[8FS_Bulau]

OK I have downloaded this plane from the site you gave.
I played with it or over and hour and it is SCREWED UP...

Well, I did not download this to try it, but just a quick look at the download page saw this:

installation:
File Su26_FM put to MODS\SU_26M\gui\game\

It appears this other download comes with its own FM file (rather than using the Diff-FM mod) which gets put in a non-standard location.