I have written a panel method of my own, 1st order, flat panels, for the analysis of sailplanes. Everything works fine, except that the induced drag calculated from panel pressure forces is way too high. Many publications recommend instead to get the induced drag from far field analysis, and that's OK, I've done it for a VLM. However for a panel method, I don't know how to calculate the strip's circulations from the doublet strengths.

Can anybody help ?

Thanks

For flat wings it is sometimes easier to use the leading edge suction force instead of a direct calculation of induced drag.

A good test of your code is a circular wing.

The exact lift slope is CL/alpha = 1.79002303, and the moment is CM/alpha = -0.466174.

For the induced drag I get Cdi/alpha^2 approx = 0.802, and CS/alpha^2 approx =0.982 for the leading edge suction force.

Don't expect reliable results from VLM for the circular wing. Some panel densities will give Ok results, others may give nonsense.

If you need more detail, search for the PhD thesis of D.W.F. Standingford.

Good luck, Leo.