Artificial and Natural Flight

by Hiram S. Maxim

My experiments have demonstrated that relatively narrow aeroplanes lift more per square foot than very wide ones; but as an aeroplane, no matter how narrow it may be, must of necessity have some thickness, it is not advantageous to place them too near together. Suppose that aeroplanes should be made ¹⁄₄-inch thick, and be superposed 3 inches apart—that is, at a pitch of 3 inches—one-twelfth part of the whole space through which these planes would have to be driven would be occupied by the planes themselves, and eleven-twelfths would be air space ([Fig. 86]). If a group of planes thus mounted should be driven through the air at the rate of 36 miles an hour,[12] the air would have to be driven forward at the rate of 3 miles an hour, or else it would have to be compressed, or spun out, and pass between the spaces at a speed of 39 miles an hour. As a matter of fact, however, the difference in pressure is so very small that practically no atmospheric compression takes place. The air, therefore, is driven forward at the rate of 3 miles an hour, and this consumes a great deal of power; in fact, so much that there is a decided disadvantage in using narrow planes thus arranged.

[12] The arrows in the accompanying drawings show the direction of the air currents, the experiments having been made with stationary planes in a moving current of air.

In regard to the curvature of narrow aeroplanes, I have found that if one only desires to lift a large load in proportion to the area, the planes may be made very hollow on the underneath side; but when one considers the lift in terms of the screw thrust, I find it advisable that the planes should be as thin as possible, and the underneath side nearly flat. I have also found that it is a great advantage to arrange the planes after the manner shown in [Fig. 87]. In this manner the sum of all the spaces between the planes is equal to the whole area occupied by the planes; consequently, the air neither has to be compressed, spun out, nor driven forward. I am, therefore, able by this arrangement to produce a large lifting effect per square foot, and, at the same time, to keep the screw thrust within reasonable limits.

Fig. 87.—The position of narrow aeroplanes arranged in such a manner that the air has free passage between them, and this arrangement has been found superior to arranging one above the other after the manner of a Venetian blind.

A large number of experiments with very narrow aeroplanes have been conducted by Mr. Horatio Philipps at Harrow, in England. [Fig. 88] shows a cross-section of one of Mr. Philipps’ planes. Mr. Philipps is of the opinion that the air, in striking the top side of the plane, is thrown upwards in the manner shown, and a partial vacuum is thereby formed over the central part of the plane, and that the lifting effect of planes made in this form is therefore very much greater than with ordinary narrow planes. I have experimented with these “sustainers” (as Mr. Philipps calls them) myself, and I find it is quite true that they lift in some cases as much as 8 lbs. per square foot,[13] but the lifting effect is not produced in the exact manner that Mr. Philipps seems to suppose. The air does not glance off in the manner shown. As the “sustainer” strikes the air two currents are formed, one following the exact contour of the top, and the other that of the bottom. These two currents join and are thrown downwards, as relates to the “sustainer,” at an angle which is the resultant of the angles at which the two currents meet. These “sustainers” may be made to lift when the front edge is lower than the rear edge, because they encounter still air, and leave it with a downward motion.

[13] In my early experiments I lifted as much as 8 lbs. per square foot with aeroplanes which were only slightly curved, but very thin and sharp.

Fig. 88.—The very narrow aeroplanes, or sustainers, employed by Mr. Philipps. It has been supposed that the air in striking at A was deflected in the manner shown, but such is not the case. The air in reality follows the surface, as shown in the dotted line in the second illustration.

In my experiments with narrow superposed planes, I have always found that with strips of thin metal made sharp at both edges and only slightly curved, the lifting effect, when considered in terms of screw thrust, was always greater than with any arrangement of the wooden aeroplanes used in Philipps’ experiments. It would, therefore, appear that there is no advantage in the peculiar form of “sustainer” employed by this inventor.