Controls on steep banks

The turning of an aeroplane is also interesting from the control point of view because as the bank becomes steeper the rudder gradually takes the place of the elevators, and vice versa. This idea, however, needs treating with a certain amount of caution because, in a vertical bank for instance, the rudder is nothing like so powerful in raising or lowering the nose as are the elevators in normal horizontal flight. Incidentally, the reader may have realised that a ver­tical bank, without sideslip, is theoretically impossible, since in such a bank the lift will be horizontal and will provide no contribution towards lifting the weight. If it is claimed that such a bank can, in practice, be executed, the explanation must be that a slight upward inclination of the fuselage together with the propeller thrust provides sufficient lift.

This only applies to a continuous vertical bank in which no height is to be lost; it is perfectly possible, both theoretically and practically, to execute a turn in which, for a few moments, the bank is vertical, or even over the vertical. In the latter case the manoeuvre is really a combination of a loop and a turn.

Generally speaking, the radius of turn can be reduced as the angle of bank is increased, but even with a vertical bank there is a limit to the smallness of the radius because, quite apart from the question of side-slipping, the lift on the wings (represented by CL. jpV2. S) must provide all the force towards the centre, i. e. m. Г-/Г or WV2lgr.

Thus WW/gr = CL. ipV2. S

or r = 2W/(CL. pS. g)

Now, in straight and level flight the stalling speed (V) is given by the equation W = L = CL max. CL. ipV2. S

If we substitute this value of W into our formula for the radius we get

Г = (2 . CL max. CL. |pV2. S)/(CL. pS. g) i. e. r = (V2/g) X (CLmax/CL)

This shows that the radius of turn will be least when CL is equal to CL max, i. e. when the angle of attack is the stalling angle, and radius of turn = V2/g. It is rather interesting to note that the minimum radius of turn is quite inde­pendent of the actual speed during the vertical banks; it is settled only by the stalling speed of the particular aeroplane. Thus, to turn at minimum radius, one must fly at the stalling angle, but any speed may be employed provided the engine power is sufficient to maintain it. In actual practice, the engine power is the deciding factor in settling the minimum radius of turn whether in a ver­tical bank or any other bank, and it must be admitted that it is not usually possible to turn on such a small radius as the above formula would indicate.

This formula applies to some extent to all steep turns and shows that the aeroplane with the lower stalling speed can make a tighter turn than one with a higher stalling speed. (We are referring, as explained above, to the stalling speed in straight and level flight.) But in order to take advantage of this we must be able to stand the g’s involved in the steep banks, and we must have engine power sufficient to maintain turns at such angles of bank.