# XFOIL - Modify the gap at the trailing edge of the airfoil

I'm working on inviscid analysis flow in XFOIL. The trailing edge of my airfoil is too sharp, therefore leading to non-convergence of the analysis. Modifying the trailing edge thickness may do the trick. How to adjust the trailing edge thickness in XFOIL? I read online that you have to use the command GDES > TGAP. However, I couldn't find anything regarding of what to input the command. The documentation was not helpful either. I would like to change the thickness to 20% of the chord length.

• Where do those 20% come from? Is that a typo, or is it a root airfoil for a wind turbine? The latter can be truncated for transport reasons. Feb 12, 2023 at 3:28

XFOIL is happy with a sharp trailing edge and will tolerate a gap of a few percent. However, 20% is way beyond of what it is meant to receive as input, so I would not trust the result.

How to adjust the trailing edge thickness in XFOIL?

From the starting point you should first load an airfoil (for my example I used the command NACA 2412 to get a simple NACA four-digit airfoil).

Then I changed into the geometry design subset with gdes and continued like this:

.GDES   c>  tgap

Current gap =  0.00252

Enter new gap   r>  0.02

Enter blending distance/c (0..1)   r>  0.1
Max thickness =     0.120041  at x =   0.297
Max camber    =     0.019999  at x =   0.403

.GDES   c>


The result is an overlaid plot of the old and the new geometry, like this:

To set a sharp trailing edge, set tgap to zero. For this example, I continued into the operating subset and calculated the inviscid flow at an AoA of 4°:

.GDES   c>  tgap

Current gap =  0.02000

Enter new gap   r>  0.0

Enter blending distance/c (0..1)   r>  0.1
Max thickness =     0.120031  at x =   0.297
Max camber    =     0.019999  at x =   0.403

.GDES   c>

Buffer airfoil is not identical to current airfoil

XFOIL   c>  pane

Sharp trailing edge

Paneling parameters used...
Number of panel nodes       160
Panel bunching parameter    1.000
TE/LE panel density ratio   0.150
Refined-area/LE panel density ratio    0.200
Top    side refined area x/c limits  1.000 1.000
Bottom side refined area x/c limits  1.000 1.000

XFOIL   c>  oper

.OPERi   c>  alfa 4.0
Calculating unit vorticity distributions ...

.OPERi   c>