Amplitude Effects On Dynamic Stall Of An Oscillating Airfoil PDF Download

This study's aim was to obtain a basic understanding of the effect of compressibility on the phenomenon of dynamic stall under typical flight conditions encountered by a helicopter in forward flight, so that eventually a means for its control can be devised and thus, its flight envelope can be expanded. The first phase of the study was devoted to building a drive system to produce the necessary unsteady airfoil motion. A novel design was arrived at and built. It uses a four-bar chain mechanism of which the airfoil is one of the links. The drive can produce a sinusoidal variation of the angle of attack as : alpha = alpha sub 0 + alpha sub m(sin omega+), with the mean angle of attack alpha0, continuously variable from 0 to 15 deg, the amplitude of oscillation alpha sub m, from 2 to 10 degs and the frequency from 0-100 Hz, in an oncoming flow Mach number M, from 0-0.5. The drive was installed in the indraft wind tunnel at NASA Ames Research Center. Stroboscopic schlieren studies and interferograms as well as holographic interferometry studies were conducted for a wide range of flow conditions, amplitudes and frequencies. Results show that compressibility effects appear at M = 0.3, that a dynamic stall vortex forms for all Mach numbers, and that for M> or = 0.3, the dynamic stall angle decreases as M increases. On the other hand, increasing the degree of unsteadiness, delays deep stall monotonically at all Mach numbers. Amplitude has a dominant effect, and dynamic stall occurs at lower angles of attack at lower amplitudes, yet this angle of attack is always higher than the static stall angle. (edc).