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Is Wing Flex Ineffecient?  
User currently offlineDocLightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Posted (5 years 5 months 3 days 16 hours ago) and read 17670 times:

So I know that on a low-mounted wing, an upward sweep of the wing provides stability by increasing the lift of the wing that is downwards during a bank.

So the 787 and 748 as described by Boeing have this crazy wing flex. In fact, most modern Boeing A/C have some pretty crazy wing flex. But the A380 doesn't.


A380's wings droop down on the ground and straighten out in flight.

I would think that the more flexed the wing, the less lift. So why this nutty wing flex on modern wings?


37 replies: All unread, showing first 25:
 
User currently offlineWILCO737 From Greenland, joined Jun 2004, 9054 posts, RR: 76
Reply 1, posted (5 years 5 months 3 days 15 hours ago) and read 17691 times:
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Quoting DocLightning (Thread starter):

Wing flex cannot be wrong. We try to copy the wings of a bird somehow and if you see a huge bird in the sky just soaring above us, then the wingtips are flexed up as well:



wilco737



It it's not Boeing, I am not going.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17054 posts, RR: 67
Reply 2, posted (5 years 5 months 3 days 15 hours ago) and read 17679 times:

I think you're talking more about curvature than flex. As I see it, curvature refers to the shape while flex refers to (variable) bending (flapping).

If the wings are built strong enough to flex significantly less, they will be much heavier. This means a loss of efficiency. I assume any loss of efficiency from flex is less than that loss.


Note also that the last few years of promo drawings from Boeing have shown a dramatic bend which may not quite correspond to reality.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineRwessel From United States of America, joined Jan 2007, 2353 posts, RR: 2
Reply 3, posted (5 years 5 months 3 days 14 hours ago) and read 17661 times:
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The idea would be to make sure that the flexed wing is in its most efficient shape. Think about bending an object, perhaps a plastic ruler, that has some inherent flex. If you supply a significant load, it will bend some, but still hold the load (up to the point where you break it, of course). If you made the ruler rigid enough to take the same load without any bending, it would be far heavier. Structurally this allows you to build some of the strength into the top part of the spar, which is then loaded in compression.

That being said, Boeing's art department appears to be taking a bit of license here, as regards the degree of flex.

This became an issue in the soaring community in the late eighties and early nineties, when a number of sailplanes were built with unusually (for the time) flexible wings. These bent to the desired shape when loaded in flight, but drooped very considerably when they weren't holding up the airplane - for example, when sitting on the ground. Endless debates raged over how to measure the wingspan of these aircraft. The "drooped" position on the ground, or the position the wings actually assumed in flight (or with the help of a burly pair of wing benders). Since most contest classes have rules on wingspan (15m being by far the most common), a few inches of droop related span could disqualify an aircraft.

The fashion for single cantilevered wings doesn’t exactly help the structural side here either.  Wink

The world is full of structures which don't take their design shape until loaded to their design points. One presumes that Boeing reaches a good compromise between weight, flexibility and aerodynamic efficiency.


User currently offlineDh106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 4, posted (5 years 5 months 3 days 14 hours ago) and read 17637 times:

Of greater efficiency impact than the bending is the way the wing twists in a wash out/in sense as that alters the wing's load distribution which in turn affects issues such as generated vortex strength and which area of the wing stalls first at high angles of attack. As Rwessel says, the wing is designed to be at it's most efficient aerodynamically under load.


...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineDocLightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Reply 5, posted (5 years 5 months 3 days 1 hour ago) and read 17497 times:



Quoting Rwessel (Reply 3):

That being said, Boeing's art department appears to be taking a bit of license here, as regards the degree of flex.

Not necessarily. Some of the engineering diagrams that I've seen with the 788 have shown that the wing flex is actually portrayed accurately.


User currently offlineVikkyvik From United States of America, joined Jul 2003, 10047 posts, RR: 26
Reply 6, posted (5 years 5 months 2 days 19 hours ago) and read 17403 times:
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Quoting DocLightning (Thread starter):

I think you're confusing a few different things here.

Most low-mounted wings have some dihedral built in - that is, in flight, they will be pointed upward at some angle from root to tip. Dihedral has to do with at what angle the wing is mounted on the fuselage.

The wings will flex upward under load. On swept wings. this will typically result in some amount of "washout". Washout refers to the angle of attack at the tip being less than the angle of attack at the root. When flexed under load, a swept wing twists in this way.

Washout has at least one advantage - since the root is at a higher angle of attack, it will stall before the tip. Thus, when the wing is first stalling, you'll still have roll control with the ailerons, which are out at the wingtip.

The A380 appears to have both dihedral and wing flex. Although the wings appear to droop downward when on the ground, in flight (as shown in the photo you posted) the wingtips are higher up than the root.

One last clarification - in a bank, both wings are still producing the same lift. The lift vectors are tilted, but they still add up to a vector that is pointing out of the top of the fuselage. However, in a bank, the tilted lift vector will induce a sideslip. The resultant velocity vector, in turn, will produce more lift on the downward wing, producing a restoring roll moment.

Edited to add stuff I forgot:

In the most basic structural sense, wing flex is necessary. If wings were brittle, they'd just snap under high loading. Their ability to flex under load allows them to carry said load with less supporting structure (which equals less weight).

[Edited 2009-05-01 20:50:44]


"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineDocLightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Reply 7, posted (5 years 5 months 2 days 18 hours ago) and read 17385 times:



Quoting Vikkyvik (Reply 6):


In the most basic structural sense, wing flex is necessary. If wings were brittle, they'd just snap under high loading. Their ability to flex under load allows them to carry said load with less supporting structure (which equals less weight).

Well yes, obviously. But now it seems like it's being designed in on purpose.


User currently offlineViscount724 From Switzerland, joined Oct 2006, 25457 posts, RR: 22
Reply 8, posted (5 years 5 months 2 days 16 hours ago) and read 17351 times:



Quoting DocLightning (Reply 7):
Quoting Vikkyvik (Reply 6):


In the most basic structural sense, wing flex is necessary. If wings were brittle, they'd just snap under high loading. Their ability to flex under load allows them to carry said load with less supporting structure (which equals less weight).


Well yes, obviously. But now it seems like it's being designed in on purpose.

That was true many years ago also. Manufacturers had different ideas. You may not have flown on a 707 or DC-8 but one of the most noticeable differences as a passenger was the very flexible wings on the 707 compared to the DC-8. In turbulence it was actually somewhat disturbing if you were sitting near the front of the 707's wing where you could see the engines bouncing around in all directions while the wing was also flexing very noticeably. That was much less apparent on the DC-8 which seemed to have significantly stiffer wings than the 707. Obviously Boeing and Douglas had different ideas about wing stiffness.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 9, posted (5 years 5 months 2 days 5 hours ago) and read 17247 times:



Quoting DocLightning (Reply 7):
Well yes, obviously. But now it seems like it's being designed in on purpose.

It is designed in on purpose. If you did your aerodynamics to the jig position (the position of the wings when they're built) your aerodynamics would be completely off once the thing took to the air. No commercial jet has a wing stiff enough that you can ignore the effect of flex and still get good aero data, so you have to design for it.

Tom.


User currently offlinePPVRA From Brazil, joined Nov 2004, 8964 posts, RR: 39
Reply 10, posted (5 years 5 months 2 days 1 hour ago) and read 17193 times:



Quoting Vikkyvik (Reply 6):

One last clarification - in a bank, both wings are still producing the same lift. The lift vectors are tilted, but they still add up to a vector that is pointing out of the top of the fuselage. However, in a bank, the tilted lift vector will induce a sideslip. The resultant velocity vector, in turn, will produce more lift on the downward wing, producing a restoring roll moment.

Would it be right to say that a more flexible wing would reduce this "sideslip" effect? Would a reduction of this be something desirable on a wing?



"If goods do not cross borders, soldiers will" - Frederic Bastiat
User currently offlineVikkyvik From United States of America, joined Jul 2003, 10047 posts, RR: 26
Reply 11, posted (5 years 5 months 1 day 23 hours ago) and read 17162 times:
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Quoting PPVRA (Reply 10):
Would it be right to say that a more flexible wing would reduce this "sideslip" effect? Would a reduction of this be something desirable on a wing?

I can't see any reason why a more flexible wing would reduce the sideslip.

The sideslip is caused by the resultant lift vector (taking both wings into account) being tilted to the side in a bank. Some of the force that was holding the plane up is now pulling it to the side as well. That won't change with a more flexible wing.

Besides which, the existence of the sideslip is what gives the airplane stability in the roll axis. In one sense, commercial airplanes don't actually have direct roll stability - flying at a bank angle by itself doesn't cause the airplane to return to zero bank. Whereas, flying at some yaw angle or pitch angle relative to the airflow will cause the airplane to want to return to straight, level flight.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineMovingtin From United States of America, joined Oct 2006, 183 posts, RR: 0
Reply 12, posted (5 years 5 months 1 day 6 hours ago) and read 17054 times:

Lockheed has proved that a stiff wing is not desirable over the long term. Their wings are notorious for cracks.

User currently offlinePPVRA From Brazil, joined Nov 2004, 8964 posts, RR: 39
Reply 13, posted (5 years 5 months 1 day 5 hours ago) and read 17034 times:

Quoting Vikkyvik (Reply 11):

What I had in mind is an aircraft with, say, a 5 degree wing flex doing a 5 degree bank, and the downward wing essentially being parallel to the ground, meaning the vectors would be perpendicular to the ground (thus no sideslip from this wing?). . . but then as you said, they are not fixed parallel to the ground, but more with the banking. I think that's where I went wrong.

So I assume the wing flex does not affect the lift vectors? I.e, when the 787 is flying straight, there is no counteracting sideslip from both wings because of their flex?

[Edited 2009-05-03 09:58:57]


"If goods do not cross borders, soldiers will" - Frederic Bastiat
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 14, posted (5 years 5 months 1 day 3 hours ago) and read 16990 times:



Quoting PPVRA (Reply 13):
So I assume the wing flex does not affect the lift vectors? I.e, when the 787 is flying straight, there is no counteracting sideslip from both wings because of their flex?

Locally, lift is essentially normal to the wing surface. Overall lift is just the integration of all the local lift vectors over the wing...since wing flex changes the normal direction, it also changes the lift direction. As the wing flexes more, you get more sideways force from each wing but, in straight and level flight, they balance so you don't get any sideslip.

Tom.


User currently offlineDocLightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Reply 15, posted (5 years 5 months 1 day 1 hour ago) and read 16956 times:



Quoting Tdscanuck (Reply 14):
As the wing flexes more, you get more sideways force from each wing but, in straight and level flight, they balance so you don't get any sideslip.



Quoting Tdscanuck (Reply 14):

Locally, lift is essentially normal to the wing surface.

Interesting, I hadn't thought of it this way. I viewed the underside of the wing as a high-pressure area and the upward bend as allowing this pressure to act less.

Either way, the less horizontal a section of wing is, the less upward force it generates. If a given section of wing at 45% to horizontal is locally producing 2 N of force normal to the wing, it will only produce √2 N of true upward force and another √2 kn inwards, which will be balanced by the same inward force on the opposite wing.

This, to me, strikes me as inefficient. It means that you're dumping upward force and allowing it to produce an inward force that does nothing except require additional structure to withstand that "squeeze" force.

Now, I'm obviously wrong or Boeing would have stiffened the wing more. So why am I wrong?


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 16, posted (5 years 5 months 20 hours ago) and read 16896 times:



Quoting DocLightning (Reply 15):

This, to me, strikes me as inefficient. It means that you're dumping upward force and allowing it to produce an inward force that does nothing except require additional structure to withstand that "squeeze" force.

If you look at the wing purely from a lift-generation point of view, you're absolutely right.

Quoting DocLightning (Reply 15):
Now, I'm obviously wrong or Boeing would have stiffened the wing more. So why am I wrong?

Because the objective of the wing isn't maximum lift, it's maximum L/D for the entire aircraft.

Reducing wing flex requires stiffening the wing, which adds weight. That drives you to a larger and draggier wing. A perfectly flat wing also provides no roll stability, so you end up spending more weight/cost somewhere else to restore the lost stability.

The "sweet spot" for the entire design is somewhere between the extremes of a perfectly flat wing and a massively bent U-shaped one, which is what we see in service.

Tom.


User currently offlineVikkyvik From United States of America, joined Jul 2003, 10047 posts, RR: 26
Reply 17, posted (5 years 5 months 20 hours ago) and read 16893 times:
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Quoting DocLightning (Reply 15):
This, to me, strikes me as inefficient. It means that you're dumping upward force and allowing it to produce an inward force that does nothing except require additional structure to withstand that "squeeze" force.

Now, I'm obviously wrong or Boeing would have stiffened the wing more. So why am I wrong?

In addition to what Tom said, the amount of material you need to add to prevent a long, relatively thin beam from bending is pretty prohibitive.

And then you have to provide even more material so that it can meet the required safety factor for accelerations (such as turbulence) without snapping off.

It's not a linear relationship, because for every pound of material you add to prevent bending at a certain location along the span, you have to add more material at all points inward (spanwise) just to support it.

At the flex angles we're talking about, that would far outweigh whatever structure is required to allow the wing to bend.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineWingedMigrator From United States of America, joined Oct 2005, 2214 posts, RR: 56
Reply 18, posted (5 years 5 months 20 hours ago) and read 16891 times:



Quoting Tdscanuck (Reply 16):
The "sweet spot" for the entire design is somewhere between the extremes of a perfectly flat wing and a massively bent U-shaped one, which is what we see in service.

I think people are reacting to a perceived change towards the U-shape, prompted by computer renderings of the 787 and 747-8 in flight. In reality, I doubt we'll see any discernible difference between the 787 and, say, the 777. In testing, the 777's wingtips deflected 24 feet at 3.85 g; the 787's were predicted to go 26 feet at 3.75 g. That makes the 787's wings only about 10% more flexy than the 777's.

That won't prevent folks from seeing what they want to see... pictures of first flight are sure to elicit comments about that insane wing flex! (never mind that it will be a gentle take-off well below MTOW)


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 19, posted (5 years 5 months 19 hours ago) and read 16864 times:



Quoting WingedMigrator (Reply 18):
I think people are reacting to a perceived change towards the U-shape, prompted by computer renderings of the 787 and 747-8 in flight. In reality, I doubt we'll see any discernible difference between the 787 and, say, the 777.

I'm not sure about that...whatever optimization is done between stiffness, weight, and aerodynamic efficiency has got to take the material properties as an input. Since the 787 wing uses a fundamentally different material than the 777 wing, it wouldn't surprise me much to find out that the optimal point for a 787 is a different amount of flex.

Tom.


User currently offlineJetMech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 20, posted (5 years 5 months 18 hours ago) and read 16864 times:

Quoting DocLightning (Thread starter):



Quoting Movingtin (Reply 12):
Lockheed has proved that a stiff wing is not desirable over the long term. Their wings are notorious for cracks.



Quoting Vikkyvik (Reply 17):
And then you have to provide even more material so that it can meet the required safety factor for accelerations (such as turbulence) without snapping off.

I think this is the key to it all. The wings of an airliner need to be flexible to a certain degree to minimise the stresses caused from absorbing the energy due to sudden displacements.

If for example the wing was very stiff, any flexing of the wing would be arrested in a very short time period. This would probably generate quite high stresses in the wing structure. If a certain amount of flexibility is allowed, the time period over which energy can be absorbed is extended, thus, the stresses generated in the structure should be reduced.

A good analogous example to this would be the difference between falling 5 metres onto a reinforced concrete slab compared with a trampoline. Obviously, too much flex is also undesirable not only from the aerodynamic point of view, but possibly from the structural point of view, as you would then have too little structural damping in the wing.

Regards, JetMech

[Edited 2009-05-03 20:55:04]


JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineDocLightning From United States of America, joined Nov 2005, 19786 posts, RR: 59
Reply 21, posted (5 years 4 months 4 weeks 1 day 6 hours ago) and read 16692 times:



Quoting JetMech (Reply 20):

I think this is the key to it all. The wings of an airliner need to be flexible to a certain degree to minimise the stresses caused from absorbing the energy due to sudden displacements.

Well, that's reasonable, I agree. Having the wings snap off an aircraft generally makes the manufacturer look pretty silly.

But Airbus went for a straight wing inflight with the A380. They did this by designing the wing to droop downwards on the ground. The wing is flexible, but its flight-neutral position is roughly straight, as shown in the pic I posted. I'm sure it flaps like a bird in turbulence, but in straight and level flight, it is straight.

For the A350, they have the same upward sweep that Boeing has on the 787 and 748. And I can't figure out why the change.


User currently offlineVikkyvik From United States of America, joined Jul 2003, 10047 posts, RR: 26
Reply 22, posted (5 years 4 months 4 weeks 1 day 5 hours ago) and read 16675 times:
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Quoting DocLightning (Reply 21):

But Airbus went for a straight wing inflight with the A380. They did this by designing the wing to droop downwards on the ground. The wing is flexible, but its flight-neutral position is roughly straight, as shown in the pic I posted. I'm sure it flaps like a bird in turbulence, but in straight and level flight, it is straight.

As is stated in another thread, the "gulling" effect of the A380's wing on the ground (the outward droop) is probably due to the need to have adequate space for the 4 giant engines.

A side effect of that would be that the wing flexes to a relatively straight position once loaded inflight (instead of being straight on the ground and flexing to an upward-curved shape once loaded).

As long as the wing is adequately flexible, and has adequate dihedral, it doesn't really matter if it's straight or curved inflight. I'm sure the design engineers knew how it would act on the ground and inflight.



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User currently offlinePPVRA From Brazil, joined Nov 2004, 8964 posts, RR: 39
Reply 23, posted (5 years 4 months 4 weeks 19 hours ago) and read 16568 times:

Quoting Vikkyvik (Reply 22):

Any chance it could have been a way to perhaps shorten the wingspan a bit? On a straighter wing, if your lift vectors are less sideways, I would think you could have a smaller wing. . though not necessarily a shorter one.

Would it be significant, though? Maybe as part of an overall effort to reduce wingspan?

[Edited 2009-05-05 20:30:38]


"If goods do not cross borders, soldiers will" - Frederic Bastiat
User currently offlineVikkyvik From United States of America, joined Jul 2003, 10047 posts, RR: 26
Reply 24, posted (5 years 4 months 4 weeks 18 hours ago) and read 16558 times:
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Quoting PPVRA (Reply 23):

Any chance it could have been a way to perhaps shorten the wingspan a bit? On a straighter wing, if your lift vectors are less sideways, I would think you could have a smaller wing. . though not necessarily a shorter one.

Would it be significant, though? Maybe as part of an overall effort to reduce wingspan?

You know, I really have no idea. I doubt it would be significant, as such. At least, that'd be my first guess, for the following reason:

The A380's wingspan (and that of any other future commercial airplane under current standards) is governed by the 80 meter (262 foot) box. The A380's wiingspan is less than a foot under this limit.

However, consider that the wingspan limit is in place due to ground maneuvering obstacles (i.e. taxiway clearance, gate space). So whatever flexing happens in-flight ought not to affect it.

That said, in the game of advanced airplane design, you want to make sure you use whatever benefits you possibly can to maximize efficiency.



"Two and a Half Men" was filmed in front of a live ostrich.
25 Roseflyer : There is not side slip caused, but everyone is ignoring a very important factor that is related. In commercial airplane design, you want both wings t
26 Post contains links and images WingedMigrator : On a somewhat unrelated note: an equally interesting phenomenon that is easily visible on the A380 is that the wing also twists under load. On the gr
27 Vikkyvik : That's not the way I've always learned it, in terms of roll stability. Far as I can tell, there pretty much has to be a side-slip caused. If the lift
28 Flipdewaf : Most flex doesnt occur at MTOW, its normally at MZFW as the fuel in the wings acts as bending relief. Fred
29 Max Q : Interesting subject, the 727 has very flexible wings and a stiff fuselage whereas the MD80 has a very flexible fuselage with a stiff wing. The 72 had
30 Airbuske : I think he means that lateral stability rolls the plane to stop sideslip. (which is correct)[Edited 2009-05-06 16:51:42]
31 Starlionblue : I suppose it makes sense. Since the wings are the main interface with the air, if they flex this will tend to smooth out vibrations. Like shocks in a
32 SEPilot : I do not believe this is correct. The 707, I know, had two sets of ailerons, one inboard and one outboard. The outboard ones are only used at low spe
33 Vikkyvik : I'm not entirely sure how that contradicts what I said. Stalls tend to happen at slower speeds - at which point you'll be using the outboard ailerons
34 Faro : The more flex I would think so too, from a simple geometrical point of view. The more flex, the less vertical component of lift; for a given amount of
35 SEPilot : Actually, at high altitude stalls are a real concern-ever hear about the "coffin corner"? The inboard ailerons would be in use there.
36 Vikkyvik : Yes, I have. But airliners don't fly anywhere near the coffin corner. Your approach speed has to be 1.3*V_stall. I'd imagine your cruise speed is gen
37 SEPilot : As I understand it at maximum altitude they are often very near it, the 727 in particular. I believe it only had about a 20kt window at maximum altit
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