Miller 



The ac outputs are passed through adjustable -gain amplifiers, where the signal 

 levels are set to the optimum level for magnetic-tape recording. Near the fly- 

 wheel are two toothed wheels that generate 1 and 60 pulses per shaft revolution. 

 Figure 13 shows these pulses for a portion of a revolution before and after pass- 

 ing through a pulse-shaper. The 60 Hz square wave controls the digitalization 

 of the signals, and the single pulse acts as a start signal and as a phase refer- 

 ence. These pulses are recorded on the magnetic tape, along with the six chan- 

 nels of data from the propeller. A digital time code is recorded on the tape to 

 identify the data and to permit the use of an automatic tape -search unit during 

 digitalization. A voice announcement is also used to record additional informa- 

 tion concerning the tests. Each of the six data signals is also photographically 

 recorded from an oscilloscope screen, along with the single reference pulse. 



Fig. 13 - Reference pulses before 

 and after shaping 



For on-the-spot analysis, a two-channel constant -bandwidth wave analyzer 

 and a phase meter are used. The analyzer consists of a common local oscilla- 

 tor, two mixers, and two 20.5-kHz crystal filters. These filters have a band- 

 width of 5 Hz and are matched for frequency and phase. One channel is con- 

 nected to the phase reference pulse. Since this is a narrow symmetrical pulse, 

 it contains many strong harmonics of shaft frequency that are in phase with each 

 other. Whenever the analyzer is tuned to one of the unsteady signals, there will 

 be a reference signal that has a fixed phase relationship to the angular position 

 of the propeller. The phase meter is connected to the two outputs of the ana- 

 lyzer to measure their phase angle, and the signal channel is connected to a 

 voltmeter to measure the amplitude. The input of the analyzer can be switched 

 to analyze any of the six components. To simplify setting the analyzer to the 

 desired signal frequency, the local oscillator frequency is mixed with that of a 

 20.5-kHz crystal oscillator to obtain the frequency component being analyzed. 

 This frequency is compared with that of a sine -wave generator connected to the 

 drive motor. Since all the frequencies of interest are simple multiples of shaft 

 frequency, it is easy to tune the local oscillator by setting it for a stationary 

 pattern on the oscilloscope. 



266 



