RE: Airplane rudder gyro
I used a Futaba GY-401 gyro on a couple of airplanes that I have. I always use it to control the rudder and matched it with a Futaba digital servo. It makes for easy ground handling during takeoff. It also makes ground handling during landing easier (switch it on AFTER you touch down). On a 3D airplane, it can also be used for rudder control during hover.
The most important item to remember about using gyros on an airplane is to be able to disable the gyro easy and fast. I got into trouble once when the rudder controlling AVCS gyro was accidentally enabled during flight. This caused the rudder to move to full deflection during an aileron turn, which causes a hard stall and spin in a 3D aircraft. Since I did not know what was going on, I almost lost the airplane. Finally, I noticed that the gyro was enabled and I disabled it. Since then, I moved the gyro controlling switch on my 14MZAP to the trainer switch. That way, it was spring loaded and required me to hold the switch to enable the gyro. Now, I fly without any gyros simple because they are too dangerous.
In summary, a rudder controlling gyro is fun and educational on an airplane, but only for a short while. I had quite a bit of fun learning about rudder controlling gyros and how they operate in an aircraft during various maneuvers (like Immelmans). However, in my opinion, they are too dangerous to be there in the airplane all the time. If they are enabled and the pilot is unaware, then you can easily lose an aircraft.