A Systematic Study Of Helicopter Rotor Stall Using Model Rotors

A wind tunnel test was conducted to systematically determine the effects of twist, camber, taper ratio, number of blades, and torsional frequency on model helicopter rotor stall characteristics. Model rotor performance, blade response, and boundary layer separation data were acquired at low Reynolds and Mach numbers to determine the occurrence and extent of blade stall. Boundary layer information was obtained by means of hot-film gages installed over the outer 25-percent span on the suction surface of two blade designs. The data obtained indicated consistently the azimuthal, chordwise, and radial location of stall onset and recovery and the laminar/turbulent transition of the boundary layer. The rotor lift at the stall threshold, indicated by hot-film measurements, was generally coincident with a change in rate of lift growth, vibratory flapping, and vibratory edgewise stress with increasing rotor shaft angle. Based on the average of these stall indicators, the effects of the various design parameters on stall were determined for advance ratios of 0.15, 0.30 and 0.45. Camber and taper provided the greatest improvement in delaying stall and stalled rotor performance. Lowering the torsional frequency tended to delay the onset of stall flutter.