In the early photographs of the aerodrome frame, especially that of January 31, 1900, Plate [45], it will be noted that the two transmission shafts, which extend from the propeller-shaft bed plates towards the center, are not in line, the port transmission shaft being at the center of the transverse frame, while the starboard shaft is three inches to one side. This arrangement was necessary in order to connect the shafts to the rotary cylinder engine which was being constructed under contract, and which was almost momentarily expected for more than a year after its original promise of delivery on February 28, 1899. Later, when this engine was finally found to be a failure, and the writer constructed the engine in the Institution shops, the starboard transmission shaft was moved over to the center line and the crank shaft of the engine, which was carried through on the center line of the transverse frame, was then connected directly to the inner ends of the transmission shafts.

These shafts, as well as the propeller shafts, were originally constructed of steel tubing 1.5 inches in diameter and 116 of an inch thick, but on account of the increased power of the large engine it was found necessary to increase the thickness of the shafts to 18 of an inch. Difficulty was also found with the tubing of which the shafts were made. This, though not exactly straight when received from the factory, could be pretty accurately straightened in the lathe by exercising proper care, but the moment any real strain was put upon it in the transmission of power, it again went out of shape and caused serious damage to the bearings by whirling, buckling, and so forth. As the skin of the tubing is really the strongest part, owing to the cold-drawing process to which it has been subjected, great care was taken to secure shafts which were sufficiently straight for use without machining, but it was finally found impossible to rely on the unmachined shafts, and all the later shafts for the aerodrome were made by getting tubing a sixty-fourth of an inch thicker than was calculated to be necessary and turning off this extra metal in a lathe.

PL. 58. BEDPLATE, GEARS, ETC. [◊] [lgr]

Suitable flanges and collars were brazed to the propeller shafts; but, for convenience in assembling, the flanges by which the main transmission shafts were connected to the crank shaft of the engine were at first fastened to the shafts by screw-threads, the threads being in the proper direction to cause the flanges to jam against the shoulders of the shafts when the engine turned in its normal direction. This method of fastening, however, caused serious trouble, owing to the flanges jamming so tight that it became impossible to unscrew them after they had once been used in driving the propellers. The usual provisions of keys and key-ways adopted in general engineering practice, where solid shafts are employed, were, of course, out of the question, since the shaft would have to be greatly increased in thickness throughout its entire length [p177] merely to provide the extra metal at the small place in which the key-ways were formed. Taper pins either sheared off or very soon stretched the holes so badly as to leave the parts loose, and were otherwise very unsatisfactory. The method finally adopted, which proved very successful, was that of forming integral with the couplings shallow internal tongues and grooves which fitted corresponding tongues and grooves either in the exterior surface of the shafts or in collars brazed to them at the proper point. The form of flange coupling, in which bolts draw the two flanges tightly together, was also a source of considerable trouble and delay, which was finally overcome by forming shallow tongues and grooves in the faces of the flanges, the tongues taking up the torsion and relieving the bolts which held the flanges together of all strain except one of slight tension. The same difficulties experienced in mounting the couplings on the shafts were met with in connection with the gears, both on the propeller and transmission shafts, and were finally obviated in a manner similar to that described above.

The bevel gears originally constructed for transmitting the power from the transmission shafts to the propeller shafts, were made of case-hardened steel and were eight-pitch, twenty-five teeth, with three-quarter inch width of face. The gears were very accurately planed to give as perfect a form of tooth as possible, in order to avoid loss of power in transmission, and although the manufacturer who cut the teeth on them asserted at the time they were made that they would not be capable of transmitting more than five horse-power, yet they actually did transmit considerably more than twelve horse-power on each set; but they were not strong enough to transmit the full power of the large engine which was finally used. The gears that were finally used were similarly constructed of mild steel which was case hardened 164 of an inch deep after they were finished, there being thirty-one teeth in the gear on the transmission shaft and forty teeth in the one on the propeller shaft, the teeth being eight-pitch, three-quarters of an inch face. These light gears proved amply strong, and several times stood the strain which they accidentally received when one of the propellers broke while the engine was under full power, and thus threw the entire fifty horse-power over on the other propeller, which was consequently driven at a greatly increased speed.

Plain bronze bearings had been used throughout on the model aerodromes, but in the construction of the large aerodrome ball-bearings were used on all of the propeller and transmission shafts, not only on account of the decreased loss through friction, but also because ball-bearings can be built much lighter than solid bronze ones, and, furthermore, do not present such great difficulties in lubrication. However, owing to the limited size which it was possible to secure for these bearings, because of their having been originally designed for only twenty-four horse-power, and without any margin for a later increase of the [p178] space in which they had to be applied, they were never really large enough for the work they had to do when transmitting the full power of the large engine. They gave continual trouble, and were the source of delay which, while it cannot be accurately measured, since there were often other causes, yet might be conservatively estimated at not less than three or four months. Such a delay, when reckoned in retrospect, can easily be seen to have caused an expense which would have sufficed for almost any change in the bearings, bed plates, etc., had the change been made immediately after the bearings were found to give trouble. With the better steel which it is now possible to obtain for the races of the bearings, and with the high-grade balls now obtainable, the bearings could be readily replaced without changing any other parts and still be amply strong for the work.

PROPELLERS

Both the tests on the whirling-table and the actual results with the models had shown that propellers which were true helices formed out of wood were rather more efficient than those constructed by the use of a hub in which were inserted wooden arms, forming a framing over which cloth was tightly drawn. But the very great difference in the cost of construction and the facility with which the latter type could be repaired in case of damage—the wooden ones were practically of no use if once they were much injured-—made it seem advisable to construct all the propellers for the large aerodrome in the manner just explained. Several pair of small propellers had been built on this plan, some as early as 1895, and one very important advantage had been found to be possessed by this type besides cheapness and facility of repair. Wooden propellers of even so small a diameter as one metre had been found to suffer a quite appreciable bending of the blades, due to the thrust produced by them, even though the blades had been made of considerable thickness. In planning a propeller 2.5 metres in diameter for the large aerodrome it was seen that in order to make the blade sufficiently strong to withstand its own thrust it would be necessary to make it inordinately thick, which, of course, would mean a considerable increase in weight. In fact, it was seen that the weight of the larger propellers would increase practically as the cube of the diameter; which, for the 2.5-metre propeller, would involve a weight of something over fifteen times the weight of those one metre in diameter. The other type, which for convenience we will call “canvas covered,” permitted the bending moment produced on the blade by the thrust to be taken up by guy-wires running from the corners of the blades to a central post projecting from the hub of the propeller, and it was found that in this way a considerable saving in weight could be effected. [p179]

In November, 1897, in order to obtain by actual test some data on propellers, such as it was planned to use on the large aerodrome in case it was later built, it was decided to construct one propeller 2.5 metres in diameter and 1.25-pitch ratio with two blades, each covering the sector of 36 degrees on the projected circle. About this same time an engine builder, who some years before had made some experimental model engines in the Institution shops, proposed to construct a gasoline engine for the proposed large aerodrome. As past experience, not only with such engines but with all other forms of explosive motors, had not been very reassuring it was thought best to make brake tests of one of the heavier engines which he was at this time building, and at the same time make tests with one of these large propellers. A first series of tests was made at several different speeds, and then a second series was made with the engine driving the propeller at the same speeds. The engine varied so much, however, in the power developed at any speed that the data obtained were of little value. As it was also desired to learn just how much thrust could be obtained from these propellers, when driven by a given horse-power, a special hand car was fitted up to carry the engine, which was connected to a shaft on which the propeller was mounted. The propeller was raised above the floor of the car and projected over the rear end of it so as to be as little disturbed as possible by the deflection of the air currents caused by the car. This car, with the engine and propeller, was tested on a track near Mount Holly, N. J., in November, 1897, but the results were very unsatisfactory. In the first place, the car with the engine mounted on it was so very heavy and offered such a strong tractive resistance that very little speed of propulsion could be obtained. In the second place, the engine, which was said to have furnished over six horse-power on Prony-brake tests, evidently did not furnish anything like this amount of power at this time. And in the third place, the propeller was evidently far too large to permit the engine to run at the speed at which it would develop a reasonable amount of power unless some reduction gearing were interposed between it and the propeller. As the tests, for various reasons, had to be made at a great distance from Washington, and the supervision of them had to be entrusted by Mr. Langley to others, who either did not understand or appreciate the value of obtaining accurate data, it was found impracticable to continue them.