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 "fan" itself). For this reason, the propfan has been variously described as an "unducted fan" or an "ultra-high-bypass (UHB) turbofan".