Helicopter tail rotor instability

Champneys, A. (2003) Helicopter tail rotor instability. European Study Group with Industry > 46th ESGI [Bristol 31/3/2003 - 4/4/2003].

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EPrint Type:	Study Group Report
Study Group:	European Study Group with Industry > 46th ESGI [Bristol 31/3/2003 - 4/4/2003]
Company Name:	Westland Helicopters
Industrial Sector:	Aerospace and defence
Additional Contributors:	Aigner, A. and Balanov, S. and Champneys, A. and Eyres, R. and Friswell, M. and Gravesen, J. and Halse, C. and Hjorth, P. and Janson, N. and Lloyd, D. and Ockendon, J.R. and Silchenko, S. and Wagg, D. and Wang, C. and Wood, D. and Wright, J. and Xu, X.
ID Code:	25
Deposited By:	Gordon White
Deposited On:	10 June 2004

Problem Statement

Data obtained by Westland Helicopters from a simulation of a teetered tail rotor shows instability at su ciently high forward velocity of the aircraft, and also indicates a rotor natural frequency at nearly three times the rotor rotation frequency. The Study Group was asked to provide an explanation of these observations. Using a linear model for the teetering motion and the umbrella ap mode of the tail rotor blades, the Study Group showed that parametric terms, containing the forward velocity, provide excitation at frequencies which are once and twice the blade rotation frequency. Taking the classical damped Mathieu equation as a toy model of the system, and observing the near 3:2 ratio between the natural rotor frequency and the second excitation frequency, suggested that the observed instability arises as the forward velocity passes into the 3:2 resonance tongue of the stability map of the Mathieu equation. The current snapshot eigenvalue method, in use at Westland Helicopters, does not capture this instability. The Study Group recommended instead that Floquet theory be applied to a time-dependent linearised model.

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