Modeling the Effect of Yawed Flow on Dynamic Stall with Secondary Lift Peak - Experimental Correlation

For a helicopter in forward flight, the rotor blade is unswept at only two locations, 90° and 270°; and the blade section undergoes wide variation in sweep angle during each revolution. Furthermore, as advance ratio (i.e. the forward flight speed) increases, the sweep angle becomes large enough to create significant effects on the aerodynamic forces and, consequently, on the dynamic stall characteristics. In linear aerodynamic analysis, it is assumed that the radial component of the velocity does not affect the sectional loads; and the airloads are calculated considering only the velocities in a plane containing the airfoil chord. However, experimental research shows that, although this assumption is quite acceptable in the low angle-ofattack region, in the high angle-of-attack, stall region, a much higher lift coefficient is obtained for larger sweep angles. Therefore, conventional, unswept theories may not be sufficient for rotor aerodynamic analysis in a three-dimensional flow field. The purpose of this study is to add the effect of yawed flow to the dynamic stall model of Ref. [1].