1、i iINASA NASA-TP-248119860002055 !i iJ !Technical _ ,Paper2481October_ 1985A Study of the CorneringForces Generated byAircraft Tires on a Tilted,Free-Swiveling Nose GearRobert H. Daughertyand Sandy M. StubbsProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS
2、-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NASATechnicalPaper24811985A Study of the CorneringForces Generated byAircraft Tires on a Tilted,Free-Swiveling Nose GearRobert H. Daughertyand Sandy M. StubbsLangley ResearchCenterHampton, Virginia
3、National Aeronauticsand Space AdministrationScientific and TechnicalInformation BranchProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Summary rollout, u
4、nwanted cornering forces were developed at thenose gear.An experimental investigation was conducted at the The purpose of this paper is to present results of testsNASA Langley Research Center to study the effects of conducted at the NASA Langley Research Center tovarious parameters on the cornering
5、forces produced by a determine the cause of this cornering-force phenomenon.rolling aircraft tire installed on a tilted, free-swiveling The paper also examines the effects of various param-nose gear. The parameters studied included tilt angle, eters on the magnitude of cornering forces produced bytr
6、ail, tire inflation pressure, rake angle, vertical load, and a free-swiveling nose-gear tire rolling in a tilted attitude.in the case of twin tires, whether or not they corotate. These parameters include tilt angle, trail, tire inflationThese parameters were evaluated by measuring the cor- pressure,
7、 rake angle, vertical load, and in the case of anering force produced by an aircraft tire installed on the twin-tire arrangement, whether the wheels are lockednose gear of a modified vehicle as it was towed slowly, together to rotate as a unit or are allowed to rotateAlthough more readily apparent i
8、n a corotating twin- independently.tire system, this cornering-force phenomenon occurs in asingle tire when a tilt angle causes one side of the tire to Apparatushave a smaller rolling radius than the other side and thuscreates a differential slip in the tire footprint. Test Vehicle and TiresIn gener
9、al, the cornering-force coefficient increases as The vehicle used in this investigation was a modifiedtilt angle increases. Increasing trail decreases the airboat shown in figure 1. A retractable tricycle landingcornering-force coefficient nonlinearly for a given tilt gear was added to the airboat a
10、nd provided a means forangle. Tire inflation pressure has no effect on the producing roll attitudes up to l0 . The airboat weighedcornering-force coefficient, whereas nose-gear rake 4425 lb, and the vertical load on the nose gear wasdecreases cornering-force coefficient. Increasing vertical typicall
11、y 480 lb, except for tests examining the effect ofload dramatically decreases the cornering-force coef- vertical load, during which weights were added to theficient at a fixed tilt angle. Having a twin-tire system does nose gear. A more detailed description of the vehicle cannot affect the cornering
12、-force coefficient if the tires can be found in reference 2. The lateral distance between therotate independently. However, if the twin-tire system main gears was approximately 11 ft, and the spacing be-corotates, the cornering-force coefficient for a given tilt tween the nose gear and the main gear
13、 was 13.4 ft.angle can be greatly increased. Several different nose-gear configurations werestudied in this investigation. Inserts were added to theIntroduction nose-gear piston to modify the standard zero-trail con-figuration to configurations of 1.5 and 10 in. of trail.The mechanical characteristi
14、cs of tires sometimes Photographs of the zero-trail and 10-in.-trail configura-cause them to produce forces not normally anticipated, tions are shown in figure 2. In addition, tests were con-One such phenomenon involves a tire producing side or ducted on a twin-tire arrangement shown in figure 3. Th
15、ecornering forces when it rolls in a tilted condition and is distance between the wheel centerlines was 8 in. for thefree to pivot or swivel about the steering axis. This twin-tire arrangement. An insert for the nose-gear dragphenomenon was observed in 1966 during landing-gear link was fabricated, a
16、nd when installed, it produced atests conducted on a model of an HL-10 manned lifting forward rake angle of 10 on the nose gear.entry vehicle (ref. 1). During these tests it was learned The nose-gear tires used in this investigation werethat asymmetrical main-gear strut deflection, which pro- 6.00 6
17、 TT 8-ply type III aircraft tires with a rated loadduced a tilt on all three landing-gear struts, led toof 2350 lb. The main-gear tires were 6.50 l0 TT 8-plydevelopment of side forces at the nose gear if it was free type III aircraft tires with a rated load of 3750 lb.to swivel. It was demonstrated
18、that this phenomenoncould be used as an alternate steering method. Since Towing Systempublication of reference 1, however, this steeringphenomenon has not been recognized by the aviation A schematic of the towing system used in this in-community in general, vestigation is shown in figure 4. The lead
19、 (tow) tug pulledRecently, excessive and unexplained differential brak- both the test vehicle and the instrumentation tug; bothing forces were required on some Space Shuttle orbiter tugs followed straight expansion joints on a flat concretelandings to keep the orbiter aligned with the runway surface
20、. A cable with an integral load cell was attached tocenterline. It was determined that the combination of the nose-gear cylinder and to the instrumentation tug.crosswinds and runway crown caused the orbiter to The test vehicle was tilted away from the instrumentationassume a roll or tilt angle, and
21、since the nose gear is tug, so that the instrumented cable from the nose geartypically operated in a free-swiveling mode during was always in tension. A portable generator providedProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-electrical power for t
22、he recording system mounted on the cornering-force-generation phenomenon and to obtain ainstrumentation tug. more accurate linear curve fit of the data at the lower tiltangles.InstrumentationA strain-gauge-type load cell was used to measure the Nose-gear isolation tests. A series of tests was con-du
23、cted to determine if the main gears of the test vehicleload in the cable between the test vehicle nose gear and affected the force readings obtained at the nose gear. Forthe instrumentation tug. For four tests, a strain-gauge-type load cell was mounted in the left main-gear scissor these tests, a do
24、lly was placed under the nose-gear tire, sothat no cornering forces could be generated by the tire.assembly to obtain a measure of torque. The signals from Then tests were conducted at tilt angles up to 10% Thethese strain gauges were recorded in real time on a stripchart recorder, square symbols in
25、 figure 5 denote these forces as a func-tion of tilt angle. Small cornering forces were observedfor these test conditions, with a maximum force of aboutTesting Technique 25 lb at a tilt angle of 10.Before each towed run, the test vehicle and the two To verify the presence of these main-gear forces,t
26、ugs were positioned, and all cables were secured. Next, another set of tests was conducted with a dolly underthe tilt angle for the test was set by raising or lowering each main-gear tire, so that cornering forces could not beeach gear while maintaining the proper rake angle. The generated by those
27、tires, and any force measurements ob-rake angle was normally set at 0 . Finally, an instrumen- tained would be those generated strictly by the nose-geartation calibration was performed, and the tow test began, tire. The results are also presented in figure 5 and areAll tow tests were conducted at a
28、speed of approximately denoted by the triangular symbols. The resultant corner-5 ft/sec and covered a distance of approximately 200 ft ing forces are, in fact, lower than the original ones by theon a level concrete apron, same 25 lb at a tilt angle of l0 . All following tests wereconducted without d
29、ollies, and the measured nose-gearResults and Discussion forces reported in this paper were adjusted to remove theeffect of the main-gear forces at the rate of 2.5 lb/deg ofThe test conditions and results for each of the towed tilt.runs are summarized in table I. The first series of testresults pres
30、ented in the following paragraphs provides in- Differential slip within the tire footprint. The follow-sights into the basic phenomenon of cornering-force ing free body diagram of the test vehicle during the test atgeneration by tilted tires and sheds light on the underly- a tilt angle of 10 with a
31、dolly under the nose gear in-ing mechanisms responsible for these forces. The effects dicates that the main gears are producing a total force,of various parameters on the cornering force produced 2Fm, of 25 lb in the direction of the tilt:by an aircraft tire installed on a free-swivelingnose gearare
32、 also evaluated in the following paragraphs. Theseparameters include tilt angle, trail, tire inflation pressure,rake angle, tire vertical load as a function of rated load, Fand in the case of a twin-tire arrangement, whether or nnot the tires corotate. Definitions of some of theparameters and forces
33、 discussed in this paper are given in dappendix A.Generation of CorneringForces by a Tilted Tire on aFree-SwivelingNose Gear Fm m Initial tests. The first set of data, represented by thecircular symbols in figure 5, was obtained to evaluate the 217 = 17 T = F d/2effect of tilt angle on tire cornerin
34、g force. The tire was in- m n nflated to 24 psi and was loaded vertically to 480 lb, whichrepresents 20 percent of the tire rated load. A linear The measured force, Fn, at the nose gear and thecurve-fitting technique was applied to the experimental force produced by the main gears produced a couple
35、ondata. A maximum cornering force of approximately the test vehicle of 25 lb times the spacing between the310 lb was obtained for a tilt angle of 10. Although the nose gear and the main gear, d, of 13.4 ft, or 335 ft-lb.higher tilt angles, beyond 5o, would not be expected to The test vehicle did not
36、 yaw during the test. The absenceoccur on an aircraft under normal operations, they were of yaw indicates that the main tires produced an oppos-included in this investigation to illustrate more vividly the ing torque, 2T. The assumption was made that each main2Provided by IHSNot for ResaleNo reprodu
37、ction or networking permitted without license from IHS-,-,-tire produced half of the opposing torque, which varied side force which acts behind the steering axis, as shown inlinearly from zero at no tilt to 167.5 ft-lb at a tilt angle of appendix B. The tires continue to steer in the direction of10o
38、. the torque until the moment produced by the side forceA calibrated load cell was placed in one of the main- acting behind the steering axis is balanced by the torquegear scissor assemblies to provide a measurement of the due to differential slipping. When this torque equilibriumtorque produced by
39、one of the main-gear tires. Figure 6 is reached, an unbalanced force, the cornering force duepresents plots of the measured main-gear torque and the to tilt, is present. This cornering force can be responsiblepredicted torque based on the free body diagram as a for uncommanded steering inputs during
40、 aircraft groundfunction of tilt angle. The prediction agrees very well operations. It should be noted that significant corneringwith the experimental data. The data show that there is forces due to tilt arise only when the nose gear is free toindeed a torque produced in the footprint of a tilted, r
41、oll- swivel. If the nose gear is locked or nose-gear steering ising tire and that the mechanism that produces this torque engaged, the torque produced by differential slipping isis the basis for the generation of cornering forces due to counteracted by the gear and aircraft structure, and cor-tilt.
42、nering forces due to tilt are essentially eliminated.Another observation provided additionalinformation As a result of this investigation, it was determinedon this phenomenon. This observation involved the ap- that substantial torques due to differential slipping can bepearance of the tire tracks of
43、 the main gear on the con- developed by a single tire. The “common axle“ is thecrete test area after the vehicle was tested, especially at wheel itself. Reference 3 suggests that a single tire can bethe larger tilt angles. The tires left rubber deposits, and in treated as two thin tires separated by
44、 the tire width. Foreach track, the edge toward the direction of tilt was ob- this analysis, the tire is treated as an infinite number ofviously darker. Thus, increased tire wear is indicated in thin tires, each having a different rolling radius based onthis region of the tire contact area. This obs
45、ervation sug- its lateral location in the tire footprint. A mathematicalgests that differential slip occurs across the footprint of a description of this approach can be found in appendix B.tilted, rolling tire.To illustrate this differential slip more clearly, a piece Effects of Various Parameters
46、on Cornering Forcesof graph paper was lightly coated with grease, and a thinGenerated by Tilted Tiresstring was laid along one of the lines of the graph paper,as shown in figure 7. The vehicle was tilted to 10 and Trail. The effect of trail on the cornering forcetowed so that a main-gear tire rolled
47、 across the paper generated by a tilted, rolling tire is illustrated in figure 8.perpendicular to the string. The grease prevented the Note that for the remainder of this paper, cornering forcestring from moving after the tire had rolled off the paper, will be expressed as cornering-force coefficien
48、t,/%, deter-and a visual indication was obtained of the differential mined by subtracting 2.5 lb/deg from each measured cor-slipping that developed in the tire footprint. Both the ini- nering force at the various tilt angles and dividing by thetial and final positions of the string are shown in figu
49、re 7. normal load on the nose gear for that test. The cornering-The final position of the string near the left edge of the force coefficients were largest at zero trail and decreasedfootprint indicated that this portion of the footprint was with increasing trail over the range of tilt angles tested.in a slipping or braking condition. The final position of This trend of decreasing cornering-force coefficient
