Effects of rotor-rotor interactions on aerodynamic loads for two helicopters in forward flight

Abstract

This paper presents an investigation of the aerodynamic interactions between two UH-1H helicopter rotors in forward flight. The wake flow structure and performance characteristics of the rotors are investigated when the rear rotor is operating within the wake of the front rotor. A 3-D unsteady vortex-panel method potential flow solver based on a free-vortex-wake methodology is used for this purpose. The solver is validated using the experimental data from the Caradonna-Tung experiments. The interactional analyses are performed for 10 different positions of the rear rotor with respect to the front rotor. The wake of the front rotor, which is generated due to the mutual interactions of the wake and tip vortices, is significantly asymmetric and non-uniform. Therefore the rear rotor operates within a complex and vortical free-stream flow. The wake-rotor interactions cause substantial variations in the forces generated by the rear rotor. The asymmetries in the wake geometry in forward flight cause changes in performance characteristics of the rear rotor when it is operating at the retreating and advancing sides of the front rotor. The decrease in vertical lifting is more when the rear rotor is operating within the wake of the advancing side. A frequency spectrum analysis of the force and moment fluctuations of the rear rotor shows that for the vertical force component the rear rotor experiences a dominant peak at two times the blade passing frequency, except when the rear rotor is operating within the wake of the advancing side of the front rotor. Other peaks at different harmonics are also observed and the location of these peaks in the frequency spectrum depends on the lateral position of the rear rotor with respect to the front one. Moment fluctuations generally are similar for the front and rear rotors except for the pitching moment which show fluctuations at two times the blade passing frequency illustrating that the pitching moment gets influenced by the wake interactions more than the other two moment components. © 2010 Association Aeronautique et Astronautique de France. All rights reserved.