actual cavity thickness was never measured, this is not a scientific estimate. The 

 leading-edge cavity thickness selected for Model 3770 results in an almost infinite 

 cavity length at every section of the blade, except near the tip and the hub. How- 

 ever, because the design cavitation number of Model 3870 is not very small, the 

 selected leading-edge cavity thickness is not large enough to induce a smooth sheet 

 cavity all over the blade. Therefore, the leading-edge cavity thickness of Model 

 3870 was corrected to give a cavity length at least 50 percent longer than the chord. 

 To do this, an extra leading-edge point drag was added to the cascade theory used in 

 the present design method, as explained previously. The design thrusts used in the 

 present lifting-line computations are the experimental values listed in Table 2, 

 where Case 1 is for cases without angle of attack and Case 2, with preset angle of 

 attack. It is seen that the efficiencies of the propellers, as predicted by the 

 present method, are very close to the measured efficiencies, even in the preliminary 

 design stage of calculations. 



Pitch distributions obtained from the preliminary design and lifting-surface 

 design computations, according to the two design approaches, Case 1 (without angle 

 of attack) and Case 2 (with angle of attack) are shown in Figures 18 through 21 

 together with the pitches of the two propeller models. The pitch values obtained 

 from the preliminary design calculations are higher than those obtained from lifting- 

 surface calculations because, in preliminary design, the effect of flow retardation 

 is not considered. The pitch distribution is also related to the leading-edge 

 cavity thickness. In general, when the leading-edge thickness increases, the pitch 

 also increases; however, the efficiency decreases slightly. The pitch distributions 

 for the predictions and for the models are noticeably different. This is because 

 the optimum lift distribution and the pitch angle, which are influenced by the blade- 

 cavity interference in the present method are quite different from those of the 

 models. If these factors are taken into account, all results appear reasonable 

 compared with those of the models. 



The lifting-surface corrections to the source distribution for the two pro- 

 pellers are shown in Figure 22. Although the correction for Model 3770 is close to 

 1, it is about 20 percent greater near the trailing edge than at the leading edge for 

 Model 3870. If the correction factor is unity, this means that the cascade source 

 strength is the same as the blade cavity source strength. This source strength has 



28 



