174 SMITHSONIAM CONTRIBUTIONS TO KNOWLEDGE VOL. '27 



and solders which could be made to work with the aluminum-bronze, but the 

 final result was that the aluminum-bronze thimbles had to lie abandoned. They 

 were replaced by similar castings of gun-metal of a slightly heavier section, 

 which at the time were thought to he very suitable for the purpose. 



But, in finally assembling the frame after the changes described above had 

 been made, steel thimbles, built up of short pieces of tubing, as had originally 

 been planned, were substituted for the gun-metal thimbles. This change was 

 made not only because of the great increase in strength, hut more particularly 

 because many of the gun-metal fittings had been imperfectly constructed, so 

 that it was extremely difficult to align the frame. The steel thimbles, which 

 were made in the Institution shops proved thoroughly satisfactory and gave no 

 trouble of any kind. Many of these thimbles and the method of attaching the 

 guy-wire fittings to them are shown in Plates 56 and 57, as well as in Plate 55. 



Transverse Frame 



It will be recalled from the description of the models Nos. 5 and 6, in Part 

 I, that the position of the line of thrust, with respect to the positions of the 

 center of pressure and center of gravity in the vertical plane was, theoretically, 

 very much better in No. (5 than in No. 5. In designing the large aerodrome, 

 it was desired to reproduce as nearly as possible the relative position of the line 

 of thrust with reference to the center of pressure and center of gravity which ex- 

 isted in No. 6, but for constructional reasons it was found impossible to do so. In 

 fact it appeared that without seriously complicating the construction of the frame 

 it was impossible to raise the line of thrust with respect to the center of gravity 

 materially higher than it was in No. 5. In No. 6 the line of thrust was 12 centi- 

 metres above the midrod, this being effected by placing the engines some dis- 

 tance from the boiler, and at the extreme ends of the transverse frame where 

 they were connected directly to the propellers. In the case of the steam engine 

 the weight of the engine proper is a relatively small portion of the entire 

 weighl of the power plant, and it is, therefore, possible to put the engine al- 

 most anywhere without materially affecting the center of gravity. But where 

 a gas engine is used the engine itself constitutes the greater part of the weight 

 of the power plant, and any raising of the engine, therefore, materially raises 

 the .■enter of gravity of the whole machine. The line of thrust in the large 

 aerodrome was. therefore, practically in the plane of the main frame, and con- 

 sequently very little higher than the center of gravity. 



The use of one engine to drive two propellers mounted at opposite ends 

 of the transverse frame, and in a direction perpendicular to the crank shaft of 

 the engine, aecessitates the use of a pair of bevel ,n'oars between each of the 

 propeller shafts and the shafts by which the power is conveyed to them from 



