Blown flaps, or jet flaps, are powered aerodynamichigh-lift devices used on the wings of certain aircraft to improve their low-speed flight characteristics. They use air blown through nozzles to shape the airflow over the rear edge of the wing, directing the flow downward to increase the lift coefficient. There are a variety of methods to achieve this airflow, most of which use jet exhaust or high-pressure air bled off of a jet engine's compressor and then redirected to follow the line of trailing-edge flaps.
Blown flaps may refer specifically to those systems that use internal ductwork within the wing to direct the airflow, or more broadly to systems like upper surface blowing or nozzle systems on conventional underwing engine that direct air through the flaps. Blown flaps are one solution among a broader category known as powered lift, which also includes various boundary layer control systems, systems using directed prop wash, and circulation control wings.
Internal blown flaps were used on some land and carrier-based fast jets in the 1960s, including the Lockheed F-104, Blackburn Buccaneer and certain versions of the Mikoyan-Gurevich MiG-21. They generally fell from favour because they imposed a significant maintenance overhead in keeping the ductwork clean and various valve systems working properly, along with the disadvantage that an engine failure reduced lift in precisely the situation where it is most desired. The concept reappeared in the form of upper and lower blowing in several transport aircraft, both turboprop and turbofan.
Mechanism
In a conventional blown flap, a small amount of the compressed air produced by the jet engine is "bled" off at the compressor stage and piped to channels running along the rear of the wing. There, it is forced through slots in the wing flaps of the aircraft when the flaps reach certain angles. Injecting high energy air into the boundary layer produces an increase in the stalling angle of attack and maximum lift coefficient by delaying boundary layer separation from the airfoil. Boundary layer control by mass injecting (blowing) prevents boundary layer separation by supplying additional energy to the particles of fluid which are being retarded in the boundary layer. Therefore, injecting a high velocity air mass into the air stream essentially tangent to the wall surface of the airfoil reverses the boundary layer friction deceleration thus the boundary layer separation is delayed.[1]
The lift of a wing can be greatly increased with blowing flow control. With mechanical slots the natural boundary layer limits the boundary layer control pressure to the freestream total head.[2] Blowing with a small proportion of engine airflow (internal blown flap) increases the lift. Using much higher quantities of gas from the engine exhaust, which increases the effective chord of the flap (the jet flap), produces supercirculation,[3] or forced circulation[4] up to the theoretical potential flow maximum.[3] Surpassing this limit requires the addition of direct thrust.[4]
Development of the general concept continued at NASA in the 1950s and 1960s, leading to simplified systems with similar performance. The externally blown flap arranges the engine to blow across the flaps at the rear of the wing. Some of the jet exhaust is deflected downward directly by the flap, while additional air travels through the slots in the flap and follows the outer edge due to the Coandă effect. The similar upper-surface blowing system arranges the engines over the wing and relies completely on the Coandă effect to redirect the airflow. Although not as effective as direct blowing, these "powered lift" systems are nevertheless quite powerful and much simpler to build and maintain.
A more recent and promising blow-type flow control concept is the counter-flow fluid injection which is able to exert high-authority control to global flows using low energy modifications to key flow regions. In this case the air blow slit is located at the pressure side near the leading edgestagnation point location and the control air-flow is directed tangentially to the surface but with a forward direction. During the operation of such a flow control system two different effects are present. One effect, boundary layer enhancement, is caused by the increased turbulence levels away from the wall region thus transporting higher-energy outer flow into the wall region. In addition to that another effect, the virtual shaping effect, is utilized to aerodynamically thicken the airfoil at high angles of attack. Both these effects help to delay or eliminate flow separation.[5]
In general, blown flaps can improve the lift of a wing by two to three times. Whereas a complex triple-slotted flap system on a Boeing 747 produces a coefficient of lift of about 2.45,[6] external blowing (upper surface blowing on a YC-14) improves this to about 7,[6] and internal blowing (jet flap on Hunting H.126) to 9.[7]
History
Williams[8] states some flap blowing tests were done at the R.A.E. before the Second World War and that extensive tests were done during the war in Germany including flight tests with Arado 232, Do-24 and Bf 109 aircraft. Lachmann[9] states the Arado and Dornier aircraft used an ejector-driven single flow of air which was sucked over part of the trailing edge span and blown over the remainder. The ejector was chemically powered using high pressure vapour. The Bf 109 used engine-driven blowers for flap blowing.
Rebuffet and Poisson-Quinton[10] describe tests in France at O.N.E.R.A. after the war with combined sucking at le of first flap section and blowing at second flap section using a jet engine compressor bleed ejector to give both sucking and blowing. Flight testing was done on a Breguet Vultur aircraft.[11]
Tests were also done at Westland Aircraft by W.H. Paine after the war with reports dated 1950 and 1951.[8]
In the United States a Grumman F9F Panther was modified with flap blowing based on work done by John Attinello in 1951. Engine compressor bleed was used. The system was known as "Supercirculation Boundary Layer Control" or BLC for short.[12]
Between 1951 and 1955 Cessna did flap blowing tests on Cessna 309 and 319 aircraft using the Arado system.[13]
During the 1950s and 60s, fighter aircraft generally evolved towards smaller wings in order to reduce drag at high speeds. Compared to the fighters of a generation earlier, they had wing loadings about four times as high; for instance the Supermarine Spitfire had a wing loading of 24 lb/ft2(117 kg/m2) and the Messerschmitt Bf 109 had the "very high" loading of 30 lb/ft2(146 kg/m2), whereas the 1950s-era F-104 Starfighter had 111 lb/ft2(542 kg/m2).
One serious downside to these higher wing loadings is at low speed, when there isn't enough wing left to provide lift to keep the plane flying. Even huge flaps could not offset this to any large degree, and as a result many aircraft landed at fairly high speeds, and were noted for accidents as a result.
The major reason flaps were not effective is that the airflow over the wing could only be "bent so much" before it stopped following the wing profile, a condition known as flow separation. There is a limit to how much air the flaps can deflect overall. There are ways to improve this, through better flap design; modern airliners use complex multi-part flaps for instance. However, large flaps tend to add considerable complexity, and take up room on the outside of the wing, which makes them unsuitable for use on a fighter.
The principle of the jet flap, a type of internally blown flap, was proposed and patented in 1952 by the British National Gas Turbine Establishment (NGTE) and thereafter investigated by the NGTE and the Royal Aircraft Establishment.[14] The concept was first tested at full-scale on the experimental Hunting H.126. It reduced the stall speed to only 32mph (51km/h), a number most light aircraft cannot match. The jet flap used a large percentage of the engine exhaust, rather than compressor bleed air, for blowing.[15]
A Buccaneer pictured with the blowing slots visible on the leading edges. The extended flaps are contributing to the Coanda airflow over the wing.
Starting in the 1970s the lessons of air combat over Vietnam changed thinking considerably. Instead of aircraft designed for outright speed, general maneuverability and load capacity became more important in most designs. The result is an evolution back to larger planforms to provide more lift. For instance the F-16 has a wing loading of 78.5 lb/ft2(383 kg/m2), and uses leading edge extensions to provide considerably more lift at higher angles of attack, including approach and landing. Some later combat aircraft achieved the required low-speed characteristics using swing-wings. Internal flap blowing is still used to supplement externally blown flaps on the Shin Meiwa US-1A.
Some aircraft currently (2015) in service that require a STOL performance use external flap blowing and, in some cases, also use internal flap blowing on flaps as well as on control surfaces such as the rudder to ensure adequate control and stability at low speeds. External blowing concepts are known as[15] the "externally blown flap" (used on the C-17 Globemaster ), "upper surface blowing" (used on the An-72 and An-74) and "vectored slipstream", or "over the wing blowing",[19] used on the An-70 and the Shin Meiwa US-1A and ShinMaywa US-2.
Powered high-lift systems, such as externally blown flaps, are not used for civil transport aircraft for reasons given by Reckzeh,[21] which include complexity, weight, cost, sufficient existing runway lengths and certification rules.
A fluid flowing around an object exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force parallel to the flow direction. Lift conventionally acts in an upward direction in order to counter the force of gravity, but it can act in any direction at right angles to the flow.
A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expressed as its lift-to-drag ratio. The lift a wing generates at a given speed and angle of attack can be one to two orders of magnitude greater than the total drag on the wing. A high lift-to-drag ratio requires a significantly smaller thrust to propel the wings through the air at sufficient lift.
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil, and Reynolds number.
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the turbo portion refers to a gas turbine engine which achieves mechanical energy from combustion, and the fan, 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.
The Coandă effect is the tendency of a fluid jet to stay attached to a convex surface. It is named after Romanian inventor Henri Coandă, who described it as "the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surroundings so that a region of lower pressure develops."
A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface or a rotor blade of a wind turbine. VGs may also be attached to some part of an aerodynamic vehicle such as an aircraft fuselage or a car. When the airfoil or the body is in motion relative to the air, the VG creates a vortex, which, by removing some part of the slow-moving boundary layer in contact with the airfoil surface, delays local flow separation and aerodynamic stalling, thereby improving the effectiveness of wings and control surfaces, such as flaps, elevators, ailerons, and rudders.
In aircraft design and aerospace engineering, a high-lift device is a component or mechanism on an aircraft's wing that increases the amount of lift produced by the wing. The device may be a fixed component, or a movable mechanism which is deployed when required. Common movable high-lift devices include wing flaps and slats. Fixed devices include leading-edge slots, leading edge root extensions, and boundary layer control systems.
The Boeing YC-14 is a twinjet short take-off and landing (STOL) tactical military transport aircraft. It was Boeing's entrant into the United States Air Force's Advanced Medium STOL Transport (AMST) competition, which aimed to replace the Lockheed C-130 Hercules as the USAF's standard STOL tactical transport. Although both the YC-14 and the competing McDonnell Douglas YC-15 were successful, neither aircraft entered production. The AMST project was ended in 1979 and replaced by the C-X program.
A flap is a high-lift device used to reduce the stalling speed of an aircraft wing at a given weight. Flaps are usually mounted on the wing trailing edges of a fixed-wing aircraft. Flaps are used to reduce the take-off distance and the landing distance. Flaps also cause an increase in drag so they are retracted when not needed.
A leading-edge slot is a fixed aerodynamic feature of the wing of some aircraft to reduce the stall speed and promote good low-speed handling qualities. A leading-edge slot is a spanwise gap in each wing, allowing air to flow from below the wing to its upper surface. In this manner they allow flight at higher angles of attack and thus reduce the stall speed.
The Avro Canada VZ-9 Avrocar was a VTOL aircraft developed by Avro Canada as part of a secret U.S. military project carried out in the early years of the Cold War. The Avrocar intended to exploit the Coandă effect to provide lift and thrust from a single "turborotor" blowing exhaust out of the rim of the disk-shaped aircraft. In the air, it would have resembled a flying saucer.
Boundary layer control refers to methods of controlling the behaviour of fluid flow boundary layers.
In aeronautics, ice protection systems keep atmospheric moisture from accumulating on aircraft surfaces, such as wings, propellers, rotor blades, engine intakes, and environmental control intakes. Ice buildup can change the shape of airfoils and flight control surfaces, degrading control and handling characteristics as well as performance. An anti-icing, de-icing, or ice protection system either prevents formation of ice, or enables the aircraft to shed the ice before it becomes dangerous.
In aerodynamics, pitch-up is an uncommanded nose-upwards rotation of an aircraft. It is an undesirable characteristic that has been observed mostly in experimental swept-wing aircraft at high subsonic Mach numbers or high angle of attack.
A podded engine is a jet engine that has been built up and integrated in its nacelle. This may be done in a podding facility as part of an aircraft assembly process. The nacelle contains the engine, engine mounts and parts which are required to run the engine in the aircraft, known as the EBU. The nacelle consists of an inlet, an exhaust nozzle and a cowling which opens for access to the engine accessories and external tubing. The exhaust nozzle may include a thrust reverser. The podded engine is a complete powerplant, or propulsion system, and is usually attached below the wing on large aircraft like commercial airliners or to the rear fuselage on smaller aircraft such as business jets.
A circulation control wing (CCW) is a form of high-lift device for use on the main wing of an aircraft to increase the maximum lift coefficient. CCW technology has been in the research and development phase for over sixty years. Blown flaps were an early example of CCW.
Slats are aerodynamic surfaces on the leading edge of the wings of fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack. A higher coefficient of lift is produced as a result of angle of attack and speed, so by deploying slats an aircraft can fly at slower speeds, or take off and land in shorter distances. They are usually used while landing or performing maneuvers which take the aircraft close to a stall, but are usually retracted in normal flight to minimize drag. They decrease stall speed.
This article briefly describes the components and systems found in jet engines.
A powered aircraft is an aircraft that uses onboard propulsion with mechanical power generated by an aircraft engine of some kind.
Sweeping jet actuators are a type of active flow control technology based on fluidic oscillators used to produce sweeping jets. The first use of fluidic oscillators in the form of sweeping jets for flow control was demonstrated by Raman et al., 1999.<Cavity Resonance Suppression Using Miniature Fluidic Oscillators, G. Raman, S. Raghu and T.J. Bencic' AIAA-99-1900, 5th AIAA/CEAS Aeroacoustics Conference, Seattle, WA, May 10–12, 1999> and later by several authors working in the area of flow control. Many organizations have been working on the use of such actuators for flow control. Boeing, NASA and the University of Arizona Department of Aerospace and Mechanical Engineering, Illinois Institute of Technology, [Advanced Fluidics], Technical University of Berlin are a few of them. They are slots built into the control surface of an airfoil that build on the same principles as that of blown flaps; that by actively blowing air over the surface of an airfoil the effective lift produced by it is increased.
↑ Control of High-Reynolds-Number Turbulent Boundary Layer Separation Using Counter-Flow Fluid Injection, B.E. Wake, G. Tillman, S.S. Ochs, J.S. Kearney, 3rd AIAA Flow Control Conference, 2006
1 2 "Aerodynamic issues in the Design of High-Lift Systems for Transport Aircraft" Figure 1. Trends in Boeing Transport High Lift System Development, Agard CP-365
↑ "U.S. Naval Air Superiority Development of Shipborne Jet Fighters 1943-1962" Tommy H. Thomason, Midland Publishing, Hincklet 2007, ISBN978-1-58007-110-9, page 81
↑ "Cessna Wings for the World, the Single-Engine Development Story" by William D. Thompson, 1991
↑ "United States Army and Air Force Fighter 1916-1961" produced by D.A. Russell, Harleyford Publications Limited, Letchworth 1961, Library of Congress Card No.61-16739(United States) page 132
↑ American Military Training Aircraft' E.R. Johnson and Lloyd S. Jones, McFarland & Co. Inc. Publishers, Jefferson, North Carolina
↑ "TSR2 with Hindsight" edited by Air Vice-Marshal A F C Hunter CBE AFC DL, Royal Air Force Historical Society 1998, ISBN0-9519824 8 6, page 181
↑ "Aerodynamiic Design of Airbus High-Lift Wings in a Multidisciplinary Environment" Daniel Reckzeh, European Congress on Computational Methods in Applied Sciences and Engineering ECCOMAS 2004
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