Analytical and a numerical ground resonance analysis of a conventionally articulated main rotor helicopter

[摘要] The helicopter is a prime example of a nonlinear multi-body dynamic system that is subjected tonumerous forces and motions to which the system must react. When a helicopter, with aconventionally articulated rotor head, is resting on the ground with its rotor spinning, a conditioncalled ground resonance can develop. Ground resonance is a specific self-excited oscillation ofthe helicopter and is caused by the interaction between the main rotor blades and the fuselagestructure. Inertia forces of the blades perform an out-of-phase lagging motion, which reacts withthe elastic landing gear of the helicopter. For certain values of the main rotor angular velocity, thefrequency of these inertia forces coincides with a natural vibration frequency of the fuselagestructure. If this occurs, the inertia forces of the lagging blades produce oscillations of thefuselage, which then further excite the lagging motion of the blades. This interaction isresponsible for an instability of conventionally articulated main rotor helicopters, which is calledground resonance.The ground resonance phenomenon is investigated by means of a classical analytical approachin which the ground resonance equations are derived from Euler-Bernoulli beam theory andverified with results in literature. These equations are required to discuss ground resonancestability in further detail and determine the specific regions in which the phenomenon occurs.These results are incorporated in a simplified numerical model using an elastic multiple-bodydynamics analysis program called DYMORE to simulate the South African Rooivalk CombatSupport Helicopter. DYMORE is a program that offers nonlinear multi-body dynamic analysiscode, using the finite element method, which was specifically developed for helicopter modelling.The complexity of helicopter modelling generally requires large amounts of computing power toensure reasonable processing time. In order to prevent excessive computational time, thenumerical model will be simplified in terms of aerodynamic and structural aspects. The scope ofthe numerical investigation is, therefore, limited to the ground resonance phenomenon without theeffect of aerodynamic forces and representing the fuselage as multi-body beam structures ofspecified stiffness.

[发布机构] Stellenbosch University