For Propeller 4769, two of the pressure transducers in Helmholtz cavities were 

 replaced by flush-mounted transducers, one to each surface of the blade at 50 percent 

 chord at the 0.70 radius. These transducers are the recently-developed extremely 

 small type. The total diameter is 0.050 in. (1.3 mm) and the diameter of the sensing 

 element is 0.023 in. (0.58 mm). These flush-mounted transducers were found to be 

 unsatisfactory in terms of durability and accuracy. 



The instrumentation components used in a single pressure measurement channel 

 are shown in Figure 6.* 



The pressure transducer sensing elements are contained in a resistance 

 Wheatstone bridge circuit which produces an output signal voltage proportional to the 

 applied pressure. Power is supplied to the pressure transducer through a slip ring 

 on the propeller shaft and is regulated by four integrated circuit voltage 

 regulators, one for each group of ten pressure transducers. Only two arms of the 

 Wheatstone bridge are actually located at the pressure transducer; there are two 

 bridge completion resistors per transducer located inside the propeller hub. 



The bridge output voltage is fed through a semiconductor analog switch into a 

 voltage controlled oscillator (VCO) . The VCO produces a frequency modulated (FM) 

 output signal in which a deviation in the VCO center frequency is proportional to the 

 voltage level of the input signal. 



The FM output signal from 20 of the VCO modules, corresponding to the 20 pres- 

 sure signals on one blade, are fed into a common mixer circuit module. The FM 

 multiplexed signal at the output of the mixer is then telemetered via a single slip- 

 ring and a single coaxial cable from the propeller hub to a bank of 20 discriminator 

 channels located on the towing carriage. There are two identical but separate mixer 

 circuit modules and banks of discriminators, each of which carries the signals from 

 20 pressure transducers. Therefore, the signals from all 40 pressure transducers 

 are transmitted simultaneously using only two coaxial cables and two slip rings. 



Each discriminator channel converts the associated FM signal into a high level 

 dc voltage proportional to a corresponding pressure transducer signal; i.e., it de- 

 multiplexes the signal. The resulting voltages are digitized, averaged, and stored 

 by an Interdata Minicomputer, as discussed later. 



^Details of the instrumentation are shown on DTNSRDC Drawings C-543-1, C-543-2, 

 C-543-3, and C- 543-4. 



