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Ducted fan
From Wikipedia, the free encyclopedia
A ducted fan is a propulsion arrangement whereby a propeller is mounted within a cylindrical shroud or duct. The duct prevents losses in thrust from the tips of the propeller and, if the duct has an airfoil cross-section, it can provide additional thrust of its own. Ducted fan propulsion is used in aircraft, airboats and hovercraft. A kind of ducted fan, known as a fantail or by the trademark name Fenestron, is also used to replace tail rotors on helicopters.
In aircraft applications, ducted fans normally have more and shorter blades than propellers and thus can operate at higher rotational speeds. The operating speed of an unshrouded propeller is limited since tip speeds approach the sound barrier at lower speeds than an equivalent ducted propeller. The higher rotational speed of a ducted fan may require a gearbox when used with piston engines, which adds weight and negates some of the advantages. Instead, electric or Wankel rotary engines are the preferred method of power, and efficient home-made examples exist for both. A turbine can also be used to power the fan; in this configuration the ducted fan is referred to as a turbofan. Ducted fans usually have an odd number of blades to prevent resonance in the duct.
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- By reducing propeller blade tip losses and directing its thrust towards the back only, the ducted fan is more efficient in producing thrust than a conventional propeller at low speeds (normally considered under 80 knots, 40 m/s or approximately 90 miles per hour for an aircraft).
- For the same static thrust, a ducted fan has a smaller diameter than a free propeller.
- Ducted fans are quieter than propellers: they shield the blade noise, and reduce the tip speed and intensity of the tip vortices both of which contribute to noise production.
- 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 tiltrotors in some applications.
- At higher speeds (above about 80 knots), the presence of the duct may create more drag than the extra thrust it provides and therefore its comparative advantage to an open prop is canceled.
- Good efficiency needs very small gap between tip blade and duct; Ducts are heavy and expensive.
- In general, a large propeller is more efficient than a small propeller, and ducted fans usually have very small propellers, and so can be inefficient compared to a larger standard propeller arrangement.
Ducted fans are favored in VTOL (such as Paul Moller's Sky Car) and other low-speed designs such as hovercraft for their higher thrust-to-weight ratio. A fictional example is the Egg Hawk boss from Sonic Heroes, a hawk-shaped VTOL craft that you fight early on in the game.
Among model aircraft hobbyists, the ducted fan is popular with builders of high-performance radio controlled model aircraft. Internal-combustion glow engines combined with ducted-fan units were the first achievable means of modeling a scaled-size jet aircraft. Despite the introduction of model-scale (miniature jet engine) turbines, ducted fans remain popular today as they are more affordable, and because ducted fans are compatible with electric-powered flight systems.