Measurements of mid-chord pitching moment derivatives at high speeds

Abstract

Measurements of the pitching moment derivative coefficients for a 7½ per cent bi-convex aerofoil oscillating about the mid-chord axis were made in a high-speed wind tunnel by the method of decaying oscillations. The tests were made at Mach numbers of 1.275, 1.455 and 1.515 for supersonic flow, and covered a range extending from 0.4 to 0.9 at subsonic speeds. The effect of variation of frequency parameter was also investigated, and conditions giving rise to sustained or growing oscillations at subsonic speeds were examined. Comparison with existing flat plate theories for supersonic flow shows complete disagreement in the trend of the damping with Math number change, the linearized theory for a flat plate giving an increasing negative value as M is reduced below 1.41, whereas experiment gives an increasing positive value. A recent theory which takes into account the shape of the profile agrees in trend with experiment, suggesting that profile is of vital importance in this field. The results of the subsonic tests exhibit a narrow region of Mach number extending from approximately 0.87 to 0.89 within which negative damping can arise. It is thought that this effect is bound up with the formation of shock-waves at the surface of the model.