BULBOUS TIP 



A wing tip bulb is defined as any selective increase in the wing tip 

 thickness, e.g., aircraft wing tip tanks or pods. The thicker tip viscous 

 boundary layer associated with the bulb increases the viscous mass flow 

 entering the vortex core, thus destabilizing or dissipating the vortex core 

 energy. In addition, the bulb may act in a manner similar to an endplate 

 and retard the tip vortex rollup process. 



The bulbous tip concept has been applied, with varying degrees of 



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 success, to both model and full-scale marine propellers. The benefit of 



this device is shown in Figure 9, which presents cavitation inception and 



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 efficiency data for a pair of model propellers, one without and one with 



a tip bulb of thickness approximately 2 percent of the propeller diameter. 

 As shown, the variation of cavitation inception index a as a function of 

 advance coefficient J is substantially lower for the bulbous tip propeller; 

 e.g., for J ~ 0.65, Aa ~ 7.5, which corresponds to a ratio of free-stream 

 speeds of 1.4. The bulbous tip propeller suffers a maximum decrease in 

 efficiency r\^ - 4.5 percent. 



The results from the bulbous tip work appear promising, and the bulb 

 may prove to be a viable concept for delaying tip vortex cavitation incep- 

 tion. However, the bulb must be carefully designed to minimize both local 

 cavitation and efficiency loss. 



TIP DUCT 



The tip duct consists of a faired tube attached to the transverse or 

 chordwise edge of the wing tip. The duct outer surface acts similar to the 

 bulbous tip, while the inner surface tends to destabilize the vortex core 

 by retarding the core entry flow. Also, reverse swirl vanes can be added 



inside the duct in an attempt to induce rotational velocities to oppose the 



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 vortex rollup. Tip duct investigations report only modest increase in 



effectiveness with a large increase in drag. Thus, this device does not 



appear to be suited for the marine propeller. 



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