1、NASA Technical Paper 1569 Wear, Friction, and Temperature Characteristics of an Aircraft Tire Undergoing Braking and Cornering John L. McCarty, Thomas J. Yager, and S. R. Riccitiello DECEMBER 19 79 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-. NA
2、SA Technical Paper 1569 TECH LIBRARY KAFB, NM OL3477b Wear, Friction, and Temperature Characteristics of an Aircraft Tire Undergoing Braking and Cornering John L. McCarty and Thomas J. Yager Langley Research Cellter HamptotZ, Virgitlia S. R. Riccitiello Ames Research Celzter Moffett Field, Califorui
3、a National Aeronautics and Space Administration Scientific and Technical Information Branch 1979 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SUMMARY An experimental investigation was conducted to evaluate the wear, friction, and temperature chara
4、cteristics of aircraft tire treads fabricated from differ- ent elastomers. Braking and cornering tests were performed on size 22 X 5.5, type VI1 aircraft tires retreaded with currently employed and experimental elastomers. The braking tests consisted of gearing the tire to a driving wheel of a groun
5、d vehicle to provide operations at fixed slip ratios on dry surfaces of smooth and coarse asphalt and concrete. The cornering tests involved freely rolling the tire at fixed yaw angles of O0 to 24O on the dry smooth asphalt surface. The results show that the cumulative tread wear varies linearly wit
6、h distance traveled at all slip ratios and yaw angles. The wear rate increases with increasing slip ratio during braking and increasing yaw angle during cor- nering. The extent of wear in either operational mode is influenced by the character of the runway surface. Of the four tread elastomers inves
7、tigated, 100-percent natural rubber was shown to be the least wear resistant and the state-of-the-art elastomer, comprised of a 75/25 polyblend of cis-polyisoprene and cis-polybutadiene, proved most resistant to wear. The results also show that the tread surface temperature and the friction coeffici
8、ent developed during braking and cornering is independent of the tread elastomer. A comparison of tire-tread data obtained during the cornering tests with those from the braking tests, on the basis of equivalent slip velocities, suggests that the amount of tread wear is comparable but friction and s
9、urface temperatures are greater dur- ing braking operations. The difference is attributed to the tire being softer in the lateral direction which would tend to reduce the relative slippage between the tire and the pavement and therefore provide a lower effective slip ratio in cornering. INTRODUCTION
10、 Tire replacement is of major economic concern to the aviation industry. The reasons for tire replacement include cutting, which is generally attributed to characteristics of the runway surface and to the presence of foreign objects; tearing and chunking, where strips or chunks of rubber are separat
11、ed from the tire; and tread wear, which results from braking, yawed rolling maneuvers, and wheel spin-up at touchdown. The primary reason is tread wear, particularly that due to the braking and yawed rolling required during the landing roll-out and taxi phases of the normal ground operations of an a
12、irplane. In view of the economic and inherent safety considerations, NASA undertook a program in the early 1970s to examine the effects of tire tread wear attributed to the vari- ous ground operations of an airplane. Reference 1 presents the results from the initial study which explored the wear and
13、 related characteristics of fric- tion and tread surface temperatures for an aircraft tire during braking. The purpose of the investigation reported in this paper is to extend that initial study (ref. 1) to include (1) different tread materials with known elastomeric composition, (2) different runwa
14、y surfaces, (3) more complete temperature coverage both on and beneath the tread surface, (4) higher slip ratios in the I“ Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-braking mode, and (5) operations in the yawed rolling mode. The tires for this
15、study were size 22 x 5.5, type VI1 aircraft tires retreaded with both currently employed and experimental elastomers. The experimental elastomers are part of a program being conducted by the Chemical Research Projects Office at the NASA Ames Research Center to seek new elastomeric materials which wo
16、uld provide improved tire tread wear, traction, and blowout resistance. This program and some of 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 revolu
17、tions of braked wheel over a measured distance NO number of revolutions of free-rolling (unbraked) myawed wheel Over a measured distance RS slip ratio % Vt tire circumferential velocity test speed of ground vehicle vr,b resultant slip velocity of a braked, unyawed tire Vr 19 9 yaw angle resultant sl
18、ip velocity of a free-rolling yawed tire APPARATUS AND TEST PROCEDURE: Tires The tires of this investigation were size 22 x 5.5, 12-ply rating, type VII, aircraft tires which were retreaded with stocks which used the four different elastomers defined in the following table: Elas t- Canposition A- 10
19、0% natural rubber B 75% rubber (85% natural, 15% synthetic) 25% cis-polybutadiene C 75% natural rubber 25% vinyl polybutadiene D 75% natural rubber 25% trans polypentenemer - 2 . Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Elastomer A was selecte
20、d for 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- cally comprised of a 75/25 polyblend of cis-polyisoprene, either as natural rubber or “syn
21、thetic“ natural rubber, and cis-polybutadiene. One such tread stock, in an optimized formulation developed to satisfy various airline carrier requirements, is identified in the table as elastomer B. Elastomers C and D were experimental and the formulation of the tread stock from those materials was
22、not optimized. All retreads were cured in the same mold: thus, all tires had an identical tread pattern of three circumferential grooves. During the retreading process, half of the tires were equipped with two thermocouples each, which were installed on the same shoulder of the carcass beneath the t
23、read. The purpose of these thermocouples was to measure the hysteretic heat- ing within the carcass of the tire at a location where tire flexing is great. For all tests, the tires were vertically loaded to 17.8 kN (4000 lb) which was the approximate maximum loading available with the test fixture an
24、d somewhat below the rated loading of 31 .6 kN (71 00 lb) for this tire size. The tire inflation pressure for all but one 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, an
25、d temperature data were obtained for each of the fixed-slip-ratio conditions, generally through- out the life of each tire tread, that is, until most of the tread rubber was removed. In several of these tests, one tire was used to provide data for two test conditions because of the limited number of
26、 available tires. Test Surf aces The fixed-slip-ratio tests were conducted on one concrete and two asphalt surfaces at the NASA Wallops Flight Center. Close-up photographs of the sur- faces are presented in figure I. The surface identified as coarse asphalt had an average texture depth of 0.39 mm (0
27、.0152 in.) as measured by the grease technique described in reference 3. The texture depth of the smooth asphalt averaged 0.27 mm (0.01 04 in.) . The concrete had the lowest average texture depth (0.24 mm (0.009 in.) but had a very abrasive sandpaperlike surface. All tire yaw tests were conducted on
28、ly on the smooth asphalt. Both the fixed-slip ratio and the yaw tests were performed with the surfaces dry. Ground Test Vehicle and Instrumentation Figure 2 is a photograph of the powered ground test vehicle employed in this investigation and figure 3 shows the wheel test fixture in the fixed- slip-
29、ratio mode. Figure 4 is a close-up of the instrumented wheel test fixture in the yaw test operational mode. Also identified in figures 3 and 4 are the key components to each test operation. Vertical load was applied to the tire by means of two pneumatic cylinders and this load, together with the dra
30、g and side loads on the tire, was measured by strain gage beams in the wheel test fixture. To provide operations at fixed slip ratios, the test tire was driven through a universal coupling by interchangeable gears, which in turn were chain- driven by a driving wheel on the vehicle. Changing the slip
31、 ratio entailed merely replacing and positioning the gear at the driving end of the universal 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-coupling. This technique for changing slip ratio proved to be far superior to that employed in reference 1
32、 for similar tests. For the yaw tests, the univer- sal coupling was disconnected and, as shown in figure 4, the entire fixture rotated and clamped 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
33、 in fig. 4 but removed in fig. 3) was mounted on the fixture in a position to monitor tread temperature after the tire had rotated approxi- mately 3/8 of a revolution out of the footprint. This location of the pyrometer was necessary to avoid contaminating the sensor element. The output from the pyr
34、ometer and those from the two tire-carcass theromocouples, as collected through slip rings, were recorded on an oscillograph mounted in the vehicle driving compartment. The instrumented trailing wheel, seen in all photographs of the vehicle and identified in figures 3 and 4, provided an accurate mea
35、sure- ment of vehicle speed and distance, and a cam-operated microswitch transmitted a signal for each test wheel revolution to the oscillograph as well as to a visual counter. Test Technique The testing technique involved driving the ground vehicle at a speed of 32 km/hr (20 mph) over a known dista
36、nce 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, the various tire loadings, and tire tread and carcass temperatures were moni- tored. Tread wear was obtain
37、ed by weighing the tire and wheel assembly prior to testing and at frequent intervals during the tire test life. Since the vehi- cle had to be stopped and the tire and wheel assembly removed for weighing, a number of passes over a known distance were made on each surface between weigh- ings. In gene
38、ral, the pass lengths were set at 305 m (1000 ft) , but at the high slip ratios and the large 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 other conditions where th
39、e tread wear was slight, numerous passes were made between weighings. Several free- rolling tests were also conducted on unyawed tires at distances up to 5 km (3.1 miles) to better understand the temperature buildup in the tire carcass. During a typical test run, either at one slip ratio or at one y
40、aw angle, the tire was first lawered to the surface and loaded to 17.8 kN (4000 lb) while the vehicle remained stationary. The vehicle was then driven at 32 km/hr (20 mph) over the fixed distance, with the acceleration and deceleration phases as brief as possible. During the pass an oscillograph rec
41、orded the vehicle speed and distance, the test wheel revolutions , the outputs from the load beams , and, when available, the response of the two thermocouples and the optical pyrometer. Figure 5 is a reproduction of a typical oscillograph record taken during a test at a fixed slip ratio and figure
42、6 is typical of those taken dur- ing yaw tests of a freely rolling tire. At the conclusion of the pass, the vertical load on the tire was removed, the tire was raised from the surface, and the vehicle was realined for another pass. If a tire-wear data point was desired the tire and wheel assembly wa
43、s removed and weighed at this time and then reinstalled for additional passes. Testing of a tire was concluded when most of the tread rubber had been removed. Because of the limited number of 4 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-availabl
44、e 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 whenever possible. Before and after each discrete fixed-slip-ratio test, the driving gear was disengaged and the test tire, while fully loaded, was driven ov
45、er the pass length in the free-rolling or unbraked state and the number of wheel revolutions recorded. A knowledge of the braked and unbraked wheel revolutions was neces- sary to define the test slip ratio of the braked tire. Similarly, to obtain an equivalent braking slip ratio during the yaw tests
46、, the tire was freely rolled at zero yaw angle prior to and following each yaw test. RESULTS AND DISCUSSION General Data from the fixed slip-ratio tests included a revolution count of the freely rolling and gear-driven test wheel over the pass distance, tire weights at various distance intervals, an
47、d time histories of the vertical and drag tire loadings plus the output from the various temperature sensors, when avail- able. These data defined the test slip ratio, the tire-tread wear rate, the drag-force friction coefficient, the approximate tread surface temperature, and the temperature of the
48、 tire carcass beneath the tread in the shoulder area. The test slip ratio was computed from the relationship No - Nb Slip ratio = NO where No and Nb are the number of unbraked (free-rolling) and braked test wheel revolutions, respectively, over the same distance on the test surface. The tire tread w
49、ear was determined from the cumulative loss in tire weight as a function of distance. The drag-force friction coefficient was defined as the ratio of the measured drag load to the applied vertical load. Similar data, together with the side loading on the tire, were collected during the yaw tests. This side loading (perpendicular to the wheel plane) was converted to a side-force friction coefficient by dividing it by
