44 SMITHSONIAN' CONTRIBUTIONS TO KNOWLEDGE VOL. 27 



data of " Aerodynamics " (obtained in constrained horizontal flighl with the 

 whirling-table) are here insufficient. They arc insufficienl also because these 

 values arc obtained with small rigid piano, while the surfaces we are now to use 

 cannot be made rigid under the necessary requirements of weight, without the 

 use of guy wires and other adjuncts which introduce head resistance. 



Againsl all these unfavorable conditions we have the favoring one that, other 

 things being equal, somewhat more efficiency can be obtained with suitable curved 

 surfaces than with planes/' 



1 have made numerous experiments with curves of various forms upon the 

 whirling-table, and constructed many such supporting surfaces, some of which 

 have been tested in actual flight. It mighl be expected that fuller results from 

 these experiments should be given than those now presented here, but 1 am 

 nol yet prepared to offer any more detailed evidence at present for the perform- 

 ance of curved surfaces than will be found in Part 111.' I do not question that 

 curves are in some degree more efficient, hut the extreme increase of efficiency in 

 curves over planes understood to he asserted by Lilienthal and by Wellner, ap- 

 peal's to have been associated either with some imperfect enunciation of condi- 

 tions which gave little more than an apparent advantage, or with conditions 

 nearly impossible for us to obtain in actual flight. 



All these circumstances considered, we ma> anticipate that the power re- 

 quired (or the proportion of supporting area to weighl ) will he very much greater 

 in actual than in theoretical (that is, in constrained horizontal) flight, and the 

 early experiments with rubber-driven models were in fact successful only when 

 there were from three to four feet of sustaining surface to a pound of weight. 

 When such a relatively large area is sought in a large aerodrome, the construc- 

 tion of light, yet rigid, supporting surfaces becomes a nearly insuperable diffi- 

 culty, and this must be remembered as consequently affecting the question of the 

 construction of boiler, engines and hulls, whose weight cannot be increased with- 

 out increasing the wing area. 



■ More recent experiments conducted under my direction by Mr. Huffaker Rive similar results, 

 but confirm my earlier and cruder observations that the curve, used alone, for small angles, is much 

 more unstable than the plain-. 



♦As stated in the Preface, Part III has not yet been prepared for publication. 



