Development of an aeroelastic stability boundary for a rotor in autorotation

Trchalík, J., Gillies, E.A. and Thomson, D.G. (2008) Development of an aeroelastic stability boundary for a rotor in autorotation. In: AHS Specialist's Conference on Aeromechanics, San Francisco, USA, 23-25 January 2008,

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<p>For the present study, a mathematical model AMRA was created to simulate the aeroelastic behaviour of a rotor during autorotation. Our model: Aeroelastic Model of a Rotor in Autorotation (AMRA) captures transverse bending and teeter, torsional twist and lag-wise motion of the rotor blade and hence it is used to investigate couplings between blade flapping, torsion and rotor speed. Lagrange’s method was used for the modelling of blade flapping and chord-wise bending. Torsional twist of the rotor blade was modelled with the aid of finite element method (FEM), and blade transverse bending could also be modelled in FEM. The model can switch between using a full FEM model for bending and torsion, or a FEM model for torsion and simple blade teeter, depending on the complexity that the user requires.</p> <p>The AMRA model was verified against experimental data obtained during a CAA sponsored flight test programme of the G-UNIV autogyro. Published results of modal analysis of helicopter rotor blades and other data published in open literature were used to validate the FEM model of the rotor blade. The first torsional natural frequency of the ’McCutcheon’ rotor blades was measured with the aid of high-speed camera and used for validation of the FEM model of blade torsional twist. As a further verification of the modelling method, Aérospatiale Puma helicopter rotor blade data were compared on a Southwell plot showing comparison between experimental results and AMRA estimation.</p> <p>The aeromechanical behaviour of the rotor during both axial flight and forward flight in autorotation was investigated. A significant part of the research was focused on investigation of the effect of different values of torsional and flexural stiffness, and the relative positions of blade shear centre/elastic axis and centre of mass of the blade on stability during the autorotation.</p> <p>The results obtained with the aid of the model demonstrate the interesting, and unique, characteristics of the autorotative regime - with instabilities possible in bending and torsion, but also in rotorspeed. Coupled rotor speed/flap/twist oscillations (flutter and divergence) occur if the torsional stiffness of the blade is lower than a critical value, or if the blade centre of mass is significantly aft of the blade twisting axis, as is the case in helicopter pitch-flap flutter. The instability shown here, however, is specific to the autogyro, or autorotating rotor, as it is coupled with rotorspeed, and so differs from both helicopter rotor flutter and fixed-wing flutter. The coupling with rotorspeed allows a combined flutter and divergence instability, where the rotor begins to flutter in rotorspeed, teeter angle and torsional twist and, once the rotorspeed had dropped below a critical value, then moves into divergence in flap and rotorspeed. It was found that the aeroelastic behaviour of a rotor in autorotation is significantly affected by the strong coupling of blade bending stiffness and teeter angle with rotorspeed, and the strong coupling between blade aeroelastic twist and rotor torque.</p>

Item Type:Conference Proceedings
Keywords:Aeroelasticity, autorotation.
Glasgow Author(s) Enlighten ID:Thomson, Dr Douglas and Gillies, Dr Eric
Authors: Trchalík, J., Gillies, E.A., and Thomson, D.G.
Subjects:T Technology > TL Motor vehicles. Aeronautics. Astronautics
College/School:College of Science and Engineering > School of Engineering > Autonomous Systems and Connectivity
Research Group:Flight Dynamics
Copyright Holders:Copyright © 2008 American Helicopter Society
First Published:First published in AHS Specialist's Conference on Aeromechanics
Publisher Policy:Reproduced with permission of the publisher

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