J42 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 27 



the delays caused by the lack of such small accessories which are now so easily 

 procurable were very exasperating, and it was finally necessary to insulate these 

 wires by covering them with several thicknesses of ordinary rubber tube of 

 different diameters telescoped over each other. 



In the early tests of this new engine, which were made with it mounted on 

 a special testing frame and delivering its power to the water-absorption dyna- 

 mometers, the engine was operated without any fly wheels, and, so far as its 

 smoothness of operation was concerned and its ability to generate its maximum 

 power, it did not require any. 



After the completion of the tests on the testing frame the engine was as- 

 sembled in the aerodrome frame, which was first mounted on the floor of the 

 launching car. The car itself was mounted on a short track in the shop, which 

 arrangement provided a smoothly rolling carriage which could be utilized for 

 measuring the thrust of the propellers by merely attaching a spring balance be- 

 tween the rear of the car and a proper holding strap on the track. In the first 

 tests of the engine under these conditions, it was found that while the engine 

 itself did not require any fly wheels, yet the lack of them caused trouble with 

 the transmission and propeller shafts, which, while it. had never been antici- 

 pated, was easily understood when it was encountered. This difficulty was 

 caused by the " reverse torque," which fluctuated from a maximum to a mini- 

 mum five times during each double revolution of the engine, and which set up 

 fluctuating torsional strains of such magnitude in the transmission and propel- 

 ler shafts that the shafts themselves became exceedingly hot after a few min- 

 utes operation of the engine, and under more prolonged periods of operation 

 these fluctuating torsional strains caused a permanent twisting and bending of 

 the shafts. The transmission and propeller shafts were at first made of tubing 

 one-sixteenth of an inch thick, but these were abandoned both on account of the 

 necessity of abandoning the screw-thread method of attaching the flange coup- 

 lings and gears, and also because these shafts had been designed when it was 

 expected to transmit only twelve horse-power to eacli propeller, while the in- 

 crease of power in the large engine necessarily required much stronger shafts. 

 The first shafts which were actually tested in the frame were, therefore, one 

 and one-half inches in diameter by three-thirty-seconds of an inch thick, (he tub- 

 ing having been one-thirty-second of an inch larger originally and turned down 

 to this size to Insure a straight shaft. When these shafts twisted under the 

 action of the reverse torque of the engine, a very much heavier set, practically 

 twice as thick, were constructed. When used in the tests these heavier shafts, 

 while much stronger, still showed a large amount of heating due to the fluctu- 

 ating torsional strains. 



Upon calculation it was found that by providing specially light fly wheels 

 the major portion of this reverse torque could be eliminated for a less increase 



