Propeller -Induced Appendage Forces 



Fig. 7 - Test appendage, exten- 

 sions, and balance unit 



Fig, 8 - Dynamic balance 



system in terms of millivolts per pound of applied force as a function of the fre- 

 quency at which the shaker was operating. Figure 9 is a block diagram of the 

 dynamic calibration setup and wiring of the electronic instrumentation. 



The dynamic balance calibrations during this series of investigations were 

 undertaken in the following ways. In the earlier studies the foil was completely 

 submerged during the transverse calibration as the shaker was attached to one 

 end of the support rod which extended outside of the tunnel. However for the 

 axial calibration the shaker was attached to the center of the foil and the cali- 

 bration undertaken in air. The response of the system in the axial direction as 

 a function of frequency was then plotted. The foil was then completely sub- 

 merged, and with the output of the dynamic balances fed to an optical galvanom- 

 eter the foil was struck a sharp blow in the axial direction. The resonant fre- 

 quency was then determined by examining the optical galvanometer record of the 

 response trace. The calibration curve which had been obtained in air was then 

 shifted by this slight change in the resonant value to obtain the curve used for 

 data evaluation. 



During the later studies the transverse calibration was performed in the 

 same way as for the earlier investigations, but for the axial calibration the 

 transducer was encapsulated in a rubber protective cover so that this calibra- 

 tion was also performed while the foil was submerged. Thus all calibration data 

 were obtained with the foil completely submerged in the operating medium. 



The system was calibrated for each test condition prior to taking data. This 

 technique permitting establishment of the exact calibration (±0.02 Hz) at the sub- 

 sequent desired frequency of interest. By calibrating each system prior to test- 

 ing, any change in the system response due to foil extension attachment, etc., 

 was included. Flow background levels were also taken at this time. A typical 

 plot of system calibration and flow background levels is shown in Fig. 10. 



177 



