16 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. '_ I 



EXFEBIMENTS WITH AeBODBOMES NoS. 30 AND 31. 



Remembering thai the principal object of all these experiments is to be able 

 In predicl that setting of the wings and tail with reference to the center of 

 gravity which will secure horizontal flight, we must understand that in 1lie fol- 

 lowing tables (see No. 30) the figures (7Pm=1516.5 cm. mean a prediction that 

 the center of pressure of the sustaining surfaces in motion (CP,,,) is to be found 

 in a certain position 1516.5; that is, 16.5 cm. in advance of the line joining the 

 propeller shafts. This prediction has been made by means of previous calcula- 

 tion joined with previous experimental adjustment. We know in a rongh way 

 where the CP will fall on the wings when they are exposed independently if flat, 

 and at a certain angle, and where it will fall on t lie tail. From these, we can 

 find where the resulting CP of the whole sustaining surface will be. 



It would seem that when we have obtained the center of gravity by a simple 

 experiment, we have only to slide the wings or tail forward and back until the 

 (calculated) center of pressure falls over this observed center of gravity. But 

 in the very act of so adjusting the wings and tail, the center of gravity is itself 

 altered, and the operation has to be several times repeated in order tit get the 

 two values (the center of pressure and center of gravity) as near each other as 

 they are found in the above-mentioned table, our object being to predict the posi- 

 tion which will make the actual flight itself horizontal. How far this result has 

 been obtained, experiment in actual flight alone can show, and from a comparison 

 of the prediction with the results of observation, we endeavor to improve the 

 formula. 



The difficulties of these long-continued early experiments were enhanced by 

 the ever-present difficulty which continued through later ones, that it was almost 

 impossible to build the model light enough to enable it to fly, and at the same 

 time strong enough to withstand the strains which flight imposed upon it. The 

 models were broken up by their falls after a few flights, and had to be continu- 

 ally renewed, while owing to the slightness of their construction, the conditions 

 of observation could not he exactly repeated; and these flights themselves, as has 

 already been stated, were so brief in time (usually less than six seconds), so 

 limited in extent (usually less than twenty metres), and so wholly capricious and 

 erratic, owing to the nature of the rubber motor and other causes, that very 

 many experiments were insufficient to eliminate these causes of mal-observation. 



It is not necessary to take the reader through many of them, but not to pass 

 over altogether a labor which was so great in proportion to the results, but whose 

 results, such as they were, were the foundation of all after knowledge, I will, as 

 illustrations, take from an almost unlimited mass of such material the observa- 

 tions of November 20, 1891, which were conducted with Model No. 30 with a 

 single pair of wings, shown in Plate 1, and with another one, No. 31, also shown 



