292 



FIGURE 8. Schematic diagram of 

 experiment. 



■=J>F 



AFT VIEW IN PROPELLER PLANE 



AFTERBODY 



PLAN VIEW 



sional amplitude and phase of the blade frequency 

 force F3, are given by 



F 9 1, 



1 ,- -1 

 tan 



+ hi 



(ba/as) 



(61) 



(62) 



F 3 



where the phase angle, Gp, is the position of the 

 reference blade when the force is a positive maximum 

 or, from Figure 8, Sp is the angle by which the 

 force leads the blade position. 



difficult to process. An example of this type of 

 run and comparison with a good data run is shown 

 in Figure 12. Generally, the low amplitude data 

 resulted in force coefficients much below the 

 values obtained from the higher amplitude data. 

 Second, for certain runs the data were overscale on 

 the individual records, but not in the averaged 

 plot. These overscales, if abundant, produced 

 anomolies. Third, structural resonances of 18-20 

 Hz and 55-60 Hz grossly distort data for blade 

 frequencies with these values. To the extent 

 possible, data contaminated by these problems were 

 discarded and are not in the results presented. 



Experimental Results 



Force measurements with propeller 4118 located 16.0 

 in. (6.3 cm) aft of the nose of the body and with 

 a nominal tip clearance of 3.0 in. (1.18 cm) are 

 given in Figure 9. The force generally increases 

 in amplitude and lags further with higher propeller 

 loading. The data points at design J (0.83) for 

 speeds of 4 and 8 knots show good agreement. In 

 Figure 10, the blade frequency pressure induced on 

 the body in the plane of the propeller [x = 16.0 in. 

 (5.3 cm)] shows a monotonic increase in amplitude 

 with increased propeller loading and repeats well 

 for different speeds. 



Force measurements with propellers 4118 and 4119 

 positioned 10.0 in (3.94 cm) aft of the nose of 

 the body [4.5 inc. (1.77 cm) tip clearance] are 

 shown in Figure 11. Over the range of propeller 

 advance coefficient, the force amplitude tends to 

 increase with increased propeller loading and the 

 effect of thickness is demonstrated. 



The data exhibit some scatter for reasons not 

 yet fully understood and further calibration experi- 

 ments and data runs are needed. The variation in 

 the data for different speeds (and hence different 

 propeller excitation frequencies) is particularly 

 disturbing. It may be noted that a post-test 

 examination of the raw (unaveraged) data for the 

 flexure, displacement, and afterbody accelerometers 

 revealed three specific sources of difficulty. 

 First, low amplitude data, particularly for speeds 

 of 6 knots and a blade frequency of 35 Kz, was 



20 



-201 



4.0- 



O 



- 3.0- 



2.0- 



1.0 



DESIGN ZERO THRUST 



(J = 0.83) (J = 1-16) 

 / 



^. - 



CALC. METHOD 



1 = Breslln (1964-1971) 



2 = Vorus (1974) 



_L. 



oTo 072 oil O O To 1T2 Ti 



J = U/nO 



FIGURE 9. Calculated and measured blade frequency 

 force for propeller located at 1 = 16.0 in. with 

 tip clearance C = 3.0 in. 



