NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 89 



this may largely be accounted for by the effect of the walls/ M. 

 Eiffel's tests in the open air with large plates dropped from the Eiffel 

 Tower indicate still larger values of K, but here velocities were as 

 great as 90 miles per hour. In addition to the extreme difficulty of 

 obtaining precise measurements of resistance and speed with this 

 method, a further complication is injected into the problem from the 

 fact that the velocity of fall was accelerating. 



It seems obvious that the resistance to accelerated motion should 

 be greater than that due to uniform motion, since the energy system 

 accompanying the disk is being built up. 



In conclusion, then, within the limits of these tests, the resistance of 

 a flat disk normal to the wind for speeds above a certain minimum may 

 be correctly represented by 



The existence of a critical velocity for disks has not been detected 

 in previous experiments. M. Eiffel's tests were not run at sufffciently 

 low velocity, Riabouchinski's tests were not very precise ; but the 

 experimental work of Stanton or Foppl might have been expected to 

 bring out a critical velocity. 



One explanation may lie in the fact that the critical velocity at 

 which eddy making begins to become a stable phenomenon depends 

 largely on the quality of the wind in the tunnel. It is reasonable to 

 suppose that a turbulent wind would cause the critical velocity to be. 

 reached sooner than it would be with a more steady wind. 



Experiments at Gottingen on spheres showed a marked critical 

 velocity at a certain point. When the wind was made more turbulent 

 by a fish net up stream, the critical velocity came at about two-thirds 

 of the former value.'' 



The Gottingen tunnel in which Foppl's tests were made is a closed 

 circuit in which the wind is guided around the corners by vanes, 

 strained through diaphragms and wire mesh and forced to flow in 

 horizontal lines where the model is placed. The result of such forc- 

 ing the air to take unnatural lines of flow must introduce a degree of 



^ Lord Rayleigh, " On the Resistance Experienced by Small Plates Exposed 

 to a Stream of Fluid," Phil. Mag., July, 191 5. A simple yet delicate experi- 

 ment showed that the resistance of two small plates was equal to that of a 

 single large plate of the same area for the same speed. This is in agreement 

 with the results presented above for disks, and indicates that if there be a 

 critical velocity, this velocity is the same for both large and small areas. Lord 

 Rayleigh used disks cut from cardboard, which had presumably a square edge 

 resembling that of the brass disks. 



^ Zeitschrif t fiir Flugtechnik und Motorluftschiffahrt, May 16, 1914. 



