Back in 2008, Boeing published a video of an isolated 50-foot section of a 787 wing being flexed to the point of failure. In the test, the wings were flexed upward of 25 feet (~150% of the maximum expected forces during normal operation). On a recent trip to Taiwan I happened to sit in perfect view of the entire left wing span of the 747 we were flying aboard, so I decided to take a look and see just how much the wings actually flexed during 'normal operation'.

While taxing to the runway, the wing was near completely flat. Looking at how large the turbojet engines are, you must think about how much constant stress the wing is under.

In the air, the wing rose some 7-12 feet (based on visual estimation). It seems the highest point of flex occurs between the wingtip and left-most engine.

Interestingly back on land as the plane taxied to the gate, the wing actually appeared to be a little higher than it was prior to flight. Even accounting for the different camera angles, the angle of that last flex point is apparent.