GURNEY FLAP By: KASYAP T V S7 M2 27222

Introduction Principle behind gurney flap Experimental setup Results of experiment Hysterisis effect Advantages & Disadvantages Applications Research areas

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 devices The 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 flap Due 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 Setup Rectangular plan form wing with span=0.457m. Chord=0.154m Maximum 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 coefficient Increase 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 coefficient Coefficient of drag increases with the flap height. Coefficient of lift increases faster then drag coefficient till a particular height of flap.

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

Height optimization From the overall performance curve the optimum height is found to be 90% of the boundary layer thickness. Hopt=0.9*t where t=boundary layer thickness Hopt=0.9*0.383x/(Re^0.2) Hopt=37.155*(c^0.8)*(u^-0.2)

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 hysterisis Occurs at the stall angle After 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 hysterisis Occurs below the stall angle As 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 angles hence improved glide ratio at during down stroke.

Features of hysteresis effects Strongly depends on the geometry of aerofoil Reynolds number has pronounced effect on them Generally unfavorable to aircrafts

Glide ratio curves with hysterisis Clean aerofoil Gurney flap 5.5mm Re = 65,000

Gurney flap 5.5mm Clean aerofoil Re=110,000

Advantages of gurney flaps Up to 40% increase in lift Improved glide ratio No moving parts Can easily be fitted Cheap Hysterisis effects are reduced to some extend

Disadvantages Cannot be used for high speed and supersonic applications Gurney flap causes increased vibration on wings.

Applications Rear spoilers for race cars Micro air vehicles Wind turbines Gliders

Research areas include…… Application to helicopter rotors Application to delta wings Active gurney flaps for race cars

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

Bibliography The aeronautical journal(sept.2003) Aerodynamics - L J Clancy The aerodynamic design of aircraft - D Kuchemann F R S http://aerodyn.org/HighLift/gurney.html www.allamericanracers.com http://www.as.go.dlr.de/Transsonium4 http://www.cfd.tu-berlin.de

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