I had to postpone work on the topic for a long time. Now I am back to the forum and would like to thank the colleagues for their help by communicating useful references.
Thanks!
G. Kretzschmar
unsteady subsonic aerodynamics
We have just the same problem. Seeking for a code (preferentially but not necessarily FORTRAN), we were delighted to find the forum. All we know is that "a big European aircraft manufacturer" has recently given a contract to a software developper to write a code...