Accuracy in the measurement of carriage speed and propeller rotational speed 

 would also affect the overall accuracy of the pressure measurements. No determina- 

 tion was made to evaluate the accuracy of these measured quantities, but given the 

 good results of the best-case test error, it was felt these measurements were 

 accurately made. 



The only remaining assumption in the measurement process that could be ques- 

 tioned was the equating of the carriage speed to the advance speed V through the 

 propeller disk. This assumption is always made in basin testing; however, with the 

 large size of the dynamometer, small amplitude, low-frequency standing waves were 

 setup after a few runs. These standing waves caused small additional velocities in 

 the basin. It was assumed that this effect would average out over a run, and given 

 the accuracy of the best case, was neglected. 



The accuracy of the measured fluctuating pressures was generally good. Repeat- 

 ability was the only indication of accuracy in this case because no consistent 

 governing trend existed for unsteady pressures. Error bands with a 95 percent con- 

 fidence level were calculated for first harmonic amplitude C 1 and first harmonic 

 phase (J> 1 from the repeat runs conducted at each given test condition in inclined 

 flow. Propeller 4718 produced an average error band of AC = +0.002 and A(j> 1 = 

 +4 deg, while Propeller 4679 produced expected larger average error values of AC - = 

 +0.005 and A(j> 1 = +8 deg. This average error band was relatively small for typical 

 first harmonic amplitudes in a range greater than C 1 = 0.0150, but in some cases on 

 the pressure side of the propeller blade, values of C ^ were less than 0.0050, thus 

 causing uncertainty in the measured amplitude and also in the measured phase. 



FLUCTUATING PRESSURE MEASUREMENTS AT DESIGN J 



Periodic pressure measurements were obtained when the propellers were operated 

 in inclined flow. The 7.5 deg shaft inclination produced a first harmonic, once per 

 revolution variation in the measured pressure. A typical variation of pressure with 

 gage angular position is shown in Figure 23. As expected, the pressure variation 

 was primarily first harmonic, with negligible higher harmonics observed, attributed 

 to noise. The fluctuating pressure is represented as the first harmonic pressure 



31 



