

208 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. '27 



taining this equilibrium under the varying conditions of flight and for restor- 

 ing it if for any reason il was disturbed, and, finally, provision had to be made 

 for the quick and accurate control of the flight by the aviator. These prob- 

 lems, while intimately related, had to be met one by one and solved separately. 



The general type of machine adopted was that which had been developed in 

 the years of experiment with the steam-driven models. From the very first 

 consideration of the large aerodrome, it seemed advisable to follow this type, 

 which not only had shown itself to be distinguished by remarkable longitudinal 

 and lateral stability in the tests, but was actually the only type in the world 

 which had at that time shown any possibility of successful flight. There was, 

 of course, a question whether single surface or superposed wings would be used, 

 and in spite of the negative results obtained in the tests of the models with 

 the superposed wings, it was felt that a considerable field for development was 

 open in this direction. However, in spite of the advantages which theoretical 

 considerations showed might be obtained through the introduction of this and 

 various other modifications of the original type, the whole teaching of past ex- 

 perience in the construction of the model aerodromes had been that success was 

 more certain to be achieved by following the course in which genuine practical 

 results had been achieved. It was decided, therefore, that in the construction 

 of the large aerodrome the design should follow as closely as constructional con- 

 ditions would permit the lines of the successful model Aerodromes Nos. 5 and 

 6, which have already been fully described. 



The longitudinal stability of an aerodrome is largely dependent upon the 

 relation of three chief factors; the center of pressure, the center of gravity and 

 the line of thrust. For an aerodrome of the " Langley " type, the relative po- 

 sitions of these which give the greatest degree of stability had been determined 

 as far as possible through the years of experiment with the models. How- 

 ever, while it is the usual experience in designing machinery, or even scien- 

 tific apparatus, thai what appears theoretically to be the best plan has to be 

 considerably modified for constructional reasons, yet in the design of an aero- 

 drome this is particularly true, for not only must all the various parts function 

 properly, both separately and as a whole, but this result must be secured for 

 the very minimum of weight. Experience alone can enable one to appreciate 

 thoroughly how seriously this consideration of weight complicates the problem. 



Tn making the original designs for the large aerodrome it had been recog- 

 nized that the relative positions of the line of thrust, center of pressure, and 

 center of gravity were much better in model No. (i than in model No. 5. From 

 Data Sheet No. 1, for Aerodrome No. 5 when it made its (light on May 6, 1896, 

 it will be noted that the line of thrust being assumed to be at the point 1500,* 



'See explanation of system of locating points in Part I, Chap. II, p. 15. 



