A
ducted fan is a propulsion arrangement whereby a fan, which is a type of
propellerA propeller is a type of fan which transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and air or water is accelerated behind the blade...
, is mounted within a cylindrical
shroudShroud usually refers to an item, such as a cloth, that covers or protects some other object. The term is most often used in reference to burial sheets, winding-cloths or winding-sheets, such as the famous Shroud of Turin or Tachrichim that Jews are dressed in for burial...
or
ductA duct may refer to:* Duct , various ducts in anatomy and physiology* Duct , for transfer of air between spaces in a structure* Duct tape, a kind of adhesive tape* Ducted fan, motor for aircraft...
. The duct reduces losses in thrust from the
tip vorticesWingtip vortices are tubes of circulating air which are left behind a wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and are regions of very low pressure. To first approximation, these low-pressure regions form with...
of the fan, and varying the cross-section of the duct allows the designer to advantageously affect the velocity and pressure of the airflow according to
Bernoulli's PrincipleIn fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy...
. Ducted fan propulsion is used in
aircraftAn aircraft is a vehicle which is able to fly by being supported by the air, or in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil An aircraft is a vehicle which is able to fly by being supported...
, airboats and
hovercraftA hovercraft is a craft capable of traveling over relatively smooth surfaces supported by a cushion of slow moving, high-pressure air, ejected against the surface below, and contained within a "skirt." Although supported by the air, they are not usually considered to be aircraft.Hovercraft are...
.
Applications
In aircraft applications, ducted fans normally have more and shorter blades than propellers and thus can operate at higher rotational speeds. The operating speed of an unshrouded propeller is limited since tip speeds approach the
sound barrierIn aerodynamics, the sound barrier usually refers to the point at which an aircraft moves from transonic to supersonic speed. The term came into use during World War II when a number of aircraft started to encounter the effects of compressibility, a collection of several unrelated aerodynamic effects...
at lower speeds than an equivalent ducted fan. The most common ducted fan arrangement used in full-sized aircraft is a
turbofanA turbofan is a type of aircraft gas turbine engine that provides propulsion using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses...
engine, where the power to turn the fan is provided by a
gas turbineA gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Energy is added to the gas stream in the combustor, where air is mixed with...
. Turbofan engines are used on nearly all airliners,
fightersA fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets by dropping bombs. Fighters are small, fast, and maneuverable...
, and bombers. However, a ducted fan may be powered by any source of shaft power such as a
reciprocating engineA reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
,
Wankel engineThe Wankel engine is a type of internal combustion engine which uses a rotary design to convert pressure into a rotating motion instead of using reciprocating pistons. Its four-stroke cycle takes place in a space between the inside of an oval-like epitrochoid-shaped housing and a rotor that is...
, or
electric motorAn electric motor uses electrical energy to produce mechanical energy, usually through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on...
. A kind of ducted fan, known as a fantail or by the trademark name
FenestronA Fenestron is a shrouded tail rotor of a helicopter that is essentially a ducted fan. The housing is integral with the tail skin, and, like the conventional tail rotor it replaces, is intended to counteract the torque of the main rotor. It was original conceived by Sud Aviation in the 1960s...
, is also used to replace
tail rotorThe tail rotor, or anti-torque rotor, is a smaller rotor mounted so that it rotates vertically or near-vertically at the end of the tail of a traditional single-rotor helicopter. The tail rotor's position and distance from the center of gravity allow it to develop thrust in a direction opposite of...
s on helicopters. Ducted fans usually have an odd number of blades to prevent
resonanceIn physics, resonance is the tendency of a system to oscillate at larger amplitude at some frequencies than at others. These are known as the system's resonant frequencies . At these frequencies, even small periodic driving forces can produce large amplitude vibrations, because the system...
in the duct.
Ducted fans are favoured in
VTOLVTOL is an abbreviation for Vertical Take-Off and Landing aircraft. See also V/STOL. This classification includes fixed-wing aircraft that can hover, take off and land vertically as well as helicopters and other aircraft with powered rotors, such as tiltrotors...
aircraft such as the
Lockheed MartinLockheed Martin is a multinational aerospace manufacturer, global security and advanced technology company formed in 1995 by the merger of Lockheed with Martin Marietta. It is headquartered in Bethesda, Maryland, in the Washington Metropolitan Area. Lockheed Martin employs 146,000 people...
F-35 Lightning IIThe Lockheed Martin F-35 Lightning II is a fifth-generation, single-seat, single-engine, stealth-capable multirole fighter, that can perform close air support, tactical bombing, and air defense missions...
, and other low-speed designs such as
hovercraftA hovercraft is a craft capable of traveling over relatively smooth surfaces supported by a cushion of slow moving, high-pressure air, ejected against the surface below, and contained within a "skirt." Although supported by the air, they are not usually considered to be aircraft.Hovercraft are...
for their higher thrust-to-weight ratio.
Among
model aircraftModel aircraft are flying or non-flying models of existing or imaginary aircraft, often scaled down versions of full size planes, using materials such as balsa wood, foam and fiberglass. Designs range from simple gliders, to accurate scale models, some of which can be very large.Models may be built...
hobbyists, the ducted fan is popular with builders of high-performance radio controlled model aircraft. Internal-combustion glow engines combined with ducted-fan units were the first achievable means of modeling a scaled-size jet aircraft. Despite the introduction of model-scale
turbojetTurbojets are the oldest kind of general purpose jet engines. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s, although credit for the first turbojet is given to Whittle who submitted...
engines, electric-powered ducted fans remain popular on smaller, lower-cost model aircraft. Some electric-powered ducted fan airplanes can reach speeds of over 200 miles per hour.
Advantages
- By reducing propeller blade tip losses and directing its thrust towards the back only, the ducted fan is more efficient in producing thrust than a conventional propeller.
- By sizing the ductwork appropriately, the designer can adjust the air velocity through the fan to allow it to operate more efficiently at higher air speeds than a propeller would.
- For the same static thrust, a ducted fan has a smaller diameter than a free propeller.
- Ducted fans are quieter than propellers: they shield the blade noise, and reduce the tip speed and intensity of the tip vortices
Wingtip vortices are tubes of circulating air which are left behind a wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and are regions of very low pressure. To first approximation, these low-pressure regions form with...
both of which contribute to noise production.
- Ducted fans can allow for a limited amount of thrust vectoring
Thrust vectoring is the ability of an aircraft or other vehicle to direct the thrust from its main engine in a direction other than parallel to the vehicle's longitudinal axis. The technique was originally envisaged to provide upward vertical thrust as a means to give aircraft vertical or short ...
, something normal propellers are not well suited for. This allows them to be used instead of tiltrotorA tiltrotor aircraft utilizes a pair or more of powered rotors mounted on rotating shafts or nacelles at the end of a fixed wing for lift and propulsion, and combines the vertical lift capability of a helicopter with the speed and range of a conventional fixed-wing aircraft...
s in some applications.
Disadvantages
- Good efficiency requires very small clearances between the blade tips and the duct.
- Ducts are heavy and expensive.