...... -...--..-..-...-...."...... ,...... , , , , . I1 '

Figure 6.- Reproduction of typical oscillograph record showing instrumentation response duringyaw operations of freely rolling tires. Yaw angle = 16O; smooth asphalt runway surface. Slip ratio “-0.396 ”0.187 d -0.0084 c5 -g 125 E c

;r;i I. 0 0 V c 0 .-c .-U .k .5

I Q I I I

.5 I. 0 I. 5 2.0 2. 5 Distance, km

I I I I I 0 .4 .8 I. 2 I. 6 Distance, miles

Figure 7.- Typical test results from tiresoperating atfixed slip ratios. Tire treads with elastomer C onsmooth asphalt runway surface.

21 Asphalt,coarse Elastomer

Y OA

Asphalt,smooth

E I. 5 -...Y m Y a- cE 1.0- L co a L P m a .-L c .5- aF .-L L

0 .I .2 .3 .4 .5 Slip ratio

Figure 8.- Variation of tire-wear rate with slip ratio during fixed-slip-ratio operations with test tread elastomers on three runway surfaces.

22 Elastomer OA OB oc AD

-/ coarse Asphalt,

I I I I I I 0 .I .2 .3 .4 .5 Slip ratio

Figure 9.- Variation of drag-force friction coefficient with slip ratio during fixed-slip-ratio operations with test tread elastomerson three runway surfaces.

23 c, c W .r .8 V cc.I- cc W 0 V S .6 0 *I- c, V *r L + .4 Asphal t, course W V L Asphalt, smooth cc0 0 I g .2 L n

E Y '2D, Y

n W c, l-0 L L 03 W F I!! *I-I- 1

0 .1 .2 .3 .4 .5 Slipratio

Figure 10.- Effect ofrunway surface on tire-tread wear rate and drag-force friction coefficient developed duringfixed- slipratio operations. Elastomer A.

24 25 Fa ired

concrete

J D Asphalt, smooth 1 1 1 Asphalt , coarse Concrete

.1 .2 .3 .4 Slip ratio

Figure 11.- Maximum approximate tire-tread surface temperatures measured during fixed-slip-ratio operationson test surfaces.

25 32 t 500

400 YE 24C aJ \ --r V 0 E \ n V aJ 0 c, IU n L aJ P 300 3 L P 2*r 16C 5 3 -0 n 7.? aJ 3 L n 3 c, E IU !OO L 5 aJ c, a IU E L aJ QJ n I- I- E 80 aJ l- IO0

I I I I I 0 .3 .1 .2 .4 .5 Slip ratio Figure 12.- Effect of slip ratio on tire-carcasstemperature buildup rate. Elastomer C on smooth asphalt runway surface. Numbers in symbols referto pass sequence at each slip ratio. - 100

V 0 af 75- L S w m L

I I I I I I 1 I 0 2 4 6 8 Distance, km

I I I I I I 0 1 2 3 4 5 Distance, miles

(a)Test speed.

,,, ,,, Figure 13.- Effect of severaltest parameters on carcasstemperature of freelyrolling unyawed tire. 4 1 oc

80 0 0

W L 3 60 fu L W Q E, c, 2 40 fu V L fu 0

0 2 4 6 8 10 Distance, km

I I I I I I I I 0 1 2 3 4 5 6 7 Distance,miles

(b) Test distance. : Figure 13.- Continued. 801- p = 641 kPa (93 psi)l

c, p = 738 kPa (107 psi ) /

0 2.5 .5 2.01 .o 1.5 Distance , km

0 .5 1 .o 1.5 Di stance , mi1 es

Figure 1 3. - Concluded. 40 k/” ”””””” ””””” ./”--/

“L. 0 “L. J

Distance, km

1 I I ” .L.” J 0 .2 .4 .6 .8 Distance,miles

Figure 14.- Typical results from freely rolling tires at several yaw angles. Elastomer C on smooth asphalt runway surface.

30 1 .o El astomer / 0 A P "-0 B P/'/ "0 C 1' / .75 - """

E Y \ 0, Y

n aJ c, a L .5 L a

?aJ L I-

,2E

0 5 15 20 25 Yaw angle, deg

Figure 15.- Variation of tire-wear rate with yaw anglefor test tread elastomers on smooth asphalt runway surf ace. W h)

.8 El astomer 0 A 0 B .6 0 C D D

10 15 5 10 20 25 Yaw angle,deg

Figure 16.- Variationof cornering and drag friction coefficients with yaw angleon smooth asphalt runway surface. 0

0 0 n W I L 2 c, - L 150 W E W c, -0 m W - El as tomer c,L 100 5 0 B E 0 C X zm 0 D 50 -

0 5 10 15 20 25 Yaw angle, deg

Figure 17.- Variation of maximum approximate tire-tread surface temperature with yaw angleon smooth asphalt runway surface.

W W 1

W IP

.10

.08

Elastomer A .06 0 0 B 0 C D D .04

.02

0 5 10 15 20 25 Yaw angle, deg

Figure 18.- Equivalentbraking slip ratio from freely rolling yawed tires. Braking, JI = Oo (fromfig. 8) -5

"Cornering, unbraked (from fig. 15) 1.25 -

E 1.0 Y \ oi Y " W c, (d L .75 5- (d W 7 a, 3' L .r I- .5

.25

' -( 0 .1 .2 .3 .4 .5 Resultant sl ip ratio

Figure 19.- Effect of operating modeon tire wearon smooth asphalt runway surface. i

1 .o

Elastomer .4 Cornering,unbraked 0 A 0 B 0 C .2 D D

0 .1 .2 .5 .3 .4 Resultant slip ratio

Figure 20.- Effect of operating mode on tire friction coefficient onsmooth asphalt runway surface. Brakin (from fig. 11)

Cornering, unbraked

El as tomr 0 B 0 C 0 D

0 .1 .2 .3 -4 .5 Resultant slip ratio

Figure 21.- Effect of operating modeon maximum approximate tire-tread surfacetemperature on smooth asphalt runway surface.

37 . . - . ~. - ." .. .. - 1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. NASA TP-1569 . . ". . " ." " 1 - - 4. Title and Subtitle 5. Report Date WEAR, FRICTION, AND TENPERATURE CHARACTERISTICS OF December 1979 AN AIRCRAFT TIRE UNDERGOING BRAKING AND CORNERING 6. PerformingOrganization Code

"" " -~."- - . . . " . .. 7. Author(s) 8. PerformingOrganization Report No. L-13239 John L. McCarty, Thomas J. Yager, ..

and S. R. Riccitiello " .. ~ 10. Work UnitNo. 9. Performing Organization Name and Address 505-44-33-01 NASA Langley Research Center 11. Contractor Grant No. Hampton, VA 23665

13. Type of Report and Period Covered . . . "~"" . . . ". .. 12. Sponsoring Agency Name and Address Technical Paper NationalAeronautics and Space Administration - Washington, DC 20546 14. Sponsoring Agency Code

." - ". .___ "" " 15. Supplementary Notes

~~ ~.~ ~ "" ~ . . ..~ . .~ ~ .~ . ~." ~ "" ~ 16. Abstract An experimentalinvestigation was conducted toevaluate the wear, friction, and temperaturecharacteristics of aircraft tire treads fabricated from different elastomers.Braking and cornering tests were performed on size 22 x 5.5, type VI1 aircrafttires retreaded with currently employed and experimentalelastomers. The results shaw thatthe tread wear rateincreases with increasingslip ratio during braking and increasing yaw angleduring cornering, and the extent of wear in either operational mode is influenced by the character of the runway surface. Of thefour treadelastomers investigated, 100-percent natural rubber wasshown to be the least wear resistant and thestate-of-the-art elastomer, comprisedof a 75/25 polyblend of cis-polyisoprene and cis-polybutadiene,proved most resistantto wear. The results also show thatthe tread surface temperature and the friction coefficient developed duringbraking and cornering is independentof the tread elastomer.

.. .. . ~~ ". ~ -- 7. Key Words (Suggested by Author(s1 1 18. DistributionStatement Aircrafttires Unclassified - Unlimited Tire wear Braking Cornering Ground performance SubjectCategory 03 " ~ ~~~ . -~ . - -~-~ ...... " - .. 9. Security Classif. (of this report) 20. Security Classif. (of this page) 21. NO. of Pages 22. Price' U nclas sified Unclassified Unclassified

" ~ . . " I 37: I. $4.50 - - *For sale bv the National Technical Information Service, Swinnfield..- Virninta - 22161 NASA-Langley, 1979 THIRD-CLASS BULK RATE Postage and Fees Paid National Aeronautics and National Aeronautics and Space Administration Space Administration NASA451 Washington, D.C. 20546

Official Business Penalty for Private Use, $300

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