The wing flex of a big airline type plane is engineered in for several reasons,
1. The plane would be too heavy to build a solid wing with no flex.
2. The ride for the passengers would be extremely rough in any kind of turbulence if the wings didn't flex at all (the are like shock absorbers)
3. Using today's building materials (aluminum) it is most "cost effective" to build -for- the flex instead of trying to build -out- the flex.
But, the new 787's are being designed and built of Carbon Fiber, to save weight. I'm not fully aware of what they are doing with the wing on that, but less flex at takeoff would give it more lift, ie. higher payload, which is what it's all about in comercial aviation.
Wings do flex quite a bit at takeoff, ever seen a fully loaded 747 take off? WOW, those wings really flex up!!
But some amount of lift is lost as they bend up, no doubt. Once the plane levels off at cruising altitude, the wings are not so bent, only when you hit turbulence will you see the wingtips flapping up and down! Quite funny to see really, but necessary to give a smoother ride.
I know there are some guys on this board who actually work for Boeing so maybe they can help me out here with a fuller explanation on how they engineer them.
