194 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. _/ 



shifts when the velocity of advance or angle of incidence is changed, and to the 

 further fact that a wide surface docs not support proportionately as much per 

 square foot as a long and narrow one, it was thought that some advantage 

 might be gained by making the covering of the wing in the form of strips, 

 the edges of which would be perpendicular to the direction of motion, or by 

 making this covering in more or less slat-like form, which would permit the air 

 which had already been acted upon by the leading slat to slip through between 

 the rear edge of the first slat and the leading edge of the succeeding one. In 

 the tests on the whirling-table, however, it was found that this type of construc- 

 tion not only did not possess any advantages, but was even less effective than 

 a similar one in which the covering was continuous. The difference was prob- 

 ably due to the fact that the air which passed between the slats reduced the 

 suction on the upper side of the following slat, and also to the fact that the 

 distance between the slats was not sufficient to gain the effect of having each slat 

 act on air which had not already been partially deflected by the preceding one. 



In view of the results of these tests on various types of wings, it was de- 

 cided that in constructing the first set of wings for the large aerodrome it would 

 be best to employ the " single-tier " type, which had proved successful with 

 the models, and that after getting a successful flight with these the super- 

 pose. 1 wings would be tried in order to get, if possible, the advantage which 

 they possessed of being structurally stronger and more compact. It was there- 

 fore clear that any gain in the strength and rigidity of the first set of wings, 

 as a whole, would have to be obtained by improvements in the construction of its 

 integral parts, that is, in the main and cross-ribs which made up its framework. 



Before attempting to proportion the parts of the necessary wooden wing 

 frame, which it was expected would probably undergo many changes before 

 a final design was secured which would embody maximum strength for min- 

 imum weight, various tests were made to determine just how Iighl a cloth cov- 

 ering could be obtained which would be strong enough and sufficiently im- 

 pervious to the air. In the construction of the wings for the models a good 

 grade of China silk had been employed, but on account of the greatly increased 

 quantity of cloth required for the large wings, it was hoped that something 

 approximately as good as the silk could be secured at a much less cost, and 

 various grades of percaline were therefore tested. The weight of the various 

 -rades of percaline ranged from three grammes to ten grammes per square 

 foot, the lighter samples being of a rather coarse mesh, while the heaviest 

 ones were not only close mesh but some specimens contained a large amount 

 of " sizing." The particular grade which was finally adopted weighed seven 

 grammes per square foot. This material was practically impervious to air at 

 a pressure of one pound per square foot, which, of course, was considerably 



