STICK-FREE AND STICK-FIXED STABILITY

In full-sized aviation, designers must consider what an aircraft will do if the pilot lets go of all the controls and allows them to float freely under the various air loads. Stick-free stability is the art of designing aircraft so that they will fly themselves, at least for some little time, without a pilot at all. In model aviation this is not of concern. The controls are normally fixed, as in free flight models, or they are held in whatever position they are placed by means of servo motors, control rods or wires. This is the case even if the pilot lets go of the radio transmitter sticks, because these are spring-loaded to centralise automatically (except for the throttle, as a rule). This is the condition known as ‘stick fixed’, although of course it does not imply that the controls do not move. On the contrary, they move, but only in response to command.

Another factor in full-sized stability calculations is ‘stick force per G’. This refers to the force which has to be exerted by the pilot on the control column, in order to produce an acceleration force of one ‘G’ (either positive or negative) on the aircraft In a loop or a turn there is always some ‘G’ force. If an aircraft has very light controls a small stick pull or push can produce a large ‘G’ force and with clumsy handling this can overstress the structure. For ordinary aeroplanes, not intended for aerobatics, stick force per ‘G’ is made large so that a pilot will have to pull very hard to produce a ‘tight’ loop, for instance, and the danger of overstressing is less. Aerobatic aircraft, flown by experts, are lighter on the controls.

With radio controlled models the stick forces felt by the pilot are those of the transmitter springs and do not relate directly to the forces felt by servos and pushrods in the model. There is, however, a similarity in that for a given stick force at the transmitter, a model with relatively small stability will respond more sharply. The same applies; clumsy handling may produce a disaster.