Artificial and Natural Flight

by Hiram S. Maxim

Having designed and put my boiler and engine in hand, I commenced a series of experiments for the purpose of ascertaining the efficiency of screw propellers working in the air, and the form and size that would be best for my proposed machine. The illustration [Fig. 9] shows a photographic group of the screws and other objects with which I experimented. [Fig. 10] shows some of the leading types which, as will be seen, have blades of different shape, pitch, and size. [Fig. 11] shows three of the best screws employed. It will be observed that one has uniform pitch, another increasing pitch, and the third compound increasing pitch. In order to test the efficiency of my screws I made the apparatus shown in [Fig. 12]. The power for running the screw was transmitted by means of a belt to the straight cylindrical pulley c, c. Shaft b, b was of steel, rather small in diameter, and ran smoothly, and practically without friction, through the two bearings d, d. When the first screw, a, a, was run at a high velocity, the axial thrust pushed the shaft b, b back and elongated the spiral spring e. The degree of screw thrust was indicated in pounds by the pointer g. The power was transmitted through a very accurate and sensitive dynamometer, so that the amount consumed could be easily observed by a pointer similar to the one employed for indicating the screw thrust. A tachometer was also employed to observe the number of turns that the screw was making in a minute. The whole apparatus was carefully and accurately made and worked exceedingly well. I was thus enabled, with my various forms of screws and other objects, to make very accurate measurements, some of which are exceedingly interesting.

Fig. 12.—Apparatus for testing the thrust of screws—a, a, the screw; b, b, shaft sliding freely in the bearing d, d; c, cylindrical pulley; e, spiral spring; f, steel rod; g, pointer for indicating the thrust in pounds.

Fig. 13.—Apparatus for testing the direction of air currents caused by a rapidly rotating screw. Silken threads were attached to the wire c, c, which indicated clearly the direction in which the air was moving.

In many of the treatises and books of that time it was stated that a screw propeller, working in the air, was exceedingly wasteful of energy on account of producing a fan-blower action. Some inventors suggested that the screw should work in a stationary cylinder, or, better still, that the whole screw should be encased in a rotating cylinder, to prevent this outward motion of the air. In order to ascertain what the actual facts were, I attached a large number of red silk threads to a brass wire, which I placed completely around my screw (see [Fig. 13]). Upon starting up I found that, instead of the air being blown out at the periphery of the screw, it was in reality sucked in, as will be seen in the illustration. I was rather surprised to see how sharp a line of demarkation there was between the air that was moving in the direction of the screw and the air that was moving in the opposite direction. The screw employed in these experiments was 18 inches in diameter and had a pitch of 24 inches. It was evident, however, if the pitch of the screw was coarse enough that there would be a fan-blower action. I therefore tried screws of various degrees of pitch, and found when the pitch was a little more than three times the diameter, giving to the outer end of the blade an angle of 45°, that a fan-blower action was produced—that is, part of the time when the screw was running, the air would alternate; sometimes it would pass inwards at the periphery and sometimes outwards. The change of direction, however, was always indicated by a difference in the pitch of the note given out, and also by the thrust. In [Fig. 14] I have shown the extremities of the blades of some of the different forms of screws experimented with, in which a shows a plain screw, the front side being straight and of equal pitch from the periphery to the hub; b is a screw of practically the same pitch, but slightly curved so as to give what is known as an increasing pitch; c shows the extremity of a screw in which the curve is not the same throughout—that is, it is what is known as a compound increasing pitch; d is the shape of the screw that gave the angle of 45° above referred to.

Fig. 14.—This drawing shows the ends of screw blades in which a is a plain screw; b, screw with increasing pitch; c, screw with compound increasing pitch; d, end of screw blade 45°; e, screw with very thick blade; f, blade with no pitch at all; g, blade which gave a thrust in the direction of the convex side, no matter in which direction it was revolved; h, screw said to have been used in the French Government experiments.

The first screw experimented with was a. This screw was run at a high velocity—about 2,500 revolutions per minute—until a screw thrust of 14 lbs. was obtained, and then the governor of the engine was set so that all screws of the same diameter could be run at the same speed. Wishing to ascertain the efficiency of the screw and how much was lost in skin friction, I multiplied the thrust in pounds by the pitch of the screw in feet and by the number of turns it was making in a minute. This, of course, gave the exact number of foot-pounds in energy that was being imparted to the air. I was somewhat surprised to find that it corresponded exactly with the readings of the dynamometer. I thought at first that I must have made some mistake. Again I went very carefully over all the figures, tested everything, and made another experiment and found, even if I changed the number of revolutions, that the readings of the dynamometer were always exactly the same as the energy imparted to the air. This seemed to indicate that the screw was working very well and that the skin friction must be very small indeed. In order to test this, I made what we will call, for the moment, a screw without any pitch at all—that is, the blades were of wood and of the exact thickness and width of the blades of the screw a, but without any pitch at all. The extremity of the blade is shown at f. I placed this screw on my machine in place of a, and although my dynamometer was so sensitive that the pointer would move away from the zero pin by simply touching the tip of the finger to the shaft, it failed to indicate, and thus the screw appeared to consume no power at all. These experiments were repeated a considerable number of times. I then obtained a sheet of tin the same diameter as the screws, 18 inches, and upon running it at the same speed, I found that it did consume a measurable amount of power, certainly more than the two blades f. This no doubt was due to the uneven surface of the tin. Had it been a well-made saw blade without teeth, perfectly smooth and true on both sides, it probably would not have required power enough to have shown on the dynamometer. However, it is quite possible that there is a little more skin friction with a polished metallic surface, than with a piece of smooth evenly lacquered wood. The screws which I employed were of American white pine such as used by patternmakers. This wood was free from blemishes of all kind, extremely light, uniform, and strong. When the screw had been formed, it was varnished on both sides with a solution of hot glue, which greatly increased the strength of the wood crosswise of the grain. When this glue was thoroughly dry, the wood was sand-papered until it was as smooth as glass; the whole thing was then carefully varnished with shellac, rubbed down again and revarnished with very thin shellac something like lacquer. In this way the surface of the screw was made very smooth. The screws, of course, were made with a great degree of accuracy and as free as possible from any unevenness. Having tested screw a, I next tested screw b. I found with the same number of revolutions per minute that this screw produced more thrust, but it required more power to run it, and when the energy imparted to the air was compared with the readings of the dynamometer, it was found that it did not do quite so well as a; still as the thrust was greater and the efficiency only slightly less, it appeared to be the better screw. Upon trying screw c, under the same conditions, the thrust was very much increased, but the power required was also increased to a still greater degree, showing that this form was not so favourable as either a or b. All the screws experimented with had very thin blades, and it occurred to me that the difference between a and b might arise from the fact that, when a was running at a very high velocity, the working side instead of being flat might have become convex to a slight extent, whereas with b, a slight bending back of the edges of the blade would still leave the working side concave. I therefore made the screw shown at e, which had the same pitch as the other three, but the working side was of the same shape as a. Of course the additional thickness of the blades made it impossible to give an easy curve to the back. Curiously enough I found that e did nearly as well as a, and quite as well as b. The additional thickness did not interfere to any appreciable extent with its efficiency. I then made another propeller, shown at g, which was of the same thickness in the middle as e. Upon running this, I found that it required considerable power, and no matter which way it was run, the thrust was always in the direction of the convex side, which was quite the reverse from what one would have naturally supposed.