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In aeronautics a ducted fan is a thrust-generating mechanical fan or propeller mounted within a cylindrical duct or shroud.Other terms include ducted propeller or shrouded propeller.When used in vertical take-off and landing (VTOL) applications, it is also known as a shrouded rotor.Ducted fans are used to propel or directly lift many types of vehicles, including airplanes, airships, hovercraft, and powered lift VTOL aircraft.A very successful and popular example of a ducted fan design is the high bypass ratio turbofan engine used on many modern airliners.In most cases, the duct improves thrust efficiency by up to 90% compared to a similarly sized propeller in free air.Ducted fans are quieter and provide a good opportunity for thrust vectoring.The shroud provides good protection for ground personnel from accidental contact with the spinning blades and protects the blades themselves from external debris or objects.By varying the cross-section of the duct, the designer can favorably influence the velocity and pressure of the airflow according to Bernoulli's principle.Disadvantages include increased weight due to the added shroud structure, the need to precisely control the tolerances of the tip-shroud clearance, the need for better vibration control compared to free-air propellers, and complex piping design requirements.Finally, when at high angles of attack, the shield stalls and creates high drag.
A ducted fan has three main components; the fan or propeller that provides thrust or lift, the ducts or shroud around the fan, and the engine or motor that powers the fan.
Like any other fan, propeller or rotor, a ducted fan is characterized by the number of blades.The Rhein Flugzeugbau (RFB) SG 85 has three blades, while the Dowty Rotol Pipe Thruster has seven. The blades may be fixed or variable pitch.
The duct or shroud is an aerodynamic ring that surrounds the fan and fits snugly around the blade tips.It must be strong enough not to deform under flight loads and not touch the blades as they turn.Pipelines perform several functions:Primarily, it reduces the turbulence created by the air flowing over the tips of the blades.This reduces aerodynamic loss or drag, which increases the overall efficiency of the fan.Because of this, fans can be used to both provide greater thrust and aircraft performance,and can also be made smaller than an equivalent free propeller.It provides acoustic shielding while reducing energy waste and significantly reduces propeller noise emissions.It acts as a protection, both to protect objects such as ground crew from being hit by the spinning blades, and to protect the blades from damage during such impacts.Reduced tip vortex also means less turbulence in the fan wake.With careful design, heat exhaust from the engine cooling system can be injected into the low-turbulence fan wake for increased thrust.
Ducted fans can be powered by any type of motor capable of turning the fan.Examples include piston, rotary (Wankel) and turboshaft internal combustion engines, as well as electric motors.Fans can be mounted directly on the power unit output shaft, or driven remotely via an extended drive shaft and gearing. In remote arrangements, multiple fans can be driven by a single power unit.
The entire assembly designed as a single integrated unit is called a fan pod or ducted thruster.An advantage of the pod approach is that each component can be designed to match the others, helping to maximize performance and reduce weight.It also simplifies the vehicle designer's task of integrating with the vehicle and its systems.