Malavard 



- Lower- 

 Surface 



/-Upper- 

 Surface 



Fig, 21 - Cavity shape for a flat wing of trapezoidal planform, 

 with a strut of the same width as the central chord, for in- 

 cidence a = 5° 



lift-drag ratio with respect to the foils designed according to given pressure 

 distribution is not surprising, as the Cl corresponding to a = 5° for the flat 

 wing is very low. 



Marine Screw Propellers 



The usual aerodynamic theories of screw propellers are particularly effec- 

 tive for airscrews. They do not solve satisfactorily the problems presented by 

 the marine propeller, nor do they permit the analysis of two important factors; 

 one is geometric and concerns the low aspect ratio of the blades, and the other 

 is hydrodynamic and concerns cavitation phenomena. Indeed, the first factor has 

 destroyed the fundamental simplification of the classic theory in which the blade 

 section could be substituted conveniently by an equivalent lifting line. The sec- 

 ond factor requires a precise knowledge of pressure distributions on the blades — 

 the only means of foreseeing or avoiding cavitation — which a too general theory 



396 



