The speed correction term produces a dependency of the first harmonic pressure 



coefficient on the magnitude of the mean pressure coefficient, C . Since the ve- 



P 



locity correction terms are constants multiplied by C , . , and C . increased 



pJmxn __ pJmax 



values of C _ . and C _ will produce an increased value of C , . This trend is 

 pJmin pJmax pi 



important when observing C .. over a range of J, and when considering the accuracy of 

 the first harmonic pressure coefficients, C 1 generated from values of C with large 

 speed effects. 



The quasi-steady analysis represents an intuitive description of the fluctuating 



pressure, excluding any unsteady effects. It provides a good base for comparison of 



33 34 

 the measured data for the two propellers, and the unsteady theory by Tsakonas. ' 



The correlation between the measured and quasi-steady results can also be compared 



to similar correlations of fluctuating blade loads performed by Boswell and Jessup. 



CORRELATION OF FIRST HARMONIC PRESSURE COEFFICIENTS WITH THEORY 



The measured first harmonic pressure coefficient in Figures 22 and 23 generally 

 tend to decrease in amplitude from leading to trailing edge. This trend was gen- 

 erally approximated by the quasi-steady approach, but with an amplitude 30 percent 

 to 50 percent less than the measured result. This result matched similar correla- 

 tions of quasi-steady and measured fluctuating blade loads by Boswell and Jessup. ' 

 Intuitively, the observed trend from leading edge to trailing edge was reasonable due 

 to the higher sensitivity of the leading-edge pressures to angle-of -attack variation. 

 Good correlation with the quasi-steady predictions was due partially to the shaft- 

 rate frequency of the nonuniform tangential wake. Fluctuating effects will be small 

 for low-frequency, shaft-rate variations in the wake. Therefore, with small fluc- 

 tuating effects, a quasi-steady analysis should provide close agreement to the 

 measured result. Also, good correlation may be due to the incorporation of measured 

 mean results in the quasi-steady procedure, avoiding possible errors by the predic- 

 tion of mean pressure variation with advance coefficient. The unsteady theory by 



33 34 

 Tsakonas et al. ' produced a reduction in the first harmonic pressures in the 



first quarter chord at each radial station. The extreme nature of this trend as 



compared to both the measured and quasi-steady results produced little confidence in 



the accuracy of the method of Tsakonas et al. 



35 



