What is true of ships is true of flying machines. Good results can never be obtained by placing the screw in front instead of in the rear of the machine. If the screw is in front, the backwash strikes the machine and certainly has a decidedly retarding action. The framework, motor, etc., offer a good deal of resistance to the passage of the air, and if the air has already had imparted to it a backward motion, the resistance is greatly increased. The framework will always require a considerable amount of energy to drive it through the air, and the whole of this energy is spent in imparting a forward motion to the air, so if we place the propelling screw at the rear of the machine in the centre of the greatest atmospheric resistance, it will recover a portion of the lost energy, as in the steamship referred to. It will, therefore, be seen that when the screw is at the rear, it is running in air which is already moving forward with a considerable velocity, which reduces the slip of the screw in a corresponding degree. I have made experiments with a view of proving this, which I shall mention further on, and which ought to leave no chance for future discussion.

Fig. 21.—Small apparatus for testing fabrics for aeroplanes, the material being subjected to an air blast in order to test its lifting effect as compared with its tendency to travel with the blast.

My first experiments had shown that wooden aeroplanes did much better than any of the fabric covered aeroplanes that I was able to make at that time, but as wood was quite out of the question on my large machine on account of its weight, it was necessary for me to conduct experiments with a view of ascertaining the relative values of different fabrics. For this purposes, I made the little apparatus shown ([Fig. 21]). This was connected to a fan blower driven by a steam engine having a governor that worked directly on the point of cut-off. The speed was, therefore, quite uniform and the blast of air practically constant. I had a considerable number of little frames cut out of sheet steel, and to these I attached various kinds of fabric, such as ordinary satin, white silk, closely woven silk, linen, various kinds of woollen fabrics, including some very coarse tweeds, also glass-paper, tracing linen, and the best quality of Spencer’s balloon fabric. The blast of air was not large enough to cover the whole surface of the aeroplanes, so that the character of the back of the frames was of no account. The first object experimented with was a smooth piece of tin. When this was placed at an angle of 1 in 14, it was found that the drift or tendency to travel in the direction of the blast was just one-fourteenth part of the upward tendency, or lift. This was exactly as it should have been. Upon changing the angle to 1 in 10, a similar thing occurred; the lift was ten times the drift. I, therefore, considered the results obtained with the sheet of tin as unity, and gave to every other material experimented with, a coefficient of the unity thus established. Upon testing a frame covered with tightly-drawn white silk, a considerable amount of air passed through, and with an angle of 1 in 14, the lift was only about double the drift. A piece of very open fabric, a species of buckram, was next tried, and with this the lift and drift were about equal. With closely-woven, shiny satin the coefficient was about ·80; with a piece of ordinary sheeting the coefficient was ·90; with closely-woven, rough tweeds, ·70; and with glass-paper about ·75. With a piece of tracing linen very tightly drawn, results were obtained identical with those of a sheet of tin, and with Spencer’s balloon fabric the coefficient was about ·99. I, therefore, decided to cover my aeroplanes with this material. It will be observed that the apparatus is so arranged that both the lift and the drift can be easily measured.

Fig. 22.—Apparatus for testing the lifting effect of aeroplanes and condensers in an air blast. k, k show two aeroplanes in position for being tested.

[Fig. 22 enlarged] (110 kB)

Fig. 23.—Apparatus for testing aeroplanes, condensers, etc., in an air blast. The opening is 3 feet square. Thin brass sustainers are shown in position for testing.