Dynamic Stall Experiments on a Rotor with High Cyclic Setting in Axial Inflow

Abstract

Dynamic stall is one of the limiting aerodynamic phenomena that occur on modern helicopter rotors during fast forward and maneuvering flight conditions. During dynamic stall the effective angle of attack on the retreating side of the rotor exceeds the static stall angle and a lift peak as well as a negative peak in the pitching moment are observed. These loads can be crucial regarding the blades and the pitch links structural integrity and therefore limit the flight envelope of modern helicopters. Separated flows, especially in a rotating frame, are strongly three-dimensional, which is neglected when examining two-dimensional pitching airfoils where the flow is assumed to be two-dimensional in the mid-section of the model. Therefore experiments are conducted on model rotors, e.g. in hover chambers. Two major disadvantages apply to most (small) hover chambers: usually the rotor is operated in ground effect and the rotor inflow is disturbed due to recirculation of the rotor wake. The closest representation of the flow around a full-scale helicopter is achieved in experiments conducted in wind tunnels where it is possible to achieve fast forward flight conditions. However these are often very complex and expensive. During the design phase of the rotor test facility (RTG) at the DLR in Göttingen, three major aspects were considered: avoidance of an operation in ground effect, supply of a defined rotor inflow with prevention of a rotor wake recirculation and construction of an easy to use facility with good optical access to the rotor. These aspects resulted in the design…