The Effect of Compressibility on the Maximum Lift Coefficient of Aerofoils at Subsonic Airspeeds

It has long been known that compressibility affects the maximum lift coefficient at Mach numbers as low as 0·15. A schematic flight envelope (Fig. 1) shows that there are two main phenomena to be considered; the effects of compressibility on the low speed stall, and on the high subsonic speed stall. In the low speed stall regime there is, in general, a decrease of the maximum lift coefficient with increasing Mach number which is appreciable even at typical aircraft landing speeds. A knowledge of the lift boundary throughout the envelope is necessary if allowances are to be made for gusts and if a safe rough-air performance is to be determined.