It looks like I will not get anywhere with this without telling you everything. I'm an aeronautical engineer and DER test pilot.
The airspeed readings were verified with the GPS triangle method and were within +-1 kt. I measured angle of attack with a string attached to a boom on the wingtip and a scale next to it. The accuracy was verified by measuring the angle of the fuselage at different airspeeds in level flight. It correlates well up to about 8 deg alpha, at higher angles the string on the wingtip indicates too high, but this was mainly to get a comparison between the two airplanes. The difference in stall alpha was at least 3 deg on the wing tip gage.
The stall can be either defined by the airplane pitching down or by the stick reaching the aft stop. In this case the stick reaches the aft stop and can be held there, but the lowest speed is reached right at aft stop and then increases together with the sink rate. The nose stays up. I had increased the elevator up deflection until I got the airplane to pitch gently, but this had no effect on stall speed.
The wings are not swept and the aileron rigging was checked (within type design tolerance).
Engine speed at idle is about the same, I have to add about 3-4 inHg of manifold pressure before the stall speed decreases, but it does that for all of our airplanes.
I have run some airfoil variations through XFOIL, but the analysis showed no difference. At stall angles of attack any analysis is more of a guess anyway. We took "splashes" of the wing leading edges at various stations (wood templates bonded to the wing with filler, then removed, to get the exact airfoil shape) and compared them to plots of what the shape should be, that's how I know about the "thinner" issue. It is not apparent to the "naked eye".
Last edited by SCaro; 1st May 2006 at 18:07.