No, they wont. They'll only behave like that if the thrust line is off. Think about what happens if a stable plane with the perfect CG has the engine mounted with a lot of down incidence. When flying at full throttle, the engine is producing forward thrust as well as pulling the nose down. That will have to be compensated for with lots of up trim to get level flight. Now chop the throttle and see what happens. You've no longer got all that down thrust trying to pitch the nose down but you've still got all that up elevator trim so the plane climbs.

Now take the same plane and put the engine at a large positive thrust angle. The results will, not surprisingly, be just the opposite.

Both cases had the correct CG but both trimmed poorly.

CG and thrust line are both important adjustments that can be made to improve flight trim. But you are right that CG location should be the first step in the process.


Plans can be wrong. Construction errors can be made. Tolerances can gang up against you. Nothing is ever perfect. Thrust line adjustment is very commonly required on a new plane.

Let's take the same hypothetical plane we were discussing above and try to correct it's trim problems by moving the CG back. Eventually you'll have a plane that still won't trim at different speeds and it'll also be so pitch sensitive that you won't be able to fly it. The correct adjustment for thrust line problems is thrust line adjustment. The correct adjustment for CG problems is CG adjustment And the trim charts are developed to help you determine which part of the airplane's set up needs to be adjusted. That's why I described how to flight test for thrust line determination. Flight testing for CG is different. Again, see the charts.

Recommendations for things like thrust line and control throws are only starting points. Every plane is slightly different and for best performance each one needs to be tested and tweaked.

Dave