Dynamic Stability Analysis Of Isolated Integrated Motor Propulsor.

Abstract

Integrated Motor Propulsors (IMP) are being considered for the propulsion system of future
underwater vehicles. A key issue is the reduction of IMP mechanical vibration transmitted to the
vehicle. To this end, vibration isolation of the IMP is a potential approach to reduce these vibrations.
However, the isolation of the IMP from the vehicle body can potentially lead to a dynamic instability
problem of the system. Coupled-mode flutter of aircraft wings is an example of such dynamic
instability caused by the interaction of a flow with a flexible structure. For the particular case of the
IMP, the instability occurs because the thrust generated by the IMP creates a static load that is always
aligned with one of the IMP axes which is connected to the vehicle’s body through a compliant mount.
This results in a following force acting on a flexible system much like a loose garden hose. The
objective of this work is to develop the theoretical formulation to analyze the dynamic stability of soft
mounted IMP systems, and to develop numerical tools and software to perform numerical simulations
of the dynamic stability of soft mounted IMP systems taking into account the external fluid-loading
effects. The approach here was to develop a generic, closed form modeling tool of the isolated IMP
system to determine the dynamic stability of the system. The model includes the most important
system parameters, e.g. IMP dynamic properties, isolation properties, fluid loading, and so forth. The
formulation was numerically implemented in a computer code and examples will be presented. The
stability of the system can be determined by inspection of the time history responses and/or an
eigenvalue analysis, e.g. coalescence of natural frequencies. The data for these examples do not
correspond to any particular IMP. However, the tool developed here can be used to analyze practical
cases.