The gyroplane represents the first successful rotorcraft design and it paved the way for the development of the helicopter during the 1940s. Gyroplane rotors are not powered in flight and work in autorotative regime and hence the characteristics of a helicopter rotor during powered flight and a rotor in autorotation differ significantly. Gyroplanes in the UK have been involved in number of fatal accidents during the last two decades. Despite several research projects focused on gyroplane flight dynamics, the cause of some of gyroplane accidents still remains unclear. The aeroelastic behaviour of autorotating rotors is a relatively unexplored problem and it has not yet been investigated as possible cause of the accidents.A mathematical model was created to simulate aeroelastic behaviour of rotors in autorotation. The model can investigate couplings between blade teeter, bending, torsion and rotor speed using a finite element model combined with a blade element method and a dynamic inflow model. A set of 'McCutcheon' rotor blades was subjected to a series of experiments, yielding baseline input parameters for the model. The model was validated against published results of modal analysis of helicopter rotor blades, experimental flight measurements and other data published in open literature.Effect of selected rotor design parameters on performance and stability of autorotating rotors was analyzed. Results of the model suggest that steady autorotative flight is not possible for excessive values of blade fixed incidence angle or geometric twist of the blade, leading to an aeromechanical instability. Negative values of these parameters lead to rotor over-speed, loss of rotor thrust and increase in vehicle speed of descent. The simulations have shown that moderate values of blade geometric twist applied to the inboard region of the blade together with blade tip mass can improve stability of a rotor in autorotation.A significant part of the research was focused on investigation of the effect of different values of torsional and flexural stiffness, and the relative chord-wise positions of blade elastic axis and centre of mass on rotor stability during autorotation. The results obtained from the model demonstrate an interesting and unique characteristic of the autorotative regime. Coupled flap-twist-rotor speed oscillations of the rotor occur if the torsional stiffness of the blade is lower than a critical value and if the blade centre of mass is aft of the blade elastic axis. The new type of aeroelastic instability is specific to autorotating rotors and differs from both helicopter rotor flutter and fixed-wing flutter. An extra degree of freedom in rotor speed does not alter flutter onset point significantly and hence this instability can be classified as pitch-flap flutter, with the stability boundary of a hyperbolic shape. However, variation of rotor speed in response to coupled flexural and torsional dynamics of the rotor blades changes behaviour of the rotor during the instability. The coupling of rotor teeter, blade torsion and rotor speed with vehicle speed of descent results in a combined flutter and divergence instability.The investigation aeroelastic behaviour of rotors in autorotation has shown that although autorotation has strong autostabilizing character, catastrophic aeroelastic instability can occur. Aeroelastic instability of this type has not been previously described in open literature. The instability can be initiated by incorrect mass balance of the rotor blades together with their insufficient torsional stiffness. Alternatively, unsuitable rotor geometry causing excessive blade incidence can prevent the rotor from entering steady autorotation. Hence a rotor in autorotation with unsuitable design of rotor blades can encounter an aeroelastic instability even if it is correctly mass balanced.
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