NO. 3 LANGLEY MEMOIR ON MECHANICAL FLIGHT 243 



in weight than would be occasioned by sufficiently increasing the thickness of 

 the transmission and propeller shafts to safely stand it. Since it was desired 

 to concentrate as much as possible of the weight of the fly wheels in the rims, 

 the idea at once suggested itself of building them up like a bicycle wheel by 

 means of tangent spokes. Two steel automobile-wheel rims were therefore pro- 

 cured thirty-three inches in diameter, and these were provided with tangent 

 spokes connected to special steel hubs fitted to the crank shaft of the engine. 

 The rims themselves not being quite heavy enough, and constructional reasons 

 necessitating their being at different distances from the center of length of the 

 crank pin, the extra weight which it was desired to give to these rims was pro- 

 vided by means of steel wire wound tightly around and fastened to the rims, 

 the weight of each rim being made inversely proportional to its distance from 

 the center of the crank pin. The first spokes which were used for these wheels 

 were standard bicycle spokes three-thirty-seconds of an inch in diameter, but 

 these were soon found to be entirely too weak to withstand the sudden strains 

 due to the rapid starting of the engine. They were therefore replaced by stand- 

 ard spokes one-eighth of an inch in diameter, but these also proved too weak 

 and were later replaced with special spokes made in the shop out of No. 10 

 coppered-steel wire, which by test was found to have a tensional strength of 

 2192 pounds. As these steel rims were only one-sixteenth of an inch thick and 

 had not been made exactly true, but had been straightened before being used, 

 it was found that they very quickly went out of shape under the strain due to 

 the centrifugal force at high speeds, and also when the engine was suddenly ac- 

 celerated. As long as they did stay true, however, it was found that they were 

 sufficiently heavy to provide all of the fly-wheel effect it was necessary to have 

 in order to eliminate all trouble from the reverse torque. 



After further consideration, it was decided that the only means of con- 

 structing a fly wheel which would have a stiff rim and at the same time would 

 not be heavier than the steel ones, which had been found adequate, was by per- 

 petrating what would at first sight appear to be an absurdity. A new set of 

 rims for the fly wheels was made by constructing them of an aluminum cast- 

 ing, the section of the rim being U-shaped. After machining these rims and 

 assembling the fly wheels with them, it was found that they were many times 

 stiffer than the previous steel ones of the same weight, and after this change no 

 further trouble was experienced in keeping the fly wheels perfectly true, even 

 under the most severe strains. In fact, on one occasion when the engine broke 

 loose from the propellers, it ran to a speed, which, while not exactly known, 

 yet reached the limit of the tachometer, which was 2000 R. P. M., without in- 

 jury to the fly wheels. 



It will be recalled that in starting up the engine on the quarter-size model, 

 the initial " cranking " necessary with a gasoline engine was accomplished by 



