CONTROLLABILITY AND STABILITY

It follows from the above that stability and controllability must be considered together. A stable aircraft will obey the controls predictably because whatever their position at any instant, it will strive to obey them. A genuinely unstable aircraft will, in contrast, not settle down in any position. Every small divergence from the desired flight path will be magnified. If the model is in a shallow dive this will tend to become steeper unless the pilot corrects it. A shallow turn will very quickly become a tightening spiral. If flying inverted, an unstable model will roll over or bunt, unless the pilot makes constant corrections at every moment An unstable radio controlled aeroplane is difficult to fly, although it can be done. A momentary inattention may produce disaster but with extreme concentration the aircraft can be saved. An unstable free-flight model will almost certainly crash.

Even so, too much stability in a radio controlled model can be a liability. Since the forces involved in holding position are relatively powerful, it is evident that changing from one attitude to another requires large forces too. Unless the control surfaces are unusually effective, this makes the over-stable aeroplane sluggish in response to the pilot. If the model is flying towards a tree or some other obstruction, it is very important to have quick response to commands. Hence although a high degree of stability is very desirable for the beginner’s radio controlled model, and for all free-flight aircraft, most model fliers prefer to have only moderate stability for the sake of more immediate control reaction.