A qualitative examination of dynamic stall from flight test data

Three flight conditions from the UH-60A Airloads Program are examined where dynamic stall occurs on the rotor: a symmetric pull-up that includes non-zero angular rates and accelerations; a high-speed, diving turn that includes non-zero angular rates, but nominally zero angular accelerations; and a level flight case with zero angular rates and accelerations. The two maneuver conditions result in severe pitch-link loads and represent design conditions for this military aircraft. Dynamic stall characteristics are identified in the section lift, the section pitching moment, and the trailing edge pressure and are used to create rotor disk maps that show the location and behavior of dynamic stall for this rotor, The measured blade pressures are used to examine the dynamic stall process itself and airfoil maps are developed that show the behavior of the dynamic stall vortex on the blade and its interaction with areas of supercritical flow. It is shown that the torsional dynamics of the rotor control where dynamic stall may occur, while the flight condition and the resulting aerodynamic inflow determine whether it will occur.