Miller 



0.4 



0.3 



0.2 



^i A A 



S-i i » Q » 



^—ii m B B 



HORIZONTAL BENDING MOMENT 



a 6 » •- 



HORIZONTAL SIDE FORCE 



40 



60 80 100 



Frequency 



Fig. 11 -Dynamic calibration of horizontal side force and bending moment 



The overall accuracy of the system is approximately ±5% for blade frequency 

 and harmonics up to about 200 Hz when values from the calibration curves at 

 blade frequency are used in the calibration matrix. Thrust measurements will 

 retain this accuracy up to 600 Hz and torque measurements can be used to 400 

 Hz with corrections. Side forces and bending moments are limited to 200 Hz, 

 due to the resonance at 230 Hz. 



INSTRUMENTATION 



The instrumentation was designed for both obtaining a record on magnetic 

 tape and for making an on-the-spot analysis of the signals directly from the dy- 

 namometer during the tests. 



Figure 12 shows a block diagram of the system. Power is supplied to the 

 thrust, torque, side-force, and bending-moment strain-gage bridges by four 

 separate power supplies. These supplies are adjustable between 5 and 24 V, and 

 the excitation for each channel is set to a value that will produce a strong signal 

 but will not overload the amplifiers. The output of the gages goes directly 

 through the sliprings and brushes to zero balancing circuits and then to a dc 

 microvoltmeter for measuring the steady components of the propeller forces. 

 The signals also go to ac amplifiers contained in the drive shaft and then through 

 sliprings and brushes to ac recording and analysis instrumentation. The dc out- 

 put circuit is also used to introduce a measured ac calibration signal to the out- 

 put of the strain-gage bridges in order to measure the gain of the ac system. 



264 



