Air speed and ground speed

But our chief concern with the wind at the present moment is that we must understand that when we speak of the speed of an aeroplane we mean its speed relative to the air, or air speed as it is usually termed. Now the existence of a wind simply means that portions of the air are in motion relative to the earth, and although the wind will affect the speed of the aeroplane relative to the earth – i. e. its ground speed – it will not affect its speed relative to the air.

For instance, suppose that an aeroplane is flying from A to В (60 km apart), and that the normal speed of the aeroplane (i. e. its air speed) is 100 km/h (see Fig. 2.3). If there is a wind of 40 km/h blowing from В towards A, the ground speed of the aeroplane as it travels from A to В will be 60 km/h, and it will take one hour to reach B, but the air speed will be 100 km/h. If, when the aero­plane reaches B, it turns and flies back to A, the ground speed on the return journey will be 140 km/h (Fig. 2.4); the time to regain A will be less than half an hour, but the air speed will still remain 100 km/h – that is, the wind will strike the aeroplane at the same speed as on the outward journey. Similarly, if the wind had been blowing across the path, the pilot would have inclined his aeroplane several degrees towards the wind on both journeys so that it would have travelled crabwise, but again, on both outward and homeward journeys the air speed would have been 100 km/h and the wind would have been a headwind straight from the front as far as the aeroplane was concerned.

An aeroplane which encounters a headwind equal to its own air speed will appear to an observer on the ground to stay still, yet its air speed will be high. A free balloon flying in a wind travels over the ground, yet it has no air speed – a flag on the balloon will hang vertically downwards.

All this may appear simple, and it is in fact simple, but it is surprising how long it sometimes takes a student of flight to grasp the full significance of air speed and all that it means. There are still pilots who say that their engine is overheating because they are flying ‘down wind’! It is not only a question of speed, but of direction also; a glider may not lose height in a rising current of air (it may, in fact, gain height), yet it is all the time descending relative to the air. In short, the only true way to watch the motion of an aeroplane is to imagine that one is in a balloon floating freely with the wind and to make all observations relative to the balloon.

Ground speed is, of course, important when the aeroplane is changing from one medium to another, such as in taking-off and landing, and also in the time taken and the course to be steered when flying cross-country – this is the science of navigation, and once again the student who is interested must consult books on that subject.

The reader may have noticed that we have not been altogether consistent, nor true to the SI system, in the units that we have used for speed; these already include m/s, km/h and knots. There are good reasons for this inconsis­tency, the main one being that for a long time to come it is likely to be standard practice to use knots for navigational purposes both by sea and by air, km/h for speeds on land, e. g. of cars, while m/s is not only the proper SI unit but it must be used in certain formulae. We shall continue to use these different units throughout the book as and when each is most appropriate, and the important thing to remember is that it is only a matter of simple conversion from one to the other –

1 knot = 0.514 m/s = 1.85 km/h