First,
Thanks for everyone.
I particularly liked sewoodruff explanation,
which seems to have some good understanding
in aerodynamics.

The A/P is a VMAR PC9 .45 size engine,
having a semi-symetric low wing.

I had several crashes and did some repairs,
I probably set the firewall and/or wing incidence
incorrect.

Please correct me if i am wrong.
Is a higher wing incidence is not the same case
as a low hor-stab incidence?

in such a case, a negative incidence angle for
the stab would create a pitch up moment,
which is actually my current problem...

anyhow,
I had already lowered the wing incidence.
next week i will make a test
flight and inform you my results !!!


B.T.W
lowering the engine inclination angle
didn't show any improvement.