Ducted fan

Last updated
Bell X-22 X-22a onground bw.jpg
Bell X-22

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. [1] When used in vertical takeoff and landing (VTOL) applications it is also known as a shrouded rotor. [2]

Contents

Ducted fans are used for propulsion or direct lift in many types of vehicle including aeroplanes, airships, 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 efficiency by up to 90% in most cases, 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.

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. [3] [4] The blades may be of fixed or variable pitch.

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. [4]

It acts as a protective device, both to protect objects such as ground staff from being hit by the whirling blades, and to protect the blades from damage during such an impact.

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. [4]

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 propulsor

An assembly designed throughout as a single integrated unit is referred to as a fan pod or ducted propulsor. [3] [4]

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

Applications

Dirigible airship with ducted fans GR SK Propeller.jpg
Dirigible airship with ducted fans

In aircraft applications, the operating speed of an unshrouded propeller is limited since tip speeds approach the sound barrier at lower speeds than an equivalent ducted fan. The most common ducted fan arrangement used in full-sized aircraft is a turbofan engine, where the power to turn the fan is provided by a gas turbine. 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, 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.

Avian Gyroplane with ducted pusher prop (but no tail-boom) Avian 2-180 Gyroplane.jpg
Avian Gyroplane with ducted pusher prop (but no tail-boom)
Ducted fan on an Air Cushion Landing Craft US Navy 060817-N-1467R-042 A Sailor assigned to Assault Craft Unit Four (ACU 4) stationed at Little Creek Amphibious Base, Va., checks the propeller of a Landing Craft Air Cushion (LCAC).jpg
Ducted fan on an Air Cushion Landing Craft

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. [6]

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 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.

The Martin Jetpack personal aircraft with ducted fans Martin Jetpack Unveiling, Liftoff! (2714934801).jpg
The Martin Jetpack personal aircraft with ducted fans

See also

Related Research Articles

<span class="mw-page-title-main">Aircraft</span> Vehicle or machine that is able to fly by gaining support from the air

An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or the dynamic lift of an airfoil, or, in a few cases, direct downward thrust from its engines. Common examples of aircraft include airplanes, helicopters, airships, gliders, paramotors, and hot air balloons.

<span class="mw-page-title-main">Jet engine</span> Aircraft engine that produces thrust by emitting a jet of gas

A jet engine is a type of reaction engine, discharging a fast-moving jet of heated gas that generates thrust by jet propulsion. While this broad definition may include rocket, water jet, and hybrid propulsion, the term jet engine typically refers to an internal combustion air-breathing jet engine such as a turbojet, turbofan, ramjet, pulse jet, or scramjet. In general, jet engines are internal combustion engines.

<span class="mw-page-title-main">Propeller</span> Device that transmits rotational power into linear thrust on a fluid

A propeller 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 fluid such as water or air. Propellers are used to pump fluid through a pipe or duct, or to create thrust to propel a boat through water or an aircraft through air. The blades are shaped so that their rotational motion through the fluid causes a pressure difference between the two surfaces of the blade by Bernoulli's principle which exerts force on the fluid. Most marine propellers are screw propellers with helical blades rotating on a propeller shaft with an approximately horizontal axis.

A vertical take-off and landing (VTOL) aircraft is one that can 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-vectoring fixed-wing aircraft and other hybrid aircraft with powered rotors such as cyclogyros/cyclocopters and gyrodynes.

<span class="mw-page-title-main">Turbofan</span> Airbreathing jet engine designed to provide thrust by driving a fan

The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a combination of the preceding generation engine technology of the turbojet, and a reference to the additional fan stage added. It consists of a gas turbine engine which achieves mechanical energy from combustion, and a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.

<span class="mw-page-title-main">Aircraft engine</span> Engine designed for use in powered aircraft

An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Aircraft using power components are referred to as powered flight. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years many small UAVs have used electric motors.

<span class="mw-page-title-main">Bypass ratio</span> Proportion of ducted compared to combusted air in a turbofan engine

The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for every 1 kg of air passing through the core.

<span class="mw-page-title-main">Propfan</span> Type of aircraft engine

A propfan, also called an open rotor engine, open fan engine or unducted fan, is a type of aircraft engine related in concept to both the turboprop and turbofan, but distinct from both. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop. A propfan is typically designed with a large number of short, highly twisted blades, similar to the (ducted) fan in a turbofan engine. For this reason, the propfan has been variously described as an "unducted fan" (UDF) or an "ultra-high-bypass (UHB) turbofan".

<span class="mw-page-title-main">Contra-rotating propellers</span> Two-propeller design for improving low-airspeed maneuverability

Aircraft equipped with contra-rotating propellers (CRP) coaxial contra-rotating propellers, or high-speed propellers, apply the maximum power of usually a single piston engine or turboprop engine to drive a pair of coaxial propellers in contra-rotation. Two propellers are arranged one behind the other, and power is transferred from the engine via a planetary gear or spur gear transmission. Contra-rotating propellers are also known as counter-rotating propellers, although the term counter-rotating propellers is much more widely used when referring to airscrews on separate non-coaxial shafts turning in opposite directions.

A jet engine performs by converting fuel into thrust. How well it performs is an indication of what proportion of its fuel goes to waste. It transfers heat from burning fuel to air passing through the engine. In doing so it produces thrust work when propelling a vehicle but a lot of the fuel is wasted and only appears as heat. Propulsion engineers aim to minimize the degradation of fuel energy into unusable thermal energy. Increased emphasis on performance improvements for commercial airliners came in the 1970s from the rising cost of fuel.

<span class="mw-page-title-main">Fenestron</span> Helicopter anti-torque system based on a ducted fan

A Fenestron is an enclosed helicopter tail rotor that operates like a ducted fan. The term Fenestron is a trademark of multinational helicopter manufacturing consortium Airbus Helicopters. The word itself comes from the Occitan term for a small window, and is ultimately derived from the Latin word fenestra for window.

<span class="mw-page-title-main">Propeller (aeronautics)</span> Aircraft propulsion component

In aeronautics, an aircraft propeller, also called an airscrew, converts rotary motion from an engine or other power source into a swirling slipstream which pushes the propeller forwards or backwards. It comprises a rotating power-driven hub, to which are attached several radial airfoil-section blades such that the whole assembly rotates about a longitudinal axis. The blade pitch may be fixed, manually variable to a few set positions, or of the automatically variable "constant-speed" type.

<span class="mw-page-title-main">Powered lift</span> VTOL capable fixed-wing aircraft

A powered lift aircraft takes off and lands vertically under engine power but uses a fixed wing for horizontal flight. Like helicopters, these aircraft do not need a long runway to take off and land, but they have a speed and performance similar to standard fixed-wing aircraft in combat or other situations.

<span class="mw-page-title-main">Components of jet engines</span> Brief description of components needed for jet engines

This article briefly describes the components and systems found in jet engines.

An airbreathing jet engine is a jet engine in which the exhaust gas which supplies jet propulsion is atmospheric air, which is taken in, compressed, heated, and expanded back to atmospheric pressure through a propelling nozzle. Compression may be provided by a gas turbine, as in the original turbojet and newer turbofan, or arise solely from the ram pressure of the vehicle's velocity, as with the ramjet and pulsejet.

<span class="mw-page-title-main">Distributed propulsion</span> Engines placed along the wingspan of a plane

In aeronautics, Distributed propulsion is an arrangement in which the propulsive and related air flows are distributed over the aerodynamic surfaces of an aircraft. The purpose is to improve the craft's aerodynamic, propulsive and/or structural efficiency over an equivalent conventional design.

<span class="mw-page-title-main">VTOL X-Plane</span> American experimental aircraft

The Vertical Take-Off and Landing Experimental Aircraft program is an American research project sponsored by the Defense Advanced Research Projects Agency (DARPA). The goal of the program is to demonstrate a VTOL aircraft design that can take off vertically and efficiently hover, while flying faster than conventional rotorcraft. There have been many previous attempts, most of them unsuccessful.

<span class="mw-page-title-main">Cyclorotor</span> Perpendicular axis marine propulsion system

A cyclorotor, cycloidal rotor, cycloidal propeller or cyclogiro, is a fluid propulsion device that converts shaft power into the acceleration of a fluid using a rotating axis perpendicular to the direction of fluid motion. It uses several blades with a spanwise axis parallel to the axis of rotation and perpendicular to the direction of fluid motion. These blades are cyclically pitched twice per revolution to produce force in any direction normal to the axis of rotation. Cyclorotors are used for propulsion, lift, and control on air and water vehicles. An aircraft using cyclorotors as the primary source of lift, propulsion, and control is known as a cyclogyro or cyclocopter. A unique aspect is that it can change the magnitude and direction of thrust without the need of tilting any aircraft structures. The patented application, used on ships with particular actuation mechanisms both mechanical or hydraulic, is named after German company Voith Turbo.

<span class="mw-page-title-main">Annular lift fan aircraft</span>

An annular lift fan aircraft is a conceptual vertical takeoff and landing (VTOL) aircraft that was first systematically and numerically investigated in 2015. This concept was proposed to offer a VTOL solution for both high hovering efficiency and high cruise speed, using a large annular lift fan instead of the relatively small conventional circular lift fans used in the Ryan XV-5 Vertifan and the F-35B Lightning II (JSF).

References

  1. Anita I. Abrego and Robert W. Bulaga; "Performance Study of a Ducted Fan System", NASA, 2002.
  2. Kotwani, Kailash; "Ducted Fan or Shrouded Rotor Aerodynamics and its Application in Miniature VTOL Aerial Vehicles", 2009. (retrieved 22 March 2022).
  3. 1 2 Jane's All the World's Aircraft 1980-81, Jane's, 1980. p.704.
  4. 1 2 3 4 Jane's All the World's Aircraft 1980-81, Jane's, 1980. p.725.
  5. "Jon Longbottom - Mechanical aeronautics, thesis in PDF format" (PDF). Archived from the original (PDF) on 2014-01-02. Retrieved 2014-01-02.
  6. 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.