Optimum economy with piston engine

The fuel flow rate required by a piston engine driving a propeller is approxim­ately proportional to the power produced (Chapter 6). Over a typical range of cruising speeds below about M = 0.65 we find that the engine/propeller combination can be designed to have roughly the same efficiency irrespective of the selected cruising speed of the aircraft.

Thus, if we look at Fig. 7.5 again for a moment, and select as our operat­ing point the minimum drag speed for any wing loading curve, we can design a piston/propeller combination which will operate at the same efficiency irre­spective of the particular wing loading chosen.

The main thing that will change in the engine design as we alter the wing loading will be the engine size. If we select a wing of small area the loading will be large and the minimum drag speed high. We already know that the min­imum drag value for the airframe will be the same for all the curves in Fig. 7.5, so the increase in operating speed means that the power (equal to drag times speed) required will be greater; hence the need for a larger engine.

If we double the engine size to double the power, we also double the fuel flow rate. Thus, if we select a smaller wing area and double the minimum drag speed, we will double the engine size and use fuel at twice the previous rate. However, we shall complete the journey in half the time so the same total amount of fuel will have been used.

We have, of course, been guilty of over-simplification once more. The larger engine will increase the weight of the aircraft which we have assumed to be constant. If we recall that a large wing implied an increase in structural weight we can now see the design compromise which must be made. If we choose a high operating speed then the engine weight will be high. If, on the other hand we choose a low speed the wing will be large and the structure heavy. The designer has, therefore, to seek an optimum point between the two extremes.

Figure 7.5 also shows that an increase in altitude also means an increase in the minimum drag speed. This means a more powerful engine once again. This means an increase in weight – a good reason for limiting the cruising altitude of piston-engined aircraft. In addition, piston engines do not work particularly well at high altitude, although supercharging helps (see Chapter 6).

So far we have considered the size of the required engine only from the standpoint of cruise performance. In the real aircraft a somewhat larger engine will be required since matching the engine to the minimum drag speed at low altitude would permit the aircraft to fly only over a very limited speed range (Fig. 7.7), and extra power is required to give the aircraft an acceptable speed range and ceiling.

In order to get full benefit from the engine for a given weight it should be operated near the maximum throttle opening. Because of the reducing air density the power output of the engine falls with increasing altitude. Thus a cruising altitude is selected where the available engine power matches the required power near minimum drag.

There will be other operational requirements, such as the need to keep above severe weather, which will influence the actual choice of cruising altitude, but in general, the type of piston driven airliner of a few decades ago cruised at a comparatively low altitude compared with today’s turbo-jet driven airliners. We shall now examine reasons for the increased cruising altitude for this latter type of aircraft.