SOME AERONAUTICAL EXPERIMENTS. 145 



pcndicular to the surfuce, and the ratio of lift to drift is therefore tlie 

 same as that of the coskie to.the sine of the angle of incidence. But 

 in curved surfaces a ver^^ remarkable situation is found. The pressure, 

 instead of being uniformly normal to the chord of the arc, is usually 

 inclined considerably in front of the perpendicular. The result is 

 that the lift is greater and the drift less than if the pressure w(>re 

 normal. Lilienthal was the first to discover this exceedingly important 

 fact, which is fully set forth in his book, Bird Flight the Basis of the 

 Flying Art, but owing to some errors in the methods he used in making 

 measurements, question was raised by other investigators not only as 

 to the accuracy of his figures, but even as to the existence of any tan- 

 gential force at all. Our experiments confirm the existence of this 

 force, though our measurements difl'er considerably from those of 

 Lilienthal. While at Kitty Hawk we spent much time in measuring 

 the horizontal pressure on our unloaded machine at various angles of 

 incidence. We found that at lo the horizontal pressure was about 

 2?) pounds. This included not only the drift proper, or horizontal 

 component of the pressure on the side of the surface, but also the head 

 resistence of the framing as well. The weight of the machine at the 

 time of this test was about 108 pounds. Now, if the pressure had 

 been normal to the chord of the surface, the drift proper would have 

 been to the lift (108 pounds) as the sine of 13° is to the cosine of 13°, or 



.22 X 108 



-— —21+ pounds; but this slightly exceeds the total pull of 23 



• y i 



pounds on our scales. Therefore it is evident that the average pressure 



on the surface instead of being normal to the chord was so far inclined 



toward the front that all the head resistance of framing and wires 



used in the construction was more than overcome. In a wind of 14 



miles per hour resistance is l)y no means a negligible factor, so that 



tangential is evidently a force of considerable value. In a higher 



wind, which sustained the machine at an angle of 10°, the pull on the 



scales was 18 })ounds. With the pressure normal to the chord the 



.IT X 98" 

 drift proper would have been - — qo = IT pounds, so that, although 



the higher wind velocity must have caused an increase in the head 

 resistance, the tangential force still came within 1 pound of over- 

 coming it. After our return from Kitty Hawk we began a series of 

 experiments to accurately determine the amount and direction of the 

 pressure produced on curved surfaces when acted upon by winds 

 at the various angles from zero to 90°. These experiments are not 

 yet concluded, but in general they support Lilienthal in the claim 



c The travel of the center of pressure made it necessary to i)ut sand on the front 

 rudder to bring the centers of gravity and })ressnre into coincidence. Consequently 

 the weight of the machine varied from 98 pounds to 108 ])ounds in tiie different 



tests. 



SM 1902 10 



