Performance in turning flight

So far we have considered the aircraft to be flying in a straight line. We now turn our attention to the important manoeuvre of changing direction; the turn.

We look first at the case in which the turn is made at constant altitude. Any time a turn is made by any vehicle, whether it be a bicycle, car, train or aeroplane, a force must be provided towards the centre of the turn because there is an acceleration directed towards the centre. In the case of a car or bicycle this force is provided by the tyres; in the case of a train it is provided by the rails. For an aircraft some other means must be found and this is done by tilting, or banking the aircraft so that a component of the lift force produced by the wings acts in the required direction (Fig. 7.21). Thus the wings must produce a higher amount of lift than was required for normal straight and level flight.

This extra lift means that, for a given speed, the wing must be operated at a higher angle of attack in the turn and, in addition, the increase in the lift will be accompanied by an increase in drag. This drag will, in turn, mean that the power required to sustain a steady turn is greater than that required for flight in a straight line at the same speed. The angle of bank and increase in lift, drag and required power all increase as the turn is tightened, and it may be that the minimum radius of turn which can be achieved is limited by the amount of power that is available from the engine. Alternatively the demand for extra lift may cause the wing to stall before this point is reached, and stalling may therefore prove to be the limiting factor.

Vertical lift component balances weight

Lift component provides required centripetal force

Fig. 7.21 Forces in turning flight

Lift must increase to provide both the vertical component to balance weight and the required forces for the turn

Recommended further reading

Mair, W. A. and Birdsall, D. L., Aircraft performance, Cambridge University Press, Cambridge, 1996, ISBN 0521568366. A good general text on aircraft performance.