Dear Captain Lim,
I will quote part of an answer you gave to Chris Jeker:
You will notice that the Boeing 777 does not have winglets like the A340-600 or Boeing 747-400. Someone once asked me whether fitting winglets on the Boeing 777 would make any difference to the efficiency of the wings. I am made to believe that it does not, as the Boeing 777 is the first aircraft to be completely designed by computer. Therefore, the current wing design is the most efficient one in terms of fuel saving.
Winglets would make B 777*s wings more efficient. You are wrong thinking that solely because the plane was computer-designed, the wing is at its maximum efficiency!
When the 777 were designed, the winglets were not a "fashion" in air construction. If you are not aware of the advantages the winglets bring, of course you don*t use them as a design solution. Simulating flows around complex geometries like airplane wings is extremely time consuming-not mentioning the resources involved. Later, all redesigned wings featured winglets! B 747, B 737, B 787, not mentioning Airbus. Boeing would fall a bit into the ridiculous side if they admitted that a plane designed in late 80*s-early 90*s has an obsolete wing! So, they keep it like this...
Of course the B 777 is a tremendously efficient machine. I only say it could be even more efficient. But I think Boeing won*t redesign the wing too soon.
Ask me a question: Why the first thing that was redesigned to the 737 families was the wing, in order to make it more efficient? And it features winglets, for the -800 and -900... even huge ones. Do you remember the previous 737s? They had some small vertical plates on the upper part of the wing. That is a classical example of a not very good wing-improved so as to respond the requirements. Why I mention this? Because I want to emphasize the fact that one can find always simple, efficient solutions for improving even such complex a thing like an airplane wing! And the winglets IMPROVE the wing*s efficiency.
Even though I have not mentioned this in my past answers, I believe, one of the reasons why winglets were not introduced on the Boeing 777 was partly due to economics and structural design issues.
My argument was not that, winglets design would not contribute to fuel saving. Yes it would. Remember, the Boeing 777 has a very long wing and the outboard sections are not that heavily loaded. Because of this, the pressure differential between the top surface and bottom at the tip is small - and therefore the induced drag is also little. So any gain from creating winglets is quite modest. Hence, I believe, the *pay off point* was not in favor of having winglets as any fuel saving would not exceed the weight penalty. (Sure, the Boeing 737NG have winglets, but their wings design is such that it had higher loading at the tips and therefore, winglets were attractive in them.)
So, in the final analysis, Boeing felt that it was unnecessary to have winglets on the Boeing 777. Even on the newer Boeing 787 - they have not added these so-called fuel-efficient winglets but rather raked wings. Further, on the latest Boeing 777-200LR that holds the longest-range record in the world today (see here) - Boeing found that they do not need to install winglets on them.
However, I believe, the Boeing 777-200LR/300ER do have more heavily loaded wings. With improved performance of winglets designs, its installation would probably have some fuel-saving effect on their very long-range flights. For instance, on a 10,000 kilometers flight, half a percent fuel saving would be around 500 kilograms - that is more than what the winglets are going to weigh. In that sense, I do agree with you that winglets do play a part in fuel saving.