An Investigation of Landing Gear Shimmy: Tire Models, Tire Test Methodologies, Analysis, and Parameter Studies

This paper presents typical tire models used by the aerospace community for studying landing gear shimmy. Closed form solutions were developed for the tire models and fit to different types of laboratory tire data. The laboratory tests include the traditional windup tests on a Tire Force Machine, dynamic non-rolling tests on the dynamometer and tire force machine, and dynamic yaw and rolling tests on the dynamometer. Both bias and radial tire designs were studied in both the new and worn condition. Based on the testing performed, significantly different tire property information is obtained. For example, torsional stiffness measured for the non-rolling condition was nearly three times larger than that measured for the rolling tire. Also, very large differences in the torsional damping coefficient between tire types were only evident through dynamic yaw and rolling testing. A discrete landing gear shimmy model was developed to study the effects of tire type and other landing gear parameters. It was determined that the radial tire is less prone to self-excite in shimmy due to the larger torsional damping coefficient. In the worn condition, both the radial and bias tires become more prone to self-shimmy excitation. However, at higher speeds, effect of tire configuration becomes less significant. Effect of mass damping and friction was also studied and analytical stability simulations were performed. Full scale laboratory shimmy testing was performed on the dynamometer to study the effects of tire type, mass damping and friction. In all cases, the analysis agreed well with experimental observations. Testing revealed that the worn bias tire was the least stable since it was most prone to self excite in shimmy at relatively low speeds. Effects of adding mass and friction were shown to add significant stability to the gear.