An active back-flow flap for a helicopter rotor blade

If the angle of attack of an airfoil is increased above a critical value stall occurs. The stall is attended by a decrease of lift, an increase of drag and a change of the aerodynamic pitching moment. In order to reduce these negative effects an observation of birds in flight can inspire a technical solution. In case of stall the top feathers of bird wings begin to lift which delays the decrease in lift and prevents the back-flow, generated by the stall vortex, on the upper airfoil surface. This functional principle can be transferred and has already been demonstrated for fixed wing aircrafts. For helicopters this transfer is even more challenging. The flow field of a helicopter rotor in forward flight is very complex. During each revolution the rotor blades may be subjected to blade vortex interactions, transonic effects, reverse flow and stall. The stall that occurs on the retreating blade has a highly dynamic character and is therefore significantly differing from the quite well known static stall of airfoils. First numerical investigations have shown that a back-flow flap can improve the dynamic stall characteristics of oscillating airfoils. The flap was able to weaken the stall vortex and therefore to reduce the peak in the pitching moment. In order to validate this phenomenon a concept for the realization of a back-flow flap is needed. This paper gives a brief insight in the function principle of a back-flow flap. Further on the challenges related to the implementation on an oscillating profile will be pointed out. Finally a structural integration concept and different actuation mechanisms of a back-flow flap for a helicopter rotor blade will be presented, discussed and assessed