1、NASA TECHNICAL NOTE NASA _- TN _- D-2770 _ ca i - AN INVESTIGATION OF THE INFLUENCE OF AIRCRAFT TIRE-TREAD WEAR ON WET-RUNWAY BRAKING by Tufford J. W. Leland und Glenn R. Taylor Lungley Reseurch Center Lungley Station, Humpton, Va. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. APRI
2、L 1965 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NASA TN D-2770 AN INVESTIGATION OF THE INFLUENCE OF AIRCRAFT TIRE-TREAD WEAR ON WET-RUNWAY BRAKING By Trafford J. W. Leland and Glenn R. Taylor Langley Research Center Langley Station, Hampton, V
3、a. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 - Price $1.00 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-AN INVESTIGATION OF THE INFLUENC
4、E OF AIRCRAFT TIRE-TREAD WEAR ON WET-RUNWAY BRAKING* By Trafford J . W. Leland and Glenn R. Taylor Langley Research Center SUMMARY Wet-runway braking tests were conducted at the Langley landing-loads track in which smooth and dimple-tread tires were used to represent completely worn tires. Five circ
5、umferential grooves were then cut into the tires to depths representing varying degrees of tread wear. Two types of wear were simulated: uniform wear with all grooves cut to the same depth, and nonuniform wear with the center groove wearing completely smooth while significant depths remain in the ou
6、ter grooves. For comparative purposes tests were made on dry and damp concrete surfaces and through water depths of 1 inch. On the wet runway, a gradual degradation in braking effectiveness was experienced up to about the 80-percent-worn tire-tread condition, where the wet-runway friction coeffi- ci
7、ents dropped markedly. The completely worn tire was observed to develop, at the higher speeds, only about one-half the braking effectiveness of a new tire. INTRODUCTION Previous tire research, as exemplified by references 1 and 2, has indicated that the wet-runway braking effectiveness of an aircraf
8、t tire is highly depend- ent on the original tire-tread design. An important corollary to this fact is that even a good tread design may lose effectiveness as the tire becomes worn through normal use. It was the primary purpose of this investigation to deter- mine at what degree of wear a tire tread
9、 begins to lose braking effectiveness and to aid in determining when the tire should be removed most economically from service without compromising safety requirements. TEST APPARATUS AND PROCEDURE Test Facility The investigation was conducted at the Langley landing-loads track, which has been used
10、for several years to investigate many different facets of the * Some of the material presented in this report was originally presented at the 1st AIAA Annual Meeting at Washington, D.C., June 29-Jd.y 2, 1964, in a paper by the authors entitled “Effects of Tread Wear on the Wet Runway Braking Effecti
11、veness of Aircraft Tires. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-landing and ground-handling problems of aircraft. The landing-loads track, as shown schematically in figure 1, consists of a large hydraulic water-jet cata- pult which accelera
12、tes the 60-foot-long test carriage (fig. 2) to speeds up to 120 knots. A complete description of the catapult system is given in refer- ence 3 and a description of the operation of the track, in reference 4. Close control of such test parameters as forward speed, sinking speed, vertical load, and ru
13、nway-surface condition coupled with versatile instrumentation capabilities permits a detailed independent investigation of each of the many variables affecting landing-gear and tire performance. Excellent repeatability of test conditions can also be achieved for purposes of comparison and control. T
14、ires Tested In the tests 32 x 8.8, type VII, 22-ply-rating aircraft tires were used. One was a specially molded smooth tire and the others were standard dimple- tread and three-groove tires. Tire-tread wear was simulated by using the smooth and dimple-tread tires to represent completely worn tires a
15、nd then cutting pro- gressively deeper grooves into the tire treads to simulate various wear condi- tions. Five circumferential grooves were cut into all the tires with the saw and jig arrangement shown in figure 3. The saw was held rigidly in a special fixture, and an adapter plate on the bottom of
16、 the saw permitted closecontrol of groove depth. Photographs of the tires tested are shown in figure 4. The dimple-tread tire (tire I, fig. therefore, a stand- ard three-groove fighter-airplane tire (tire 111, fig. 4(c) was modified by cutting two additional grooves to represent a new five-groove ti
17、re. Groove depths for all tires were measured and recorded before and after each test with both a micrometer depth gage and a dial indicator. The depth of each groove was measured at approximately the same location in six different places around the circumference of the tire. The average depths reco
18、rded for each groove and the tolerances maintained are shown in table I. Tire footprints were also taken at each wear point. Test Conditions Most of the tests were made on a wet runway for a tire pressure of 150 psi. The test section of the track was provided with a sprinkler system to achieve essen
19、tially constant wetness which because of runway uneveness varied in the test section from 0.1to 0.3 inch of water. Some tests were made for a tire pressure of 90 psi, both on the wet runway and on a runway covered with 1 inch of water. The static vertical load on the tire for all runs was 10 500 pou
20、nds. Magnetic pickups placed at intervals along the track initiated braking cycles and controlled the number and location of these cycles for each run. Pressure was metered to the brake through a micrometer needle valve, which acted as a controllable orifice. This metering was done so that brake-pre
21、ssure rise time 2 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-could be varied with the forward speed and anticipated friction conditions to give approximately the same braking distance per run. with the new five-groove tire on a damp surface, in
22、which the surface closely resembled a runway as it might be in the early morning following a heavy dew with no puddles or standing water. A few tests were made on dry concrete to provide comparative data for other braking conditions. Several tests were made The test tires were mounted in the special
23、 test fixture shown schematically in figure 5, which is the same one used in previous investigations (refs. 1, 2, and 5). The vertical load and drag load were measured at the axle, and errors due to the inertia of the lower mass were corrected by accelerometers. These corrected values were used to c
24、ompute the true instantaneous tire-ground fric- tion coefficients throughout the entire brake cycle. Also recorded were wheel angular displacement, velocity, and acceleration; brake torque and brake pres- sure; wheel vertical displacement; and carriage forward velocity. TEST RESULTS Instantaneous br
25、aking-friction coefficients were computed for all.braking cycles from a freely rolling wheel to a locked wheel. However, the braking- effectiveness test results discussed in this paper are expressed in terms of pAv, 0.5 (shown schematically in fig. 6). tends to smooth any uncharacteristic peaks or l
26、ow points in individual brake cycles which may be caused by localized slippery spots or runway contaminants. As indicated in figure 6, the curve of friction coefficient p as a function of slip ratio for wet runways flattens out, so that the average friction coef- ficient pAv, the maximum friction co
27、efficient pw, and the skidding friction coefficient pSKID all tend toward a commom value. The average friction coef- ficient is also more likely to be the overall friction coefficient obtained with modern antiskid systems. the average friction coefficient developed between a slip ratio of 0.1 to The
28、 presentation of data in this manner Tire-Wear Effects Previous experience (ref. 1) had suggested little difference in wet-runway braking effectiveness between smooth and dimple-tread tires, a conclusion sup- ported by the results shown in figure 7 where tire I (dimple) and tire I1 (smooth) are comp
29、ared before any grooves were cut. Nonuniform wear.- Figure 8 summarizes the results of the nonuniform-wear investigation in which the dimple tire was used. Friction coefficients for all wear points drop rapidly with increasing forward speed, with the worn tire developing only about one-half the fric
30、tion of the new tire at the higher speeds. It should be recalled that when the nonuniformly worn tire is 100 per- cent worn, only the center groove is worn smooth while significant groove depths remain in the outer grooves, as shown in table I. This fact accounts for the relatively high friction lev
31、els at this wear condition as compared with the results obtained for the ungrooved dimple-tread tire. The effect of tread wear 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-is illustrated more clearly in figure 9, where a gradual degradation in b
32、raking effectiveness is noted as the wear progresses from a new tire to about the 80-percent-worn tire-tread condition. Further tread wear noticeably reduces the braking-friction levels at all velocities, with as much braking effective- ness being lost between the 80- and 100-percent-worn tire-tread
33、 condition as was lost between the 0- and 80-percent-worn condition. Uniform wear.- The trends noted in the nonuniform-wear investigation were also observed in the uniform-wear investigation; these results are summarized in figure 10. As in the nonuniform-wear case, a noticeable decline in braking e
34、ffectiveness occurs near the 80-percent-worn tire-tread condition. As previ- ously mentioned, total tire tread wear prevented regrooving tire I1 beyond the 20-percent-worn tire-tread condition, but apparently the results would follow the trend in figure 9, with very little change in braking effectiv
35、eness occur- ring between the 0- and 20-percent-worn conditions. A possible explanation of the poor braking performance of a worn tire is offered in figure 11, where the tire footprints taken at the five highest uni- formly worn conditions are shown. In taking the tire footprint, the entire lower su
36、rface of the tire was covered with chalk, including the grooves. The tire was then lowered onto a piece of paper stretched over an aluminum plate, and allowed to settle until full static vertical load was attained. The foot- print taken at the 95-percent-worn condition (fig. ll(b) shows heavy eviden
37、ce of chalk in all five grooves; this indicates that tire deformation has, in effect, eliminated the grooves at this wear condition. At the 90-percent-worn condition (fig. ll(c) slight chalking is still evident in all grooves, whereas at the 85-percent-worn condition (fig. ll(d) chalking is found in
38、 the outer grooves only. The tire shows no evidence of chalking in the grooves at the 80-percent-worn condition (fig. ll(e), and it is at this percent of tread wear, as shown previously in figure 10, that tire performance begins to deteriorate most rapidly with increasing wear. Thus, it seems that t
39、he effects on braking performance of the degree of tread wear of a tire cannot accurately be gaged by eye, since a continuous tread pattern is visible in figure 11 even at the 95-percent-worn condition although the tire behaves very much like a smooth tire during braking. Tire spin-up.- Excessive ti
40、re-tread wear can greatly increase the time required for the wheel to spin up following brake release, as shown in fig- ure 12. Spin-up time in this figure is understood to be the elapsed time between brake release following a full skid and the achievement of equivalent free-roll forward velocity. S
41、pin-up time for the new tire on a dry runway is seen to be below 0.1 second for all forward velocities, whereas the addition of water to the runway more than doubles the spin-up time for this tire at the higher velocities. As shown in figure 12, spin-up time at the higher velocities increases rapidl
42、y with increasing tread wear. For example, at 40 knots the 100-percent-worn tire requires about 0.1 second for spin-up, whereas at 80 knots the same tire requires a full second for spin-up. In many of the high-speed tests with the smooth tire, spin-up following brake release never occurred within th
43、e limited length of the test track. The reason for the long spin-up times noted is, of course, due +Uo the extremely low friction coefficients devel- oped under these conditions. It is of interest to note the magnitude of the time increase, however. 4 Provided by IHSNot for ResaleNo reproduction or
44、networking permitted without license from IHS-,-,-Other Effects Groove- configuration.- Because of slight differences in the tire-grooving technique and the different tires used, the groove widths and groove locations differed for the various tires tested. The importance of the difference in groove
45、width is shown in figure 13. Tire I11 having a nominal groove width of 0.375 inch developed notably better friction coefficients at all speeds than the nonuniformly 0-percent worn tire I, which had a nominal groove width of 0.290 inch. Although tire I11 had somewhat deeper grooves than tire I (table
46、 I), it is believed that the groove width was the primary factor respon- sible for the increased braking friction, since the average water depth (0.2 inch) in both tests was less than the average groove depth. This conclu- sion is further supported in figure 13 by comparing the braking effectiveness
47、 for the nonuniformly 50-percent-worn tire I (nominal groove width of 0.290 inch) with that for the uniformly 60-percent-worn tire I1 (nominal groove width of 0.220 inch), which had about the same groove depth in the three center grooves. The improved wet-runway braking effectiveness of the wider gr
48、oove is probably a result of better or more rapid escape of water from the footprint region and of higher local tire-ground bearing pressures. Although the grooves were cut into the tires symmetrically spaced about the center groove, small differences did exist in the groove spacing between tires, a
49、s shown in figure 14. These differences are thought to have less of a an effect on braking performance than groove width. Tire 111, a modified pro- duction tire, also had grooves of different shape, as shown schematically in figure 14(c), with the three central grooves having a rounded shape, whereas the other two grooves in this tire and all grooves in the other tires were rectangular. It is uncertain what effec
