Gurney Flap 3n5063

GURNEY FLAP

Introduction  Principle behind gurney flap  Experimental setup

 Results of experiment  Hysterisis effect  Advantages & Disadvantages  Applications  Research areas

TERMINOLOGY AEROFOIL Generally wing section of the aeroplane which produces aerodynamic forces when air or any fluid flows around it AILERONS 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 other

FLAPS Movable surface on the leading edge of the wing to permit higher lift during take off.

AERODYNAMICS OF AN AERO FOIL

Lift

Drag

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 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 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.

Glide ratio curves with hysterisis

Gurney flap 5.5mm

Clean aerofoil 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