Ducted fan

A ducted fan has three main components; the fan or propeller which provides thrust or lift, the duct or shroud which surrounds the fan, and the engine or motor which powers the fan. Fan Like any other fan, propeller or rotor, a ducted fan is characterised by the number of blades. The Rhein Flugzeugbau (RFB) SG 85 had three blades, while the Dowty Rotol Ducted Propulsor had seven. The blades may be of fixed or variable pitch. See: Fan (machine) Duct The duct or shroud is an aerodynamic ring which surrounds the fan and closely fits the blade tips. It must be made rigid enough not to distort under flight loads nor touch the blades as they turn. The duct performs several functions: Principally, it reduces the vortices created by air flowing round the ends of the blades. This reduces the aerodynamic losses or drag, thus increasing the overall efficiency of the fan. Because of this, the fan can either be used to provide increased thrust and aircraft performance, or be made smaller than the equivalent free propeller. It provides acoustic shielding which, together with the reduced energy waste, significantly cuts noise emissions from the propeller. ==Applications==

The most common ducted fan arrangement used in aircraft is a turbofan engine, where the power to turn the fan is provided by a gas turbine. High bypass ratio turbofans are used on civilian airliners, while military fighters need the better high-speed performance of a low bypass ratio turbofan with a smaller fan diameter. However, a ducted fan may be powered by any source of shaft power such as a reciprocating engine, Wankel engine, or electric motor. A kind of ducted fan, known as a fantail or by the trademark name Fenestron, is also used to replace tail rotors 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 for their higher thrust-to-weight ratio. In some cases, a shrouded rotor can be 94% more efficient than an open rotor. The improved performance is mainly because the outward flow is less contracted and thus carries more kinetic energy. Among model aircraft hobbyists, the ducted fan is popular with builders of high-performance radio controlled model aircraft. Glow plug engines combined with ducted-fan units were the first achievable means of modeling a scaled-size jet aircraft. Despite the introduction of model-scale turbojet engines, electric-powered ducted fans remain popular on smaller, lower-cost model aircraft. Some electric-powered ducted fan airplanes can reach speeds of more than . Most cooling fans used in computers contain a duct integrated into the fan assembly; the duct is also used for mechanically mounting the fan to other components. == See also ==