Best Selling Books by J. Davenport

The report presents the results of an experimental investigation of rotor blade dynamic stall. Forces and moments in two-dimensional flow were determined for two thin airfoil sections (NACA 0006 and Vertol 13006-.7) usually considered in advanced helicopter rotor concepts by measuring differential pressure during oscillatory pitch motions. The Mach number, Reynolds number, and angle of attack prevailing in the retreating-blade stall region were investigated up to typical first-torsion mode natural frequencies. Pitch oscillation was shown to increase the angle of attack at which stall occurred, with a corresponding increase in maximum normal force. Hysteresis effects on the pitching moment response produced regions of negative aerodynamic damping in oscillations at mean angles of attack near the steady-flow stall values for all Mach numbers from 0.2 to 0.6. It is shown that dynamic stall effects are strongly related to the reduced frequency parameter. Significant evidence of compressibility effects appears only at Mach numbers higher than 0.4. At lower Mach numbers, the data indicate that the two effects of decreased Mach number and airfoil camber are fully comparable in terms of dynamic stall patterns and increase both the usable angle of attack and the maximum normal force.