58 EXPERIMENTS IN AERODYNAMICS. 



Third, many of the curves show a tendency to reach a minimum point for 

 an inclination of the planes of about 30°, the highest angle at which these planes 

 were used. It was, therefore, seen to be desirable to extend the angles of inclina- 

 tion far enough to include the minimum point of the curve within the range of 

 observation. This was done in the case of four of the planes whose results are 

 plotted in Fig. 9. In examining these curves, it will be seen that the minimum 

 jjoint falls between 25° and 35°. It should also be noted that the change in the 

 soaring speed is quite small for settings between 25° and 40°, and that in a 

 number of individual observations the real character of the curve over this range 

 was masked by the errors introduced by wind and weather. 



Since the planes whose results are plotted in Fig. 9 all have the same 

 weight per unit area, the difference in their soaring speeds arises solely from their 

 difference of size, shape, or aspect. The effect of shape and aspect indicated in 

 Fig. 8 is beautifully exhibited and amply confirmed in the six corapartible curves 

 of Fig. 9. For low angles, viz., below 15° or 20°, the curves of soaring speed 

 for the different planes occupy the following relative positions from below 

 upward : 30 x 4.8 inches, 24 x 6 inches, 12 x 6 inches, 6 x 6 and 12 x 12, 6 x 24 

 inches. It will be observed that the planes placed in the above order are 

 symmetrically arranged. Remembering that the first written dimension is the 

 horizontal edge, perpendicular to the line of motion, which may be called the 

 spread, and that the second written dimension is the inclined edge, or the distance 

 from front to back, it Avill be seen that, in the above order, the ratio of the spread 

 to the extent from front to back is uniformly diminishing. In other words, the 

 planes whose spread is largest in comparison with their extent from front to back 

 have the smallest soaring speed, and these planes are therefore to be considered 

 as being, in shape and aspect, the most favorable for mechanical flight. Thus the 

 30 X 4.8 inch and the 24 x 6 inch planes are favorable forms and aspects, while 

 the 12 X 12 inch plane and, to a greater degree, the 6 x 24 inch plane are 

 unfavorable forms and aspects. 



Between 15° and 30°, and in general at about 30°, a reversal takes place, 

 and for higher angles the curves are all found from below upward in the reverse 

 order. Thus the 30 x 4.8 inch plane, which for low angles soars at the lowest 

 speed, for settings above 30° requires the highest speed. This relative efficiency 

 for low angles was manifested in the experiments with the Plane- Dropper, but 

 the reversal in the j^osition of the curves for higher angles is a relation which 

 those observations were not sufficiently extended to present. The interpretation 

 of this reversal will be developed by a consideration of the general relations 

 existing between these results and the total normal pressure on the planes, and 

 will also be found to be connected with corresponding changes in the relative 

 positions of the center of pressure. 



