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A ducted fan is a propulsion arrangement whereby a propeller is mounted within a cylindrical shroud or duct, the duct reduces losses at the tips of the propeller. Ducted fan propulsion is used in aircraft, airboats and hovercraft.
An unshrouded propeller is limited to the rpm at which tip speeds approach the sound barrier. To increase the RPM further the propeller diameter must be decreased, increasing losses at the tips (similar to the increased induced drag on a low aspect ratio wing). Below a certain size, the increase of efficency that can be gained by a duct outweighs the extra weight and drag of the duct.
Ducted fans usually have an odd number of blades to prevent resonance in the duct.
The most common full-scale aircraft applications of ducted fans is the front part of a turbofan engine, where the fan is driven by a gas turbine engine. These engines operate at around Mach 0.8; it would be almost impossible to desing a propeller to fly at that speed without its tips breaking the sound barrier.
- Ducted fans can be fitted inside models of jet planes, giving a more scale appearance.
- Ducted fans can allow for a limited amount of thrust vectoring, something normal propellers are not well suited for. This allows them to be used instead of tilt-rotors in some applications.
- Ducted fans can maintain a high pressure differential across the fan because there is little leakage around the blades and the gaps between the blades are smaller. This is useful for pumping air into a hovercraft's plenum chamber.
- The high pitch speed of ducted fans means they continue making thrust even at high speeds.
- The duct contains some of the noise from the fan and protects it from damage.
- A ducted fan typically has a much smaller diameter than the propeller that would be used on a similar sized prop powered model. So, to generate the same thrust from a smaller volume of air it has to more it much faster, which is less efficient. A ducted fan model typically needs twice the power to achieve the same performance as a propeller-powered model.
- The extra power and RPM of a ducted fan usually make it noisier than a prop.
- The high pitch speed of ducted fans means they make little thrust at low speed.
Among model aircraft hobbyists, the ducted fan is mainly used modeling scale jet aircraft. Nitro-fueled piston engines were used first and now electric motors are commonly used to drive the fans. Despite the introduction of model-scale (miniature jet engine) turbines, ducted fans remain popular today as they are much more affordable and considerably smaller than even the smallest model turbine currently available.