Performance in level flight

In Chapter 1 it was shown that the lift developed by the wings of an aircraft must be equal to the weight at all times for steady horizontal flight. This is also approximately true for a steady climb, provided the angle of climb is not excessive.

As the speed is reduced, the lift is kept constant by increasing the angle of attack of the wings by using the elevators to raise the nose of the aircraft, as described in Chapter 10. In order for the new speed to be maintained, the drag of the aircraft must be exactly balanced by the engine thrust and so, in general, the throttle will need to be adjusted to bring this about.

We saw in Chapter 4 how the vortex drag, the surface friction and bound­ary layer pressure drag of an aircraft flying straight and level combined to give the typical variation shown in Fig. 7.4.

It is important in understanding the graph to remember that the wing angle of attack has been adjusted to give the same total lift at each speed. The most important feature to notice is the fact that the drag has a minimum value at a particular speed; the minimum drag speed.

Normally the aircraft will be operated at a speed greater than the speed corresponding to the minimum drag value, and a change in speed may, for example, result in the operating point on the graph moving from A to B in Fig. 7.4. The increase in drag will normally mean that the engine setting will have to be changed to produce the required extra thrust.

We can get a better picture if we also plot a series of curves showing how the engine thrust varies with speed for a number of different throttle settings. In Fig. 7.4, curves for a typical turbo-jet, at constant altitude, are shown. How­ever, exactly the same argument can be used whatever the powerplant. The steady flight ‘operating point’ for the aircraft occurs at the position where the drag and thrust curves intersect (i. e. when thrust=drag), and inspection of the intersection at points A and B shows that a higher throttle setting is required at B.

So far this feels right intuitively. If we want to go faster we increase the throttle setting and put the nose down to reduce the angle of attack as the speed increases. This simple view can, however, be misleading. In many cases the operating point will be quite close to the minimum drag point, say at point C in Fig. 7.4. A change in speed will thus lead to a relatively small change in the drag of the airframe as we move to point D. Further, the change in thrust with forward speed for a turbo-jet is frequently not very great. The net result of all this is that the required change in throttle setting may, in practice, be small, even for quite a substantial speed change. In this case it is primarily the change in angle of attack, produced by the change in elevator setting, which alters the speed.

This point is discussed further in Chapter 10, where we see that trying to operate below the minimum drag speed can lead to an unstable situation.