The large propeller used in these tests was built without special regard to weight, since it was expected that it would be subjected to rather rough usage under the very sudden strains produced by the irregular working of the gas engine. Its hub was made of brass tubing, the horns being brazed to rings which were slid over a central tube, the rings being finally soldered to the tube after the arms had been adjusted to the positions which would give the blade the correct shape and dimensions. The wooden arms were 1.5 inches in [p180] diameter at the hub end, tapering to 1.25 inches at the end of the blade. The blade was exceedingly stiff as regards pressure produced by thrust, but it was found to be considerably strengthened and made very much safer when guy-wires were added, in the manner explained above. This general type of construction was adhered to in all the future propellers for the aerodrome, though slight modifications, both as to the size of the arms and the number and position of the cross-pieces which formed the framing of the blade, were adopted from time to time. A pair of heavy propellers, 2.5 metre, 1.25-pitch ratio, 36-degree blade, the hubs of which were formed of brass castings, was, however, constructed for experimental purposes, where weight was not an important factor.

When these propellers were designed, the calculations as to their size and the horse-power which would be required to drive them at a certain speed were based on the very incomplete data obtained from the various propeller tests conducted during the preceding years. When later calculations were made for them, on the data obtained in the more accurate tests made in the summer of 1898, it was found that the power of the engines with which it was proposed to equip the aerodrome would not be sufficient to drive the propellers at anything like the speed which the former calculations had shown would be possible; and that, therefore, either the ratio of the gearing between the propellers and the engine would have to be changed so as to permit the engine to run at a very much higher speed than the propellers, or that propellers, having either less pitch or a smaller diameter, and possibly both, would have to be substituted for these larger ones.

Since it was easier to change the propellers then to change the gearing, a new set of propellers was designed which were of 2 metres diameter, with a pitch ratio of unity, and with a width of blade of only 30 degrees. It was calculated that 20 horse-power would drive these two propellers at a speed of 640 R. P. M., when the aerodrome was flying at a speed of 35 feet per second and the propellers were slipping about 50 per cent, this being found to be about the speed at which the engines might be expected to develop their maximum power. As the larger propellers having the brass hubs were thought to be excessively heavy, the hubs weighing 10.25 pounds each, and as any change either in size, pitch, or width of blade necessitated a new set of patterns in case the hubs were cast, it was decided to construct the new hubs of steel tubing. The weight was further reduced by decreasing the size of the wooden arms to 114 inch in diameter at the hub, tapering to 1 inch at the end of the blade.

After the engine builder in New York had been unable to fulfil his contract on the engine, and it had been condemned, propeller tests were made with the experimental engine built in the Institution shops. These tests showed: First, that the results which might be expected from larger propellers could be very safely predicted by extrapolation from the results of the propeller tests of 1898; [p181] and, second, that in order to get a thrust which would equal fifty per cent of the flying weight of the aerodrome it would be necessary to use propellers larger than two metres in diameter unless a very large surplus of power were provided. It was accordingly decided to make a set of propellers intermediate between the two-metre, unit-pitch ratio, thirty-degree blade ones, and the original ones which were two and one-half metres, one and one-quarter-pitch ratio, thirty-six-degree blade. A set was, therefore, designed two and one-half metres in diameter, unit-pitch ratio, and thirty-degree width of blade, the hubs being made of steel tubing brazed up in the same manner as the two-metre ones, and the wooden arms of the blades being one and three-eighths inches in diameter at the hub end, and tapering to one inch at the end of the blade.

Later, when the larger engine was actually tested in the frame, the inability of the original transmission and propeller shafts to stand the extra strain caused by the engine starting up very suddenly at times, together with the unsatisfactoriness of the screw-thread method of fastening the gears and couplings to the shafts made it necessary to provide new shafts, gears, couplings, etc. It was then decided to change the ratio of gearing between the engine and the propellers, which had been one to one, so that the engine might run faster and, therefore, permit the use of larger propellers. For constructional reasons the ratio chosen was thirty-one to forty, thus making the engine run approximately one-third faster than the propellers.

In the various tests made of the engine working in the frame there were two or three instances in which the propellers were damaged either by the sudden starting of the engine or by their not being able to stand the strain to which they were subjected by the power absorbed, but in every case such breakages were found to be due to imperfections of the brazing in the joints. While, therefore, it would have been desirable to make the propellers somewhat heavier, yet since the total weight of the aerodrome had been growing so very rapidly, it was felt that this need not be done, as a pair of propellers which had stood quite severe service in shop tests might reasonably be expected to stand the strain of actually propelling the aerodrome through the air.

Nevertheless, when in the summer of 1903 the actual trials of the large aerodrome were started, it was found that the very important difference between a propeller working in a closed room and one working in the open air had not been given due consideration. Several sets of propellers, 2.5 metres in diameter, unit-pitch ratio, 30-degree blade had been constructed and were on hand, in order that no delays might be caused through a lack of such extra parts. On September 9, 1903, when the aerodrome frame without the wings was mounted on the launching car on top of the boat for some trial runs with the engine to make sure that everything was again in readiness, before the engine had made 500 revolutions, the port propeller broke; and a few minutes [p182] later, when a new propeller had been substituted for this and the engine was again started up, the starboard propeller also broke. When, upon further trials and replacements of propellers, all had been so thoroughly demolished that there was not a complete set remaining, it was seen very clearly that the strains produced on a propeller working in the open air are very much greater than those produced in shop tests, where the air is necessarily quiet. These open-air tests of the propellers had demonstrated that their weakest point was where the steel tubes which received the wooden arms of the blade terminated, and that another, though not so serious, point of weakness was where the steel arms were brazed to the central hub, the thin metal tending to tear loose even before the brazed joint would give way. It was, therefore, decided to construct immediately a new set of propellers in which the steel arms should be made of much heavier tubing, that is, a sixteenth of an inch thick at the end where it was brazed to the central hub, and tapering in thickness to one-thirty-second of an inch at the other end. These arms were further made twelve inches long in place of being only three inches long as before. This added length carried the steel out beyond the point where the first section brace joined the three arms together, and where they were further strengthened by having the cloth covering tightly stretched around them. In order to utilize such of the hubs of the former propellers as had not been seriously damaged when the propellers broke, it was also decided to try the effect of merely adding an extra length of tube to the short arms by means of a thimble slipped over and brazed to the two parts, which would make these arms twelve inches long. The construction of these propellers was pushed as rapidly as possible; and after their completion no further trouble was at any later time caused by insufficient strength of the propellers. Even in the test of October 7, 1903, when the aerodrome came down in the water at a speed of something like fifty miles an hour, and at an angle of approximately forty-five degrees, no break occurred in either propeller until, when the aerodrome was plunging through the water, a blade of one propeller was broken by the terrific blow which it received when it struck the water under the impulse of the engine driving it at full speed. The severity of this blow is attested by the fact that the shaft, which was of steel tubing one-eighth inch thick, was twisted about ninety degrees.

This experience with propellers very strongly emphasizes the fact that on any flying machine the strains which are apt to be met with in the open air must be allowed for in the proportioning of the parts of the machine. But since an indiscriminate increase of strength in all the various parts of the machine would entail a prohibitory weight, very careful judgment, based on experience, will have to be exercised in deciding just where added strength must be employed, and also where the “live strains” are not apt to exceed very appreciably the calculation for statical conditions.

PL. 59. WING CLAMPS [◊] [lgr]