Unsteady aerodynamic and stall effects on helicopter rotor blade airfoil sections.

Oscillatory tests in pitch and in vertical translation were performed on symmetrical and cambered airfoils, at full-scale Reynolds numbers, to provide dynamic stall data for rotor blade analyses. The Mach number range applicable to the retreating side of the rotor disk was covered at frequencies up to the first bending and the first torsional natural frequency. A system with a torsional degree of freedom was also tested. The following key results were found: 1) The negative aerodynamic damping due to stall is highly sensitive to Mach number. 2) Negative aerodynamic damping can be encountered in large-amplitu de plunging motions. 3) The maximum normal force encountered during oscillation is substantially higher than that for static stall. In addition, the flow separation process is discussed.