Investigation of a Dual Propeller at High-Subsonic Mach Numbers

At the present time the Langley 8-foot high-speed tunnel is testing a dual-rotating propeller having a total of eight blades. The front and rear propellers, each made up of four blades, turn in opposite directions. This propeller is designed for a very high blade angle, about 75 degrees, the object being to achieve good efficiency at high forward Mach numbers by means of a low rotational speed so that the resultant Mach numbers along the blade are only slightly greater than the forward Mach number. The design value of advance ratio V/nD is high, about 7.1, so that the tip Mach number is only 10 percent higher than the forward Mach number.