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Title: Parametric study into the shimmy dynamics of a nose landing gear-fuselage model
Author: Kewley, Sarah Elizabeth
ISNI:       0000 0004 6056 5652
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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This thesis investigates the influence of different parameters on the shimmy dynamics of a nose landing gear. This is achieved by considering a sequence of increasingly complex models. Due to the nature of landing gear dynamics, numerical continuation is used to analyse the system's shimmy characteristics. A nose landing gear model consisting of a strut, caster and single wheel is presented. It has torsional and lateral bending vibrational modes and is completed with an elastic tyre. The critical forward velocity of the aircraft shimmy onset can be determined as the equilibrium solution loses stability via a Hopf bifurcation. The bifurcating steady-state periodic solution gives the maximum amplitude of the self-sustained shimmy oscillation, which are dominated by one of the vibrational modes of the gear. The Hopf bifurcation points can be continued in two parameters where they form stability boundaries for the gear's equilibrium solution over gear vertical loading and ground speed. The nose landing gear model is then coupled to a two degree of freedom model, consisting of two mass spring damper systems acting at the nose landing gear-fuselage attachment point in the lateral and vertical directions. These represent any local displacement resulting from flexible deformation of the fuselage. Shimmy onset is found to be sensitive to the lateral, but not the vertical, motion of the fuselage. However, the amplitude and frequency of shimmy oscillations are influenced by the vertical mode. The latter part of the thesis presents an extended nose landing gear-fuselage model. Here the fuselage model includes a rigid body lateral fuselage mode that represents lateral motion of the fuselage at the nose landing gear-fuselage attachment point due to fuselage yaw; it also comprises roll and pitch rotations which present themselves as rotations in the nose landing gear. The gear model also includes axial compression of the strut, has a dual wheel configuration and includes their gyroscopic effects. The shimmy dynamics of the gear are found to be virtually unaffected by the presence of the rigid body fuselage model, whilst the two wheels and their gyroscopic effects are found to strongly influence the shimmy onset point. The gear's design and tyre parameters are investigated next. The increase in separation distance merges the two shimmy oscillations into one displaying large amplitudes in both modes. It is also found that a decreased rake angle and increased caster length stabilises the gear considerably. The gear's tyre relaxation length and contact patch length are found to stabilise the gear when increased and decreased, respectively. This effect, along with that of the flexible fuselage, however, is dwarfed by the influence of the rake angle and caster length. Moreover, the gear was found to be very sensitive to the change in magnitude of the self-aligning moment. Thus, it is concluded that the fuselage dynamics play little part in the onset of shimmy, and that when considering shimmy onset particular attention should be paid to the rake angle, caster length and self-aligning moment which appear to be highly influential in triggering shimmy.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available