A General Theory for Marine Propellers 



NACA 66(215)-216, « = 0.6 



Fig. 1 - Comparison of theoretical and 

 experimental pressure distributions for 

 the NACA 66(215)-216 airfoil; Cl = 0.23 

 (from Ref. 23) 



In view of the fact that the lift coefficient Cl of 0.23 is relatively large compared 

 with that of a propeller blade and that the agreement between the theoretical and 

 the experimental pressure distribution is excellent from a practical viewpoint, it 

 seems permissible to place the pressure source and pressure dipole distributions 

 along a chord line. 



Another interesting point to be drawn from this comparison is that the vis- 

 cosity effect does not significantly influence the normal force on the blade. This 

 important fact greatly simplifies the task of computing the viscous drag of the 

 blade. By examining the drag coefficient curves of various foil sections as 

 plotted against the section lift coefficient Cl, as given in the same reference, it 

 clearly indicates that the viscous drag is greatly influenced by the pressure dis- 

 tribution. With the same basic section at the same lift coefficient, the viscous 

 drag coefficient differs depending on whether the lift is produced by camber or 

 by angle of attack. This means that the viscous drag cannot be accurately com- 

 puted unless the lift or pressure distribution over the blade has been obtained. 

 Since the pressure distribution is not affected by the viscosity within the range 

 of lift coefficient of interest, no iteration between the lift and drag is necessary. 

 The viscous effect on propeller thrust and torque can be analyzed after the po- 

 tential problem has been solved first. 



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