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

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Wear, Friction, and Temperature Characteristics of an Aircraft Tire Undergoing Braking and Cornering https://ntrs.nasa.gov/search.jsp?R=19800004758 2020-03-21T20:55:26+00:00Z View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by NASA Technical Reports Server 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 provide operations 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 cor- nering. 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 dur- ingbraking 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 to the 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 the effects of tire tread wear attributed to the vari- ous ground operations of an airplane. Reference 1 presentsthe results from the initial study which exploredthe wearand related characteristics of fric- tion 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 - 2 . 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 inflation pressure 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 through- out 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 thesur- faces 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 univer- sal 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 position to 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.
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