"Another consideration that enters into the design of the plane is the aspect ratio, or the ratio between the depth of the plane fore and aft, and the width or span. Authorities do not agree about this latter consideration. A practical aspect ratio, one states, is 6 to 1, as, for instance, a plane 39 feet spread by 6 feet 6 inches in depth. In Santos Dumont's Demoiselle the aspect ratio is only 3 to 1. The ideal plane, however, would be a plane of great length and little depth, but this is impossible in the practical machine, as a plane of excessive length would greatly weaken the construction of the machine. Again, the different authorities do not agree as to the shape of the ends of the planes. Lanchester says that an efficient plane must be of rectangular form, and the Voisin and Curtiss planes are rectangular, whereas the wings of the Blériot and the Wright planes are decidedly curved at the tips.
"I will show in other illustrations the method of placing the planes on such machines, as made by Curtiss and some other noted aviators.
"I think I have said sufficient to give you a fair idea of the reason why an aeroplane can be made to navigate the air, but I have not told you how its direction can be controlled. No doubt, if the air were always still and not subject to change, there would be but little difficulty in controlling the direction of the machine, but, unfortunately, this is not the case, so provision has to be made to meet various changes as they occur. A downward current of air causes the plane to change its inclination to the horizontal, so that it will not support the weight, and the machine falls to the ground. To overcome this unsatisfactory state of things, small auxiliary planes are used to counteract the effect of varying air currents. They control the movements of the main planes so that they always bear the same inclination to the horizontal, and they are also used to elevate the machines so as to clear small obstacles. If any great increase in altitude is desired, the speed of the engine must be increased and the planes driven more rapidly through the air, thus giving them more lifting power.
"It may be that in a short time, additional balancing planes will not be necessary, as some other scheme may be invented that will regulate the balance of the aeroplane. Already an Australian inventor, called Roberts, has applied the gyroscope to the aeroplane in order to solve the problem of making it balance automatically. It exerts a balancing force equal to 300 pounds, placed 18 inches on either side of the centre of gravity. The gyroscope is driven by electricity, and controlled by a pendulum which swings right or left, according to the tilt of the aeroplane. Mr. Roberts is also working on a small aeroplane which is to be controlled by wireless telegraphy. His inventions are being tested by the British War Office. There are many other inventors on three continents busily employed in trying to solve the balance problem.
"A very important matter in the construction of the aeroplane is the position of the screw propeller. Sir Hiram Maxim advocates placing it at the rear of the planes, and this construction is carried out in the Wright, Curtiss, Voisin and Baldwin-McCurdy machines, while the tractor screw is used on the Blériot, Antoinette, and Roe fliers. Sir Hiram's theory is that if the screw is placed in front, the backwash strikes the machine, which offers a good deal of resistance to the passage of the air, and retards action; but if the propeller is placed in the rear, the resistance of the machine imparts a forward motion to the air with which it comes in contact, and the screw, running in air that is moving forward, has less slip, and is, therefore, more efficient than the tractor screw.
"While the construction of the aeroplane is yet in an experimental stage, it is progressing quite rapidly, and though no definite rules covering the whole ground of construction and management can yet be laid down, the following points may be well considered before any steps are taken in making or using any make of aeroplane: (1) That it is useless to construct the planes of flat vertical section, as much lift is lost in doing so, and they are best constructed after the manner shown in Figs. [44] and [44a]. (2) That the most practical aspect ratio is about 6 to 1. (3) That the angle of incidence of the inclined planes ought to be somewhere between 1 in 10, and 1 in 20 (i. e., the angle by which they are inclined to the horizontal, the forward or entering edge of the plane of course being the higher). (4) That a reliable motor, one that is immune from involuntary stoppages, is absolutely essential to prevent accidents. (5) That automatic stability of the machine is the theory of aeronautics that all inventors should study most carefully.
Fig. 45. Blériot monoplane
"The illustration I show here ([Fig. 45]) represents the monoplane in which the Frenchman, Blériot, crossed over the sea from France to England. The thick curved lines, shown at A, exhibit the main plane which gives the machine its name of "monoplane"—one plane—and B shows the rear auxiliary plane, which is also of curved section and curved ends. The plane A has an area of 150 square feet, and B has an area of 17 square feet, while the rudder C has an area of 41⁄2 square feet. The total length of the machine is 25 feet, the sweep of the rudder 6 feet 6 inches. The rudder is a plane, pure and simple, and may be constructed of any light material that is strong enough to stand a reasonable wind pressure. The planes must be covered on both sides with some light fabric, silk preferred, and all the framework made as light as possible, consistent with safety.