[Fig. 18 enlarged] (55 kB)

Fig. 19.—Shows the form of the blade of a screw propeller made of sheet metal. It is riveted at the edges and also to the arm of a screw with a stiffening piece at the extreme end. However, it is not necessary to rivet edges together. They may be welded with a name of acetylene oxygen gases.

Fig. 19a.—Shows the manner of welding and the finished edge.

Fig. 20.—A new form of hub, of great strength and lightness, for use on flying machines.

In the drawing ([Fig. 18]) I have shown screw blades of a proper shape to give the best results—that is, providing a metallic screw is employed. Instead of having the arm of the screw on the back of the blade to offer resistance to the air, the arm should be tubular, flattened, and covered on both sides with sheet metal. This particular formation not only prevents the air from striking the arm, but, at the same time, prevents the pressure of the air from deforming the blade, so, if a metallic screw is to be used, the form of blade which I have shown will be found much superior to that employed at the present time on continental flying machines. We should not lose sight of the fact that weight tells very seriously against the success of a flying machine, and that no expense should be spared to reduce the weight, providing that it is possible to do so without reducing the factor of safety. Suppose, for example, that we use an ordinary hub secured to a solid shaft by a common key. All the parts have to be made heavy in order to be sufficiently strong to withstand the strain. In the drawing ([Fig. 20]) I have shown a hub which I think is quite as light and strong as it is possible to make it. The action of the motor is often spasmodic and puts very great strain upon the parts, and there is a very strong tendency for the shaft to turn round in the hub. If a key is used, the hub has to be large and strong, and the key of considerable size, otherwise the parts would be deformed. In my own experiments, I have found considerable difficulty in securing a shaft to wooden screws. However, it will be seen in the drawings that a series of grooves is cut in the shaft and that the hub has internal projections, so that the one fits the other. This makes a very strong connection and is of extreme lightness. Both the hub and the shaft should be of tempered steel. The spokes should be hard drawn steel tubes with long fine threads, so as to withstand centrifugal force. To prevent them from rotating in the hub, the nuts d, d are provided, which compress the arms of the steel hub so as to grip the tube with any degree of force required. It will be seen that with this system the pitch of the screw may be adjusted to some extent; however, it is better to have all parts of the screw, from hub to centre, of the same pitch. A slight deviation from this is admissible in the experimental stage, so long as the deviation from a true screw, caused by rotating the arm, is not greater than one half of the slip while in flight.

Many experimenters have imagined that a screw is just as efficient placed in front of a machine as at the rear, and it is quite probable that, in the early days of steamships, a similar state of things existed. For several years there were steamboats running on the Hudson River, New York, with screws at their bows instead of at their stern. Inventors of, and experimenters with, flying machines are not at all agreed by any means in regard to the best position for the screw. It would appear that many, having noticed that a horse-propelled carriage always has the horse attached to the front, and that the carriage is drawn instead of pushed, have come to the conclusion that, in a flying machine, the screw ought, in the very nature of things, to be attached to the front of the machine, so as to draw it through the air. Railway trains have their propelling power in front, and why should it not be the same with flying machines? But this is very bad reasoning. There is but one place for the screw, and that is in the immediate wake, and in the centre of the greatest atmospheric disturbance. While a machine is running, although there is a marked difference between water and air as far as skin friction is concerned, still the conditions are the same as far as the position of the screw is concerned. With a well-designed steamship, the efficiency of the screw is so great as to be almost unbelievable; in fact, if a steamship had never been made, and the design of one should be placed before the leading mathematicians of to-day, with the request that they should compute the efficiency of the screw, none of them would come anywhere near the mark. They would make it altogether too small. As before stated, when a steamship is being driven through the water, the water adheres to its sides and is moved forward by the ship—that is, it has acceleration imparted to it which exactly corresponds to the power consumed in driving the ship through the water. This, of course, retards it and we find in a well-designed ship, not run above its natural speed, that about 80 per cent. of the power of the engine is consumed in skin friction, or in imparting a forward motion to the water. Suppose that we should take such a ship, remove the screw, and tow it through the water with a very long wire rope at a speed of, say, 20 miles an hour; we should find that the water at the stern of the ship was moving forward at a velocity of fully 6 miles an hour—that is, travelling in the same direction as the ship. By replacing the screw, and applying engine power sufficient to give the ship the same speed of 20 miles an hour, identical results would be produced. The skin friction still impels the water forward, so that the screw, instead of running in stationary water, is actually running in water moving in the same direction as the ship at a velocity of 6 miles an hour. If the slip of the screw should only be equal to this forward motion, the apparent slip would be nothing; in fact, the ship would be moving just as fast as it would move if the screw were running in a solid nut instead of in the yielding water. Curiously enough there have been cases of negative slip in which the actual slip of the screw in the water was less than the forward movement of the water, and in such cases a ship is said to have negative slip. A very noticeable case of this kind occurred in the Royal Navy in the sixties.[1] I was at the time engaged in a large shipbuilding establishment in New York, and remember distinctly the interest that the case created amongst the draughtsmen and engineers of that establishment. Of course, this apparently impossible phenomenon created a great deal of discussion on both sides of the Atlantic. It appears that this ship had been built under an Admiralty Specification which called for a screw of a certain diameter and pitch with a specified number of revolutions per minute, and for a certain number of knots per hour, also that the boiler pressure should not go above a certain number of pounds per square inch. When the ship was finished and went on her trial trip, it was found impossible to make the full number of turns called for in the specification with the boiler pressure allowable; nevertheless, the speed was greater than the specification called for, and as speed was the desideratum, and not the number of revolutions, the contractors thought their ship should be accepted. Then arose a discussion as to the diameter and pitch of the screw. It was claimed that a mistake must have occurred. A careful measurement was made in the dry dock, and all was found correct. Once more the ship was tried, and again her speed was in excess of the specification, notwithstanding that it was still impossible to get the specified number of revolutions per minute. Mathematicians then took the matter in hand, and it was found that the ship actually travelled faster than she would have done if the screw had been running in a solid nut. Instead of a positive slip, the screw had in reality a negative slip; but this was not believed at the time, and the discussion and controversy continued. The ship was tried again and again, and always with the same results. This apparently inexplicable phenomenon was accounted for in the following manner:—The hull of the ship was said to be rather imperfect and to cause a considerable drag in the water, so that, when the ship was moving at full speed, the water at the stern had imparted to it a forward velocity greater than the actual slip.

[1] The particulars relating to this event are taken from accounts published at the time in American papers.