Hadler and He eke r 



of Attack a 



Fig. 24 - Lift and drag coefficients of wedges 

 in various flow regimes 



the leading edge, can have a marked effect. Flattening or blunting the leading 

 edge reduces both the transition angle of attack and the magnitude of the shift 

 of the lift coefficient. Reducing the hydrofoil-aspect ratio increases the transi- 

 tion angle of attack. 



The absolute speed of a given hydrofoil also has a marked effect. Increas- 

 ing speed tends to reduce the transition angle of attack and reduces the magni- 

 tude of the change in lift, even at a Reynolds number well above the laminar- 

 turbulent transition region. 



With this background we are now better able to analyze the action of the 

 propellers tested. Figure 25 shows Propeller 3768 operating semisubmerged 

 in the Hydronautics variable-pressure channel, Ref. [11]. The j = 0.75 condi- 

 tion is base-vented, whereas the J = 0.35 condition is fully vented. The differ- 

 ences in both the spray and cavity patterns may be noted for those two oper- 

 ating conditions. 



The abrupt shifts in the k^ and Kq coefficients noted on Propellers 3820 

 and 3768 are quite clearly the points at which transition occurs from base- 

 vented to fully vented operation. As could be expected the efficiency drops 

 when the sections become fully ventilated. The drop in lift-to-drag ratio ac- 

 counts for this. 



1474 



