148 SOME AERONAUTICAL EXPEEIMENTS. 



the di,siidvantao"o of reciiiirino- ratliei" .sti'ono- winds or very laro-e sup- 

 porting- .surfaces. However, when oliding- operators have attained 

 greater skill, the}^ can, with comparative safet}', maintain themselves 

 in the air for hours at a time in this way, and thus by constant prac- 

 tice so increase their knowledge and skill that the}' can rise into the 

 higher air and search out the currents which enable the soaring ])irds 

 to transport themselves to any desired point l)y tirst rising in a circle 

 to a great height and then sailing off at a descending angle. The last 

 illustration shows the machine, alone, il^'ing in a wind of 35 miles per 

 hour on the face of a steep hill 100 feet high. It will be seen that the 

 machine not only pulls upward, but also pulls forward in the direction 

 from which the wind blows, thus overcoming l)oth gravity and the 

 speed of the wind. We tried the same experiment with a man on it, 

 but found danger that the forward pull would l^ecome so strong that the 

 men holding the ropes would l)e dragged from their insecure foothold 

 on the slope of the hill. So this form of experimenting was discon- 

 tinued after four or five minutes' trial. 



In looking over our experiments of the past tw^o years, with models 

 and full-size machines, the following points stand out with clearness: 



1. That the lifting power of a large machine, held stationary in a 

 wind at a small distance from the earth, is much less than the Lilien- 

 thal table and our own lalioratory experiments would lead us to 

 expect. When the machine is moved through the air, as in gliding, 

 the discrepancv seems much less marked. 



2. That the ratio of drift to lift in well-shaped surfaces is less at 

 angles of incidence of 6° to 12° than at an angle of 3"-. 



3. That in arched surfaces the center of pressure at 90° is near the 

 center of the surface, l)ut moves slowly forward as the angle becomes 

 less, till a critical angle, varying with the shape and depth of the 

 curve, is reached, after which it moves rapidly toward the rear till the 

 angle of no lift is found. 



•1. That with similar conditions large surfaces may be controlled 

 with not nuich greater difficulty than small ones, if the control is 

 effected b}' manipulation of the surfaces themselv(^s, rather than l)y a 

 movement of the bod}^ of the operator. 



5. That the head resistances of the framing can ])e hi-ought to a 

 point nuich below that usually estimated as necessary. 



6. That tails, both vertical and horizontal, may with safety l)c clinfi- 

 nated in gliding and other Hying experiments. 



7. That a horizontal position of the operator's l)ody may be assumed 

 without excessive danger, and thus the head i-(\sistan<-(^ rcxluccd to 

 a))out one-fifth that of the upright position. 



8. That a pair of superposed or tandem surfaces has less lift in pro- 

 portion to drift than either surface separately, even after making 

 allowance for weight and head resistance of the connections. 



