The Vibratory Output of Contrarotating Propellers 

 VERTICAL BENDING MOMENT 



HORIZONTAL BENDING MOMENT 



Fig. 7 - Recording of the vertical and horizontal 

 bending moment variations of the four-bladed 

 propeller 



For a proper evaluation of the thrust eccentricity, the lateral forces have to 

 be determined with an additional recording. It appears, however, that the fluc- 

 tuating lateral forces for the forward as well as the aft propeller were not meas- 

 urable in an accurate way. Apparently the amplitudes are reduced considerably 

 and are hidden by the noise of the measuring system. For further evaluation of 

 the results (determination of the thrust eccentricity) it is assumed that the fluc- 

 tuations of the lateral forces are negligible for both propellers. 



A vectorial summation of the horizontal and vertical thrust eccentricity for 

 both propellers is presented in Fig. 9. To keep the figure readable only about 

 1/4 of a revolution is presented (blade encounters Nos. 1 through 10 (Fig. 4)). 

 Besides the normal thrust eccentricity and its modulation due to the finite num- 

 ber of blades, again we can distinguish the high-frequency modulations due to 

 the mutual blade encounter. 



Additional measurements were conducted to measure the effect of a change 

 of the angular positions of the mutual blade encounter. The new angular posi- 

 tions of mutual blade encounter follow from Eq, (1). The results of these tests 

 are reviewed in Table 1 and compared with the results of the first tests. From 

 this table it can be concluded that no significant differences occur for both 

 conditions. 



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