side seem little affected by the vortex, and perhaps are desensitized to J by its 

 position on the back. This effect at the tip occurred only on Propeller 4679 which 

 is characterized by swept-back blades with wide, swept tips. 



An attempt was made to compare the sensitivity of C with J along the chord 



30 P 



with the two-dimensional theory used to predict the pressure distributions at 



design. From Figure 17, the slopes of the first-order curve fits of C versus J 

 were plotted against chord position in Figures 21 and 22. The magnitude and sign 

 of the slope are proportional to the magnitude and direction of the sensitivity of 

 C to J. At the 0.5 and 0.7 radii, the pressure coefficients on the pressure side 

 of the blade were more sensitive to J than those on the suction side, while at 0.9 

 radius, the pressure coefficients on the suction side appeared more sensitive on 

 both propellers. Also, the sensitivity reversal at the trailing edge on the suction 

 side can be seen as a negative slope. 



Similar slope distributions along the chord were approximated from the two- 

 dimensional theory. With the same propeller blade sections, pressure distributions 

 were calculated over a range of assigned angles of attack a. Slopes of these 

 approximately linear relationships between C and a were calculated . The predicted 

 slopes on each side of each section were then normalized by a constant factor so 

 that the predicted and experimental slopes were equal for the gages nearest to the 

 leading edge. This procedure was used to make simple approximate predictions of the 

 slope or sensitivity distribution of C to J along the chord, because no simple 

 relationship between effective two-dimensional angle of attack and advance coeffi- 

 cient is known. The predictions show roughly similar distributions of slope, but 

 do not predict the differences between the measured slopes on the suction and 

 pressure sides. Also, as expected, the predictions do not indicate any sensitivity 

 reversal near the trailing edge. One might conclude from the gross similarity 

 between prediction and measurements, that the effective three-dimensional camber 

 distribution is similar to that of the equivalent two-dimensional model. More 

 accurate comparisons with a lifting surface model should be made to confirm this 

 hypothesis. 



28 



