Experimental Determination of Unsteady Propeller Forces 



When these effects are considered, it is not possible to measure thrust 

 fluctuations having frequencies higher than the terminal natural frequency of the 

 system, due to the hydrodynamic interaction of the thrust and the torque system. 



A second question refers to the extremely skewed propeller, showing oppo- 

 site trends of vibratory output versus advance ratio, as compared to regular 

 propellers. Is the dynamic response of the propeller blades probably the cause 

 of this phenomenon? 



REPLY TO THE DISCUSSION 



Marlin L. Miller 



The dynamic calibrations were made in water with a mass approximately 

 equal to a propeller mounted on the balance. This did not provide the coupling 

 between the thrust and torque that is present when a propeller is being tested. 

 With a propeller, the thrust response will be limited by the torsional resonance 

 of the system. However, the frequencies measured have been well below this 

 limit, so that the results have not been affected by this coupling. 



The resonant frequencies of the blades of the skewed propeller have been 

 measured in water. The hub of the propeller was driven by a shaker and the 

 resonant frequency of each blade determined with a hydrophone held close to the 

 blade. The average resonant frequency was found to be 97 Hz. This is close to 

 the second harmonic of blade frequency, which was 90 Hz for the tests reported 

 in this paper. An examination of the test data has shown an unusually large 

 second-harmonic component of thrust and torque and a somewhat more complex 

 harmonic distortion of the side force and moment signals at the two highest val- 

 ues of advance coefficient for the skewed propeller. Therefore, these values, 

 shown in Figs. 21, 22, 23, and 24, are not reliable. However, little or no har- 

 monic response was observed for the two lower values of advance coefficient, 

 and these values are considered reliable. 



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