Turbofan engines improve efficiency by increasing the amount of cold low-pressure air they accelerate, using some of the energy normally used to accelerate hot air in the classic non-bypass turbojet. The ultimate expression of this design is the turboprop, where almost all of the jet thrust is used to power a very large fan – the propeller. The efficiency curve of the fan design means that the amount of bypass that maximizes overall engine efficiency is a function of forward speed, which decreases from propellers, to fans, to no bypass at all as speed increases. Additionally, the large frontal area taken up by the low-pressure fan at the front of the engine increases drag, especially at supersonic speeds, and means the bypass ratios are much more limited than on subsonic aircraft.