Stall Flutter of Helicopter Rotor Blades: A Special Case of the Dynamic Stall Phenomenon

Abstract

Abstract : Several conclusions were drawn with respect to stall flutter and airload prediction of high speed and/or highly loaded helicopter rotor blades. The stall of an airfoil section during rapid transient high angle of attack changes is delayed well above the static stall angle and results in a large transient negative pressure disturbance leading to large transient lift and nose down pitching moment. The magnitude of the pitching moment is such as to generate substantial nose down pitching displacements of the blade. These pitching displacements can substantially alter the angle of attack distribution of the rotor blade. The dynamic stall phenomenon of a helicopter rotor blade can be separated into three major phases: (1) A delay in the loss of blade leading edge suction to an angle of attack far above the static stall angle, with associated airloads of the type described by classical unsteady airfoil theory. (2) A subsequent loss of leading edge suction accompanied by the formation of large negative pressure disturbance (due to the shedding of vorticity from the vicinity of the blade leading edge) which moves aft over the upper surface of the blade. Associated with this phase are high transient lift, drag, and nose-down pitching moment associated with the greatly altered pressure distribution on the airfoil. (3) Complete upper surface separation of the classic static type, characterized by low lift, high drag, and moderate nose- down pitching moment.