chord, and greater towards the trailing edge. There, a large negative pressure 

 coefficient implies an acceleration of flow at the trailing edge on the pressure 

 side, in somewhat better agreement with the suction-side pressure coefficients at 

 the trailing edge. 



A possible explanation for some of the irregularities in the measured pressures 

 at the 0.5 radius involves the flow perturbation produced by the fairwater and hub of 

 the propeller. The propellers were operated on a downstream shaft, with a hemi- 

 spherical fairwater ahead of the blades. The blades were mounted to a protruding 

 spherical section intersecting the cylinder to which the fairwater was attached, as 

 shown in Figures lb and 2. The hub would disturb the flow into the blades near the 

 root and the perturbation velocity would depend on the axial position of the blades 

 relative to the hub and fairwater. The increased axial speed would increase the 

 advance coefficient locally at the blade-section leading edge near the hub, thus 



causing a decrease in -C on the suction side and an increase in -C on the pressure 



P P 



side. Potential flow calculations on a hemispherical headform predicted a maximum 



axial velocity increase of 3 percent of free stream speed at the 0.5 propeller 



radius. A 3 percent increase in advance coefficient changed the measured pressures 



at the leading edge only slightly, but in the direction of the theoretical result. 



Therefore, this effect could only partially explain the trends of the measured 



pressures near the leading edge at the 0.5 radius. 



Image effects of the hub are another possible explanation for the trends of the 

 measured pressure distribution on the suction side at the 0.5 radius. Unfortunately, 

 there is no simple way to approximate the effect. Any influence would be tied to the 

 roll-up process of the hub vortex and the boundary condition on the hub. At present, 

 there is no available lifting-surface design computer code that takes the hub into 

 account. 



At the 0.7 radius, data from Propeller 4718 match the theory reasonably well, 

 but there is a theoretical over-prediction of pressure magnitudes in the midchord 

 regions. 



At the 0.7 radius, the flow over Propeller 4679 produces irregular pressure 

 distributions. On the suction side, the measured pressure distribution has a suction 

 peak at the leading edge followed by a drop across the midchord, and an increase 



17 



