An investigation into the phenomenon of helicopter blade sailing

Abstract

This thesis has been completed as a requirement for a higher degree of the University of Southampton. Blade sailing is an aeroelastic phenomenon affecting helicopter rotors when rotating at low speeds in high wind conditions. This is a potentially dangerous blade motion and the excessive flapwise tip deflections generated endanger the airframe, the flight crew and any personnel working close to the aircraft. This phenomenon is particularly applicable to naval helicopters or those operating off exposed sights such as oil rigs. The research covered an experimental investigation into the effect of an abeam wind flow over a simulated flight deck of a Rover Class Royal Fleet Auxiliary vessel on a Westland Lynx sized helicopter. A theoretical rigid blade method was derived to allow blade sailing to be predicted and a comparison with the experimental results was used as the verification. Blade sailing was obtained and the influence of the deck location of the helicopter was found to be of paramount importance. Agreement of the theory was only possible when a detailed Laser Doppler Anemometer (LDA) survey of the deck flows was performed and used as an input to the theory. The helicopter rotor system, in reality, is far more complicated than the model used for the wind tunnel tests and the verified simpler theory was extended. Blade flexibility and rotor hub mechanical features were introduced into the theory and the resulting method applied to the Westland Lynx and Sea King aircraft. The semi-rigid rotor is relatively well controlled but the rotor hub construction of an articulated rotor and the interaction with the flexing blades allows blade tip deflections to be generated of an order to strike the fuselage. Results of this analysis was compared with data from a rotor engagement and disengagement of a full scale Puma aircraft. The present study described in the thesis has lead to an improved understanding of the blade sailing phenomenon. This potentially violent rotor blade aeroelastic behaviour has been investigated in the past but to no great degree considering its importance. Research work has addressed the origins of the aerodynamic forcing and examined simple approximations to the blade dynamic motion. However, the research described herein, to the author's knowledge, is the first time that a detailed qualitative and quantitative survey of the airflow over a ship's flight deck has been married to detailed aeroelastic modelling of the helicopter rotor blade. A rigid blade teetering rotor and ship's flight deck have been tested in wind tunnels and the behaviour of the rotor blades has been accurately predicted using a theoretical model developed for this research. The effect of blade flexibility has been introduced to the theoretical model and the results concur with the reports of the blade sailing occurrences and the operational conditions which are likely to trigger this phenomenon. The development of theoretical models may be used to assess the importance or otherwise of this effect on future rotor designs or the operational limitations of the aircraft.

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