INSTRUMENTATION 



The propellers were instrumented with 40 semiconductor pressure transducers 

 (gages); 20 transducers were mounted on the pressure side (face) of one blade 

 (Blade 2) and 20 transducers were mounted in the same radial and chordal positions 

 on the suction side (back) of another blade (Blade 3); see Figure 4 and Table 4. 



The pressure transducers were mounted in Helmholtz cavities. A hole 0.040 in. 

 (1.0 mm) in diameter connected each cavity to the surface of the blade. This 

 configuration was accomplished in one of two ways. Where the thickness of the blade 

 permitted, the transducer was installed from the opposite side of the blade 

 (Figure 5a) . 0-rings isolated the transducer from the blade in order to essentially 

 eliminate the effects of blade deflection, as discussed in the section on Experi- 

 mental Results. A set screw secured the assembly, and the remainder of the hole 

 was filled with a clear resin. 



Near the leading and trailing edges, and at the 90 percent radius, there was 

 not sufficient thickness to permit such an installation. At these locations the 

 transducer was set into a 0.625 in. (15.9 mm) diameter cavity which was milled into 

 the blade from the surface on which the pressure was to be measured (Figure 5b). 

 The transducer was held in place with teflon tape and the tape was epoxied in place 

 at its edges, so that the transducer was not directly attached to the blade. A 

 faired coverplate with a 0.040 in. (1.0 mm) diameter hole was cemented over the 

 cavity. 



Dynamic calibrations were conducted using an enclosed column of liquid vibrated 

 by a shaker. The natural frequency of the pressure gage cavity arrangement was above 

 1600 Hz. This ensured a flat response with no dynamic effects in the 5 to 20 Hz 

 shaft rate frequency range investigated during the experiment. 



Transducers recessed in Helmholtz cavities were used rather than flush-mounted 

 transducers for the following reasons: 



1. Recessed transducers are better protected from damage arising from 

 cavitation collapse pressures or from accidental damage due to handling the blades 

 than are flush-mounted transducers. 



2. Recessed transducers produce less disturbance to the blade contour than do 

 surface mounted transducers. However, this advantage of recessed transducers has 

 been partially mitigated by smaller transducers which have become available since 

 these recessed transducers were installed (July 1976). 



