Main Rotor - Empennage Interaction and its Effects on Helicopter Flight Dynamics

The longitudinal flight dynamic phenomenon known as ‘pitch-up’ has caused significant problems to helicopter designers during the past 40 years. It is widely believed that pitch-up is due to the impingement of the main rotor wake on the horizontal stabilizer over a narrow range of low-speed flight conditions. Accurate simulation of pitch-up requires an accurate model of the structure and evolution of the main rotor wake as it passes close to the empennage of the helicopter. Brown’s Vorticity Transport Model has been used to analyse the aerodynamics of various typical main rotor - horizontal stabilizer configurations. The model is capable of predicting the development of the loads on the stabilizer that would induce pitch-up in a real helicopter. The range of pre-transitional advance ratios over which pitch-up is predicted correlates well with flight test data for helicopter configurations that are similar to those simulated. The overall magnitude of the down-load on the stabilizer is dependent on both the magnitude of the local velocity and the effective stabilizer angle of attack, leading to some counterintuitive behaviour, whilst the spanwise distribution of the loads along the stabilizer is modified by asymmetries in the flow-field that are induced by the main rotor wake. It is demonstrated how the physical effects that are predicted by high-resolution aerodynamic models can be incorporated into a simple flight dynamic simulation, allowing realistic representation of the effects of the interaction between the main rotor wake and stabilizer on the longitudinal dynamics of the helicopter.