quasi-steady loading correction. Also there was a negligible variation in C with 

 speed. This implies good credibility in measurement at this gage location. 



The quasi-steady prediction at this gage location (0.5R, 0.5x/c) supported the 

 measured result by agreeing with both the first harmonic phase angle and amplitude. 

 The good quasi-steady correlation relates to the reversed slope of the C versus J 

 curve in Figure 17. As J decreases, the pressure coefficient C decreases rather 

 than increasing as with the other gages on the pressure side at the 0.5 radius. 



This anomaly in the fluctuating pressure was supported by the steady measure- 

 ments conducted over a range of advance coefficients. An obvious polarity error at 

 this gage location would not explain this anomaly due to the correct sign of the mean 

 pressure coefficient, C . The evidence indicates that a real-flow phenomenon pro- 

 duced the unexpected behavior at this location. Separation is not an obvious hypoth- 

 esis due to its location on the pressure side of the blade, but is a possibility due 

 to the adverse pressure gradient starting at the 50 percent chord position. 

 Physically, in uniform flow, the inclined flow, increased loading increases the 

 pressures on the pressure side of the blade and thereby decreases the local velocity. 

 At this gage location (0.5R, 0.5x/c), the local velocity increased instead of 

 decreasing. The anomaly could be due to a severe local effect of various induced 

 velocities in the flow regime, possibly causing large cross flows. 



UNSTEADY PRESSURE MEASUREMENTS OVER A RANGE OF J 



The first harmonic pressure coefficients and phases were measured over a range 

 of advance coefficients for both propellers, and are shown in Figures 28 and 29. 

 Quasi-steady predictions of the first harmonic amplitudes were calculated over 

 similar J ranges and plotted in Figures 30, 31, and 32. 



A general trend on the suction side of the blades indicated a decrease in the 

 measured first harmonic amplitudes in the midchord region, with decreasing values of 

 J. The 0.9 radius position did not follow this trend. Propeller 4679, at 0.9 

 radius, produced a sharp increase in the first-harmonic amplitude at the leading edge 

 with decrease in J, while on Propeller 4718 the same trend occurred to a lesser 

 degree. This trend at the tip could be due to the separation of the tip vortex as 

 J decreases. 



38 



