Sources of Unsteady Aerodynamic Loading
Figure 8.1 summarizes the various sources of unsteady effects that may affect the blade airloads. The AoA environment of a typical blade element is the resultant of a combination of forcing from collective and cyclic blade pitch, twist angle, elastic torsion, blade flapping velocity, and elastic bending. The induced downwash effects from the trailed wake system and the locally high velocity field perturbations produced by discrete tip vortices are also of primary importance, and their effects on the airloads must be considered if the unsteady aerodynamics effects on the rotor are to be fully understood and predicted.
At the blade element level, the various effects described in Fig. 8.1 can be decomposed into perturbations to the local AoA and velocity field, as shown in Fig. 8.2. At low angles of attack with fully attached flow, the various sources of unsteady effects manifest primarily as moderate amplitude and phase variations relative to the quasi-steady airloads. However, at higher angles of attack when time-dependent flow separation from the airfoil may be involved, a phenomenon that has become known as dynamic stall may occur. This phenomenon is manifest by large overshoots in the values of the lift, drag, and pitching moment relative to the quasi-steady stall values. Dynamic stall is also accompanied by much larger phase variations in the unsteady airloads as a result of significant hysteresis in the flow developments; that is, the values of the airloads at the same AoA may be very different depending on whether the flow is separating or reattaching. As will be discussed in Chapter 9, the amplitude and phase effects produced by the stalled airloads can lead to various aeroelastic problems on the rotor that may seriously limit its performance compared to that assumed or predicted on the basis of making quasi-steady flow assumptions.