ORIGINAL: otrcman



Yes, the rudder of the DRI is cable controlled. But remember, there is no fin. Just a rudder. The plane will sideslip one way or the other all by itself and has no natural tendency to come back to center. It requires active pilot input to keep the plane anywhere near straight. That's a difficult concept to wrap your mind around if all you have flown is modern airplanes (say, 1920 or later) with actual stability.

The advent of a fixed fin on full scale airplanes was a total game changer for the pilot. Once there is a fixed fin (of sufficient size) the airplane will fly with little or no sideslip all by itself. Before fixed fins came along, maintaining zero sideslip was the pilot's job. Normally, the way the pilot sensed sideslip was to feel whether the wind was equal on both sides of his face.

In pitch, there is a big difference between an elevator that can ''float'' versus an irreversible elevator. That's why we see reference to stick-fixed versus stick-free stability in aerodynamic testing. Stick-fixed is more stable in almost all cases, because you are essentially making the fixed surface larger by locking the elevator.

In a typical light airplane, the pilot isn't maintaining the control surfaces rigidly in place at all. The surfaces are pretty much floating in the slipstream and the pilot is just applying pressure one way or the other when he exerts control. Not the same as an R/C plane where the servo keeps the surface from floating in the slipstream.

An easy way to think of the concept is this: A fixed tail surface (fin, stabilizer) is where the stability comes from. A movable tail surface (rudder, elevator) is where control comes from.

Dick