Propfan
A propfan is a type of Jet engine, a device that produces thrust by using a jet turbine engine. It is based on the turbofan design of a jet engine, and consists of a jet engine which has a large fan (many bladed propeller) attached to the front, on the same axis as the compressor blades, only without any ductwork.The ducting of the normal turbofan has the side effect of containing the sonic boom of the fan inside the engine where it is largely muted. Such is not the case on a propfan. Propfans were at one time thought to be the next logical step in engine development for subsonic aircraft, but their very high noise levels made them unattractive, and work on them has since stopped.
Propfans are also known as ultra high by-pass (UHB) engines.
History
The reason propeller engines lose efficiency at high speed is the same reason that airplanes find it difficult to fly at supersonic speeds: an effect known as wave drag significantly increases drag just below the speed of sound, and led to the concept of the sound barrier.
In the case of a propeller this effect can happen any time the prop is spun fast enough that the tips of the prop start travelling near the speed of sound, even if the plane is sitting still. This can be controlled to a large degree by adding more blades to the prop, using up more power at a lower speed. This is why most WWII fighters started with two-blade props and were using five-blade designs by the end of the war. As their engines increased in power, they couldn't just spin the prop faster. However this solution does not help as the plane itself accelerates; at some point the forward speed of the plane combined with the rotational speed of the propeller will once again result in wave drag problems.
A method of decreasing wave drag was discovered by German researchers in WWII: sweeping the wing backwards. Today almost all aircraft designed to fly much above 450 mph (700 km/h) use a swept wing. In the 1970s NASA started researching propellers with similar sweep. Since the inside of the prop is turning more slowly than the outside, the blade became progressively more swept toward the outside, leading to a curved shape.
Example:
