ACCURACY OF MEASURED DATA 



At the time of earlier evaluations, attempts to quantify the accuracy of the 



measured mean pressures were hindered by small variations in advance coefficient. 



Because the carriage speed and the propeller rotational speed are set manually, a 



precise value of J cannot be set. The prescribed test matrix produced a series of 



runs at values close to the specified test conditions. The dependence of certain 



gages on speed or Reynolds number further hindered an quantification of the accuracy 



of the measurement system. 



To overcome these problems, an error analysis was conducted based on the C 



P 

 versus J curves in Figure 17. The mean pressure coefficient represented in these 



curves had been corrected for Reynolds number dependence, as described earlier. 

 Therefore, this accepted Reynolds number effect, whether being an instrumentation 

 error or a real-flow phenomenon, had been eliminated in these figures. First- and 

 second-order curves were least squares fit to these speed-corrected pressure coeffi- 

 cients over a range of J, and then a standard error for each curve was calculated. 

 The standard error represents the standard deviation of the measured pressure 

 coefficients from the least squares curve-fit values. The standard error was 

 multiplied by 1.96 to represent the standard error at a 95 percent confidence level. 

 This implies that, if one assumes a normal distribution of the variation of measured 

 pressure coefficients from the curve fit values, then 95 percent of the measured 

 pressure coefficients fall within plus or minus the value of the standard error from 

 the curve-fit result. This procedure permitted the use of the entire test matrix, 

 over a range of J and carriage speed, in calculating a statistical error band. Also, 

 small variations in J, for a given test condition, were properly accounted for. The 

 resulting nondimensional error bands in +C , are shown in Table 7. 



These results were extended to provide a dimensional error band in terms of a 

 dimensional pressure. The standard error process was modified to calculate dimen- 

 sional pressures and arrive at a 95 percent confidence level error band in psi that 

 could be compared to the approximated error band of the measured pressures during 

 calibration. These results, shown in Table 8 for the two propellers tested, indi- 

 cate, in the best case of Propeller 4718 in uniform flow with a second-order curve 

 fit, an average error band very close to the predicted error from the calibrations. 

 Most other cases indicate a test error band up to twice the predicted error based on 

 calibration error. 



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