After four days of investigation we made the following discoveries:
1. The two aircraft had a difference of 1.5 degrees in elevator deflection. After we increased the elevator deflection in one aircraft by 1.5 degrees the stall speed was 3 kts lower!
2. The trim tab position seemed not to have a noticeable effect on the stall speed. I did the stall speed testing once with the trim tab at the full up position and once at the full down position. We could not find a noticeable difference in the stall speeds.
3. We found a difference of about 100 rpm in the idle rpm of the two aircraft. After increasing the idle rpm of one aircraft the stall speed was 1 kts lower!
4. I did the stall speed flights in different altitudes once in 8000 ft and once in 3000 ft. As expected there was no difference in the stall speeds.
5. We also found out, that the two aircraft have different pitot tubes, but we did not investigate the effect yet. Today we will change the pitot tube of one aircraft and do the test again. I will keep you posted about the outcome.
I have on more question. Does anybody remember how the deceleration rate in the different stalls can be mathematically corrected to 1kt/s?
Thank you very much for your help again.