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THE GURNEY FLAP
A Gurney Flap is the simplest trailing edge device which can be used as a high lift device for low speed applications like micro air vehicles, gliders, wind turbines etc. The Gurney Flap is named after American aero dynamist Dan Gurney who introduced it in the form of a vertical tab attached to the trailing edge of an ordinary aerofoil. This modification makes the flap capable of producing higher lift force at lower velocities.
This paper is based on the experimental analysis of Gurney Flap and from the results of the experiment an empirical relation for the optimum flap height has been proposed. The paper contains a vivid description of the hysteresis effects of the flow on the flap. The paper also mentions the advantages, disadvantages and applications of the flap.
High lift devices are one of the most important aerodynamic devices attached to aircrafts and other flying machines. As the name indicates these are intended to produce higher lift force than conventional wings or aerofoils.Generally two types of high lift devices are used in practice. The first type works on the principle of increasing the aerofoil geometry, which is the camber of aerofoil. The second kind of devices work on the principle of energizing the boundary layer. A gurney flap is a typical and simple high lift device which works on the principle of changing the effective camber of the aerofoil.
Gurney flaps are generally intended to perform at lower speeds. It was proposed by American aero dynamist Dan Gurney. Actually the invention was in late 1970s but only now this flap has been subjected to detailed experimental analysis.
2. NEED FOR HIGH LIFT DEVICES
From the basic principles of aerodynamics, the lift force produced by an aerofoil is directly proportional to the velocity of flow. For an aircraft when landing or take off, the velocity is desirable to be lower to reduce the length of runway required .But for this some additional high lift devices has to be incorporated in the wings to generate the necessary lift force ,at lower velocities.
More over the application of high lift devices reduces the stalling speed of the aircraft. Stalling speed of the aircraft is the minimum speed required to produce the necessary lift, so that the aircraft is in equilibrium. A reduced stalling speed makes the aircraft to land, take off or even fly at lower speeds.
3. STRUCTURE AND WORKING OF GURNEY FLAP
A gurney flap is in the form of a simple vertical tab attached to the pressure side of the trailing edge of the wing as shown below.
Fig 1 construction of Gurney flap
In the picture it has been shown that a metal plate bolted to the aerofoil serves as the vertical flap. Apart from the conventional flaps the gurney flap is not movable which simplifies its application.
The working principle of the gurney flap is based on the formation of two counter rotating vortices as shown in fig .2 For the proper explanation of the working of the flap let us consider the Kutta-Joukovski equation for the lift force developed by an aerofoil. That is the lift force developed L=*V*G
Where =density of fluid
V=velocity of fluid
G=circulation around the flap
Fig 2 charting the flow
Thus it is clear from the equation that as circulation or number of vortices increases, the lift force also increases. But in this case the vortices are counter rotating. Thus one vortex will produce circulation such that the lift produced by it gets added to the total lift. The other vortex generates lift in the opposite direction. But the fact is that the useful lift producing vortex overshadows the other one. Thus producing a net higher lift force. The following figure reveals this.
Fig 3 Vortices behind the flap
In the picture, the useful lift producing vortex is the bigger one at the bottom. The size of the opposite lift producing vortex is small. Thus the useful lift producing vortex predominates. Actually the reason behind its dominancy is that the air flowing past the bottom side of the flap suffers more downward deflection than the air flowing through the upper surface due to the presence of flap.
4. PRESSURE DISTRIBUTION OVER THE GURNEY FLAP
Fig 4 Pressure distribution of Gurney flap and plain aerofoil
In the figure white dotted curve indicates the pressure distribution over an ordinary aerofoil while black dotted curve shows pressure distribution over a gurney flap of height 1% of the chord length. The X axis of the curve represents distance along the flap as a fraction of the chord length and Y axis shows the pressure coefficient. If we are taking the pressure difference between upper and lower surfaces for a gurney flap, it is found to be greater than the ordinary aerofoil. Since pressure difference between upper and lower surfaces is responsible for the lift, the above curves prove that a gurney flap produces higher lift than ordinary aerofoil.
5. EXPERIMENTAL SETUP
The experiment on the flap was conducted in a low speed open jet wind tunnel. Open jet wind tunnel was preferred because of the ease of taking measurements from it. The test section was a 0.457m square section and was 1.2m long.The velocity range for the air in tunnel was ranging from 4m/s to 15m/s.The aerofoil was made of balsa wood and its surface was polished and coated with water proof paint The aerofoil was rectangular in plan. Following were the important dimensions of the aerofoil.
Â¢ Maximum thickness =10mm at 15% chord
Â¢ Maximum thickness to chord ratio=0.065
The experimental setup was incorporated with a pyramidal balance with digital read out to measure the forces acting on the flap accurately.
6. EXPERIMENTAL PROCEDURE AND RESULTS
As far as an aerofoil is considered, the focus will be on the lift coefficient, drag coefficient and the glide ratio or the lift to drag ratio. The above parameters are defined below.
Coefficient of lift CL=L/(1/2*V^2*S)
Coefficient of drag CD=L/(1/2*V^2*S)
Glide ratio=L/D= CL/ CD
Where L=lift force
=density of fluid
V=velocity of flow
S=surface area of the flap
We are concerned with the variation of the lift coefficient, drag coefficient and glide ratio with the angle of attack. For that the angle of attack was changed in step and for each value of angle of attack, the required parameters were calculated and the curves showing their variation with respect to the angle of attack was plotted.
6.1 Variation of coefficient of lift
The coefficient of lift shows an increasing trend with respect to the angle of attack till a particular angle of attack. After that the coefficient of lift drops as the angle of attack increases which is due to the separation of the flow and formation of eddies. The figure below shows the variation of the lift coefficient for Gurney flaps with different heights.
Fig 5 Lift curve for plain aerofoil and Gurney flap.
In the figure the lift curves are plotted for plain aerofoil, 4mm gurney flap and 9mm gurney flap. From the figure it can be understood that the lift coefficient increases as the flap height increases. Maximum lift coefficient is obtained as 2.1 for the 9mm flap at an angle of attack of about 10deg. After this point stalling occurs and coefficient lift decreases. The curve was plotted for a Reynolds number of 110,000.
One important fact that can be understood this curve is that even after stalling condition a gurney flap produces higher lift coefficient than a plain aerofoil.
6.2 Effect on drag coefficient
As the angle of attack increases the drag coefficient also increases. The rate of increase is low till the stalling angle of attack and after that the coefficient of drag drastically increases. This is due to the separation effects and formation of eddies. Figure below shows the variation of drag coefficient.
Fig 6 Drag curves for plain aerofoil and Gurney flap(Re=65000)
As the flap height increases, the coefficient of drag also increases. Thus the 9mm flap has the largest drag coefficient.
6.3 Overall performance
If any kind of aerofoil has to be applied onto flying objects, the value of coefficients of drag and lift must be considered simultaneously because .So to have an idea about the overall performance of the flap ,we go for the ratio between lift and drag coefficients which is called glide ratio.
Also the aim of evaluating the overall performance of the flap is to have an idea about the optimum height of the flap. Thus for plotting the overall performance curve the X axis variable is flap height and the dependent variables are lift coefficient, drag coefficient and the glide ratio. Figure below shows the performance curves.
Fig 7 Overall performance curves
From the curves it is clear that coefficient of lift increases more faster than coefficient of drag .Hence till a particular flap height we get glide ratio increasing with flap height. After that optimum flap height the glide ratio decreases and hence performance degrades.
6.4 Flap height optimization
This step provides an empirical formula for determining the flap height according to the velocity of flow, the chord length of flap etc.From the analysis of the performance curves and the boundary layer height at the trailing edge of the flap,it was found that the optimum flap height is about the 90% of the boundary layer thickness at the trailing edge.
The boundary layer thickness can be evaluated from the following relation.
t=0.383x / (Re^0.2)
Where x=distance along chord.
After substituting for Reynoldâ„¢s number in terms of the approximate value of the viscosity and density of air and the chord length we get the following relation for optimum flap height.
Hopt = 37.155*c^0.8 U^-0.2*1000 mm
Where c = chord length
U=free stream velocity
Now it is possible to design the gurney flap for any given chord length and flow velocity.
6.5 Hysteresis effect
Hysteresis effect cause the slopes of coefficient curves to be different for increasing and decreasing angle of attacks. Actually the experiment is conducted in two stages. In the first stage angle of attack is increased in a stepwise manner and the lift curve is plotted. In the second stage the angle of attack is decreased in a stepwise manner and another lift curve is plotted in the same graph. The two curves will not coincide due to hysteresis effect. The hysteresis effect is due to a separation phenomenon of the flow called laminar separation bubble.
6.5.1 Laminar separation bubble
A laminar separation bubble may be defined as a region of locally separated flow.
When fluid flows past an aerofoil, after some distance from the leading edge, due to the building of adverse pressure gradient , the flow separates. The separation occurs in the transition region of the boundary layer near to the end of laminar region.
After moving some distance the flow reattaches in the turbulent region of the boundary layer. Thus in effect there exists a locally separated region with a mass of fluid rotating within it called laminar separation bubble. Figure below shows a laminar separation bubble.
Fig 8 Laminar separation bubble
At the stalling angle of attack, the flow fails to reattach and complete separation of flow occurs.
Depending on the size of bubble, it may be called as a long bubble or short bubble. For a long bubble, the separated area will be larger and it causes higher drag on the flap.
6.5.2 Short bubble hysteresis
It generally occurs at the stalling angle of attack. We already have the glide ratio curve in which the angle of attack was increasing and the flow separates completely at stalling angle of attack. Now if we decrease the angle of attack, the reattachment of flow takes place at a lesser angle of attack. Thus a hysteresis loop is formed at the region of stalling angle of attack as shown in the figure below.
Fig 9 Short bubble hysteresis
In the figure the glide ratio curve has been selected for explanation. But the lift curve and drag curve also follow similar trends. In the fig. upstroke means increasing angle of attack and down stroke means the decreasing angle of attack.
6.5.3 Long bubble hysteresis
The long bubble hysteresis can occur at any angle below the stalling angle of attack. In the upstroke as the angle of attack is increased, the long bubble grows larger in size due to very adverse pressure gradient. But when the angle of attack reaches near the stalling value the long bubble bursts to a cluster of short bubbles. Since the size of bubble becomes smaller, a slight improvement in the lift coefficient and glide ratio has been observed.
During the down stroke the reunion of short bubbles to long bubble occurs only at lesser angle of attacks which produces a long bubble hysteresis loop below the stalling angle of attack. The figure below clearly shows the different regions of operation of separation bubbles.
Fig 10 Distinction between long bubble and short bubble hysteresis
6.5.4 Features of hysteresis effects
Although the prediction of hysteresis effects is cumbersome; it has been observed that the hysteresis effect strongly depends on the geometry of aerofoil and the Reynoldâ„¢s number of flow, the figures 11 to 14 reveal that.
Fig. 11 Glide ratio for clean aerofoil (Re=65000)
Fig. 12 Glide ratio for 5.5mm Gurney flap (Re=65000)
Fig. 13 Glide ratio for clean aerofoil (Re=110000)
Fig. 14 Glide ratio for 5.5mm Gurney flap (Re=110,000)
It is clear from the above figures that as Reynoldâ„¢s number increases hysteresis effects become severe and the size of hysteresis loops increases. Also the sizes of hysteresis loops are larger for a plain aerofoil than a gurney flap. Thus the gurney flap is effective in reducing the harmful hysteresis effects. If hysteresis effects are there in the wings of an aero plane it may be harmful to its stability.
7. ADVANTAGES OF GURNEY FLAPS
Up to 40% increase in lift
Improved glide ratio
No moving parts
Can easily be fitted
Hysteresis effects are reduced to some extent
A gurney flap cannot be applied for very high speed and supersonic applications.
Gurney flap causes increased vibration:
Actually the above disadvantage is due to a phenomenon called vortex shedding. It has already been stated that two counter rotating vortices are produced due to the flap. In fact, those vortices have some oscillatory characteristics also. So alternatively lift forces are produced in either direction which causes vibration. But this can be eliminated by having some additional modifications in the trailing edge.
9. APPLICATIONS OF GURNEY FLAPS
Rear spoilers for race cars:
This is a typical application of the gurney flaps. Spoilers in race cars are used for pressing the car onto the road, otherwise the car may fly off from the ground due to high speed. This is accomplished by a bottom up placed aerofoil which develops downward lift force. If gurney flap is used for this purpose the downward lift produced is higher and it ensures more maneuverability to the car.
Micro Air Vehicles
Micro air Vehicles are those flying objects with very small size .They are essentially low speed flyers. So a high lift device which is free from any moving parts is required for it to have good lift force. A gurney flap serves this purpose well.
If a gurney flap is fitted to the wing of a wind turbine, more lift force is produced on the wings which results in higher torque and higher power output.
Gliders are essentially low speed flying objects and also they do not have an engine. So it is desirable for them to have a high lift device like a gurney flap.
10. CURRENT RESEARCH AREAS IN GURNEY FLAPS
Application to helicopter rotors
If a Gurney flap can be incorporated on a helicopter rotor successfully, the speed of the rotor can be reduced to produce the same lift.
Application to delta wing aircrafts
Active Gurney flaps for race cars
For race cars the speed will be varying throughout the track. So the optimum height of the flap keeps on changing. By the special material of Gurney flap it can be rendered as an active one that is a flap which is capable of changing the height according to the real time velocity of car and thus keeping the optimum height for all the time.
From the experiment conducted it has been proved that a Gurney flap successfully works as a high lift device at lower velocities. Since it is very easy to be fitted and does not contain any moving parts, it promises a bright future for micro air vehicles, wind turbines, gliders etc.
L.Brown and A.Filippone (2003),Aerofoil at low speeds with Gurney Flaps, The Aeronautical Journal, No.2800, pages 539 to 546
L J Clancy, Aerodynamics, Longman Group, 1996 Edition