Prediction and Fundamental Understanding of Stall Loads in UH-60A Pull-Up Maneuver

This paper isolates the physics governing the aerodynamics and structural dynamics of UH-60A rotor in an unsteady maneuvering flight and proposes a hypothesis for the mechanism of advancing blade stall observed during pull-up maneuvers. The advancing blade stall observed during the Counter 11029 pull-up maneuver is in addition to the two conventional dynamic stall events observed on the retreating side of the blade. Both lifting-line as well as computational fluid dynamics analyses predict all three stall cycles with calculated deformations. The advancing blade transonic stall, observed from revolution 12 onward, is a twist stall triggered by 5/rev elastic twist deformation that increases the angle of attack beyond the static stall limit, resulting in shock-induced flow separation culminating in stall. The 5/rev elastic twist is triggered by the two retreating blade stalls from previous revolution, which are separated by 1/5th rev. The accurate prediction of both stall cycles on retreating blade holds the key to prediction of advancing blade stall. In analysis, advancing blade stall is triggered by a correct combination of control angles and 5/rev elastic twist.