MODELS 



The following is a physical description of the tip vortex cavitation delay 

 models. The criteria used in determining these specific designs will be discussed 

 later. 



The hydrofoil models used for the present study are all identical except the 

 various local tip modifications: 



Parent 



The parent hydrofoil had an elliptical planform with a semispan and root chord 

 length of 12 in. (0.305 m) , giving an aspect ratio AR = 2.55 for full-span flow, as 

 approximated by the test section wall. The cambered airfoil section of the hydro- 

 foil consisted of NACA-66 (DTNSRDC modified) thickness distribution with a NACA a = 

 0.8 meanline camber distribution. The hydrofoil had a constant spanwise thickness 

 to chord ratio t/c = 0.10 and a constant spanwise camber to chord ratio f/c = 0.025. 

 A sketch of the airfoil section and a listing of nondimensional section offsets is 

 given in Figure 6. In addition, the hydrofoil had a design lift coefficient (all 

 lift due to camber) C = 0.186 at an angle of attack a = 0.567 deg. The models were 

 cast with an aluminum alloy and precision machined using numerical control tech- 

 niques. Grids, consisting of a series of chordwise and spanwise lines of known 

 thickness and spacing, were penciled on the anodized foil surfaces. 



Bulb 



The two bulb modifications were limited to the tip region of the hydrofoil; 

 i.e., spanwise 2.4 in. (0.061 m) and 2.0 in. (0.051 m) inboard from the tip for the 

 large and small bulb, respectively. The bulbs consisted of thickened airfoil sec- 

 tions similar to the parent with chord lengths equal to the parent foil. The maxi- 

 mum thickness of the bulbs was 1.6 in. (0.041 m) and 0.8 in. (0.020 m) , respectively, 

 at a spanwise distance inboard from the tip of 0.8 in. (0.020 m) where the corre- 

 sponding parent thickness was 0.43 in. (0.011 m) . Figure 7 shows a photograph and a 

 plot of the thickness distribution of the two bulbs. 



Roughness 



The roughness was applied to the extreme tip area of the parent foil pressure 

 and suction sides. For the optimum distribution, the pressure side application was 



10 



