Gurney flap

Source: Wikipedia, the free encyclopedia.
A Gurney flap shown on the underside of a Newman airfoil[1]

The Gurney flap (or wickerbill) is a small tab projecting from the trailing edge of a wing. Typically it is set at a right angle to the pressure-side surface of the airfoil[2] and projects 1% to 2% of the wing

chord.[3]
This trailing edge device can improve the performance of a simple airfoil to nearly the same level as a complex high-performance design.[4]

The device operates by increasing pressure on the pressure side, decreasing pressure on the suction side, and helping the boundary layer flow stay attached all the way to the trailing edge on the suction side of the airfoil.[5] Common applications occur in

horizontal stabilizers, and aircraft where high lift is essential, such as banner-towing airplanes.[6]

It is named for its inventor and developer, American race car driver Dan Gurney.[7][8]

History

The "variable-lift airfoil" shown in Figure 1 of the 1935 E. F. Zaparka patent U.S. patent RE19412. It is a movable microflap, similar to the fixed Gurney flap.
A Gurney flap on the trailing edge of the rear wing of a Porsche 962

The original application, pioneered by American

traction.[5] He field-tested it and found that it allowed a car to negotiate turns at higher speed, while also achieving higher speed in the straight sections of the track.[9]

The first application of the flap was in 1971,[10] after Gurney retired from driving and began managing his own racing team full-time. His driver

understeering. All that was needed was to balance this by adding downforce in front.[12]

Unser realized the value of this breakthrough immediately and wanted to conceal it from the competition, including his brother Al. Not wanting to call attention to the devices, Gurney left them out in the open.[13] To conceal his true intent, Gurney deceived inquisitive competitors by telling them the blunted trailing edge was intended to prevent injury and damage when pushing the car by hand. Some copied the design and some of them even attempted to improve upon it by pointing the flap downward, which actually hurt performance.[14]

Gurney was able to use the device in racing for several years before its true purpose became known. Later, he discussed his ideas with aerodynamicist and wing designer Bob Liebeck of Douglas Aircraft Company. Liebeck tested the device, which he later named the "Gurney flap" and confirmed Gurney's field test results using a 1.25% chord flap on a Newman symmetric airfoil.[15] His 1976 AIAA paper (76-406) "On the design of subsonic airfoils for high lift" introduced the concept to the aerodynamics community.[16]

Gurney assigned his patent rights to Douglas Aircraft,[12] but the device was not patentable, since it was substantially similar to a movable microflap patented by E. F. Zaparka in 1931, ten days before Gurney was born.[12][17] Similar devices were also tested by Gruschwitz and Schrenk[18] and presented in Berlin in 1932.[19]

Theory of operation

The Gurney flap increases the maximum lift coefficient (CL,max), decreases the angle of attack for zero lift (α0), and increases the nosedown pitching moment (CM), which is consistent with an increase in camber of the airfoil.[5] It also typically increases the drag coefficient (Cd),[20] especially at low angles of attack,[21] although for thick airfoils, a reduction in drag has been reported.[22] A net benefit in overall lift-to-drag ratio is possible if the flap is sized appropriately, based on the boundary layer thickness.[23]

The Gurney flap increases lift by altering the

von Kármán vortex street.[24]
In addition to these spanwise vortices shed behind the flap, chordwise vortices shed from in front of the flap become important at high
angles of attack.[6]

The increased pressure on the lower surface ahead of the flap means the upper surface suction can be reduced while producing the same lift.[24]

Helicopter applications

Double Gurney flaps on a Bell 222U helicopter

Gurney flaps have found wide application on helicopter horizontal stabilizers, because they operate over a very wide range of both positive and negative angles of attack. At one extreme, in a high-powered climb, the negative angle of attack of the horizontal stabilizer can be as high as −25°; at the other extreme, in

autorotation, it may be +15°. As a result, at least half of all modern helicopters built in the West have them in one form or another.[25]

The Gurney flap was first applied to the

Bell JetRanger to correct an angle of incidence problem in the design that was too difficult to correct directly.[14][25]

The

hover to forward flight.[25]

See also

References

  1. ^ Liebeck, Robert H. Design of Subsonic Airfoils for High Lift. Journal of Aircraft, Vol. 15, no. 9, September 1978, pp. 547-561.
  2. doi:10.2514/2.2461
    . These devices provided an increased region of attached flow on a wing upper surface relative to the wing without the flaps.
  3. doi:10.2514/3.46528
    . One candidate technology is the Gurney flap, which consists of a small plate, on the order of 1–2% of the airfoil chord in height, located at the trailing edge perpendicular to the pressure side of the airfoil.
  4. ^ Giguere, P.; Lemay, J.; Dumas, G. (1995). "Gurney flap effects and scaling for low-speed airfoils". AIAA Applied Aerodynamics Conference, 13 th, San Diego, CA, Technical Papers. Pt. 2. pp. 966–976. through the proper use of Gurney flaps, the aerodynamic performance of a simple design, easy-to-build airfoil can be made practically as well as those of a modern, high performance, complex design.
  5. ^
    doi:10.2514/2.2309
    . Race-car driver Dan Gurney used this flap to increase the downforce and, thus, the traction and potential cornering speeds generated by the inverted wings on his race cars.
  6. ^
    S2CID 53316694
    . ...the intermittent shedding of fluid recirculating in the cavity upstream of the flap, becomes more coherent with increasing angle of attack.... Comparison of flow around 'filled' and 'open' flap configurations suggested that [this] was responsible for a significant portion of the overall lift increment.
  7. ^ "The Gurney Flap – Dan Gurney's All American Racers". allamericanracers.com. Retrieved 23 April 2018.
  8. ^ SEAS. "Gurney Flap". www.formula1-dictionary.net. Retrieved 23 April 2018.
  9. ^
    doi:10.1016/S1369-8869(98)00010-X
    . Retrieved 2007-07-06. Liebeck stated that race car testing by Dan Gurney showed that the vehicle had increased cornering and straight-away speeds when the flap was installed on the rear wing.
  10. ^ Troolin, Daniel R.; Ellen K. Longmire; Wing T. Lai (2006-06-26). "The Effect of Gurney Flap Height on Vortex Shedding Modes Behind Symmetric Airfoils". 13th Int. Symp. on Applications of Laser Techniques to Fluid Mechanics. Lisbon, Portugal.
  11. ^ Wagner, Jan R. (2004). "The 2004 Art Center Car Classic (Part Two): Dan Gurney on Racing and the "BLAT" Effect". Auto Matters. Archived from the original on 2007-10-06. Retrieved 2007-07-06. And I remembered having spent a lot of time with these little tabs on the back, or spoilers and so forth, and I thought to myself – well, I wonder if one would work on a wing? We already had wings on these in 1971. Sure enough, that was the beginning of the Gurney flap.
  12. ^ a b c Howard, Keith (September 2000). "Gurney Flap". Motor Sport Magazine. Once Gurney had confirmed they were alone, Unser told him the rear was now so well planted that the car was pushing (understeering) badly, hence the poor lap times.
  13. ISBN 0-471-64745-4
    . Dan told me to relax. Leave them in the open. Don't bring attention to them.
  14. ^
    ISBN 0-7506-5111-3
    . So successful was this deception that some of his competitors attached the tabs projecting downwards to better protect the hands.
  15. doi:10.4271/961316. Retrieved 2007-07-08. Liebeck conducted wind tunnel tests on the effect of a 1.25% chord height Gurney flap. He used a Newman-type airfoil, which had an elliptic nose and a straight line wedge for the rear section.[permanent dead link
    ]
  16. ISBN 978-1-62410-030-7
    . The first theoretical investigations were published by Liebeck who introduced the concept of trailing edge devices to aircraft aerodynamics.
  17. ISBN 1-4020-1608-5
    . Gurney flaps are known already since 1931, when they were first patented by Zaparka (USA).
  18. doi:10.2514/2.1396. Retrieved 2007-07-07.[permanent dead link
    ]
  19. ^ Gruschwitz, Eugen; Oskar Schrenk (1932-10-28). "Über eine einfache Möglichkeit zur Auftriebserhöhung von Tragflügeln (A simple method for increasing the lift of airplane wings by means of flaps)" (PDF). Zeitschrift für Flugtechnik und Motorluftschiffahrt. Wissenschaftliche Gesellschaft für Luftfahrt (21st 1932 Berlin). Vol. 23 (Translation by Dwight M. Miner ed.). Washington, June 1933: National Advisory Committee for Aeronautics. pp. 597–601. NACA-TM-714. The problem is to create, in landing, a region of turbulence on the lower side of the wing near the trailing edge by some obstacle to the air flow.{{cite conference}}: CS1 maint: location (link)
  20. ^ Jang, C. S.; Ross, J. C.; Cummings, R. M. (1992). "Computational evaluation of an airfoil with a Gurney flap". AIAA Paper: 92–2708. Retrieved 2007-07-07.
  21. doi:10.2514/2.2210
    .
  22. ^ Neuhart, Dan H.; Pendergraft, Odis C. Jr. (1988-11-01). "A water tunnel study of Gurney flaps" (PDF). NASA Langley Research Center. NASA-TM-4071. Retrieved 2007-07-07. {{cite journal}}: Cite journal requires |journal= (help)
  23. doi:10.2514/2.49
    . Retrieved 2007-07-07.
  24. ^
    doi:10.2514/1.14294
    . When hot-wire anemometry is used, a tonal component in the spectrum of the velocity fluctuations downstream of the Gurney flap is shown. This points to the existence of a von Kármán vortex street.
  25. ^ a b c Prouty, R. W. (2000-03-01). "Aerodynamics : The Gurney Flap, Part 2". Rotor & Wing. Access Intelligence. One of the critical flight conditions is a high-powered climb. The negative angle of attack of the horizontal stabilizer can be as high as −25°, whereas in autorotation it may be +15°.

External links

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