Originally Posted by SCaro
I should have posted more details, sorry. I checked the airspeed indications on both airplanes with GPS, they are right on.
How? This could be relevant, there are many ways of checking IAS against GPS, some can be very accurate, some (most?!) can be very misleading.
<snip> I measured the angle of attack at which they stall, the one with the higher stall speed also stalls at a lower angle of attack.
How measured?
The stall is aft stick limited. I have eliminated the horizontal tail and elevator authority, it is sufficient to get it stalled properly
Surely you contradict yourself here. If the stall is aft-stick (rather than aircraft response) limited then elevator authority is extremely relevant to stall speed. What are the actual stall warning and stall characteristics?
We checked the flap rigging, and set it to the largest flap deflection within the tolerance, no change in stall speed.
Have you also checked aileron rigging? Drooped ailerons can decrease stall speed, or conversely reflexed ailerons can increase it. (Particularly if the aircraft has a swept wing, this can also be evidenced by a change in apparent LSS).
I tufted the flap upper surface (low wing), the flow on the flaps was attached at full flaps before the wing leading edge modification,
Leading edge shape is very very relevant to stall alpha, and this might be the issue. If you've got a tame aerodynamicist about the place, it might be worth their taking the two wing forms and running them through something like X-foil for a quick and dirty look at the differences.
afterwards it was not, even though the stall speed was lower (another mystery, normally this would be a loss of lift). The wing contour is now as close to type design as we could get it, although on one side it semms to be slightly thinner than it should be. Stalls are straight. I'm out of ideas.
Hmm, "seems to be slightly thinner" is interesting - anything that is (presumably) apparent to the naked eye in this regard could be very significant. I'd spend some time carefully measuring it up.
In the meantime, could you give us a couple of clues...
- Composite / metal / other?
- High/low/mid wing?
- High/low/mid tailplane?
- What sort of values are we looking at in the weights and stall speeds?
G