Hello guys,

I have a question about airplane longitudinal stability in those common foamy with compensators built into elevator. For compensator I mean that control surfaces that extends ahead of the hinge line - i.e. the control surface found on nearly all the 3D planes. I don't know how they are called precisely... I hope you understand!

With my foamy and other small 3D planes I notice a weird pitch behaviour: they cannot be trimmed for level flight. In other words, this is what happens to me.

You trim it for level flight. Ok... but after some manouvers, you level the plane at the same throttle and speed, and now it dives slightly. Ok, let's give a little up-trim... and after some time you'll discover that it has a tendency of climbing up slightly...

I assure you that the CG is not the cause - I fly pretty nose heavy. So, after some inspection, I found little play in my linkage, so I fixed it. Now the wear is gone, the surface is solid, and the little play is only due to the geartrain (VERY small, almost nonexistant). The problem still exists though, even if it is of very less entity.

So... I arrive to the question: can that compensator be the cause of this little unstability in pitch? Remember that I'm referring to HS-55 servos, they are not the best as far as precision goes... What I'm thinking about is that the compensator can destabilize the control surface.

A side question: the fact that they are not used in pattern airplanes, makes me think that they are the cause.


Let me know what do you think!