and reduce the strength of the forming tip vortex. However, these gains 



are accompanied by a large drag Increase and a loss in overall efficiency. 



The drooped wing is a similar concept which represents one-half of an end- 



1 6 

 plate. Experimental studies with this concept indicate an optimum drooped 



wing attachment angle, with respect to the parent wing, of 90 degrees. 



The winglet (shown in Figure 4) is a recent sophisticated adaptation 

 of the endplate which employs highly efficient lifting surfaces at the wing 

 tip. This concept has received attention in the aircraft industry as a 

 means of increasing the wing cruise efficiency through reductions in the tip 

 vortex- induced drag. Winglet model studies have reported reasonable in- 

 creases in performance and reduced strength of the near-field tip vortex. 

 These investigations have also shown that, to be effective, the winglets 

 must be designed with extreme care. The present winglet design for aircraft 

 is quite fragile and, thus, may not be structurally suited for application 

 to marine propellers. 



The fence is a form of vertical endplate which is repositioned inboard, 

 of the wing tip. Its characteristics are similar to those of the endplate; 



however, the effects on the tip vortex decrease as it is moved inboard or 



18 

 away from the wing tip. Reports show that the fence increases the level 



of turbulent flow entering the tip vortex and, thus, reduces the far-field 

 vortex tangential velocities by as much as 70 percent. However, due to the 

 inboard location, the fence would have less effect on the tip vortex near- 

 field region — the region of interest for propeller tip vortex cavitation. 

 Contravanes (shown in Figure 4) generally refer to a localized group 



of small fences which are specifically oriented in such a way as to re- 



14 

 direct the incoming flow and oppose the vortex rollup. Past studies 



indicate that contravanes are more effective than fences and maintain good 

 efficiency. 



In addition to the reported performance limitations, all of the 

 endplate- type concepts discussed above would be susceptible to local cavita- 

 tion. Also, secondary vortices would be likely to emanate from the various 

 surface intersections. Therefore, the use of these devices on marine 

 propellers appears questionable. 



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