Part 1 (1/2)
Natural Stability and the Parachute Principle in Aeroplanes.
by W. LeMaitre.
PREFACE
Since there is nothing new under the sun, it is useless to pretend that there is anything new in the design here advocated or the theories advanced. Both are rather the result of a commonsense consideration of the different points of all flying machines, natural and artificial, and an endeavour to select from the great number of good points those which seem most likely to blend together into a practical machine. The conclusions reached are the result of a quite independent investigation, carried on over three years by means of numberless experiments, and the writer has endeavoured to make no single statement which he cannot by some experiment amply prove.
NATURAL STABILITY IN AEROPLANES
CHAPTER I.
THE IMPORTANCE OF STABILITY.
In considering the whole question of aviation, it becomes evident that the one point to strive for at the present juncture is stability. If we are ever to have a practical flying machine, that is, a machine which we can use as we do a yacht, a motor car, or a bicycle, it must be one that we can trust to keep its balance by reason of the natural forces embodied in it, and without any effort of control on the part of the pilot. It may be objected that a bicycle does not do this, and this is true, but, on the other hand, the upsetting of a bicycle is a very small matter, whereas the tilting of an aeroplane mostly means sudden death to its occupant, and it is probable that if the same consequences followed the tilting of a bicycle, bicycles would soon have been made with four wheels.
At present aeroplanes are the most unstable of all things. The least gust, the least s.h.i.+fting of weight, the slightest difference in the density of the strata of the supporting air, and the machine sways, and if not instantly corrected by the pilot the sway becomes a tilt, the tilt a dive, and the rest is silence. The first aeroplanes, the Wrights' for instance, were so unstable that twenty minutes in one of them was as much as the most iron-nerved man could stand, the continual strain being too exhausting to keep up for any length of time. By throwing out extensions and outriggers in all directions we have altered that to a certain extent, but only to an extent--we have not yet got rid of it. The monoplane is probably the most unstable, as might be expected from its smaller surface, but the bi-plane runs it pretty closely.
And the difficulty seems to arise chiefly from the fact that the machines are built round the propeller. In the case of a yacht or a car, the machine is built first and the propelling means is fitted on as an auxiliary. The consequence is that an aeroplane which is safe enough while the propeller is exerting a tractive force of some 250 lbs., becomes, the moment this power is for any reason stopped, merely a shapeless construction at the mercy of the wind and the force of gravitation. It is true that most machines may be made to glide if the pilot is clever enough and quick enough to steer them into the proper gliding angle, but the machine that will naturally and by reason of its design a.s.sume its proper gliding angle when the propelling force is withdrawn, has not yet been built.
Such a machine would have ”Natural Stability.”
It will be recognized that this natural stability, which depends on the design of the machine, is something entirely different from ”automatic stability” of which there are many systems, all having this one defect; that, depending upon working devices, movable planes, gyroscopes, compensating balancers, pendulums, etc., they are all liable to go wrong and refuse to act the moment a sudden strain makes their perfect action most important.
Considering that the propeller is the only means the aeroplane has of keeping in the air at all, the question arises: Is it possible to design a machine that will be stable to the extent of descending safely when the propeller stops, and that will yet be a good and speedy flyer?
That is the problem we have to solve.
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 ill.u.s.tration 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 pa.s.sage 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 mult.i.tude 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.