Lateral and directional stability

Now we are, at last, in a position to connect these two forms of stability – the sideslip essential to lateral stability will cause an air pressure on the side sur­faces which have been provided for directional stability. The effect of this pressure will be to turn the nose into the relative wind, i. e. in this case, towards the direction of sideslip. The aeroplane, therefore, will turn off its original course and in the direction of the lower wing. It is rather curious to note that the greater the directional stability the greater will be the tendency to turn off course in a sideslip. This turn will cause the raised wing, now on the outside of the turn, to travel faster than the inner or lower wing, and therefore to obtain more lift and so bank the aeroplane still further. By this time the nose of the aeroplane has probably dropped and the fat is properly in the fire with all three stabilities involved! The best way of seeing all this happen in real life is to watch a model aeroplane flying on a gusty day; the light loading and slow speed of the model make it possible to watch each step in the proceedings, whereas in the full-sized aeroplane it all happens more quickly, and also the pilot usually interferes by using his controls. If, for instance, the left wing drops and he applies rudder so as to turn the machine to the right, he will probably prevent it from departing appreciably from its course.

We can now explain the technique of turning an aeroplane. Suppose, when we want to turn to the left, instead of applying any rudder we simply bank the aeroplane to the left, as we have already seen it will slip inwards and turn to the left. That is all there is in it. So effective is this method that it is unneces­sary to use the rudder at all for turning purposes. So far as the yaw is concerned – and a turn must involve a yaw – the rudder (with the help of the fin) is still responsible, just as (with the help of the fin) it always was. The dif­ference is simply that the rudder and fin are brought into effect by the inward sideslip, instead of by application of rudder which tends to cause an outward skid. The pilot may do nothing about it, but the stability of the aeroplane puts a force on the rudder for him. It should also be emphasised that although it may be most practical, and most sensible, to commence a turn in certain air­craft without application of rudder, such a turn cannot be absolutely perfect; there must be an inward sideslip. The pilot may not notice it, the sideslip indi­cator may not detect it; but it is there just the same.

Just as a slight roll results in a sideslip and then a yawing motion so if an aircraft moves in a yawed position, as in Fig. 9.11, that is if it moves crabwise (which is really the same thing as slipping or skidding) lateral stability will come into play and cause the aircraft to roll away from the leading wing. Thus a roll causes a yaw, and a yaw causes a roll, and the study of the two cannot be separated.

If the stability characteristics of an aeroplane are such that it is very stable directionally and not very stable laterally, e. g. if it has large fin and rudder and little or no dihedral angle, or other ‘dihedral effect’, it will have a marked tend­ency to turn into a sideslip, and to bank at steeper and steeper angles, that it may get into an uncontrollable spiral – this is sometimes called spiral insta­bility, but note that it is caused by too much stability (directional).

If, on the other hand, the aeroplane is very stable laterally and not very stable directionally, it will sideslip without any marked tendency to turn into the sideslip. Such an aircraft is easily controllable by the rudder, and if the rudder only is used for a turn the aircraft will bank and make quite a nice turn.

The reader will find it interesting to think out the other characteristics which these two extremes would cause in an aeroplane, but the main point to be emphasised is that too much stability (of any type) is almost as bad as too little stability.