Propfan

A propfan is a type of aircraft engine related in concept to both the turboprop and turbofan, but distinct from both. The engine uses a gas turbine to drive an unshielded propeller like a turboprop, but the propeller itself is designed with a large number of short, highly twisted blades, similar to a turbofan's bypass compressor The propfan engine is designed to have the speed and performance of a turbofan, but the fuel efficiency of a turboprop. The propfan is also known as the unducted fan (UDF) engine because the fan is not enclosed like that of a turbofan. The propulsive efficiency of a propfan engine is increased by the high bypass ratio achieved using external fan blades. The efficiency boost is maintained even at high speeds because the fan blades are curved like scimitars to prevent shockwave formation on the outer tips of the blades. The increased propulsive efficiencies translate into overall propulsive efficiency gains of as much as 20-25% over turbofans.