DURAND— MECHANICAL ASPECTS. 183 



of those responsible for the design and construction of the prime 

 mover. 



It seems appropriate to note at this point, that until the margin 

 of uncertainty or of unreliability is reduced far below where it now 

 stands, the navigation of the air will be closed to the great mass of 

 people who will prefer the safer if somewhat less thrilling mode by 

 way of the solid earth or the water-borne boat. 



Propulsion. 



The screw propeller has been universally adopted as the means 

 for transforming the work developed by the prime mover into 

 propulsive work. 



In spite of its simple form the operation of the propeller depends 

 on an astonishingly large number of variables, interrelated in com- 

 plex and baffling ways, and thus far transcending all effort to bring 

 them into practicable expression through the application of aero- 

 dynamic theory. The chief variables or conditions thus entering 

 into the operation of a propeller may be Hsted as follows : 

 (o) Characteristics of the propeller as a geometrical body, 

 (i) The diameter or general determining dimension. 



(2) The pitch of the helicoidal surface employed for the driv- 



ing face. This may have two different modes of 

 specification, viz. : 



(a) The single value of the pitch if uniform, or the mean 

 value if variable. 



(b) The distribution of values if variable. 



(3) The form of the contour bounding the blade or helicoidal 



surface employed. 



(4) The area of the blade on the driving face. 



(5) The cross section or thickness of the blade. This may 



have two mode of specification, viz. : 



(a) Areas of cross sections and their distribution radially. 



(b) Forms of cross sections. 



(6) The character and finish of the blade surfaces. 



(7) The form and dimensions of the hub or central body 



carrying the blades. 



