Aeromechanics Modelling of Tiltrotor Aircraft

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Abstract

Tiltrotor aircraft offer the hover and low-speed capabilities of helicopters coupled with the high-speed performance of fixed-wing aircraft. A unique operating regime of this configuration is the transition from rotor-borne to wing-borne flight. This flight envelope, expressed through the rotor tilt degree of freedom, is termed the conversion corridor. This thesis investigates the impact of the rotors/airframe interactions on the predicted conversion corridor, trim behaviour and aircraft performance. A generic aeromechanics model is developed that consists of a flight mechanics module and aerodynamic modules for each component. Three methods are rationalised to predict the conversion corridor with a simple trim sweep adopted due to the parallelisation capability for large domain investigations. A robust Newton-Raphson scheme that implements a variable damping factor is also presented to find the unknown trim quantities. A new formulation for the equations of motion of a gimballed rotor are presented that are applicable throughout the conversion corridor. The rotors/airframe interaction can be classified as rotors-on-wing, rotors-on-empennage and wing-on-empennage. The overall effect of the interactions on the predicted corridor boundaries was fairly small but generally acted favourably to widen the corridor. The wing-on-empennage interaction was found to have the most impact on the predicted boundaries, decreasing the minimum-speed boundary towards aeroplane mode. The aerodynamic interactions were most pronounced on the trim characteristics, particularly the pitch attitude and stick position. The combined downwash and upwash at the empennage resulting from the rotors and wing was most important in helicopter mode to correlate the pitch and stick trim against literature data. As the rotors were tilted forwards, the increased pitch attitude at low-speeds increased the wing downwash and the wing-on-empennage interaction became dominant. The wing downwash at the tailplane had little effect on the pitch trim past a forward rotor tilt of 30 deg but significantly affected the stick trim. The rotors-on-wing interaction had a small effect on the trim behaviour for the configuration considered but did significantly increase the thrust and power required at low speeds due to the imposed download force on the airframe.

Keyword(s)

Aeromechanics; Conversion; Corridor; Interaction/s; Tiltrotor

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