Boeing's New 787 Wings — Amazingly Flexible

An anonymous reader writes "Boeing is making the wings of its new 787 out of carbon fiber instead of metal. That means the wings are so strong and flexible that they could bend upward and touch above the fuselage — or come close. The company is expected to deliver the first 787 to All Nippon Airlines in May 2008. 'Boeing has completed static testing of a three-quarter wingbox, but engineers are still considering whether to limit testing of the full wing to a 150% load limit held for 3 sec. or to continue bending it to see when it breaks. 'There's a raging debate within the engineering team to see if we should break it or not,' says [787 General Manager Mike] Bair.'" They have come a long way in wing flexibility.

Comments

Why (not)?

[borizz]

You could, instead of downright trying to see how much it will take, try to get it up to 200% (or something, I'm not an aerospace engineer) and see for how long it can hold up to extremes like that. Might be more valuable data. Maybe someone more in the know can elaborate.

Old News: Flexible wings on the Boeing B-47

[G4from128k]

Thin flexible wings date back to the Boeing B-47. Up until this plane appeared in 1947, planes tended to have thick rigid wing structures. Advances in aeronautics, fluid dynamics, and structure design enabled engineers to create thin flexible swept wings that offered lower drag at high speed without flutter or breakage. The wings of B-47 (and B-52) were so floppy, they needed outrigger wheels to keep the wings from dragging on the ground during landings and take-offs.

Well...

[StressGuy]

The actual requirement from Title 14, Code of Federal Regulations, Part 25, Subpart C, paragraph 303 is where ultimate load definition comes from:

Unless otherwise specified, a factor of safety of 1.5 must be applied to the prescribed limit load which are considered external loads on the structure. When a loading condition is prescribed in terms of ultimate loads, a factor of safety need not be applied unless otherwise specified

The three second requirement comes out of paragraph 305(b):

(b) The structure must be able to support ultimate loads without failure for at least 3 seconds. However, when proof of strength is shown by dynamic tests simulating actual load conditions, the 3-second limit does not apply. Static tests conducted to ultimate load must include the ultimate deflections and ultimate deformation induced by the loading. When analytical methods are used to show compliance with the ultimate load strength requirements, it must be shown that--
(1) The effects of deformation are not significant;
(2) The deformations involved are fully accounted for in the analysis; or
(3) The methods and assumptions used are sufficient to cover the effects of these deformations.


If our intrepid engineers manage to test to 200% for 3 second, then somebody is going to come along and say, "let's see if we can make the wings lighter"

Good thing or bad thing?....depends upon your point of view I guess.

As it turns out, validating airframe structures with respect to FAA airworthiness requirements is kinda what I do for a living.

Re:I hope they test it!

[TheRaven64]

A deformed wing may not be aerodynamic enough to fly with, but it may slow your descent enough to turn a fatal crash into a near-fatal crash. A shattered wing is unlikely to do any good at all.

Re:I hope they test it!

[NeilTheStupidHead]

While this sounds good, looking at the video of the 777 wings, I doubt they would hold up under windspeed after that kind of damage. The aluminium panels buckled and ripped free of the rivits and the way the aircraft sagged as they did suggests that the panels are a signifigant structural component of the wings. This 150% number they keep throwing around is 150% greater than the maximum load the aircraft wings would be expected to face (which probably has it's own safety margin thrown in). Given that aircraft can and have been flown into hurricanes, and hurricanes can have maximum sustained winds of over 300 kph, I don't think I'll be worrying about the wings failing the next time I get on a Boeing aircraft.

It's probably designed to different criteria

[Solandri]

The fact that the wing is so strong suggests that it may be being over-designed.

It's probably not be overdesigned per se. Composites tend to exhibit much more strain (deflection under stress) than traditional materials. So a lot of times, the maximum deflection becomes the prevailing design criteria, not the maximum sustainable load. Most likely, the specifications for how much the wing is allowed to deflect under normal load is a more stringent criteria than how much load the wing can support without breaking. So they have to add more material to reduce the deflection, which adds strength as a side effect. (They could probably put additional stringers inside or switch to a sandwich structure to gain stiffness without additional material, but that could complicate fuel capacity and inspections.)

The first time this was really driven home to me was in undergraduate school in '88. A classmate was working on a portable carbon-fiber bridge project for the Army. It had to support the weight of a main battle tank crossing it. In the full-scale test demo, the general overseeing the project commented that you'd get one and only one tank crew to cross the bridge. He felt that after the other tank crews saw how much the bridge flexed, there was no way they'd want to drive on it.

Re:missed the best part...

[arivanov]

Comfortable? Forget it. I hate flying on any of the new Boeings. Have you flown on a 777 in a storm? You can actually see the fuselage bend and buckle and the luggage compartment above the central seats move by nearly a foot left and right. While the engineer in me knows that this way it is actually more likely to survive through turbulence and load, the little scared mammal in the depth of my brain (which everyone has) screams "run for your life". No thanks, had that twice and enough is enough. From there on I try to chose long haul flights by Iberia or one of the other airlines which operate "boeings and dogs not allowed" policy and use A340 on transatlantic routes. It is considerably more comfortable.