= tan 1 (V A /2TTrn) 



= tan 1 [J/(irr/R)] 



From this relationship, one can determine, approximately, the corresponding shifts 

 in upper cavity surface that result from small changes in angle of attack. For 

 example, in Figure 29, at the design J (1.037), 3 is determined to be 42.44 deg. 

 For a J value of 1.0, 3 = 41.40 deg. Therefore, a change of J corresponding to 

 0.037 has caused, approximately, a one-deg change in angle of attack, which shifts 

 the cavity surface upwards as shown. 



Theoretically, the section lift and cavity thickness for Propeller 4717C are 

 generated entirely by camber and point drag (note the blunt nose in Figures 29 

 through 31). That is, no incidence was used in the design to generate lift or cavity 

 thickness. In Figures 29 though 31, the theoretical prediction of cavity height 

 agrees fairly well with the experimental data. Near the leading edge, however, the 

 theory appears to overpredict cavity thickness. Also, visual observations indicated 

 that the backs of the blades at all radial sections on Propeller 4717C were wetted 

 to about 2- or 3-percent of chord from the leading edge. At this point, separation 

 was caused by a locally flat area that was inadvertently machined onto the back of 

 the blade. Although this local flat was almost microscopic, it effectively caused 

 separation. Apparently, very near the leading edge some portion of the blade metal 

 was interfering with the upper cavity streamline. 



Figures 32 to 34 compare linear theory predictions of cavity height with experi- 

 mental results for Propeller 4738A. Note in these figures the large amount of point 

 drag or blunt nose indicated by the theory. This results because both Models 4738A 

 and 4717C were designed to have approximately the same full-scale stress levels. 



This dictated that the maximum, theoretical, cavity thicknesses for Models 4738A and 



22 

 4717C would be almost the same. To obtain the same maximum thickness in a shorter 



distance, we used a large amount of point drag together with incidence and camber to 



generate the theoretical cavity. 



Figures 32 to 34 show three experimental upper cavity surfaces corresponding to 



three values of J. Note that at r/R = 0.361, Figure 32, the blade was fully wetted 



at the design value of J (1.037); therefore, cavity heights for three other values 



32 



