Book excerpt: Summary: An investigation of the effects of compressibility on the hovering-performance characteristics of two 10-foot-diameter helicopter rotors having solidities of approximately 10 percent has been conducted in the Langley full-scale tunnel. One rotor, having NACA 0012 airfoil sections, a plan-form taper ratio of 3/1, and -8° of twist, was tested to a tip Mach number of 0.95 and a disk loading of 16 pounds per square foot. The other rotor had NACA 64-series airfoil sections, tapering to a 6-cent-thick tip, a plan-form taper ration of 3/1, -16° of twist, and was tested to a tip Mach number of 1.0 and a disk loading of 20 pounds per square foot.