Note that some aircraft (767-400, 787 etc) have a additional wing angle near the tip, which forces the vortex behind the lifting portion of the main wing, removing the requirement for having a fence (to stop the flow around the top of the wing). Nice explanation and drawings here..

http://www.aerospaceweb.org/question...cs/q0148.shtml

Mt comment was intended simply to dispel the notion that that winglets cannot be installed on larger aircraft. THat not all Boeing 777 will have them, although is a piece of factoid, it does not add any thing to the discussion of why winglets are installed. :)

What you are saying is that winglets help at higher speeds. It does not, however prove that the way they operate make the aircraft less efficient. Of course the extra weight of the winglets, would cost in more fuel consumption during take off. It is not that winglets do not help at all at slower speed but that their advantage,which increases with speed, is overcome by the weight penalty. The slow speeds during take off are not really all that slow to guarantee that there will be no leakage of high pressure air.

For the senior project for my undergraduate aero engineering degree, my group designed a delta-wing configuration while most of the other groups, given the same general specs, went with traditional wing. We introduced our project to the audience, which included the entire faculty of the department plus some high-profile invited guests, by saying "Our design represents a return to the most basic principles of aircraft design." At that point we pulled several paper airplanes from under the desk and flew them at the audience. (At this distance of several decades, I don't recall if they had winglets or not.)

Fortunately, our department chair didn't rescind my grad school recommendation.

(One of the other group members later became director of aircraft design for a major defense aircraft contractor, from which he has since retired. We must have learned something, though I didn't use it as much as he did.)

Exactly. The adoption of the winglet to improve flying efficiency is modeled after studying soaring bird species such as the golden eagle according to a documentary on the A380 by National Geographic.

Winglets to get a lot weirder
 

Aviation Partners targets later summer for second-generation spiroid winglets

http://www.flightglobal.com/articles...n-spiroid.html

LOL :D
^