16 SMITHSONIAN CU.NTKIIH TIu.N'S TO KNOWLEDGE VOL. 27 



The solution would be comparatively simple if the position of the CP could 

 be accurately known beforehand, but how difficult the solution is may be realized 

 from a consideration of one of the facts just stated, namely, that the position 

 of the center of pressure in horizontal flight shifts with the velocity of the flight 

 itself, much as though in marine navigation the trim of a steamboat's hull were 

 to be completely altered at every change of speed. It may be remarked here 

 that the ceuter of pressure, from the symmetry of the aerodrome, necessarily 

 lies in the vertical medial plane, but it may be considered with reference to its 

 position either in the plane XY {cp x ) or in the plane YZ (cp z ). The latter cen- 

 ter of pressure, as referred to in the plane YZ, is here approximately calculated 

 on the assumption that it lies in the intersection of this vertical plane by a hori- 

 zontal one passing through the wings half way from root to tip. 



Experiments made in Washington, later than those given in " Aerodynam- 

 ics," show that the center of pressure, {cp ± ) on a plane at slight angles of inclina- 

 tion, may be at least as far forward as one-sixth the width from the front edge. 

 From these later experiments it appears probable also that the center of pres 

 sure moves forward for an increased speed even when there has been no percep 

 tible diminution of the angle of the plane with the horizon, but these considera- 

 tions are of little value as applied to curved wings such as are here used. Some 

 observations of a very general nature may, however, be made with regard to the 

 position of the wings and tail. 



In the case where there are two pairs of wings, one following the other, the 

 rear pair is less efficient in an indefinite degree than the front, but the action 

 of the wings is greatly modified by their position with reference to the propel- 

 lers, and from so many other causes, that, as a result of a great deal of experi- 

 ment, it seems almost impossible at this time to lay down any absolute rule with 

 regard to the center of pressure of any pair of curved wings used in practice. 



Later experiments conducted under my direction by Mr. E. C. Huffaker, 

 some of which will appear in Part III, indicate that upon the curved surfaces I 

 employed, the center of pressure moves forward with an increase in the (small) 

 angle of elevation, and backward with a decrease, so that it may lie even behind 

 the center of the surface. Since for some surfaces the center of pressure moves 

 backward, and for others forward, it would seem that there might be some other 

 surface for which it will he fixed. Such a surface in fact appears to exist in the 

 wing of the soaring bird. These experiments have been chiefly with rigid sur- 

 faces, and though some have been made with elastic rear surfaces, these have 

 not been carried far enough to give positive results. 



The curved wings used on the aerodromes in late years have a rise of one 

 in twelve, or in some cases of one in eighteen, 3 and for these latter the following 

 empirical local rule has been adopted: 



1 See footnote page 47. 



