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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.
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.
"how can they make accurate judgments and calculations without knowing exactly how much stress the wings can take before snapping?"
You don't need to. You test to 150% of the rated load factor.
I think for for airliners it is +3 -2 Gs. It has been a few years since I needed to know it.
So you would test the wing to 4.5 Gs.
If it passes it is good to go.
Testing to destruction is good data to have but not required. If they get to to a 9 g load and the wing doesn't break I really think they could stop. Any airliner pulling a sustained 3 Gs will end up on the nightly news.
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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.
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Airplane wings flex quite a bit more than you'd expect. Airliners.net has a great head-on shot of a 747 taking off that shows the wingtips flexed up higher than the fuselage. Kinda freaky looking.
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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.
A goal is a dream with a deadline
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.
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I am an aerospace engineer, however i am a propulsion engineer and not a structures guy. Ill try to add some light on the subject.
First off the requirement is a 1.5 saftey factor, ie 1.5 times greater load on the wings then they would encounter during operations. In the past, wings were always broken on new planes. Not only is this fun (engineers do like breaking things, its true), but it provides very useful data to validate your computer models and test methodology. Not often does an engineer get to shatter such an expensive and large article! Predicting before hand when a wing will snap can be very useful on future airplane designs to optimize the structural layout. Remember, any load past the 1.5 saftey factor just means you made the wing too strong, and thus it has extra weight!
Now a days, the structural FEMs (finite element models) and load definitions from CFD (Computational Fluid Dynamics) have become so good, that its not necessary to validate the tools. They have been validated before, and there is a high level of confidence. Someone mentioned above me that these wings were different since they are composite, but in fact commercial airplanes have had composites in the wings for a long time. The military has been making nearly all composite airplanes for even longer.
The A380 from Airbus ran into trouble a few years ago, as they designed the wing for 1.5 load factor, but on testing it only made it to 1.48. Hence they had to add extra weight and strengthen it. But being that they aimed for 1.5 and got to 1.48 shows you how accurate the tools have become.
There might also be a cost element in this decision. I believe Boeing could potentially use that model for some other purpose, whether it be passenger escape tests, wing fuel fire tests, wing fatigue tests, or maybe even just for a model to sit in a hanger somewhere and generate PR. Personally, im hoping to see a great video on YouTube of those wings splintering into pieces!
The question is do you test up to that 150% - which is all you need to do to certify the airplane - or do you test until the wing breaks? What, exactly, do you learn from an engineering standpoint by testing beyond the 150% limit? That you've over-engineered the plane? There's nothing really to be gained from testing so far beyond the structural loads that any aircraft will ever encounter - even an extra 50% is, by definition, already 50% more of a load that the wing will ever have to withstand.
Up until you get something that exceeds their theoretical load limit - either they misguessed or something else happened. There's a reason for the 150% requirement.
Personally, I'd test it up to 2-300%, just so they can slap it up as a 'safety' feature; Wings that are stronger than ever before. Twice as strong as FAA requirements!
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Take a look here. There was a 1.5 inch difference in the diameter of the Section 41 (nose and cockpit) and the Section 43 (forward fuselage, where the forward entry door is). The parts are made in Wichita and Charleston, SC. They have managed to join them now, but the job was "challenging".
Now I am an engineer at an aircraft MRO. Once these things hit more than 15 years old, there are going to be a million problems with this fuselage. Carbon fibre is a very different beast to aluminium, or even fibreglass. For one, the carbon is a conductor of electricity, which can lead to galvanic corrosion (the circumferential frames are still aluminium, there are still metallic fasteners going through the skin to attach them). Also, repairs are going to be an absolute bitch.
Twice in the last month, we have had to fix large holes in the side of aircraft due to trucks driving into the side of them. These incidents happened at outstations (where there were no major repair facilities) and we had to send out a small team to assess and repair the damage. In both instances these were done by a repair engineer, inspector and a couple of sheet metal workers in a couple of days. They took a sheet of metal, an air compressor and a bucket of rivets.
Currently, composites are used on a number of components on almost all aircraft. Invariably they are removable components, like flight control surfaces, or fairings. In order to repair them, they are usually removed from the aircraft and repaired in a composites shop, where temperature and humidity can be controlled - preferably in an autoclave.
Now, how the hell is anyone going to remove a fuselage section to drag it into a shop?
In fact, there is a seperate "limit load" test that is performed at 100% and must show no detrimental permanent deformation. It is not unheard of that a structure will pass the ultimate load test yet fail the limit load test because of this criterion even though the limit load is smaller.
A goal is a dream with a deadline
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.
Lose: misplace or fail || Loose: not bound together
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.
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.
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