Gurney Flap

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A gurney flap is nothing but a vertical tab attached to the pressure side of trailing edge of an aerofoil.



  • IntroductionPrinciple behind gurney flapExperimental setupResults of experimentHysterisis effect Advantages & DisadvantagesApplicationsResearch areas

  • TERMINOLOGYAEROFOIL Generally wing section of the aeroplane which produces aerodynamic forces when air or any fluid flows around itAILERONS Ailerons are control surfaces at the outer part of the wing which operate differently to raise the lift on one side of the wing and to lower the otherFLAPS Movable surface on the leading edge of the wing to permit higher lift during take off.



  • What is a Gurney Flap? A gurney flap is nothing but a vertical tab attached to the pressure side of trailing edge of an aerofoil.It is used as a high lift device intended to perform at low speeds.This has been proposed by American aero dynamist DAN GURNEY as a spoiler in the rear of racing cars.

  • Need for high lift devicesThe lift developed by an airfoil is directly proportional to the velocity of flow.A high lift device can produce higher lift for the same velocity.A high lift device reduces the stalling speed(minimum speed) of aircrafts.

  • Principle of gurney flapDue to the sharp corner flap two counter rotating vortices are formed.Total circulation around the aerofoil gets increased which adds to the lift.

  • Pressure distribution with Gurney flap

  • Experimental SetupRectangular plan form wing with span=0.457m.Chord=0.154mMaximum thickness=10mm at 15% chord.Thickness chord ratio=0.065.Open jet low speed wind tunnel.Velocity range 4.1m/s to 15m/s.

  • Experimental Results

  • Effect on lift coefficientIncrease in the lift is proportional to the flap height.Even after stall gurney flap produces lift greater than plain aerofoil.maximum lift coefficient is increased by 25%.

  • Effect on drag coefficientCoefficient of drag increases with the flap height.Coefficient of lift increases faster then drag coefficient till a particular height of flap.

  • Overall performanceFor Re=110,000 the maximum performance is obtained at 5.5mm.Performance curve shows a negative trend thereafter.

  • Hysteresis Effect Causes the lift curve slopes to be different for increasing and decreasing angle of attacks.It is caused by separation bubble effect.

  • What is a separation bubble?Strong adverse pressure gradient causes the flow to be separated in the laminar region itself.Flow reattaches in the turbulent region.

  • Space between detachment and attachment points is called a SEPARATION BUBBLE.If the flow is unable to reattach full separation or stalling occurs

  • Short bubble hysterisisOccurs at the stall angleAfter the stall,in the while decreasing the angle of attack(downstroke)flow reattaches at a lower angle than the stall angle.A clockwise hysterisis loop is formed in the lift curve.

  • Long bubble hysterisisOccurs below the stall angleAs angle of incidence is increased (upstroke) long bubble grows larger.Just below the stall angle long bubble bursts to a short bubble and lift drag ratio is improved.In the down stroke short bubble is transformed to a long bubble only at lower angleshence improved glide ratio at during down stroke.

  • Features of hysteresis effectsStrongly depends on the geometry of aerofoilReynolds number has pronounced effect on themGenerally unfavorable to aircrafts

  • Glide ratio curves with hysterisisClean aerofoilGurney flap 5.5mmRe = 65,000

  • Clean aerofoilGurney flap 5.5mmRe=110,000

  • Advantages of gurney flapsUp to 40% increase in liftImproved glide ratioNo moving partsCan easily be fittedCheapHysterisis effects are reduced to some extend

  • DisadvantagesCannot be used for high speed and supersonic applicationsGurney flap causes increased vibration on wings.

  • ApplicationsRear spoilers for race carsMicro air vehiclesWind turbinesGliders

  • Research areas includeApplication to helicopter rotorsApplication to delta wingsActive gurney flaps for race cars

  • ConclusionThe gurney flap is found to be a very useful element in aerospace and automobile industry which promises a bright future for MAVs, race cars.

  • BibliographyThe aeronautical journal(sept.2003)Aerodynamics - L J ClancyThe aerodynamic design of aircraft - D Kuchemann F R S