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Complete sets of time-dependent solutions 

 for Stokes' slow-motion equations are con- 

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 of vortices. 



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Wing tip modifications, such as spinning 

 blades, obtain benefits similar to mass 

 injection without the use of additional 

 power. 



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Investigation of several devices to reduce 

 vortex core tangential velocities. Porous 

 tip found to be the most effective (factor 

 of 10). 



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