Outwash flow measurement around the subscale tandem rotor in ground effect

Abstract Numerous issues are yet to be discovered about the small tandem rotor aerodynamics in the vicinity of ground. In this paper, a multipurpose test rig has been used to test outwash patterns of a sub-scale model of a generic tandem rotor helicopter in ground effect using two total-static rakes at different positions ahead and aft of the rotor(s). The results showed that in tandem rotor, the change in the rakes distance from the rotors has mild influence on the outwash flow velocity. The interaction between rotor wakes and formation of the fountain flow impressed the outwash of the rotors and changed their behavior in comparison with a single rotor. For the elevation higher than z/2R = 0.8, the outwash averaged velocity of the rotors due to flow recirculation was negative. As the rotor elevation decreased, outwash accelerated by moving away from the rotor within the measurement area. Comparing the results of present study with the experiments for the CH-47D helicopter indicated that the overlap between the rotors was caused the outwash velocity be more than non-overlapping rotors. Tuft flow visualizations showed the evidences of a fountain flow between the rotors and that, in the vicinity of tandem rotors, the resultant outwash velocity was inward due to formation of the fountain flow and the consequent suction.