Effects of height – propeller propulsion
So far as the aeroplane is concerned, we will get the same range and we should fly at the same indicated speed, whatever the height. Now, although the drag is the same at the same indicated speed at all heights, the power is not. This may sound strange, but it is a very important fact. If it were not so, if we needed the same power to fly at the same indicated speed at all heights, then the advantage would always be to fly high, the higher the better, because for the same power the higher we went, the greater would be our true speed. However, it can hardly be considered a proof that an idea is incorrect simply because it would be very nice if it were correct. The real explanation is quite simple. Power is the rate of doing work. Our fuel gives us so many newton metres, however long we take to use it. But if we want the work done quickly, if we want to pull with a certain thrust through a certain distance in a certain time, then the power will depend on the thrust and the distance and the time, in other words, on the thrust and the velocity. But which velocity, indicated or true? Perhaps it is easier to answer that if we put the question as, which distance? Well, there is only one distance, the actual distance moved, the true distance. So it is the true air speed that settles the power. The higher we go, the greater is the true air speed for the same indicated speed and therefore the greater the power required, although the thrust and the drag remain the same.
Now a reciprocating engine can be designed to work most efficiently at some considerable height above sea-level, if it is supercharged. If we use it at sea-level, and if we fly at the best speed for range, the thrust will be a minimum, that is what we want, but, owing to the lower speed, little power will be required from the engine. That may sound satisfactory, but actually it is not economical; the engine must be throttled, the venturi tube in its carburettor is partially closed, the engine is held in check and does not run at its designed power, and, what is more important, does not give of its best efficiency; we can say almost literally that it does not give best value for money. In some cases this effect is so marked that it actually pays us, if we must fly at sea-level, to fly considerably faster than our best speed and use more power, thereby using the aeroplane less efficiently but the engine more efficiently. But to obtain maximum range, both aircraft and engine should be used to the best advantage, and this can easily be done if we choose a greater height such that when we fly at the correct indicated speed from the point of view of the aeroplane, the engine is also working most efficiently, that is to say, the throttle valve is fully open, but we can still fly with a weak mixture. At this height, which may be, say, 15 000 ft (4570 m), we shall get the best out of both aeroplane and engine, and so will obtain maximum range.
Here the reader may be wondering what governs the operating height the designer chooses. This may be selected for terrain clearance, cruise above likely adverse weather conditions or the engine may be sized for take-off performance and the cruising altitude follows as a by-product to give full throttle cruise.
What happens at greater heights? At the same indicated speed we shall need more and more power; but if the throttle is fully open, we cannot get more power without using a richer mixture. Therefore we must either reduce speed and use the aircraft uneconomically, or we must enrich the mixture and use the engine uneconomically.
Thus there is a best height at which to fly, but the height is determined by the engine efficiency (and to some extent by propeller efficiency) and not by the aircraft, which would be equally good at all heights. The best height is not usually very critical, nor is there generally any great loss in range by flying below that height. It may well be that considerations of wind, such as are explained in the next paragraph, make it advisable to do so.