Ducted propeller is a propeller that turns within a cylindrical device called a duct. Putting a propeller inside a duct makes the propeller more efficient. The duct prevents air from flowing around the ends of the propeller. As a result, the air pressure increases behind the propeller blades, and the propeller’s driving force increases. The thrust provided by a ducted propeller permits manufacturers to reduce the propeller’s size. Ducted propellers produce more thrust at low vehicle speeds than unducted propellers. Ducted propellers also operate more quietly than do unducted ones.
Ducted propellers are used in some V/STOL (Vertical/Short Take-Off and Landing) aircraft, which require a large amount of thrust for take-off (see V/STOL). Many air cushion vehicles use ducted propellers and fans to produce the air layer on which the vehicle travels or to drive the vehicle forward, or both (see Air cushion vehicle ). A ducted fan is similar to a ducted propeller, but it has more blades and generally rotates less quickly than a propeller. Modern jet airliner engines, called turbofan engines, have ducted fans.
