Expert: Paul Soderman - 10/28/2008

Question
Dear Paul,

Great day!

I believe when an aircraft in flight condition, the twin vortex will be generated behind the wing tip. And Winglet did serve the purpose to reduce the induce drag.

Question:-
1) In steady flight condition, lift force = weight = Function ( vortex strength). if the weight of aircraft remain unchange, how could winglet reduce the vortex strength of wing-tip vortex? Some books say is "redistribute" wing-tip vortex or vertical diffusion of wing-tip vortex. Which one is correct?

2)Airbus is using wing fence while Boeing is using blended winglet. Is there any rationale behind? I have read up some reference but i could not get satisfying answer.

3)Richard Whitcomb has invented Winglet. Why Whitcomb winglet is not popular in modern commercial air transport ?

4)Is there any recommended Text to study specific topic of Winglet/Wingtip?

5) Sorry for asking too much,hope to hear your advice soon.

Have a nice day!

Regards,
How Run

Answer
How Run
1.  The winglet does not change the total integrated vortex strength, it merely spreads the vortex velocity field in such a way as to reduce upwash at the wing, which reduces induced drag.  You are correct, weight, lift and total vortex strength are unchanged.  Only the integration area changes.
2.  The wing fence looks to me like a winglet pair - one winglet extending upward, one extending downward.  Airbus must have wind tunnel data that show that the fence is better for some reason. I don't know what that reason is, but there is often more than one way to get the same effect.  It is possible that structural requirements or weight or economics was a factor.
3.  The winglet developed by Whitcomb is very popular on commercial aircraft.
4. Sorry, I would google winglet and see what pops up.
Paul