Wereldsma 



DIRECTION OF ROTATION 



Fig. 4 - Angular position and numbered sequence 

 of mutual blade encounters of a four-bladed and 

 five-bladed contrarotating propeller combination 

 with synchronous drive and equal rpm. The 1st, 

 21st, and 41st encounters are made by the same 

 two blades. 



The angular position of the 41st encounter coincides with the first encounter and 

 occurs after one revolution of each propeller. 



A completely different angular position of the two propellers occurs when the 

 angular positions of the mutual blade encounters are between the radial lines 

 shown in Fig. 4. That means 



9 / ISO ,277 



77: (n-1) 277 + — 



40 



80 



(1) 



It is expected that this small difference of the relative angular position does not 

 result in a significant change in the vibratory output of the combination. 



Since the periodicity of the propeller combination equals one revolution due 

 to the unequal blade numbers, the fundamental frequency of the signals equals 

 the speed of rotation of the propellers (number of revolutions per second). The 



240 



