A propfan is a modified turbofan engine, with the fan placed outside of the engine nacelle on the same axis as the compressor blades. Propfans are also known as ultra-high bypass (UHB) engines. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop. The problem Turboprops have a fairly strict sweet spot at speeds below about 450 mph (700 km/h). The reason is that all propellers lose efficiency at high speed, due to an effect known as wave dr… (More on Propfan)