210 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. '-( 



With a gasoline engine, however, the conditions are very greatly altered. 

 Eere the engine constitutes practically the entire weight of the power plant, 

 only such accessories as the ignition coil, batteries, and carburetor being avail- 

 able for lowering the center of gravity, unless the fuel, cooling water tanks and 

 radiator be placed below the engine and the liquids forced up by means of a 

 pump. In making the first designs for the large aerodrome, therefore, it was 

 found that it would be practically impossible to make the relative positions of 

 the center of gravity and line of thrust the same as had existed in model No. 

 6, however desirable it might be. The center of gravity could be brought ap- 

 preciably lower than the line of thrust only by placing the gasoline engine in 

 a plane considerably below that of the propellers, and this necessitated the ad- 

 dition of at least two more sets of gears with heavy hearings and braces. Be- 

 sides this almost prohibitive factor of weight, it was also foreseen that great 

 difficulty would be experienced in keeping even the two sets of bevel gears al- 

 ready necessary aligned and in proper condition for efficiently transmitting the 

 power to the propellers unless the frame and other parts were made prohibitively 

 heavy. It was, therefore, found necessary to .bring the center of gravity prac- 

 tically in the same plane with the line of thrust, which made its general features 

 as regards equilibrium more nearly resemble those of model No. 5 than of 

 No. 6. 



The weight of the aviator, it is true, constituted an appreciable part of the 

 flying weight of the large machine, and it at first seemed possible to lower the 

 center of gravity by placing him at a considerable distance below the line of 

 thrust. But it was recognized from the beginning that the aviator would prob- 

 ably have to give a great deal of attention to any form of engine in order to 

 insure its working properly, and his position must, therefore, be selected with 

 a view to the proper supervision of the engine and without regard to its effect 

 on the center of gravity. 



Although the repeated successful flights of model No. 5 under varying con- 

 ditions of wind and power inspired the belief that the minor adjustments, as 

 well as the general plan of the large aerodrome, were such as to give highly 

 stable equilibrium, nevertheless, more direct corroboration of this opinion was 

 desired, and it was largely for this reason that tlie quarter-size model was con- 

 structed. In it every detail of the larger machine which in any way affected its 

 equilibrium was exactly reproduced to scale, and the greatest care was taken 

 that the same relative positions of the center of pressure, the center of gravity 

 and the line of thrust which it was proposed to employ for the large aero- 

 drome should be used on the model in its flight of August 8, 1903, which is 

 later described. The entire success of this flight, so far as the balancing was 

 concerned, in spite of the fact that the engine worked erratically and that the 

 launching speed was much less than it should have been, removed every doubt 



