Do you own the full fraps version or do you have a demo version.
In the full version there's a switch for full/half screen.
If you're explicitly talking about recording methods.. You need to do 2 recordings, exactly the same.
1) One at normal IL2 speed to record the sound track.
2) The second recording you slow the IL2 replay speed to 1/4 or 1/2. (big files)
On video editing, you separate the audio track from 1) and delete the audio from 2).
Using the video track from 2), Quadruple or double the playback speed and syncronise the result with the audio track from 1).
you have a movie..
edt: you might be able to get away with one (slow) recording with the audio when re-compressed, but I haven't tried this.