In aeronautics, a ducted fan is a thrust-generating mechanical fan or

propeller A propeller (often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon a working flu ...

mounted within a cylindrical duct or shroud. Other terms include ducted propeller or shrouded propeller. When used in vertical takeoff and landing (

VTOL A vertical take-off and landing (VTOL) aircraft is one that can takeoff and landing, take off and land vertically without relying on a runway. This classification can include a variety of types of aircraft including helicopters as well as thrust- ...

) 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 airplanes,

airship An airship, dirigible balloon or dirigible is a type of aerostat (lighter-than-air) aircraft that can navigate through the air flying powered aircraft, under its own power. Aerostats use buoyancy from a lifting gas that is less dense than the ...

s, 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 Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...

, 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 advantageously affect the velocity and pressure of the airflow according to

Bernoulli's principle Bernoulli's principle is a key concept in fluid dynamics that relates pressure, speed and height. For example, for a fluid flowing horizontally Bernoulli's principle states that an increase in the speed occurs simultaneously with a decrease i ...

. 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.

Design

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.''Jane's All the World's Aircraft 1980-81'', Jane's, 1980. p.704.''Jane's All the World's Aircraft 1980-81'', Jane's, 1980. p.725. The blades may be of fixed or variable pitch. See:

Fan (machine) A fan is a powered machine that creates airflow. A fan consists of rotating vanes or blades, generally made of wood, plastic, or metal, which act on the air. The rotating assembly of blades and hub is known as an ''impeller'', ''Rotor (electric) ...

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. 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.

Powerplant

A ducted fan may be powered by any kind of motor capable of turning the fan. Examples include piston, rotary (Wankel), and turboshaft combustion engines, as well as electric motors. 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.

Ducted propulsion

An assembly designed throughout as a single integrated unit is referred to as a fan pod or ducted propulsor. 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.

Limitations

* Good efficiency requires very small clearances between the blade tips and the duct. * Efficiency advantages are reduced, and may even be reversed, at lower rotation speeds, thrust levels and airspeeds. * Requires reduced vibration levels compared to a free propeller or rotor. * Complex duct design, and weight increase even if constructed from advanced composites. * At high

angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a Airfoil#Airfoil terminology, reference line on a body (often the chord (aircraft), chord line of an airfoil) and the vector (geometry), vector representing the relat ...

, parts of the duct will stall and produce

aerodynamic drag In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...

Applications

In aircraft applications, the operating speed of an unshrouded propeller is limited since tip speeds approach the

sound barrier The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound. When aircraft first approached the speed of sound, th ...

at lower speeds than an equivalent ducted fan. The most common ducted fan arrangement used in full-sized aircraft is a

turbofan A turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a combination of references to the preceding generation engine technology of the turbojet and the add ...

engine, where the power to turn the fan is provided by a

gas turbine A gas turbine or gas turbine engine is a type of Internal combustion engine#Continuous combustion, continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas gene ...

. High bypass ratio turbofan engines are used on nearly all civilian airliners, while military fighters usually make use of 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 A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common features of al ...

Wankel engine The Wankel engine (, ) is a type of internal combustion engine using an eccentric (mechanism), eccentric Pistonless rotary engine, rotary design to convert pressure into rotating motion. The concept was proven by German engineer Felix Wankel, f ...

, or

electric motor An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...

. A kind of ducted fan, known as a fantail or by the trademark name Fenestron, is also used to replace tail rotors on helicopters. Avian 2-180 Gyroplane

Ducted fans are favored in

aircraft such as the

Lockheed Martin F-35 Lightning II The Lockheed Martin F-35 Lightning II is an American family of single-seat, single-engine, supersonic Stealth aircraft, stealth strike fighters. A multirole combat aircraft designed for both Air superiority fighter, air superiority and att ...

, 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 The University of Maryland, College Park (University of Maryland, UMD, or simply Maryland) is a public land-grant research university in College Park, Maryland, United States. Founded in 1856, UMD is the flagship institution of the Univ ...

'', 2008. Accessed: 28 August 2015. 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 The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...

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 320km/h (200mph). 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.

References

{{DEFAULTSORT:Ducted fan Aircraft propulsion components Aircraft engines Ventilation fans