If an aeroplane be made perfectly flat on the bottom side and convex on the top, and be mounted in the air so that the bottom side is exactly horizontal, it produces a lifting effect no matter in which direction it is run, because, as it advances, it encounters stationary air which is divided into two streams. The top stream being unable to fly off at a tangent when turning over the top curve, flows down the incline and joins the current which is flowing over the lower horizontal surface. The angle at which the combined stream of air leaves the plane is the resultant of these two angles; consequently, as the plane finds the air in a stationary condition, and leaves it with a downward motion, the plane itself must be lifted. It is true that small and narrow aeroplanes may be made to lift considerably more per square foot of surface than very large ones, but they do not offer the same safeguard against a rapid descent to the earth in case of a stoppage or breakdown of the machinery. With a large aeroplane properly adjusted, a rapid and destructive fall to the earth is quite impossible.
THE EFFICIENCY OF SCREW PROPELLERS, STEERING, STABILITY, &c.
Before I commenced my experiments at Baldwyn’s Park, I attempted to obtain some information in regard to the action of screw propellers working in the air. I went to Paris and saw the apparatus which the French Government employed for testing the efficiency of screw propellers, but the propellers were so very badly made that the experiments were of no value. Upon consulting an English experimenter, who had made a “life-long study” of the question, he assured me that I should find the screw propeller very inefficient and very wasteful of power, and that all screw propellers had a powerful fan-blower action, drawing in air at the centre and discharging it with great force at the periphery. I found that no two men were agreed as to the action of screw propellers. All the data or formulæ available were so confusing and contradictory as to be of no value whatsoever. Some experimenters were of the opinion that, in computing the thrust of a screw, we should only consider the projected area of the blades, and that the thrust would be equal to a wind blowing against a normal plane of equal area at a velocity equal to the slip. Others were of the opinion that the whole screw disc would have to be considered; that is, that the thrust would be equal to a wind blowing against a normal plane having an area equal to the whole disc, and at the velocity of the slip. The projected area of the two screw blades of my machine is 94 square feet, and the area of the two screw discs is 500 square feet. According to the first system of reasoning, therefore, the screw thrust of my large machine, when running at 40 miles an hour with a slip of 18 miles per hour, would have been, according to the well-known formula,
V² × ·005 = P
18² × ·005 × 94 = 152·28 lbs.
If, however, we should have considered the whole screw disc, it would have been 18² × ·005 × 500 = 810 lbs. However, when the machine was run over the track at this rate, the thrust was found to be rather more than 2,000 lbs. When the machine was secured to the track and the screws revolved until the pitch in feet, multiplied by the turns per minute, was equal to 68 miles an hour, it was found that the screw thrust was 2,164 lbs. In this case, it was of course, all slip, and when the screws had been making a few turns they had established a well-defined air-current, and the power exerted by the engine was simply to maintain this air current. It is interesting to note that, if we compute the projected area of these blades by the foregoing formula, the thrust would be—68² × ·005 × 94 = 2,173·28 lbs., which is almost exactly the observed screw thrust.
When I first commenced my experiments with a large machine, I did not know exactly what sort of boiler, gas generator, or burner I should finally adopt; I did not know the exact size that it would be necessary to make my engines; I did not know the size, the pitch, or the diameter of the screws which would be the most advantageous; neither did I know the form of aeroplane which I should finally adopt. It was, therefore, necessary for me to make the foundation or platform of my machine of such a character that it would allow me to make the modifications necessary to arrive at the best results. The platform of the machine is, therefore, rather larger than is necessary, and I find if I were to design a completely new machine, that it would be possible to greatly reduce the weight of the framework, and, what is still more, to greatly reduce the force necessary to drive it through the air.
Fig. 89.—One of the large screws being hoisted into position. Its size may be judged by comparison with the man.
At the present time, the body of my machine is a large platform, about 8 feet wide and 40 feet long. Each side is formed of very long trusses of steel tubes, braced in every direction by strong steel wires. The trusses which give stiffness are all below the platform. In designing a new machine, I should make the trusses much deeper and at the same time very much lighter, and, instead of having them below the platform on which the boiler is situated, I should have them constructed in such a manner as to completely enclose the boiler and the greater part of the machinery.[14] I should make the cross-section of the framework rectangular and pointed at each end. I should cover the outside very carefully with balloon material, giving it a perfectly smooth and even surface throughout, so that it might be easily driven through the air.