VARIATION IN PRESSURE COEFFICIENTS OVER A RANGE OF J 



As the propeller advance coefficient is reduced, the increased loading will be 

 a result of increasing pressures on the pressure side of the blade and decreasing 

 pressures on the suction side. Analogous to a planar wing, pressure coefficients 

 toward the leading edge will be most influenced by angle-of -attack changes. For a 

 propeller, variations in J are closely related to variations in blade section 

 angle-of -attack. 



Graphs were constructed presenting the variation of pressure coefficient with 

 advance coefficient for each pressure gage location. Initial plots revealed pro- 

 blems on certain gages due to the effect of speed on the measured pressure coeffi- 

 cient. The uncorrected pressure coefficients plotted in Figures 16a and 16b demon- 

 strate how the speed effect shifted C values based on the operating speed of each 

 J condition. By matching speed runs from Table 5 to the plotted C values, it was 

 apparent that the values of C corresponding to higher speed runs at low advance 

 coefficients were uniformly shifted below the values extrapolated from measurements 

 of C corresponding to high J, low speed runs. 



To better describe the relationship between C and J, it was hypothesized that 



the variation of C with speed was independent of J and only dependent upon the local 



inflow speed, V T . This assumption permitted the pressure coefficient to be corrected 



for the observed speed effect. Third-order, least-squares polynomials were fitted 



to the C versus V relationships shown in Figure 14 representing the observed speed 



P K 

 effect at design J. All measured pressure coefficients were corrected to a baseline 



condition at design J of 7.88 rps for Propeller 4718 and 8.20 rps for Propeller 4679. 



The correction was made by subtracting from the measured C the difference in C from 



P P 



Figure 10. This difference is between that at the base condition and that corre- 

 sponding to the local speed of the C to be corrected. Again, this assumed that the 

 speed effect was independent of J. 



The improvement to the C versus J relationship can be seen iu the plots 

 corrected for speed; see Figure 16. The pressure coefficients shown with relatively 

 large speed effects tend to collapse to approximately straight lines indicating a 

 generally linear relationship. Similar relationships resulted with gages with little 

 or no speed effects, differing primarily by the slope of the line. Because of the 

 substantial improvement obtained, the assumption was considered accurate. 



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