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REPORTS ON WIND TUNNEL EXPERIMENTS 

 IN AERODYNAMICS 



I. THE WIND TUNNEL OF THE MASSACHUSETTS 

 INSTITUTE OF TECHNOLOGY 



By J. C. HUNSAKER, Assistant Naval Constructor, U. S. Navy 



INSTRUCTOR IN AERONAUTICS, MASSACHUSETTS INSTITUTE OF TECHNOLOGY 



An aeroplane or airship in flight has six degrees of freedom, three 

 of translation and three of rotation, and any study of its behavior 

 must be based on the determination of three forces — vertical, trans- 

 verse, and longitudinal — as well as couples al)out the three axes in 

 space. Full scale experiments to investigate the aerodynamical 

 characteristics of a proposed design naturally become mechanically 

 difficult to arrange. The experimental work is much simplified if 

 tests be made on small models as in naval architecture, and a further 

 simplification is made by holding the model stationary in an artificial 

 current of air instead of towing the model at high speed through still 

 air to simulate actual flying conditions. 



The use of a wind tunnel depends on the assumption that it is 

 immaterial whether the model be moved through still air or held 

 stationary in a current of air of the same velocity. The principle of 

 relative velocity is fundamental, and the experimental discrepancies 

 between the results of tests conducted by the two methods may be 

 ascribed on the one hand to the eft'ect of the moving carriage on the 

 flow of air about the model and to the efl:"ect of gusty air, and on the 

 other hand to unsteadiness of flow in some wind tunnels. 



The wind tunnel method requires primarily a current of air which 

 is steady in velocity both in time and across a section of the tunnel. 

 The production of a steady flow of air at high velocity is a delicate 

 problem, and can only be obtained by a long process of experimenta- 

 tion. A study was made of the principal aerodynamical laboratories of 

 Europe from which these conclusions were reached: (i) That the 



Smithsonian Miscellaneous Collections, Vol. 62, No. 4 



