Sensitivity Study of Helicopter Vibrations and Loads with Elastic Fuselage Coupling and Dynamic Empennage Loads from Free Wake Analysis

Abstract

T-tail configurations are a promising approach to increase vertical tail efficiency, reduce fuselage download and hub load cycle amplitudes in low speed transition. However, the horizontal tail can be subject to rotor wake impingement in cruise flight which might lead to high dynamic loads and structural fatigue. The involved aerodynamics are in addition highly complex and hence difficult to be predicted by simulation. In this work a simulation approach for empennage structural loads and vibration prediction is established based on free-wake analysis and modal fuselage approximation, focusing on the expectedly most dominant aerodynamic interaction effects at the T-tail. The results are compared to flight test data to evaluate the approach, and sensitivities of the framework are assessed. The results indicate that the motion of the horizontal tail is characterized only by a few modeshapes, predominantly driven by rotor wake influence, rather than rotor loads via the structural load path. At the same time, high sensitivities are associated with these particular modes and are evaluated in this work to identify the driving mechanisms of T-tail vibrations of the investigated configuration. Discrepancies in the structural model are identified against bang test data. Taking these discrepancies into account, the simulation approach yields reasonable results for T-tail vibrations and loads in comparison to flight test data. In the front part of the fuselage, flight test data is significantly underpredicted as expected and attributed to the employed simplifications in the main rotor blade model.