Q: How much wing could wing flex flex, if wing flex could flex wings?
A quick look at any artist's rendering of the 787 in flight, the eye is immediately drawn to the wings which look, by many accounts, ready to flap like a bird. One common question that has frequently been posed is whether or not when this aircraft takes to the sky, will wing flex do for 787 what the hump did for 747?
Only time will tell on that question, yet, I wanted take a closer at the wing flex issue.
The first issue I wanted to tackle was to ask the question, "Just how much are those wings really flexing in those pictures?"
I found my answer with a little help from Photoshop and the Boeing 787 Airport Planning Document. I overlaid the neutral position of the wings on top of the stylized rendering seen above. The results were surprising, the wing flex we see in the renderings of the Dreamliner comes out to roughly 10 feet.
There is no universal amount of flex to be expected in the wing, though a deflection of 10 feet doesn't appear beyond the realm of possibilities. However, this is really dependent on a number of factors, including (but not limited to) the g loads on the wing, as well as the weight and attitude of the aircraft.
Taking the idea one step farther, last week 787 Vice President of Engineering and Technology Randy Harley told a group of reporters that the wings would deflect a full 26 feet when 150% of the maximum loads were applied to the 787 wings. As a required by the FAA, all commercial aircraft must be able to withstand at least three seconds of 150% expected maximum loads on all major structures. For a historical comparison in January 1995, the 777's wings deflected 24 feet at 154% max load before they snapped in spectacular fashion.
Boeing hasn't said publicly how much they expect the 787 wings to flex before they break. A final decision is expected early next year as to whether or not to break the full-scale wings on ZY997, the static airframe, which is parked at 40-23 for testing.
Yet, a full-scale breakage of ZY997 could be very revealing. If the wing withstands loads well above 150%, then it's an important indicator that the wing could be too strong and not require as much internal structure allowing for both future growth and valuable weight savings.
Former 787 Program Manager Mike Bair speculated in jest that during a wing break test the wingtips could possibly touch above the fuselage. Whatever the result of the test is, wingtip contact or not, the world is anxiously awaiting the abandonment of artist renderings in favor of the sight of flexing wings lifting the 787 Dreamliner into the sky for the first time.
Original image courtesy of Boeing, edits by FlightBlogger
A quick look at any artist's rendering of the 787 in flight, the eye is immediately drawn to the wings which look, by many accounts, ready to flap like a bird. One common question that has frequently been posed is whether or not when this aircraft takes to the sky, will wing flex do for 787 what the hump did for 747?Only time will tell on that question, yet, I wanted take a closer at the wing flex issue.
The first issue I wanted to tackle was to ask the question, "Just how much are those wings really flexing in those pictures?"
I found my answer with a little help from Photoshop and the Boeing 787 Airport Planning Document. I overlaid the neutral position of the wings on top of the stylized rendering seen above. The results were surprising, the wing flex we see in the renderings of the Dreamliner comes out to roughly 10 feet.
There is no universal amount of flex to be expected in the wing, though a deflection of 10 feet doesn't appear beyond the realm of possibilities. However, this is really dependent on a number of factors, including (but not limited to) the g loads on the wing, as well as the weight and attitude of the aircraft.
Taking the idea one step farther, last week 787 Vice President of Engineering and Technology Randy Harley told a group of reporters that the wings would deflect a full 26 feet when 150% of the maximum loads were applied to the 787 wings. As a required by the FAA, all commercial aircraft must be able to withstand at least three seconds of 150% expected maximum loads on all major structures. For a historical comparison in January 1995, the 777's wings deflected 24 feet at 154% max load before they snapped in spectacular fashion.
Boeing hasn't said publicly how much they expect the 787 wings to flex before they break. A final decision is expected early next year as to whether or not to break the full-scale wings on ZY997, the static airframe, which is parked at 40-23 for testing.
Yet, a full-scale breakage of ZY997 could be very revealing. If the wing withstands loads well above 150%, then it's an important indicator that the wing could be too strong and not require as much internal structure allowing for both future growth and valuable weight savings.
Former 787 Program Manager Mike Bair speculated in jest that during a wing break test the wingtips could possibly touch above the fuselage. Whatever the result of the test is, wingtip contact or not, the world is anxiously awaiting the abandonment of artist renderings in favor of the sight of flexing wings lifting the 787 Dreamliner into the sky for the first time.
Original image courtesy of Boeing, edits by FlightBlogger
on May 30, 2008 7:38 AM | Reply
Is that 26 foot deflection from the neutral position or is it 26 feet from the normal cruise condition?
If you look at FAR 25.337 and do the maths, you'll see that the load factor for vertical maneuvres is 2.5, i.e. at limit load 2.5g, including the 1g of supporting the aircraft's weight. For the static testing this is taken to 3.75g (i.e 150% of the limit load).
If the 26 foot deflection is relative to the cruise deflection, and assuming the deflection is roughly linear with load, then a load of 2.75g above the cruise load means that 1g give a 9.4 foot deflection - pretty close to your ballpark.
If the 26 foot deflection is from your neutral position, then that gives a roughly 7 foot deflection in cruise.
Looking at the 777 video, the deflected wing looks more bent than your depiction of 26 feet, so I'm inclined to think that the quoted deflection may be in addition to the in-flight deflection of the wings, so I guess the rendering is accurate.
on May 30, 2008 12:04 PM | Reply
The wing flex will look far more extreme than the picture shows. The photo shows a vertical flex, but the wing will curve inboard (it will flex up a radius of 1/2 the wingspan; actually slightly more curve than the radius because the root structure is stiffer than the tip). At 26 foot deflection the wing tips will be closer together with dramatic curvature - the picture shows flat bending with the wings stretching outward.
on May 30, 2008 9:42 PM | Reply
Most interesting indeed.I keep reading that Boeing have yet to decide whether they will test the wing to failure? I had always assumed that it MUST be done.Perhaps I have misread the press releases? And of course, a test to failure is done without airflow. In the air an extreme overload to beyond Ultimate Load Factor would involve major aerodynamic factors including aero-elastic twisting/warping as the outer mainplanes bent up...I think? So the static test would not truly represent an in-flight situation. But I guess computer simulations would do all that. Can hardly expect the test crew to wear 'chutes....
on May 30, 2008 11:33 PM | Reply
Here's another simpler way of looking at it that doesn't involve Photoshop.
The 777-200A wing was basically the same span as the 787-8 wing. The 777 wing deflected 8% less at 2% greater load... ergo, the 787's wings will flex 10% more than the 777's under a given load.
In your subjective experience, does the 777 wing flex 90% as much as shown in those 787 renderings? I say, no way!!
on June 1, 2008 8:42 PM | Reply
I Liked your article, interesting way of thinking. I'll translate most of it and post on my blog with due credits. Thanks.
on June 2, 2008 9:55 AM | Reply
Unfortunately, what WingedMigrator is forgetting is any comparison between the 777 & 787 wings is an apples to oranges one, given that the majority of the 787's wing structure is composite, and the 777 is aluminum.
on June 2, 2008 5:56 PM | Reply
They need to replace the static frame wingbox with one that represents the production manufactured wingbox, not one with the non-production stiffener plates, which will never be produced.
I hope the FAA does not let them pull a fast one here, testing one with stiffener plates which might be far stronger than the composite production wingbox, which already failed once in test.
Jon - please investigate this.
on June 3, 2008 2:27 AM | Reply
Tim, there is nothing mystical about CFRP.
For the present purpose, an airplane wing is a cantilevered beam made of an elastic material that obeys Hooke's law. CFRP or aluminum are identical in that respect, and deflection will be directly proportional to load factor. The load factor vs. deflection numbers given in Jon's post are sufficient to do the math, which shows the 787's wings will not look nearly as flexy in real life as they do in the computer renderings.
(Jon, if this is a duplicate, please delete)
on June 3, 2008 7:16 AM | Reply
Anonymous,
Maybe you should take it up with the FAA. The test is as much to validate the modelling as it is to test the actual part. The results from the test are used to show that they can analyse the wing accurately.
There will be no requirement to re-test the different versions of the wing for the -9 and -3. Why do you think that is? Or why do you think Airbus didn't need to re-test the A380 wing after it failed a little below 150%? Or why was a full-scale test not even required for the 737NG wing?
The wingbox which has been tested is not the production wingbox either, so Boeing will not be putting a failed wingbox design in production aircraft at all.
There seem to be a lot of armchair experts who have never worked on an airplane program criticising both manufacturers.
on June 3, 2008 7:47 AM | Reply
Don't forget that CFRP performance degrades with temperature and moisture uptake. Because testing at ambient temperature and environment (e.g. in a hangar) doesn't cover the "worst case" required for certification, additional load is usually applied to cover this degradation. Therefore, the deflection observed in the static test may be considerably greater than what you might expect from the flight cases! Unless, of course, Boeing have found an alternative way to satisfy the authorities...
From a purely practical point of view, lots of wing flex makes it more difficult to get the load into the structure. Ideally, you want the load to be applied perpendicular to the wing surface; if the wing is bending significantly, keeping the load orientation correct can be a bit tricky! I'll be interested to see the loading rig.
By the way Jon, great blog - keep up the good work!
on June 6, 2008 6:35 PM | Reply
A video boeing put out a couple of years ago highlighted that the 787 would deflect significantly more than any other jet.
From direct observation, the 777 wing tip only deflects some 5 ft during cruise, some 1/2 of what the 787 does, for similar spans, but the 777 is far heavier.
One wag joked(?) that the only reason MCboeing put larger windows on the 787 so that the passengers would not get concerned when the lost sight of the wingtip....
on June 7, 2008 10:22 AM | Reply
This is good information regarding the wing flex. I like to see similar good information
thanks
on June 8, 2008 4:14 PM | Reply
> From direct observation, the 777 wing tip only
> deflects some 5 ft during cruise, some 1/2 of
> what the 787 does, for similar spans, but the
> 777 is far heavier.
It always amazes me how people blindly keep their faith, even in the face of quantitative evidence to the contrary. The numbers posted by Jon are quite sufficient to debunk the notion-- see my comment #4.
It doesn't matter anyway: preconceived notions will always color our perception of things, and I have no doubt that when the 787 finally takes to the air, there will be comments about how amazingly flexy those wings look... completely regardless of their actual flex, of course.