Publish Time: 2023-04-17 Origin: Site
In aircraft applications the operating speed of an unshrouded propeller is limited because the tip speed approaches the sound barrier at a lower speed than an equivalent ducted fan.The most common ducted fan arrangement in full-size aircraft is the turbofan engine, where the power to turn the fan is provided by a gas turbine.Almost all civilian airliners use high-bypass turbofan engines, while military fighter jets usually use low-bypass turbofan engines with smaller fan diameters to achieve better high-speed performance.However, ducted fans can be powered by any shaft power source, such as reciprocating engines, Wankel engines, or electric motors.A type of ducted fan known as a fantail or by the brand name Fenestron is also used to replace the tail rotor on helicopters.Ducted fans are favored in VTOL aircraft (such as the Lockheed Martin F-35 Lightning II) and other low-speed designs (such as hovercraft) because of their higher thrust-to-weight ratio.In some cases, shrouded rotors are up to 94% more efficient than open rotors.The improved performance is mainly due to the fact that the outward flow constricts less and therefore carries more kinetic energy.
Among aircraft model enthusiasts, ducted fans are popular with manufacturers of high-performance radio-controlled model aircraft. Combustion engine glow engines combined with ducted fan arrangements are the first realistic approach to modeling jumbo jets.Despite the introduction of model-class turbojets, electric ducted fans have remained popular on smaller, lower-cost model aircraft.Some electric ducted fan aircraft can reach speeds in excess of 320 km/h (200 mph).Most types of fans used in computers contain a duct integrated into the fan assembly; this duct is also used to mechanically mount the fan to other components.
Good efficiency requires a very small clearance between the tip and the duct.
At lower RPMs, thrust levels and airspeeds, the efficiency advantage is reduced and possibly reversed.
Requires lower vibration levels than free propellers or rotors.
Complex piping designs, even when made of advanced composite materials, add weight.
At high angles of attack, some ducts stall and create air resistance.