# Wing in Ground Proximity (WIG)

From the beginning, JavaFoil had the basic capability to handle multi-element airfoils. This capability was hidden, though. Based on these foundations it was relatively easy (within a few days)  to add a ground effect simulation. Internally this option works by creating a mirror image of the current airfoil below the ground, which is assumed to be at y=0. This creates a system of multiple airfoils which is symmetrical about the y=0 plane. The flow around this system can be analyzed using the panel method and the usual boundary layer analysis can be performed to calculate the friction drag.

Note: For highest performance, one would implement such  features like ground effect or a cascade option by adapting the calculation of the aerodynamic influence coefficients inside the panel method. Thus one would keep the equation system to be solved at the same size as for a single airfoil. In JavaFoil, the way of actually duplicating the airfoil for the WIG simulation has been chosen. This leads to a more straightforward coding scheme and fits the multi-element option perfectly.

 a = 0° a = -5° In order to analyze the flow at different angles of attack, the geometry has to be rebuilt for each angle of attack because the geometry of the system must stay symmetrical (see pictures at left). This requires a new panel analysis for each angle of attack, which makes the analysis slower than the plain airfoil analysis. JavaFoil does all this behind the scenes so all you have to do is to toggle the ground effect switch on the Options card. During the WIG simulation the angle of attack of the airfoil is changed by rotating the airfoil around the point (0.25/0). This point is located on the ground as shown at the left. While the example below shows a downforce application, ground effect can also be used to increase lift without increasing drag excessively. This results in an improved ratio of L/D for lifting applications. Such devices have been employed in vehicles traveling with reduced power over flat surfaces, mainly over water. Starting in the 1960s, Alexander Lippisch was probably one of the first to exploit this phenomenon in his various ground effect machines. The results of wind tunnel tests for one of his designs (the X-114) are presented at the left. They show an increase in L/D from about 6 to 10 for the complete aircraft (tests were conducted at the German Aerospace Research Center DLR in 1979). The largest machines of this type were built in the former Soviet Union, though.

## An Example: Front Wing of a Race Car

The following example shows, how JavaFoil can be used to analyze the effect of ground proximity on the downforce of an airfoil. Such a condition could occur on the front wing of a race car. As this is only a simple example, the results should not be taken as general. It is clear that an airfoil must be designed and optimized to work efficiently in ground effect. If you want to use this feature in JavaFoil, it is recommended to plan exactly what you want to analyze and which steps you have to perform in order.

## Geometry Card

• Create a NACA 4412 airfoil section with negative camber (-4%).
• JavaFoil will create the name "NACA -4412" as the camber is negative.
• Alternatively, you could also create a positively cambered section and "Flip" it in y-direction on the Modify card.

## Modify Card

• Rotate the airfoil by -10° for a more realistic setup.
Note: the current geometry always represents an angle of attack of 0° as the rotation of the airfoil has become part of the geometry.
• Translate the airfoil upwards by 20% of its chord. The 25% chord point of the airfoil is now located at the point (0.25/0.20). This creates the required free space between airfoil and ground (which is always at y=0).
• Make sure that there is no intersection with the ground.

## Flow Field Card

• Perform flow field analysis without ground effect for comparison.
• You can analyze the flow at different angles of attack - see remark about the 0° angle above (Modify card).

Note: for "free flight", the y=0 axis is always located at the center of the graph.

## Options Card

• Check the "ground effect" option. This activates the simulation of a flat floor, located at y = 0 (this is why we have moved the airfoil upwards by 20% of its chord).

## Flow Field Card

• Perform another flow field analysis with ground effect.
• The picture clearly shows the low pressure field (high velocity) between the lower surface and the ground.
• Again, you can analyze the flow at different angles of attack.

Note: with ground effect activated, the y=0 axis is always located at the lower edge of the graph.

## Velocity Distribution Card

• You can compare the velocity distribution for any angle of attack without and with ground in effect.
• The picture clearly shows the high velocity level on the lower surface in presence of the ground (square symbols). The steep velocity drop towards the trailing edge (pressure rise)  may lead to separation and increased drag.
• On the upper surface (lower curves), a small difference occurs mainly at the leading edge, as the stagnation point is shifted downstream by the ground effect.

## Polar Card

• To check the airfoil for different angles of attack, you can analyze an complete polar for different angles of attack and Reynolds numbers. The angle of attack is changed by rotating the airfoil around the point (0.25/0), which will change the height of the airfoils 25% chord point above ground somewhat.
• The comparison shows that the downforce increases from CL = -1.7 for the free airfoil to -5.7 due to the ground effect!
• It is also clearly visible that in our example the drag coefficient increased largely, though. L/D went down from about 100 for the free flow field to 60 due to the presence of the ground.

### Critical Issues:

• If you forget to shift the airfoil upwards, it will intersect with the ground plane. In this case JavaFoil will fail and probably create no results at all.
• If you perform the analysis of a Lift-Drag polar, the airfoil may intersect the ground at large angles of attack. If this occurs, JavaFoil stops the angle of attack loop and displays an error message.
• As JavaFoil does not take any displacement effects of the boundary layer into account, the results will be inaccurate if the distance of the airfoil from the ground is small (maybe smaller than the five times the displacement thickness d1, which depends on the Reynolds number - see results on the Boundary Layer card for d1).
For a full size craft, it is recommended to base the actual 3D design on more complex methods like Navier-Stokes Solvers. Also one should try to verify the results by experiments.

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