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Blown flap

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Blown flaps of theHunting H.126

Blown flaps,blown wingorjet flapsare poweredaerodynamichigh-lift devicesused on thewingsof certainaircraftto 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 thelift coefficient.There are a variety of methods to achieve this airflow, most of which use jet exhaust or high-pressure airbled offof a jet engine's compressor and then redirected to follow the line oftrailing-edge flaps.

Blown flapsmay 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 aspowered lift,which also includes variousboundary layer controlsystems, systems using directedprop wash,andcirculation control wings.

Internal blown flaps were used on some land and carrier-based fast jets in the 1960s, including theLockheed F-104,Blackburn Buccaneerand certain versions of theMikoyan-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 severaltransport aircraft,both turboprop and turbofan.

Mechanism[edit]

In a conventional blown flap, a small amount of the compressed air produced by thejet engineis "bled" off at the compressor stage and piped to channels running along the rear of the wing. There, it is forced through slots in thewing flapsof the aircraft when the flaps reach certain angles. Injecting high energy air into theboundary layerproduces an increase in the stallingangle of attackand maximumlift coefficientby delayingboundary layer separationfrom theairfoil.Boundary layer controlby mass injecting (blowing) preventsboundary layer separationby supplying additional energy to the particles offluidwhich are being retarded in theboundary layer.Therefore, injecting a high velocity air mass into the air stream essentiallytangentto 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 blowingflow 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 atNASAin the 1950s and 1960s, leading to simplified systems with similar performance. Theexternally blown flaparranges 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 theCoandă effect.The similarupper-surface blowingsystem 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 exerthigh-authority controlto global flows using low energy modifications to key flow regions. In this case, the air blow slit is located at the pressure side near theleading edgestagnation pointlocation and the control air-flow is directedtangentiallyto 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 increasedturbulencelevels away from the wall region thus transporting higher-energy outer flow into the wall region. In addition to that another effect, thevirtual shaping effect,is utilized to aerodynamically thicken theairfoilat highangles of attack.Both these effects help to delay or eliminateflow 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 aBoeing 747produces acoefficient of liftof about 2.45,[6]external blowing (upper surface blowing on aBoeing YC-14) improves this to about 7,[6]and internal blowing (jet flap onHunting H.126) to 9.[7]

History[edit]

Ball-Bartoe Jetwingused for blown-wing research. Note the "augmentor",intended to direct the discharged airflow over the wing

Williams[8]states some flap blowing tests were done at theRoyal Aircraft Establishmentbefore the Second World War, and that extensive tests were done during the war in Germany including flight tests withArado Ar 232,Dornier Do 24andMesserschmitt Bf 109aircraft. 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 atO.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 aBreguet Vulturaircraft.[11]

Tests were also done atWestland Aircraftby W.H. Paine after the war with reports dated 1950 and 1951.[8]

In the United States, aGrumman F9F Pantherwas 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 onCessna 309and 319 aircraft using the Arado system.[13]

During the 1950s and 60s,fighter aircraftgenerally evolved towards smaller wings in order to reduce drag at high speeds. Compared to the fighters of a generation earlier, they hadwing loadingsabout four times as high; for instance theSupermarine Spitfirehad a wing loading of24 lb/ft2(117 kg/m2)and theMesserschmitt Bf 109had the "very high" loading of30 lb/ft2(146 kg/m2),whereas the 1950s-eraLockheed F-104 Starfighterhad111 lb/ft2(542 kg/m2).

One serious downside to these higher wing loadings is at low speed, when there is not 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 asflow 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 BritishNational 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 thestallspeed to only 32 mph (51 km/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]

ABuccaneerwith the blowing slots visible on the leading edges. The extendedflapsare contributing to theCoandaairflow over the wing.

One of the first production aircraft with blown flaps was the Lockheed F-104 Starfighter, which entered service in January 1958.[16]After prolonged development problems, the BLCS proved to be enormously useful in compensating for the Starfighter's tiny wing surface. TheLockheed T2V SeaStar,with blown flaps, had entered service in May 1957 but was to have persistent maintenance problems with the BLCS which led to its early retirement.[17]In June 1958, theSupermarine Scimitarwith blown flaps entered service.[18]Blown flaps were used on theNorth American AviationA-5 Vigilante,theVought F-8 Crusadervariants E(FN) and J, theMcDonnell Douglas F-4 Phantom IIand theBlackburn Buccaneer.TheMikoyan-Gurevich MiG-21andMikoyan-Gurevich MiG-23had blown flaps. Petrov[19]states long-term operation of these aircraft showed high reliability of the BLC systems. TheTSR-2,which was cancelled before it entered service, had full-span blown flaps.[20]

Starting in the 1970s, the lessons of air combat overVietnamchanged 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 theGeneral Dynamics F-16 Fighting Falconhas a wing loading of78.5 lb/ft2(383 kg/m2),and usesleading edge extensionsto provide considerably more lift at higherangles of attack,including approach and landing. Some later combat aircraft achieved the required low-speed characteristics usingswing-wings.Internal flap blowing is still used to supplement externally blown flaps on theShin 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 theBoeing C-17 Globemaster), "upper surface blowing" (used on theAntonov An-72andAntonov An-74) and "vectored slipstream", or "over the wing blowing",[19]used on theAntonov An-70and the Shin Meiwa US-1A andShinMaywa 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.

See also[edit]

References[edit]

  1. ^Aerodynamics for Engineering Students, E.L. Houghton & P.W. Carpenter, Elsevier
  2. ^o. Smith, A. M. (1975). "High-Lift Aerodynamics".Journal of Aircraft.12(6): 508.doi:10.2514/3.59830.
  3. ^abhttp://naca.central.cranfield.ac.uk/reports/arc/rm/3304.pdfp.1
  4. ^abhttp://cafefoundation.org/v2/pdf_tech/Drag.Reduction/NASA.Synergistic.Airframe.1998.pdfp.22
  5. ^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
  6. ^ab"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
  7. ^ http://cafefoundation.org/v2/pdf_tech/Drag.Reduction/NASA.Synergistic.Airframe.1998.pdfp.18
  8. ^ab"Archived copy"(PDF).Archived fromthe original(PDF)on 2015-10-01.Retrieved2015-12-04.{{cite web}}:CS1 maint: archived copy as title (link)
  9. ^"1954 | 3066 | Flight Archive".
  10. ^Rebuffet, Pierre; Poisson-Quinton, P. H. (April 1952)."Investigations of the boundary-layer control on a full scale swept wing with air bled off from the turbojet"(PDF).
  11. ^Schmitt, H. (July 1985)."Discussion of the paper, Some Aspects of Propulsion for the Augmenter-Wing Concept, by D. C. Whittley"(PDF).
  12. ^"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
  13. ^"Cessna Wings for the World, the Single-Engine Development Story" by William D. Thompson, 1991
  14. ^Flight International1963 p454
  15. ^abhttp://cafefoundation.org/v2/pdf_tech/Drag.Reduction/NASA.Synergistic.Airframe.1998.pdf[bare URL PDF]
  16. ^"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
  17. ^American Military Training Aircraft' E.R. Johnson and Lloyd S. Jones, McFarland & Co. Inc. Publishers, Jefferson, North Carolina
  18. ^"British Naval Aircraft Since 1912" Owen Thetford, Putnam & Co. Ltd.London, 1962, p.318
  19. ^abICAS Archive
  20. ^"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
  21. ^"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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