Dynamic Stall in the Rotor Environment

While much of what is known about dynamic stall has been obtained from ide­alized experiments on 2-D airfoils in wind tunnels, it is important to recognize that when

dynamic stall occurs on the rotor, it has a more 3-D character than previously described and may simultaneously occur over several radial and azimuthal parts of the rotor disk. Continued advances in miniature pressure transducer technology, high-speed data acquisi­tion and telemetry systems have enabled a more detailed understanding of dynamic stall

as it manifests on helicopters during actual flight. Isaacs & Harrison (1989) and Bousman

(1998) provide good in-flight documentation of the dynamic stall phenomenon on heli­copter blades. The results shown in Fig. 9.3 are adapted from Bousman (1998) and show the time histories of the lift and pitching moment at various radial stations on the blade of a UH-60 helicopter during a pull-up maneuver at /x « 0.3 and Ст/сг « 0.17. The results are presented in terms of the nondimensional quantities M2Cn and M2C, n, be­cause these quantities give a better quantitative measure of the local airloads produced on the rotor.

Using the unsteady chordwise pressures as an indicator, Bousman (1998) has identified three locations on the rotor disk for this flight condition that show the lift overshoots and large nose-down pitching moments that are characteristic features of dynamic stall. On Fig. 9.3 these are marked by points M (moment stall) and by points L (lift stall). Remember that dynamic lift stall always occurs after moment stall. At these particular flight conditions, it is apparent that dynamic stall encompasses relatively large areas of the rotor disk. In particular, note that the occurrence of dynamic stall causes large transients in the pitching moments, especially between r = 0.77 and r — 0.92 on the blade in the first quadrant of the disk and also on the retreating side near (r = 270°. Operating the rotor at thrusts or airspeeds beyond this flight condition will result in high structural loads and stresses that can quickly exceed the fatigue or endurance limits of the rotor and/or control system (see Fig. 6.8). Even though the rotor is usually able to operate with some amount of stall, the very rapid growth in the blade torsion and other structural loads because of dynamic stall is normally a limiting factor in the overall operational flight envelope of helicopters – see also Benson et al. (1973) and Stepniewski & Keys (1984).