and 4717C. In addition, cavity heights have been measured in similar ways for a 



21 

 supercavitating flat plate and a Tulin-two-term section by Christopher and Johnson. 



DESCRIPTION AND LOCATION OF PINS 



Number-four brass machine screws with heads cut off were used as pins. These 

 pins were attached to the blade backs by drilling and tapping holes perpendicular to 

 the surface at 12 locations. The upper cavity surface location was defined as a 

 point on a line, perpendicular to the nose tail line, that runs through the center 

 of the tapped hole at the blade surface; see Figure 27. The holes were drilled and 

 tapped perpendicular to the back of the blade to cant the pins slightly away from the 

 reference line (the line perpendicular to the nose tail line) . A slight error was 

 introduced, but the machining process was greatly simplified. The locations of these 

 pins were as follows: 10, 30, 60, and 90 percent of chord at nondimensional radii 

 (r/R) of 0.361, 0.544, and 0.726; see Figure 28. Since the blade at 10 percent of 

 chord was too thin to tap, the pins at these locations were soldered in place. The 

 pins at all other locations were screwed in and secured by tiny electrical lock nuts 

 (see photos in Reference 19) . 



Three sets of pins were used for testing Propeller 4717C. When installed, the 

 first set of pins protruded above the back of the blades to a height that corre- 

 sponded to three times the distance from the back of the blades to the theoretically 

 predicted upper cavity surface. The second set of pins protruded by a factor of 1.67 

 and the third set by a factor of 1.0, the latter being the theoretically predicted 

 cavity height. However, the pins at 10 percent of chord varied from this order; 

 their heights corresponded to cavity-height factors of 3, 2.33, and 1.0. The pins at 

 the 10 percent of chord locations had to be filed by hand to the correct height, and 

 the factor of 2.33 rather than 1.67 was used to ensure that the pin-height would 

 exceed the experimental cavity thickness. The experiments have shown, however, that 

 the theory overpredicted the cavity height near the leading edge, and the pins could 

 have been filed to a height corresponding to a factor of 1.67. 



Four sets of pins were used for Propeller 4738A. The first set of pins pro- 

 truded above the back of the blade to a height corresponding to a factor of 1.8 times 

 the distance from the back of the blade to the theoretically predicted cavity sur- 

 face. The other three sets corresponded to factor of 1.4, 1.0 (theoretical) and 



30 



