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Title: A bifurcation and numerical continuation study of aircraft main landing gear shimmy
Author: Howcroft , Christopher
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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This thesis presents a bifurcation and numerical continuation study into the occurrence of shimmy instability in an aircraft main landing gear (MLG) of single side-stay, dualwheel design. The dynamics are expressed in terms of three rotational degrees of freedom (Dofs) aligned with the side-stay plane, and a fourth translational DoF representing compression of the main strut. These DoFs are modelled by oscillators that are coupled directly through the geometric configuration of the system, as well as through the tyre/ground interface. Using this representation of the MLG system we focus in this thesis on the nonlinear effects of geometric orientation and mechanical freeplay employing bifurcation analysis techniques to highlight their effects on the MLG stability. First, a changing side-stay orientation angle is investigated. Shimmy is studied by means of a two-parameter bifurcation analysis in terms of the landing gear forward velocity and loading force. For this a three-DoF model is used that does not include axial compression. This formulation along with suitable parameters allows for comparison with the existing literature, and an agreement is demonstrated with previous results for a zero side-stay angle. Subsequent variation of this angle is explored and a consistent picture presented, capturing the (transition of the two-parameter bifurcation diagram us a function of this angle. This shows a considerable increase in the complexity of the dynamics for intermediate side-stay angles. The appearance of an additional shimmy mode is observed and a region of tri-stability found where three distinct shimmy types coexist. For the study of freeplay the MLG model is extended to include axial compression; this axial DoF is required to accurately represent freeplay, introduced to the torque links of the system. Parameter values are chosen here to represent a typical mid-range civil aircraft MLG. The addition of freeplay is shown to result in shimmy oscillations that occur within the MLC operating envelope; their properties depend on both the size and 'shape' of freeplay. Freeplay and geometric coupling arc also considered together via consideration of a non-zero side-stay angle. Here, additional dynamic complexity is introduced in the presence of freeplay and, again , this coincidence with the appearance of a new shimmy mode. Further complex phenomena also appear, including multiple- frequency and chaotic-type oscillations, as well as complex transients. The non-zero geometry produce asymmetry and this results in a great sensitivity of the small-amplitude MLG behaviour to the exact shape of freeplay. Therefore, geometric orientation and freeplay found to have significant effects and, when combined, they work together to produce additional complex phenomena, not otherwise observed when considered in isolation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available