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In aeronautics, a ducted fan is a thrust-generating mechanical fan or propeller mounted within a cylindrical or shroud. Other terms include ducted propeller or shrouded propeller.Anita I. Abrego and Robert W. Bulaga; " Performance Study of a Ducted Fan System ", NASA, 2002. When used in vertical takeoff and landing (VTOL) applications it is also known as a shrouded rotor.Kotwani, Kailash; " Ducted Fan or Shrouded Rotor Aerodynamics and its Application in Miniature VTOL Aerial Vehicles ", 2009. (retrieved 22 March 2022).
Ducted fans are used for propulsion or direct lift in many types of vehicle including Airplane, , hovercraft, and powered lift VTOL aircraft. The high-bypass turbofan engines used on many modern airliners is an example of a very successful and popular use of ducted fan design.
The duct increases thrust, in comparison to a similar-sized propeller in free air. Ducted fans are quieter, and offer good opportunities for thrust vectoring. The shroud offers good protection to ground personnel from accidentally contacting the spinning blades, as well as protecting the blades themselves from external debris or objects. By varying the cross-section of the duct the designer can adjust the velocity and pressure of the airflow according to Bernoulli's principle.
Drawbacks include increased weight due to the added structure of the shroud, a need for precision in tolerances of blade-tip to shroud clearance, a need for better vibration control compared to free-air propellers, and complex duct design requirements. Lastly, when at high angles of attack, the shroud can stall and produce high drag.
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.
It acts as a protective device, to protect objects such as ground staff from being hit by the whirling blades, to protect the blades from damage during such an impact, and to protect the aircraft fuselage and contents (including crew, passengers, fuel tanks, hydraulic systems, etc.) from impact damage in the event of propeller failure.
The reduced tip vortices also mean that the fan wake is less turbulent. With careful design, the heated discharge from the engine cooling system can be injected into the low-turbulence fan wake to increase thrust.
The fan may be mounted directly on the powerplant output shaft, or driven remotely via an extended drive shaft and gearing. In the remote arrangement, several fans may be driven by a single powerplant.
An advantage of the pod approach is that the design of each component can be matched to the others, helping to maximise performance and minimise weight. It also eases the vehicle designer's task of integration with the vehicle and its systems.
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.Pereira, Jason L. Hover and wind-tunnel testing of shrouded rotors for improved micro air vehicle design p147+p11. University of Maryland, 2008. Accessed: 28 August 2015.
Among model aircraft hobbyists, the ducted fan is popular with builders of high-performance radio controlled model aircraft. glow engine 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 computer fan contain a duct integrated into the fan assembly; the duct is also used for mechanically mounting the fan to other components.
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