1、NASA Technical Paper 1863 c. 1 Static and Yawed-Rolling Mechanical Properties .of “ . Two Type VI1 Aircraft Tires . John A. Tanner, Sandy . - and John L. McCarty MAY 1981 NASA M. Stu and reference 2, relying on experimental and theoretical studies conducted prior to 1958, established empir- ical exp
2、ressions to describe most tire mechanical properties. Reference 2 is generally recognized as the basis of current knowledge of tire mechanical prop- erties; however, it is limited to static and low-speed conditions. During the past 20 years, many new aircraft tires have been introduced into the worl
3、ds fleet of commercial and military aircraft, and there have been a number of isolated tire studies. These studies, however, have been limited in scope. The University of Michigan, for example, has centered its research of tire mechanical properties primarily around the development and use of scale
4、model tires to predict full-scale tire behavior (refs. 3, 4, and 5). Refer- ences 6 and 7 are typical of NASA studies to obtain steady-state yawed-rolling data on new tire concepts, and references 8 and 9 present results from studies limited to braked-rolling and static fore-and-aft tire properties.
5、 In view of the concern that new tires may not exhibit characteristics or trends observed for earlier designs, the Society of Automotive Engineers, Inc. (Committee A-5 on Aerospace Landing Gear Systems) requested NASA to participate in a joint experimental program with the U.S. Air Force and the Uni
6、versity of Michigan to evaluate mechanical properties of two sizes of aircraft tires cur- rently in wide use. The purpose of this paper is to contribute to that request and present in detail results from the test program at NASA to measure static and dynamic mechanical properties of 18 x 5.5 and 49
7、x 1 7 type VI1 aircraft tires. During this program, tires were subjected to pure vertical load and to combined vertical and lateral loads under both static and dynamic (rolling) con- ditions. Test parameters for the static tests consisted of tire load in the vertical and lateral directions, and test
8、 parameters for the dynamic tests included tire vertical load, yaw angle, and ground speed. Effects of each of these parameters on the measured tire characteristics are discussed; and, where possible, comparisons are made with previous work. An appendix is included which defines terms and expression
9、s used in studying these tire characteristics and symbols used in presenting results of the study. APPARATUS AND TEST PROCEDURE Static tests and yawed rolling tests were conducted on two sizes of high- pressure, high-speed, bias-ply aircraft tires in this experimental investiga- L-141 25 Provided by
10、 IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-tion: size 18 x 5.5 with a 14-ply rating and size 49 x 17 with a 26-ply rating. Characteristics of the two tire designs are presented in table I. Several tires of each size were furnished by the U.S. Air Force, an
11、d each size was pro- cured fran the same manufacturer with closely spaced serial numbers and dates of manufacture. Prior to testing, all tires were broken in at rated pressure and load for three taxi runs of 3.2 km (2 miles) each on a road wheel (drum) dynamometer) at the Landing Gear Development Fa
12、cility, Wright-Patterson Air Force Base. Tire vertical load consisted of 50, 75, 100, and 125 percent of the rated load, and for the static tests, these loads were each applied at four equally spaced peripheral positions around the tire. For all tests, the inflation pres- sure was limited to the rat
13、ed pressure, which was set prior to loading. Both the static and the rolling tests were conducted with the tires installed on test carriages at the Langley Landing Loads Track. The 48 000-kg (106 000-lb) test carriage shown in figure 1 and described in reference 9 was employed in tests of the 49 x 1
14、7 tire, and the 29 500-kg (65 000-lb) carriage shown in figure 2 and described in reference 10 was used in tests of the 18 x 5.5 tire. On both carriages, the test tire was mounted within an instru- mented dynamometer (described in the appendix) to measure drag, vertical, and lateral tire forces. The
15、 dynamometer used to support the 49 x 17 tire on the large carriage is shown in figure 3. Static Vertical-Loading Tests Static vertical-loading tests were performed to measure the geometric prop- erties of the tire footprint and to determine vertical load-deflection relation- ships for the tires ove
16、r the range of test vertical load. Footprint data were obtained by coating the tire tread with chalk, applying the desired vertical load to the tire on a sheet of paper covering a flat surface, and measuring the geometric characteristics of the resulting “chalked“ footprint with a scale and a planim
17、eter. Vertical load-deflection curves were established according to the procedure recommended in reference 11 by continuously monitoring the vertical load on the tire, which was hydraulically applied, and the corresponding deflec- tion between the wheel flange and the flat bearing surface. The verti
18、cal load was measured with strain-gage beams in the dynamometer, and the tire deflection was obtained frcm a linear motion transducer. The load was increased beginning when the tire came in contact with the flat bearing surface until the desired load (50, 75, 100, and 125 percent of the rated load)
19、was reached. The load was then reduced to zero. The resulting load-deflection curve, or loop, is indica- tive of the tire vertical-loading behavior and provides information which defines the tire vertical spring rate and hysteresis loss. Both the footprint and the load-deflection tests were conducte
20、d at four peripheral positions around the tire. Static, Combined Vertical- and Lateral-Loading Tests Static tests with combined vertical and lateral loadings were performed to determine tire lateral load-deflection relationships which include lateral 2 Provided by IHSNot for ResaleNo reproduction or
21、 networking permitted without license from IHS-,-,-spring rate and hysteresis loss, lateral center-of-pressure shift, and static relaxation length. The spring rate and hysteresis loss were determined from lateral load-deflection curves which were obtained by again following the pro- cedures recommen
22、ded in reference 11. The process involved applying the desired vertical load to the tire followed by displacing the frictionless bearing plate, against which the tire rested, in a direction perpendicular to the wheel plane. The lateral displacement, imposed in the presence of the vertical load, was
23、increased until the lateral load was approximately 30 percent of the vertical load as recommended in reference 11. The lateral load was then reduced to zero, increased in the opposite direction to 30 percent of the vertical load, and finally reduced to zero again. One such hysteresis loop was genera
24、ted for each load and at each peripheral position for the 49 x 17 tire, two such loops were generated for the 18 x 5.5 tire. During the loading cycles, both the lateral load and the lateral displacement of the bearing plate were continuously moni- tored. The lateral load was measured by a load cell
25、located between a hydraulic piston and a backstop, and lateral displacements of the bearing plate were mea- sured by a linear motion transducer. The vertical load, hydraulically applied to the tire, was measured by load cells under the bearing plate. The lateral shift in the center of pressure (cent
26、roid of vertical forces in the tire footprint) was obtained during the lateral-loading tests from data pro- vided by multiple load cells which supported the bearing plate and from lateral displacement measurements of the plate. Static relaxation lengths were computed from measurements of the deforma
27、tion in the tire free-tread periphery due to the combined vertical and lateral loads. These deformation measurements were taken from linear motion transducers and dial displacement gages located along the tire centerline at known peripheral positions. Yawed-Rolling Vertical-Loading Tests Yawed-rolli
28、ng tests were performed to measure the rolling relaxation length of the tires and to establish tire steady-state characteristics over a range of ground speed. The relaxation lengths for several yaw angles at each vertical load were determined from a procedure which involved first yawing the wheel as
29、sembly, then lowering the tire onto the pavement and applying the desired ver- tical load, and subsequently rolling the tire straight ahead at the constant yaw angle while monitoring the distance traveled and the buildup of lateral force. Steady-state yawed data were obtained by either propelling or
30、 towing the car- riage over a dry concrete runway at a preselected ground speed, releasing the fixture which supported the wheel assembly to apply the desired vertical load to the tire, and monitoring the output frm the instrumented dynamometer. The dynamometer and its output are described in the ap
31、pendix. The yaw angle of the wheel assembly was held constant for each test run and consisted of lo, 3O, 6O, and go for each vertical load and ground speed. Nominal ground speeds for these tests were 5, 50, 75, and 100 knots. For a speed of 5 knots, the test carriage was towed by a ground vehicle; f
32、or higher speeds, the carriage was propelled by the hydraulic water jet catapult. During each test on the 49 x 17 tire, a discrete vertical load was applied to the tire to yield steady-state data; on the 18 x 5.5 tire, the vertical load was gradually increased to approximately 125 percent of the tir
33、e rated load, and the quasi-steady-state characteristics were measured as the load reached selected values. Early tests performed dur- 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ing this program had confirmed that characteristics measured at a
34、 particular vertical load during the course of a variable loading (quasi-steady-state con- dition) agreed with those measured during tests conducted with that load held constant. RESULTS AND DISCUSSION Static Vertical Loading This section of the paper discusses the results from static (nonrotating)
35、vertical-loading tests conducted on the two tire sizes. For all tests the tires, inflated to their rated pressure, were loaded to 50, 75, 100, and 125 percent of their rated load (see table I). The results include geometric properties of the tire footprint and relationships between the vertical load
36、 and the corresponding tire vertical deflection. Footprint Geometric Properties Footprint length.- The length of the footprint area Lf is presented as a function of tire vertical deflection 6 in figure 4 where both parameters have been nondimensionalized by the tire outside diameter d. The data from
37、 the two tire sizes can be faired in a least squares manner by a single-valued, nonlinear curve described by the expression, The numerical constant in this expression is slightly lower than the value (1.70) presented in reference 2 for other type VI1 tires. Also included in fig- ure 4 is the curve f
38、or the expression that defines the relationship between footprint length and vertical deflection if the tire were not distorted by the vertical load. Without distortion, the length of the footprint equals the length of the geometric chord formed by the intersection with the ground plane of a circle
39、having a diameter equal to that of the tire, and the numerical con- stant is 2.0. Thus, for both tire sizes the experimental footprint length is approximately 83 percent of the geometric-chord length. Footprint width.- The variation of footprint width Wf with tire vertical deflection is presented in
40、 figure 5 where both parameters have been made dimen- sionless by w, the maximum width of the undeflected tire. The data are faired by two single-valued, nonlinear curves, one for each tire size, that differ only by a multiplication factor. The width of the footprint for the 18 X 5.5 tire is closely
41、 approximated by the expression, which defines the length of the geometric chord generated at the ground plane by an undeformed circle of diameter w. It is similar to the expression in 4 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-reference 2 for
42、 other larger type VI1 tires. For the 49 x 17 tire of this pro- gram, the measured widths are roughly 86 percent of the length of such a chord and hence are somewhat lower than those presented in reference 2. Gross footprint area.- Gross footprint area AG is defined as the overall area of contact be
43、tween the tire and the pavement including spaces created by the tread pattern. If the footprint is assumed to be elliptical, then IT 4 %=- LfWf Substituting the expressions which fair the experimental data for Lf and Wf in figures 4 and 5, respectively, results in the following area equation in term
44、s of tire deflection and geometry: where kL is the constant associated with the footprint length and determined experimentally to equal 0.83 for both tire sizes, and kW is the constant for footprint width found to equal 1.0 for the 18 x 5.5 tire and 0.86 for the 49 x 17 tire. In equation (3b), the a
45、rea can be nondimensionalized and expressed in terms of dimensionless tire deflection to obtain the gross-footprint-area parameter: AG/W 6 = kLkwli (6/w) I 1 - (6/d) - (6/w) + ( 62/dw) If the fractions under the radical are neglected, equation (4) simplifies to a linear equation: Measured gross foot
46、print areas from the 16 tests conducted on each tire size (4 loadings at 4 peripheral tire positions) were nondimensionalized as in equation (5) and plotted as a function of dimensionless tire deflection in fig- ure 6. The data are faired by expressions which take the form of equation (5) where the
47、product of kLkwlT is 2.5 for the 18 x 5.5 tire and 2.1 for the 49 x 17 tire. These values are slightly lower than the values of 2.61 and 2.24 computed from constants kL and kW based on footprint length and width mea- surements (see discussion of eq. (3b). These differences can be attributed perhaps
48、to the footprint not being truly elliptical. The curves which describe the data for the two tire sizes of this test program encompass the data of reference 2 for other type VI1 aircraft tires. 5 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Net foo
49、tprint area.- Net footprint area AN of a tire is defined as the area of actual rubber contact between tire and pavement, that is, excluding the spaces created by the tread pattern. The ratio of this net area to gross area for each tire size under all loading conditions is presented as a function of dimensionless tire deflection in figure 7. Experimental data for both tire sizes are faired by lines which indicate that the area ratio increases slig
