Book Description

Experimental measurements of the unsteady separation and dynamic stall process on an oscillating three-dimensional wing are reported. The experiment was conducted at Mach numbers of 0.2-0.6, Reynolds numbers of 2-6 million, and sweep angles of 0, 15, and 30 deg. At low Mach number, as angle of attack is increased the location of transition to turbulence moves forward, the turbulent boundary layer separates near the leading edge, and a strong stall vortex is formed. At higher Mach number, compressibility causes formation of a shock, an earlier, more gradual separation, and reduced unsteady loads. Unsteady tip loads at 0 sweep are increased by the growth of a strong tip vortex. This effect is lessened by sweep-back and compressibility, and enhanced by replacing the round tip cap with a flat tip. Away from the tip, sweep effects on loads are well represented by the swept infinite wing normalization until stall. After stall, vortex propagation patterns are highly dependent on sweep and spanwise position. Sinusoidal and constant pitch rate ramp motions show similar behavior. There is significant hysteresis in both the transition/relaminarization and the separation/reattachment processes. For small amplitude motions simulating stall flutter, substantial regions of negative aerodynamic damping were found -at all studied Mach numbers, sweep angles, and reduced frequencies. The near-simultaneous stall along the span of the swept wing strengthens the resulting instability. An empirical representation of the damping characteristics was developed. Unsteady flow, Separated flow, Transition, Helicopter aerodynamic stall, Stall flutter, Supermaneuverability, Dynamics, Unsteady aerodynamics.