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.
CHAPTER III.
THE LOW CENTRE OF GRAVITY.
The first thing that occurs to the investigator on the subject of stability is that nature offers us a sure means of keeping our machines upright by adopting the simple method of placing all the heavier parts at the bottom. In all other constructions we have adopted this plan with perfect success. In boats, yachts, cars, balloons, everything man uses in fact, the simplest, best and most obvious method of keeping a thing upright is to utilize the force of gravity, place the lighter or supporting parts above and the weight below, and the thing is done.
This simple method of obtaining stability did not escape the aeroplane designers, and we have had several machines which embodied this principle, more or less. Unfortunately, however, they all proved failures. A machine would be designed, and, with the weight high, would fly well, though it was unstable. Put the weight low and you got rid of the instability, and at the same time the machine became unmanageable. It looked as if flying and instability were interchangeable terms. So, as it was a machine that would fly the designers were after, the weight was kept up and the stability was left to the pilot. The machines were made “sensitive” as it is called, that is to say, sensitive to a touch of the rudder or the balancers. They are also, it is true, equally sensitive to a gust of wind or a slight shifting of weight or pressure, and this has caused the smashing of a good many machines and some pilots; but after all this is the fortune of war, and no one is compelled to go up in an aeroplane.
The curious thing about it is that it does not seem to have occurred to our designers that if their pet design would not fly with the weight low, perhaps it might be possible to alter the design instead of altering the position of the centre of gravity, and so obtain what we are all looking for, a naturally stable machine that is yet sensitive to control.
There are two chief difficulties in the way of the low centre of gravity machine. One is that the heaviest portion of the machine being some distance below its support, it is apt to give rise to a pendulum or swaying motion. The other is that of tilting, or banking up, in turning a corner. These are really two developments of the same difficulty, i.e. pendulum motion.
If we take a strip of stiff paper to represent a plane and put a small weight in the centre of the plane, the model on being glided to earth does not tend to sway (Fig. 1). If we put our weight on a tiny piece of wire an inch or so below the plane (Fig. 2) and set the model free, it will probably acquire a swinging motion as it descends. That is the whole trouble. The trouble is real enough, but the fallacy is in supposing it to be all the fault of the low centre of gravity. All ships that were ever designed have a low centre of gravity, yet some roll dreadfully and others do not, which, in itself, should be proof sufficient that it is the design of the machine and not the position of the ballast that is at fault.