Gurney flap

The Gurney Flap (or wickerbill) is a small flat 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,cite journal
author = Van Dam, C.P.
coauthors = Yen, D.T.; Vijgen, P.
year = 1999
title = Gurney flap experiments on airfoil and wings
journal = Journal of Aircraft(0021-8669)
volume = 36
issue = 2
pages = 484–486
url = http://pdf.aiaa.org/GetFileGoogle.cfm?gID=2461&gTable=japaperimport
accessdate = 2007-07-05
quote=These devices provided an increased region of attached flow on a wing upper surface relative to the wing without the flaps. ] and projects 1% to 2% of the wing chord.cite journal
author = Storms, B.L.
coauthors = Jang, C.S.
year = 1994
title = Lift Enhancement of an Airfoil Using a Gurney Flap and Vortex Generators
journal = Journal of Aircraft
volume = 31
issue = 3
pages = 542–547
url = http://pdf.aiaa.org/GetFileGoogle.cfm?gID=46528&gTable=japaperimportPre97
accessdate = 2007-07-05
quote=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.
doi = 10.2514/3.46528] This trailing edge device can improve the performance of a simple airfoil to nearly the same level as a complex high-performance design.cite conference
author = Giguere, P.
coauthors = Lemay, J.; Dumas, G.
year = 1995
title = Gurney flap effects and scaling for low-speed airfoils
conference =
booktitle = AIAA Applied Aerodynamics Conference, 13 th, San Diego, CA, Technical Papers. Pt. 2
pages = 966-976
publisher =
url = http://www.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=200111019960MT&recid=A9536716AH
conferenceurl =
quote=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.]

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.Common applications occur in auto racing, helicopter horizontal stabilizers, and aircraft where high lift is essential, such as banner-towing airplanes.

History

The original application, by automobile racing icon Dan Gurney, was a right-angle piece of sheet metal, rigidly fixed to the trailing edge of the rear wing on his open wheel racing cars of the early 1970’s. The device was installed pointing upwards to increase downforce generated by the wing, improving traction.cite journal
author = Myose, R.
coauthors = Papadakis, M.; Heron, I.
year = 1998
title = Gurney flap experiments on airfoils, wings, and reflection plane model
journal = Journal of Aircraft
volume = 35
issue = 2
pages = 206–211
url = http://pdf.aiaa.org/GetFileGoogle.cfm?gID=2309&gTable=japaperimport
accessdate = 2007-07-05
quote=Race-car driver Dan Gurney used this flap to increase the down force and, thus, the traction generated by the inverted wings on his race cars.] He field tested it and found it allowed a car to negotiate turns at higher speed, while also achieving higher speed in the straight sections of the track.cite journal
author = Jang, C.S.
coauthors = Ross, J.C.; Cummings, R.M.
year = 1998
title = Numerical investigation of an airfoil with a Gurney flap
journal = Aircraft Design
volume = 1
issue = 2
pages = 75–88
url = http://aerosrv.atl.calpoly.edu/rcumming/jrc98.pdf
accessdate = 2007-07-06
quote=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.
doi = 10.1016/S1369-8869(98)00010-X
format = Dead link|date=May 2008]

The first application of the flap was in 1971, [cite conference
booktitle=13th Int. Symp. on Applications of Laser Techniques to Fluid Mechanics
site=Lisbon, Portugal,
date=2006-06-26
title=The Effect of Gurney Flap Height on Vortex Shedding Modes Behind Symmetric Airfoils
first=Daniel R.
last=Troolin,
coauthors=Ellen K. Longmire, Wing T. Lai] after Gurney retired from driving and began managing his own racing team full-time.His driver, Bobby Unser, had been testing a new Gurney designed car at Phoenix International Raceway, and was unhappy with the car's performance on the track. Gurney needed to do something to restore his driver's confidence before the race, and decided to try adding a "spoiler" to the trailing edge of the rear wing. [cite web
url=http://www.automatters.net/2004%20Columns/0114.htm
title=The 2004 Art Center Car Classic (Part Two): Dan Gurney on Racing and the “BLAT” Effect
date=2004
author=Wagner, Jan R.
publisher=Auto Matters
accessdate = 2007-07-06
quote='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.'] The device was fabricated and fitted in under an hour, but Unser's test laps with the modified wing turned in equally poor times. When Unser was able to speak to Gurney in confidence, he disclosed that the lap times with the new wing were slowed because it was now producing so much downforce that the car was understeering. All that was needed was to balance this by adding additional downforce in front.cite magazine
title=Gurney Flap
url=http://www.allamericanracers.com/gurney_flap.html
first=Keith
last=Howard
date=2000-09
journal=Motorsport Magazine
quote=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.]

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. [cite book | last = Unser | first = Bobby | title = Winners Are Driven | publisher = Wiley | location = New York | year = 2004 | isbn = 0471647454 | pages = p. 15 | quote = Dan told me to relax. Leave them in the open. Don't bring attention to them.] 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 “improved” it by pointing the flap downwards, which actually hurt performance.cite book | last = Houghton | first = E. | title = Aerodynamics for Engineering Students | publisher = Butterworth Heinemann | location = Boston | year = 2003 | isbn = 0750651113 | pages = pp. 500-502 | quote = So successful was this deception that some of his competitors attached the tabs projecting downwards in order to better protect the hands.]

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.cite journal
author = Myose, R.
coauthors = Heron, I.; Papadakis, M.
year = 1996
title = Effect of Gurney flaps on a NACA 0011 Airfoil
journal = AIAA Paper
pages = 96–0059
url = http://pdf.aiaa.org/GetFileGoogle.cfm?gID=8590&gTable=mtgpaper
accessdate = 2007-07-08
quote = 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.] His 1976 AIAA paper (76-406) “On the design of subsonic airfoils for high lift” introduced the concept to the aerodynamics community.cite journal
author = Schatz, M.
coauthors = Gunther, B.; Thiele, F.
year = 2004
title = Computational Modeling of the Unsteady Wake Behind Gurney Flaps
journal = AIAA Paper
volume = 2417
url = http://pdf.aiaa.org/GetFileGoogle.cfm?gID=19735&gTable=Paper
accessdate = 2007-07-06
quote = The first theoretical investigations were published by Liebeck who introduced the concept of trailing edge devices to aircraft aerodynamics.] The Gurney flap is the first aerodynamic development made in automobile racing that has been successfully transferred to aircraft engineering.

Gurney assigned his patent rights to Douglas Aircraft, 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. [cite book | last = Sobieczky | first = H. | title = Iutam Symposium Transsonicum IV| chapter = Gurney Flaps in Transonic Flows | others = Henning Rosemann and Kai Richter | publisher = Springer | location = Berlin | year = 2003 | isbn = 1402016085 | pages = p. 165 | quote = Gurney flaps are known already since 1931, when they were first patented by Zaparka (USA).] Similar devices were also tested by Gruschwitz and Schrenkcite journal
author = Zerihan, J.
coauthors = Zhang, X.
year = 2001
title = Aerodynamics of Gurney flaps on a wing in ground effect
journal = AIAA Journal
volume = 39
issue = 5
pages = 772–780
url = http://pdf.aiaa.org/GetFileGoogle.cfm?gID=1396&gTable=japaperimport
accessdate = 2007-07-07 ] and presented in Berlin in 1932. [Cite conference
title=Über eine einfache Möglichkeit zur Auftriebserhöhung von Tragflügeln (A simple method for increasing the lift of airplane wings by means of flaps)
booktitle=Zeitschrift für Flugtechnik und Motorluftschiffahrt
volume= vol. 23, no. 20
edition=Translation by Dwight M. Miner
date=1932-10-28,
pages=pp. 597-601
last=Gruschwitz
first=Eugen
coauthors=Oskar Schrenk
others =
publisher = National Advisory Committee for Aeronautics
location = Washington, June 1933
id=NACA-TM-714
conference=Wissenschaftliche Gesellschaft für Luftfahrt (21st 1932 Berlin)
url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930094703_1993094703.pdf
quote=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.]

Theory of operation

The Gurney flap increases the maximum lift coefficient (C_L,max), decreases the angle of attack for zero lift (α₀), and increases the nosedown pitching moment (C_M), which is consistent with an increase in camber of the airfoil. It also typically increases the drag coefficient (C_d),cite journal
author = Jang, C.S.
coauthors = Ross, J.C.; Cummings, R.M.
year = 1992
title = Computational evaluation of an airfoil with a Gurney flap
journal = AIAA Paper
pages = 92–2708
url = http://www.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A9245550AH
accessdate = 2007-07-07 ] especially at low angles of attack,cite journal
author = Bloy, A.W.
coauthors = Tsioumanis, N.; Mellor, N.T.
year = 1997
title = Enhanced aerofoil performance using small trailing-edge flaps
journal = Journal of Aircraft(0021-8669)
volume = 34
issue = 4
pages = 569–571
url = http://www.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=200111015428MT&recid=A9733960AH
accessdate = 2007-07-07 ] although for thick airfoils, a reduction in drag has been reported.cite paper
author = Neuhart, Dan H.; Pendergraft, Odis C., Jr
title = A water tunnel study of Gurney flaps
publisher = NASA Langley Research Center
date = 1988-11-01
url = http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890004024_1989004024.pdf
id = NASA-TM-4071
accessdate = 2007-07-07] A net benefit in overall lift to drag ratio is possible if the flap is sized appropriately based on the boundary layer thickness.cite journal
author = Giguere, P.
coauthors = Dumas, G.; Lemay, J.
year = 1997
title = T echnical N otes
journal = AIAA Journal
volume = 35
pages = 12
url = http://pdf.aiaa.org/GetFileGoogle.cfm?gID=49&gTable=japaperimport
accessdate = 2007-07-07 ]

The Gurney flap increases lift by altering the Kutta condition at the trailing edge. The wake behind the flap is a pair of counter-rotating vortices which are alternately shed in a von Kármán vortex street.cite journal
author = Meyer, R.
coauthors = Hage, W.; Bechert, D.W.
year = 2006
title = Drag Reduction on Gurney Flaps by Three-Dimensional Modification
journal = Journal of Aircraft
volume = 43
number = 1
pages = 132
url = http://pdf.aiaa.org/jaPreview/JA/2006/PVJA14294.pdf
accessdate = 2007-07-07
quote = 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.
doi = 10.2514/1.14294] In addition to the spanwise vortices shed behind the flap, chordwise vortices shed from in front of the flap become important at high angles of attack.cite journal
author = Troolin, D.R.
coauthors = Longmire, E.K.; Lai, W.T.
year = 2006
title = Time resolved PIV analysis of flow over a NACA 0015 airfoil with Gurney flap
journal = Experiments in Fluids
volume = 41
issue = 2
pages = 241–254
url = http://www.springerlink.com/index/P27074848G65538P.pdf
accessdate = 2007-07-07
quote=...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.
doi = 10.1007/s00348-006-0143-8]

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

Helicopter applications

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.cite journal
work=Rotor & Wing
publisher=Access Intelligence
date=2000-03-01
title=Aerodynamics : The Gurney Flap, Part 2
first= R.W.
last=Prouty
url=http://www.defensedaily.com/cgi/rw/show_mag.cgi?pub=rw&mon=0300&file=03rwaero.htm
quote=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°.]

The Gurney flap was first applied to the Sikorsky S-76B variant, when flight testing revealed the horizontal stabilizer from the original S-76 did not provide sufficient lift. Engineers fitted a Gurney flap to the NACA 2412 inverted airfoil to resolve the problem without redesigning the stabilizer from scratch. A Gurney flap was also fitted to the Bell JetRanger to correct an angle of incidence problem in the design that was too difficult to correct directly.

The Eurocopter AS355 TwinStar helicopter uses a double Gurney flap that projects from both surfaces of the vertical stabilizer. This is used to correct a problem with lift reversal in thick airfoil sections at low angles of attack. The double gurney flap reduces the control input required to transition from hover to forward flight.

References

ee also

*Lift (force)

External links

* the original 1935 Zaparka patent