TOE THREE AXES

Stability and control problems in aircraft are complicated because a model which is stable in pitch, i. e. in nose up and nose down senses, rotating about the crosswise or Y axis (Figure 12.2), may be unstable about the other axes, in yaw, about the vertical, Z, axis and in roll about the fore and aft or X axis. The three axes are assumed to cross one another at right angles through the centre of gravity of the aircraft This conventional system of axes does not imply that the aircraft is somehow bound to rotate, when it rotates, round the axes as if they were skewers.

The axes are in fact no more than convenient reference lines for trim, balance and stability calculations.

12.2 LONGITUDINAL BALANCE AND TRIM

Balance, or trim, and stability are closely connected, but they are distinct Consider balance first If a model is to fly at all, it must be in a balanced trim although being in balance is not enough to guarantee stability. A juggler keeps things balanced despite their instability. If he cannot achieve balance, everything comes crashing down and model aircraft are very similar. Only when balance, or trim, is achieved do questions of stability arise. Stability then may be regarded as restoring balance; the balance must be possible in the first place, if it is to be restored.

For flight in balance, or equilibrium (climbing, diving, or flying level at constant speeds) there must be no tendency of the aircraft to pitch nose up or down. Mathematically, when a model is trimmed for a particular flight attitude the longitudinal moments taken at any convenient point will total zero. On a practical aircraft this means that a state of balance must be possible in all the flight attitudes the pilot is likely to require. Even if the equilibrium is deliberately upset in order to change position (from steep climb to glide, for instance, or from level flight to inverted) if the model is to hold the new attitude for more than an instant, it must be trimmable at this position. The balanced state must be attainable, by use of the controls, over a wide range of attitudes.