184 SYMPOSIUM ON AERONAUTICS. 



(b) The characteristics of the adjacent structures such as parts of 



the aeroplane. These will influence the flow of air to and 

 from the propeller and will thus affect the force reactions 

 resulting from its operation under any stated set of condi- 

 tions. These may be primarily specified by 



(i) Dimension and form. 



(2) Location with regard to propeller. 



(c) The characteristics of the medium, 

 (i) Density. 



(2) Viscosity. 



(3) Character and extent of turbulence or departure from 



homogeneous conditions. 



(d) The characteristics of operation. 



(i) Speed of translation or speed of advance. 

 (2) Speed of rotation. 



We have thus, without going too far into detail, some 14 vari- 

 ables or conditions, any one of which may exercise an important 

 influence on the results realized from the propeller. 



For many purposes and by way of approximate working 

 formulae, the operation of the propeller is related through empirical 

 coefficients to the three most important of the above listed set of 

 conditions ; namely, diameter, pitch and the slip, which is directly 

 expressible in terms of the relation between the speed of advance 

 and the speed of rotation. 



Aside from such approximate formulae, in which the values of 

 the coefficients drawn from experience must be so selected as to care 

 for all variables other than the four directly represented, there 

 seems to be no recourse save either in direct full size experimental 

 investigation, or in model investigation. The limitations of full size 

 experimental investigation are evident, and aeronautic engineers, 

 following the lead of the naval architect, have turned to model ex- 

 periments as furnishing the most hopeful means of dealing with the 

 problem of the screw propeller. 



The use of models presupposes the application of a law or prin- 

 ciple of kinematic similitude, and regarding which it is unnecessary 

 to speak in detail on this occasion. It will aid, however, in clarify- 

 ing our present view to state the underlying assumption as follows. 



