CHAPTER II.

SPEED AS A MEANS OF STABILITY.

It is recognized on all hands that speed is a great factor in the problem of stability. To begin with, a machine going at high speed would be practically untouched by gusts of wind, different densities of air strata, holes in the air, etc. Also its greater momentum would tend to keep it in a straight line, not only relative to its course but also relative to itself. That is to say, its wings being started in a horizontal plane, would tend to keep in the same plane and would not easily tilt or sway out of it. Both these effects of natural law show that a high speed machine must be more stable than a low speed machine. How then are we to design a high speed machine?

Leaving aside the question of higher power, the first point that suggests itself is to lessen the head resistance. All fast things, boats, birds, arrows, even motor-cars, are made long and narrow. It will be objected that a bird with its wings outspread is not long and narrow, but in the sense in which this illustration is meant, the bird’s wings, being merely its propelling apparatus, do not count, and when the bird is at its fastest, as in the swoop of a hawk or an eagle, the wings are shut tightly to the body so as to offer no resistance to its lightning passage through the air. If we are to follow previous experience in Nature’s laws, our aeroplanes must be considerably reduced in span. To drive through the air at a high speed with a machine of 40 foot span is a practical impossibility, both because of the tremendous power it would require and also by reason of the great strength the plane must have to withstand the resistance of the air.

In reducing the span, however, we reduce the lifting surface of the machine. But on the other hand it must be remembered that the lifting efficiency is increased by increasing the speed. Lift is the product of supporting surface and speed. A small plane driven at a high speed will give as great a lift as a large plane driven at a low speed. Speed, again, is the difference between the propelling power and the head resistance, and we can increase the speed by decreasing the resistance. It follows, then, that we need not necessarily give up lifting power by reducing the span of the wings, since the shorter span gives greater speed, and the increase of efficiency by reason of the greater speed would go to make up for the loss of span.

It is, then, quite possible to design a short span machine which shall be as efficient for lift as a long span machine, and which will have the advantage of possessing, by reason of its speed, much greater stability.

But the span is not the only factor in the speed problem. In the low speed machines at present in use we have found it necessary to curve the planes to get greater efficiency. This efficiency is also gained at the expense of head resistance, and it is already recognized that the higher the speed the less is the need of camber. This is the same problem over again. A high speed flat plane will give as much lift as a low speed cambered plane, and we gain in stability with every additional mile per hour.

The third point to be considered in the problem of speed is the resistance caused by the multitude of struts and wires, the body of the pilot, the tanks, engine, and all the other impedimenta projecting in all directions from the body of the aeroplane. It has occurred to our builders that if the whole of these things could be collected together and enclosed in a light covered-in car of a proper shape, the skin friction of such a car would be much less than the total head resistance offered by the different obstructions so covered. And there is another advantage to be gained here, for if, at 40 miles per hour, the force of the wind is very seriously uncomfortable for the pilot, the position at such speeds as 70 or 100 miles per hour would be quite impossible.