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Supersonic Propellers

- question from Sean

NACA research during the mid-1940s did show that supersonic prop-driven aircraft were feasible, but the key was developing propellers that could operate at or near the speed of sound. NACA began a high-speed propeller development program that culminated in the supersonic propeller, but this kind of prop is substantially different from those used on the fastest propeller-driven aircraft of the day, like the P-51 Mustang. Beyond speeds of Mach 0.9, the prop blades must become significantly shorter and thinner and the blade angle (the angle of attack of the blade) must be decreased compared to blades used on previous aircraft. An example is the test propeller mounted to the nose of an XF-88 research aircraft.

Supersonic propeller used on the XF-88 research plane
(note that the blades are feathered towards the camera and not in position for flight

The propeller was powered by a turboprop engine, and the vehicle was tested up to Mach 1 during the late 1940s. You can read more about NACA's research program at the NASA History Office, Ch. 4, sec. 7 and Ch. 4, sec. 8.

The reason a supersonic prop looks so different from standard propellers is because of shock waves that form on the propeller blades as the aircraft approaches Mach 1. Even before the plane reaches the speed of sound, portions of the blades do reach or exceed Mach 1 creating "bubbles" of supersonic flow. As we discussed in a previous question on supercritical airfoils, these supersonic regions are accompanied by shock waves. These shock waves not only greatly reduce the efficiency of the propeller, but create tremendous forces that can tear the prop apart. Making the blades shorter and thinner and reducing the blade angle effectively makes the blades supercritical meaning these regions of supersonic flow form at higher speeds than is possible with standard props. By delaying the onset of shock waves and reducing their strength once they do form, the supersonic prop can operate at higher speeds.

So getting back to your question, it is probably NOT possible for a typical propeller-driven plane such as the P-47, F8F, or Spitfire to become supersonic, even in a dive, because of these shock waves that form when parts of the plane and propeller exceed Mach 1. The internal structures of these aircraft and the propeller blades were not designed to withstand the pressure forces generated by these strong shock waves and would likely disintegrate as they approached Mach 1. This is why the "sound barrier" got its name in the first place. Many early attempts to break it resulted in the airplane's destruction causing many to believe that a physical barrier prevented travel faster than the speed of sound. Despite the promising results of NACA's research, the rapid development of the jet engine doomed the supersonic propeller and the project was abandoned by 1949. However, rising fuel costs have generated renewed interest in high-speed propellers since turboprops are more fuel efficient than turbojet and turbofan engines, so we may not have heard the last of the supersonic propeller.
- answer by Joe Yoon, 17 June 2001

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