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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.

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  • $\begingroup$ 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. $\endgroup$ Feb 12, 2023 at 3:28

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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:

X11 plot from XFOIL 6.99

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>  

X11 plot of XFOIL pressure distribution

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