Perhaps the most notable aesthetic feature of 787 is its flexing wings. They look downright glorious. You can't help but stare at them if you come across a photo of a 787. Boeing used that well in its marketing materials for both 787 and 777X. The flexing wings are highlighted almost in all of their videos and ads.

Keeping looks aside, I am starting this thread because I can't help but wonder, how can Boeing just market and present this behavior to the industry as if it is a good thing from an aerodynamic point of view? This behavior by itself reduces lift and increases drag. Just like the dihedral effect, only the vertical component of lift in level flight actually supports the airplane. The horizontal component of lift is simply a waste. The wing flex of 787 factors more of its wing area into the horizontal component resulting in less effective span, more effective wing loading, less lift, and more drag ( that part is proportional to the sine of the dihedral angle + flex-induced angle). Sure, all commercial planes have a small dihedral angle for stability reasons, but this flex definitely adds more to it that some wing area is basically lost over it.


I understand that this was merely a result of having a lighter wing, and it definitely was not a design target by itself. I also understand that the gained benefit of the lighter wing would definitely outweigh the lost lift and increased drag that is equal to the sine of the angle of the flex. My question is, why is Boeing presenting this phenomena in its marketing as if it's a good thing, not only that, they also link it to more efficient cruising, which is completely not what it really does. All analysts in the industry must know this, so who's Boeing targeting with this? General public?


Thoughts?

The 787 wing is not really lighter. it is thinner ( and imu thus heavier ).
That requires more material to cope with the higher stress in the skins.
The same amount of allowable stretching for CFRP will bend the wings further up.

The trick is to create a thicker wing that does not increase drag relative to the thinner one.

I think you're overthinking it, 99.9999% of the population doesn't care, and those who do casually glance over just think "Oh cool, it's a plane."

Regarding the wing thickness/weight/drag tradeoff, ferpe had a good thread here a few years back on that topic. Search "Airbus and Boeing Wing Design Philosophies."

Just worries me that they bend almost halfway to the structural fail limit in normal flight.

LH707330 wrote:

I think you're overthinking it, 99.9999% of the population doesn't care, and those who do casually glance over just think "Oh cool, it's a plane."

Regarding the wing thickness/weight/drag tradeoff, ferpe had a good thread here a few years back on that topic. Search "Airbus and Boeing Wing Design Philosophies."

Well, my question is: why does Boeing present the flexing wings as if it's a good thing even though it reduces the L/D ratio?

I know most people would think 787 and 757 are the same , but my comment on how it looked was just an intro to the topic ..

PR wise Boeing quite often attaches "competent" reasons to things that actually have quite different causality like no time no mon, ....

Invariably you have a trail of commentators taking that (false) reasoning up as the word from the physics god.
Trickle down is that any Boeing PR feature not present on the competing product
surely shows that that product must be deficient.

WIederling wrote:

The trick is to create a thicker wing that does not increase drag relative to the thinner one.

I guess advancement in achieving a better laminar flow would answer how thick/thin wings would be in the future ..

WIederling wrote:

PR wise Boeing quite often attaches "competent" reasons to things that actually have quite different causality like no time no mon, ....

Invariably you have a trail of commentators taking that (false) reasoning up as the word from the physics god.
Trickle down is that any Boeing PR feature not present on the competing product
surely shows that that product must be deficient.

That's my point. To those who grasp the physics behind it would surely wonder why the wing is flexing this much, and they may ask: is Boeing okay with losing this much span and lift over it? But then you get Boeing starting to present it to public as if it's a good thing. Look at this photo from Boeing's site:






The wing flex is the highlight of the photo here. It looks as if Boeing is trying to say:" look how we managed to make the wing look like".

As you said, it is as if Boeing released their PR materials before showing it to the engineering department first . I mean, look at the last portion of the wing near the wingtip, in this photo it looks as if the wing is going 45 degrees upward. That's a lot of wasted span.

The B engineering department isn't involved in the share holder ecology ( well, not usually, really )
But the population there gobbles that down for breakfast, later in the day they phone their trader ...

What makes this a sensible activity from Boeing's POV is that share holder preferences punch
through to management behaviour. ( This years Nobel Prize in Economy turns the "rational decider"
into fairy tail territory anyway. )

And the wing flex looks damn sexy. But that is just in the visuals department.
( and the cockpit section panders more to cuteness than the A350 for example.)

That photo has been manipulated a bit I think, the wing doesn't bend that much, even with landing flap selected.

Eyad89 wrote:

WIederling wrote:

PR wise Boeing quite often attaches "competent" reasons to things that actually have quite different causality like no time no mon, ....

Invariably you have a trail of commentators taking that (false) reasoning up as the word from the physics god.
Trickle down is that any Boeing PR feature not present on the competing product
surely shows that that product must be deficient.

That's my point. To those who grasp the physics behind it would surely wonder why the wing is flexing this much, and they may ask: is Boeing okay with losing this much span and lift over it? But then you get Boeing starting to present it to public as if it's a good thing. Look at this photo from Boeing's site:






The wing flex is the highlight of the photo here. It looks as if Boeing is trying to say:" look how we managed to make the wing look like".

As you said, it is as if Boeing released their PR materials before showing it to the engineering department first . I mean, look at the last portion of the wing near the wingtip, in this photo it looks as if the wing is going 45 degrees upward. That's a lot of wasted span.

But in the end, the airline gets performance guarantees for fuel burn, payload and range. If the flexing wing isn't producing competitive numbers then there may be no sale.

Guaranteed performance, not inflight pictures is the bottom line to the airline.

OldAeroGuy wrote:

If the flexing wing isn't producing competitive numbers then there may be no sale.

competitive numbers wouldn't come from wings only and how it's flexing. Even if 787 loses a bit of fuel over how the wing flexes, it makes up for it in various other areas.

OldAeroGuy wrote:

Guaranteed performance, not inflight pictures is the bottom line to the airline.

Sure thing, no one's doubting that.

Eyad89 wrote:

OldAeroGuy wrote:

Guaranteed performance, not inflight pictures is the bottom line to the airline.

Sure thing, no one's doubting that.

I explained where the target audience sits and how that is effective PR.

WIederling wrote:

Eyad89 wrote:

OldAeroGuy wrote:

Guaranteed performance, not inflight pictures is the bottom line to the airline.

Sure thing, no one's doubting that.

I explained where the target audience sits and how that is effective PR.

But if the airplane isn't selling due to poor performance and the stock price is down, then a set of keen-o pictures won't make the share holders happy.

OldAeroGuy wrote:

WIederling wrote:

Eyad89 wrote:

Sure thing, no one's doubting that.

I explained where the target audience sits and how that is effective PR.

But if the airplane isn't selling due to poor performance and the stock price is down, then a set of keen-o pictures won't make the share holders happy.

You start with the pictures then you take the simpering ( over a sexy product that performs marginally better ) idiot share holders to pressure airline procurement and boost share value.

EK doesn't seem to be too concerned about the wing flex.

I agree with OldAeroGuy. Would be be having this conversation if the A350's wings looked like this? Almost sounds like jealousy.

Eyad89 wrote:

WIederling wrote:

PR wise Boeing quite often attaches "competent" reasons to things that actually have quite different causality like no time no mon, ....

Invariably you have a trail of commentators taking that (false) reasoning up as the word from the physics god.
Trickle down is that any Boeing PR feature not present on the competing product
surely shows that that product must be deficient.

That's my point. To those who grasp the physics behind it would surely wonder why the wing is flexing this much, and they may ask: is Boeing okay with losing this much span and lift over it? But then you get Boeing starting to present it to public as if it's a good thing. Look at this photo from Boeing's site:






The wing flex is the highlight of the photo here. It looks as if Boeing is trying to say:" look how we managed to make the wing look like".

As you said, it is as if Boeing released their PR materials before showing it to the engineering department first . I mean, look at the last portion of the wing near the wingtip, in this photo it looks as if the wing is going 45 degrees upward. That's a lot of wasted span.

Boeing builds the wing that way because it’s a very efficient wing. They didn’t do it because it looks good.

Balerit wrote:

Just worries me that they bend almost halfway to the structural fail limit in normal flight.

No need to be worried. It’s built to withstand extreme conditions, just like any other wing. No need for paranoia.

Balerit wrote:

Just worries me that they bend almost halfway to the structural fail limit in normal flight.

Not even close. The structural fail limit is 3.75G. In normal flight the load is 1G most of the time.

I have a suspicion that the flex actually works into some of the performance as a nonplanar aerodynamic element that may reduce the sidewash outboard. But it's just a suspicion.

FriscoHeavy wrote:

Boeing builds the wing that way because it’s a very efficient wing. They didn’t do it because it looks good.

Nobody said that. read the postings (again?).

It is a downside from wanting a thin profile together with not wanting an even heavier wing
... while PR pretends it is an advantage that also looks sexy.

Starlionblue wrote:

Balerit wrote:

Just worries me that they bend almost halfway to the structural fail limit in normal flight.

Not even close. The structural fail limit is 3.75G. In normal flight the load is 1G most of the time.

I'm talking distance traveled by the wing tip, not the G load, are there any videos of the wing test?

Balerit wrote:

Starlionblue wrote:

Balerit wrote:

Just worries me that they bend almost halfway to the structural fail limit in normal flight.

Not even close. The structural fail limit is 3.75G. In normal flight the load is 1G most of the time.

I'm talking distance traveled by the wing tip, not the G load, are there any videos of the wing test?

Distance traveled relative to distance at breaking point to is not a reliable metric of load. "Half way bent" before breaking does not mean "half way to ultimate load".

see: https://www.youtube.com/watch?v=UFSh04Zl4Yw

displacement per load increment should be linear ( no "secondary" springs or similar

With 2.5g limit load and ultimate load 150% of that you'd expect the
in "stationary flight" displacement to be ~27% of ultimate load displacement.
( Now what fuel load creates the highest wing loading? ?empty?

Very efficient wing yes...to the point of compensating for the lesser lift component due to the high wing-bending angle...it would geometrically been more efficient if it didn't bend at all and offered all its surface with a 100% vertical lift vector...but aerodynamically and structurally, the bending aspect is the best compromise that Boeing came up with...

It is interesting that another all-composite wing, that of the A350, does not bend as much and does not bend with such a curvilinear profile either...it is more linear...Airbus came up with an alternative compromise solution to CFRP wing design...

It seems to be a design philosophy thing...from artist renderings, the 777X also seems to go the way of the 787 and not the A350...very significant curvilinear bending profile...


Faro

WIederling wrote:

see: https://www.youtube.com/watch?v=UFSh04Zl4Yw

displacement per load increment should be linear ( no "secondary" springs or similar

With 2.5g limit load and ultimate load 150% of that you'd expect the
in "stationary flight" displacement to be ~27% of ultimate load displacement.
( Now what fuel load creates the highest wing loading? ?empty?

Empty wing tanks gives the highest wing loading. Which is why center tanks are always emptied first.

Faro wrote:

It seems to be a design philosophy thing...from artist renderings, the 777X also seems to go the way of the 787 and not the A350...very significant curvilinear bending profile...

IMU the 777X wing is said to be a scaled 787 wing. --> ~~20t overall higher OEW

What I do wonder is how they bring together keeping the Al center wingbox.
with those new CFRP wings and their higher bending moment ( from quite a bit more span )

Keeping in mind that CFRP is the preferred material for highly stressed dense parts
I'd have expected a change on the center wingbox too.

It is interesting to note that the B787 wing failed at 154% while the A350 went to 155% and never broke.

Balerit wrote:

It is interesting to note that the B787 wing failed at 154% while the A350 went to 155% and never broke.

Either way they were built to withstand loads beyond the regulatory limits.

154% is 3.85G. I feel safe.

Balerit wrote:

It is interesting to note that the B787 wing failed at 154% while the A350 went to 155% and never broke.

First 787 iteration failed at ?below 100%? A380 failed at ~148%

How well the target is hit is of real interest. From below or from above less so.
It is indicative of how well your CAD model is done.

What it does show is that the A350 has a lot of possibilities down the line weight wise.

The 787 wing flex is also somewhat of an illusion, Look at in-flight pictures that are from a distance. -- Only the outboard one-third of the wing seems to flex much more than typical...and the outboard wing produces only a small amount of the total lift. So the "wasted" horizontal lift component is minimal. Also, I suspect the outboard flex combined with a raked tip with a small upward winglet -- is producing some aerodynamic benefits -- perhaps a "natural" sharklet... reducing induced drag.

B also uses a lighter, "flexier" outboard wing because it also implements the inboard flaperon for roll authority...so the outboard aileron does not need as rigid a wing as an outboard aileron with more authority. Airbus requires less outboard flex for their aileron implementation. Aerodynamics is all trade-offs...I'm sure computer aided design came up with the best balance.

OEM's marketing will just use anything they can for pretty pictures.

Balerit wrote:

What it does show is that the A350 has a lot of possibilities down the line weight wise.

Not really. Heavier weight versions would have stronger wing spars.

All the 155% number shows is that there is some extra margin within the specific set of parameters defined by the ultimate load test.

Starlionblue wrote:

Balerit wrote:

What it does show is that the A350 has a lot of possibilities down the line weight wise.

Not really. Heavier weight versions would have stronger wing spars.

All the 155% number shows is that there is some extra margin within the specific set of parameters defined by the ultimate load test.

to assess the 155% performance quality would need info on the "real" target load they aimed for.
148% 150% 152% ..

Marginally undershooting the A380 wing was afaics cost "effective". fix up the damage area and no repeat test required.
Shaving down the right places in an over performing design is much more difficult than very selectively patching up local over stressing.
( the usefull margin obviously is small. you have no allowance for a defect slightly further out.)

Balerit wrote:

It is interesting to note that the B787 wing failed at 154% while the A350 went to 155% and never broke.

The 787 wing was brought to 150% and then Boeing stopped, they never broke it. We don't know the ultimate load before it breaks we just know it meets certification standards (Boeing did not want to clean up the mess a broken wing would make). They did test a section of the wing box til it broke, but I don't know at what load it was (it was above 150% according to Boeing).

It was the 777's wing that broke at 154%. Be careful- there is a video out there of the 777 wingtest claiming it is the 787 wing test, but that is not true (as evidenced by the obvious aluminum fuselage and rear end of the 777, the mid-90s date from the camcorder on parts of on the video, and the overall mid-90s video quality).

Balerit wrote:

That photo has been manipulated a bit I think, the wing doesn't bend that much, even with landing flap selected.

It does not look manipulated for me. If you fly the 787 as a passenger and look at the wing tips on the ground, at take-off, in-flight and when landing, you will notice that...

- (a) the wingtips are above the cabin ceiling when they generate a lot of lift

- (b) the wingtips are at the height of the window belt (pax windows) when they don't generate lift on the ground

Polot wrote:

Balerit wrote:

It is interesting to note that the B787 wing failed at 154% while the A350 went to 155% and never broke.

The 787 wing was brought to 150% and then Boeing stopped, they never broke it. We don't know the ultimate load before it breaks we just know it meets certification standards (Boeing did not want to clean up the mess a broken wing would make). They did test a section of the wing box til it broke, but I don't know at what load it was (it was above 150% according to Boeing).

It was the 777's wing that broke at 154%. Be careful- there is a video out there of the 777 wingtest claiming it is the 787 wing test, but that is not true (as evidenced by the obvious aluminum fuselage and rear end of the 777, the mid-90s date from the camcorder on parts of on the video, and the overall mid-90s video quality).

Thanks for the heads up

I have seen Ethiopians B787 landing with flap at JNB and the wing didn't bend as much as in the photo above. Here is a 787 photo:

LH707330 wrote:

I think you're overthinking it, 99.9999% of the population doesn't care, and those who do casually glance over just think "Oh cool, it's a plane."

Regarding the wing thickness/weight/drag tradeoff, ferpe had a good thread here a few years back on that topic. Search "Airbus and Boeing Wing Design Philosophies."

You forget that Boeing is not marketing to that 99.9999%

Balerit wrote:

I have seen Ethiopians B787 landing with flap at JNB and the wing didn't bend as much as in the photo above. Here is a 787 photo:
http://boeing.mediaroom.com/image/K64913_med.jpg

Max displacement in stationary flight "1g" should be <27% of Ultimate lLoad displacement.
( I am uncertain what load case that is. max bending should be with payload to MZFW weights and
empty tanks? )

DocLightning wrote:

I have a suspicion that the flex actually works into some of the performance as a nonplanar aerodynamic element that may reduce the sidewash outboard. But it's just a suspicion.

.


O I have a suspicion too! Not quite the same one as you so scientifically explain, however.

brindabella wrote:

DocLightning wrote:

I have a suspicion that the flex actually works into some of the performance as a nonplanar aerodynamic element that may reduce the sidewash outboard. But it's just a suspicion.

.
O I have a suspicion too! Not quite the same one as you so scientifically explain, however.

So you two think that the bird gets lift from flapping? :-=

WIederling wrote:

So you two think that the bird gets lift from flapping? :-=

Yes, when I said "nonplanar aerodynamic element that reduces sidewash" I meant "the 787 flaps its wings for lift."

The funny thing is new wing similar to 787 (namely, A350s) have very similar L/D ratio without flex wings. So I guess safe assumption is flex was not giving much in terms of wing efficiency. Difference is less than 1% (there was a topic somewhere, where numbers were shown), meaning the conventional wing would do similarly.
There are two questions anyway - is the wing lighter than the one made with more conventional concept, like A350s? And what are the costs?

Cheers,
Adam

gloom wrote:

There are two questions anyway - is the wing lighter than the one made with more conventional concept, like A350s? And what are the costs?

Cheers,
Adam

The stiffer Airbus A350 wing is heavier, but it also doesn't need an inboard aileron because the stiffness means that there is much less tendency for the wing to twist, leading to a dangerous "control reversal" when the outboard ailerons are deflected at high speed (for a flexible wing, the downward deflection of the outboard aileron could cause the outboard wing to twist downwards in response to the increased lift on the trailing edge, which would reverse the effect of the aileron and could cause catastrophic loss of control). For the 737, the shorter wing is also stiffer, which is why it lacks an inboard aileron. The benefit is that the A350 (and A330, A340, and A380) wing has no need for an inboard aileron and there is an uninterrupted line of flaps from the wing root to the outboard aileron, improving start L/D at takeoff.

For the 787, the wing is lighter, but an inboard aileron is needed. In fact, the 787 outboard wing is meant to twist aeroelastically in flight, which part of the way that a raked wingtip provides gust unloading (that's a separate topic). Boeing ameliorates the loss of the flap area inboard by using the "flaperon" function on both the 777 and 787 (and less sophisticatedly so on the 767). However, this increases the mechanical complexity of the wing, the maintenance for the aileron actuators, and the cost of design.

This difference in philosophy seems radical enough that there should be a clear winner, but there isn't. Both wing design philosophies compete nicely with each other to within a very small percentage.

DocLightning wrote:

The stiffer Airbus A350 wing is heavier ..

IMU the A350 wing is the relatively lighter design. ( Look at OEW differences between 787-10 and A350-900 : minimal. )
A350 has more fuselage and quite a bit more wing area.