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canyonblue17
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When is too much wing flex bad?

Wed Dec 25, 2019 11:54 pm

I’m not an engineer - but I was looking at a photo of a 787 recently and noticed an amazing amount of wing flex. I understand the basics that generally flex is a good thing - but at what point does it become a drawback? If a wing flexes higher and higher - doesn’t it reduce the amount of lift it can provide? Can that effect stall speeds? Can the flex become so great that the aircraft looses lift completely? Again - maybe dumb questions - but thought I’d get some good answers here. Thanks.
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SEU
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 12:00 am

When it breaks I guess?
 
B757Forever
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 12:03 am

canyonblue17 wrote:
I’m not an engineer - but I was looking at a photo of a 787 recently and noticed an amazing amount of wing flex. I understand the basics that generally flex is a good thing - but at what point does it become a drawback? If a wing flexes higher and higher - doesn’t it reduce the amount of lift it can provide? Can that effect stall speeds? Can the flex become so great that the aircraft looses lift completely? Again - maybe dumb questions - but thought I’d get some good answers here. Thanks.


All the effects you mention are considered in the design. Designers know exactly how much the wing will flex and the subsequent effects it will have on the flight characteristics.
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Rossiya747
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 12:12 am

https://www.youtube.com/watch?v=--LTYRTKV_A
here's a demonstration of wing flex
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 12:17 am

The B777 wing was tested to destruction finally breaking at 154% of the design limit.


https://youtu.be/Ai2HmvAXcU0


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DocLightning
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 12:26 am

It's basically a trade-off beween weight and lift. A lighter wing will tend to flex more, which will cost it some lift. Boeing tends to design their aircraft with lighter and more flexible wings than Airbus (at the cost of needing an inboard aileron for high-speed maneuvering). So when Boeing was designing the 787 wing, it's not difficult to imagine that they modeled the degree of flex against the amount of material that they would have to use to counter that flex and found an optimum.

Neither design philosophy is necessarily superior to the other. It's just two different approaches to the problem.
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Francoflier
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 12:45 am

I seem to remember the gist of it is flex is lighter but not as good aerodynamically whereas stiff is heavier but more efficient. There is of course much more to it.

One isn't better than the other, it's just your usual engineering trade-off / compromise, and each manufacturer seems to prefer a different approach in this case.
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mjoelnir
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 1:04 am

Wing flex can compromise the workings of the control surfaces, especially the ailerons. Flex is not only up and down, but the wing can twist, inducing undesirable aircraft movements. Designing a wing with a lot of flex, has to take that in account.
 
sf260
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 1:11 am

The flex is definitely not a design goal. It is a side effect of using lighter/different materials, the goal is to design a light wing that can withstand a certain load. Engineers can calculate how much it flexes and adopt their design. Flex is not desirable, the end-result it is a trade-off between all different parameters.
 
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 1:26 am

sf260 wrote:
Flex is not desirable, the end-result it is a trade-off between all different parameters.


Flex is certainly desirable. It's the airplane equivalent of shock absorbers and not only that, but flex is necessary to keep the wings from breaking. But like all things, there is such a thing as "not enough" and "too much."
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freakyrat
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 2:12 am

Look at the pickle fork issue with the 737 and the flex induced by the winglets on a jet that never was designed for them.
 
planecane
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 3:42 am

freakyrat wrote:
Look at the pickle fork issue with the 737 and the flex induced by the winglets on a jet that never was designed for them.

That issue hasn't been root caused yet. I'm pretty sure at least one frame didn't have winglets but had a crack in the pickle fork.
 
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DocLightning
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 4:10 am

freakyrat wrote:
Look at the pickle fork issue with the 737 and the flex induced by the winglets on a jet that never was designed for them.


Do you have evidence (not opinions) to support your contention that the pickle fork cracks are caused by the installation of winglets?
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BOSAero
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 4:47 am

DocLightning wrote:
freakyrat wrote:
Look at the pickle fork issue with the 737 and the flex induced by the winglets on a jet that never was designed for them.


Do you have evidence (not opinions) to support your contention that the pickle fork cracks are caused by the installation of winglets?


The pickle fork issues may or may not be the effect of winglets. I for one think that it’s definitely a possibility. Root cause analysts pending. If this analysis is coming from Boeing... I’m not sure how credible it might be.

When you change the original design of something, you’re also changing its limitations. There are unintended consequences that nobody accounted for. Not even the engineers making the change. This is why airbus was prudent enough to realize and limit the life of the A320s that were going to be retrofitted with sharklets. I’m not sure exactly by how many cycles... but the did this because they knew that if you changed the wing, there were bound to be issues down the road that their brightest engineers didn’t think of.

Boeing hasn’t listened to their engineers in a long tine. Everyone knows this now.
 
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 4:49 am

Too much wing flex is bad when the structural integrity of the wing is compromised and there is not enough fuel savings. Wing flex is all about fuel savings.
 
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 4:50 am

The wings that Boeing is now using some of their aircraft are made of carbon fiber if I am not wrong and as a retired aircraft mechanic I do not like that. Carbon fiber may be stronger but it may be more difficult to maintain. I think the first stage fan blades that were used on the Southwest Airlines 737 that had one disintegrate causing the death of one passenger was due to the failure of the carbon fiber at the root and is also hollow. The time that these blades are used is approaching fifteen thousand hours or more without any serious maintenance. Parts do not last forever irregardless of how well they may be engineered. Southwest has not returned that 737 to service even though the aircraft has been repaired. :old:
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 4:50 am

DocLightning wrote:
It's basically a trade-off beween weight and lift. A lighter wing will tend to flex more, which will cost it some lift. Boeing tends to design their aircraft with lighter and more flexible wings than Airbus (at the cost of needing an inboard aileron for high-speed maneuvering). So when Boeing was designing the 787 wing, it's not difficult to imagine that they modeled the degree of flex against the amount of material that they would have to use to counter that flex and found an optimum.

Neither design philosophy is necessarily superior to the other. It's just two different approaches to the problem.

Stiffer wings can be more aerodynamic, but stress more. Hence 777 limit of Validity of 60,000 cycles and 787 at 66,000 cycles. There is more than fuel burn being optimized.

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questions
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 5:06 am

DocLightning wrote:
sf260 wrote:
Flex is not desirable, the end-result it is a trade-off between all different parameters.


Flex is certainly desirable. It's the airplane equivalent of shock absorbers and not only that, but flex is necessary to keep the wings from breaking. But like all things, there is such a thing as "not enough" and "too much."


Sshhh... don’t tell him tall buildings sway!
 
VSMUT
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 11:25 am

NWAROOSTER wrote:
The wings that Boeing is now using some of their aircraft are made of carbon fiber if I am not wrong and as a retired aircraft mechanic I do not like that. Carbon fiber may be stronger but it may be more difficult to maintain. I think the first stage fan blades that were used on the Southwest Airlines 737 that had one disintegrate causing the death of one passenger was due to the failure of the carbon fiber at the root and is also hollow. The time that these blades are used is approaching fifteen thousand hours or more without any serious maintenance. Parts do not last forever irregardless of how well they may be engineered. Southwest has not returned that 737 to service even though the aircraft has been repaired. :old:


CFRP wings have been in use for decades. The F/A-18 has CFRP wings, and that has been in service since 1984. It isn't limited to Boeing either, Airbus makes plenty use of CFRP in the A350 and A220. ATR has used carbon fibre wing boxes and composite wing spars since the 80s.
 
Woodreau
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 2:00 pm

I am not an engineer, but talking to one about a non-aviation topic on flexing. He explained that it’s called elastic deformation.

The material will “flex” due to forces exerted on it and when the force is removed the material will return to its original shape. You don’t want to design something that is “too rigid” where it won’t deform elastically. There needs to be some “give”.

He gave several examples:
roller coaster tracks - there is some “give” as the car of the roller coaster travel over certain parts of the track. If the track didn’t “give” it would fracture.
aircraft carriers - standing in a longitudinal passageway looking through the water tight doors down the length of a long passageway you will see the passageway twist in certain sea states.

There is a limit though, once the deformation exceeds the ability of the material to deform and return to its original shape, it then becomes inelastic deformation, like when you bend a metal pipe - the deformation becomes permanent the pipe is bent or if continued to extremes, cause the material to fail , break, etc.

Different materials will have different limits in their ability to elastically deform and engineers use the best materials they can given the expected forces depending on they specifications they are given and trying to achieve.

So that is my lay persons interpretation of what an engineer was trying to explain to me.
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DocLightning
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Re: When is too much wing flex bad?

Thu Dec 26, 2019 3:39 pm

BOSAero wrote:
DocLightning wrote:
freakyrat wrote:
Look at the pickle fork issue with the 737 and the flex induced by the winglets on a jet that never was designed for them.


Do you have evidence (not opinions) to support your contention that the pickle fork cracks are caused by the installation of winglets?


The pickle fork issues may or may not be the effect of winglets. I for one think that it’s definitely a possibility. Root cause analysts pending. If this analysis is coming from Boeing... I’m not sure how credible it might be.


I asked for "evidence (not opinions)."
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BOSAero
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Re: When is too much wing flex bad?

Fri Dec 27, 2019 5:42 am

DocLightning wrote:
BOSAero wrote:
DocLightning wrote:

Do you have evidence (not opinions) to support your contention that the pickle fork cracks are caused by the installation of winglets?


The pickle fork issues may or may not be the effect of winglets. I for one think that it’s definitely a possibility. Root cause analysts pending. If this analysis is coming from Boeing... I’m not sure how credible it might be.


I asked for "evidence (not opinions)."


Still to early for "evidence". "Opinions" is all anyone has for now unless you're directly involved with any investigations, root cause analysis.

Do you have any opinions, theories?
 
sf260
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Re: When is too much wing flex bad?

Sat Dec 28, 2019 2:35 am

questions wrote:
DocLightning wrote:
sf260 wrote:
Flex is not desirable, the end-result it is a trade-off between all different parameters.


Flex is certainly desirable. It's the airplane equivalent of shock absorbers and not only that, but flex is necessary to keep the wings from breaking. But like all things, there is such a thing as "not enough" and "too much."


Sshhh... don’t tell him tall buildings sway!

Let's put things in the right context again, we are talking about wings. We are not talking about snowboards, bows,... or buildings. If you don't have anything useful to add, please don't.

As I have written before, you design a wing to a certain strength. How much the wing flexes, depends on the material(s) and design.

@DocLightning, flex is indeed elastic deformation, it's a property of the material & design. As long as the wing can reach ultimate load without breaking, it doesn't matter if it flexes 1 inch or 15 feet.
Flex can indeed help to dissipate turbulence, increasing comfort on board, but that's also a side effect, not a primary design goal. Good marketing on Airbus & Boeing.
Aerodynamically, you would want a wing that's not flexing very much (therefore not "desireable").
 
TSS
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Re: When is too much wing flex bad?

Sat Dec 28, 2019 2:51 am

BOSAero wrote:
DocLightning wrote:
BOSAero wrote:

The pickle fork issues may or may not be the effect of winglets. I for one think that it’s definitely a possibility. Root cause analysts pending. If this analysis is coming from Boeing... I’m not sure how credible it might be.


I asked for "evidence (not opinions)."


Still to early for "evidence". "Opinions" is all anyone has for now unless you're directly involved with any investigations, root cause analysis.

Do you have any opinions, theories?


Find a group of similarly used 737 models that have never had winglets installed (if such a group of planes still exists) and inspect them for similar pickle fork issues.
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Re: When is too much wing flex bad?

Sat Dec 28, 2019 11:52 am

One would think a minimum amount of wing flex is necessary to dampen out turbulence...if a material existed that was light, sufficiently strong while being perfectly stiff, you would get every single impulse resulting from turbulence being directly imparted to the cabin...not a very comfortable ride I would think...


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JayinKitsap
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Re: When is too much wing flex bad?

Sat Dec 28, 2019 6:44 pm

Wing flex to a point is good, energy absorption reduces forces for that loading toward the average load condition. Geometry issues come into play, do all the control surfaces operate without binding is essential. Also, a negative of flexibility is harmonics, often flexible structures have a period much closer to the natural vibration. In buildings if the natural period is close to the primary or secondary harmonic in a seismic event the structure can just be killed, same with wave states at sea. Must stay away from the harmonics, usually a stiffer structure is needed.
 
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rjsampson
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Re: When is too much wing flex bad?

Mon Dec 30, 2019 7:22 am

canyonblue17 wrote:
When is too much wing flex bad?.


SEU wrote:
When it breaks I guess?


Hats off to SEU. That succinctly, directly, and completely answered OP's question, which should have negated subsequent discussion :)
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767333ER
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Re: When is too much wing flex bad?

Wed Jan 01, 2020 4:07 pm

DocLightning wrote:
freakyrat wrote:
Look at the pickle fork issue with the 737 and the flex induced by the winglets on a jet that never was designed for them.


Do you have evidence (not opinions) to support your contention that the pickle fork cracks are caused by the installation of winglets?

Perhaps confusing it with the 737 classic where the additional wingspan and weight caused extra structural fatigue.
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IADCA
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Re: When is too much wing flex bad?

Thu Jan 02, 2020 7:09 pm

TSS wrote:
BOSAero wrote:
DocLightning wrote:

I asked for "evidence (not opinions)."


Still to early for "evidence". "Opinions" is all anyone has for now unless you're directly involved with any investigations, root cause analysis.

Do you have any opinions, theories?


Find a group of similarly used 737 models that have never had winglets installed (if such a group of planes still exists) and inspect them for similar pickle fork issues.


SAS has a fleet of old 737s, some with winglets and some without. They had at least one without winglets with the issue (LN-RPK) and also one wingletted one.
 
WPIAeroGuy
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Re: When is too much wing flex bad?

Sat Jan 04, 2020 2:58 am

JayinKitsap wrote:
Wing flex to a point is good, energy absorption reduces forces for that loading toward the average load condition. Geometry issues come into play, do all the control surfaces operate without binding is essential. Also, a negative of flexibility is harmonics, often flexible structures have a period much closer to the natural vibration. In buildings if the natural period is close to the primary or secondary harmonic in a seismic event the structure can just be killed, same with wave states at sea. Must stay away from the harmonics, usually a stiffer structure is needed.


Very good points. I don’t design airliners, but in small aircraft we look for the lightest, stiffest wing that meets strength requirements. The more flexible the wing, the more likely you are to run into aeroelastic effects at low speeds, and without Boeing’s budget and computing power, it’s best to stay as far away from that as possible.

I see wing flex not as a design goal, but as a fallout of meeting the above criteria. The stiffness is driven by two factors: the elastic modulus of the material, and the section moment of inertia (typically referred to as E*I for bending, G*J for torsion). The elastic modulus E is a material property and is generally fixed by your chosen construction material. The moment of inertia I is where you have the design freedom. In a traditional aluminum wing where the main spar carries all the bending loads, a deeper spar can have a much larger section moment of inertia for minor increases in weight. When your optimizing every last .01% on an airliner though, that additional thickness could result in more drag. So to over simplify it, once the wing loads are known, the optimum structural design for weight and drag will yield a EI for the wing (which varies spanwise) and that will result in a given deflection under load. As long as this doesn’t give any nasty aeroelastic effects then you’re good to go.
-WPIAeroGuy
 
JayinKitsap
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Re: When is too much wing flex bad?

Sat Jan 04, 2020 3:18 am

WPIAeroGuy wrote:
JayinKitsap wrote:
Wing flex to a point is good, energy absorption reduces forces for that loading toward the average load condition. Geometry issues come into play, do all the control surfaces operate without binding is essential. Also, a negative of flexibility is harmonics, often flexible structures have a period much closer to the natural vibration. In buildings if the natural period is close to the primary or secondary harmonic in a seismic event the structure can just be killed, same with wave states at sea. Must stay away from the harmonics, usually a stiffer structure is needed.


Very good points. I don’t design airliners, but in small aircraft we look for the lightest, stiffest wing that meets strength requirements. The more flexible the wing, the more likely you are to run into aeroelastic effects at low speeds, and without Boeing’s budget and computing power, it’s best to stay as far away from that as possible.

I see wing flex not as a design goal, but as a fallout of meeting the above criteria. The stiffness is driven by two factors: the elastic modulus of the material, and the section moment of inertia (typically referred to as E*I for bending, G*J for torsion). The elastic modulus E is a material property and is generally fixed by your chosen construction material. The moment of inertia I is where you have the design freedom. In a traditional aluminum wing where the main spar carries all the bending loads, a deeper spar can have a much larger section moment of inertia for minor increases in weight. When your optimizing every last .01% on an airliner though, that additional thickness could result in more drag. So to over simplify it, once the wing loads are known, the optimum structural design for weight and drag will yield a EI for the wing (which varies spanwise) and that will result in a given deflection under load. As long as this doesn’t give any nasty aeroelastic effects then you’re good to go.


Yes, if the wing is flopping around, lift will be all over the place with each 'bounce', not good.

You I am sure know this, but for others a key parameter in structural design is the Section Modulus, typically S. It is I divided by d/2 with d being the depth. S x Fb gives the moment strength of the member, which usually controls. The value of S is highest at the wing box, is almost 0 at the tip of the wing. Watching the 777 ultimate load test it is amazing how the breaking stress was reached basically along the whole wing, no excess anywhere.
 
AirBoat
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Re: When is too much wing flex bad?

Sat Jan 04, 2020 7:41 pm

The main problem with using carbon as wing structure is although it is much stronger than steel, the modulus of elasticity (or the amount of deformation per load) is the same or slightly better than steel.
This results in a wing, for the same strength as the steel one, deflecting much more.
As long as the wing does not twist under deflection (which can increase loading) its ok, but there is going to be a decrease in aero efficiency.
The optimum for aero performance is a wing that does not deflect.
There is also the scale effect. Consider an ant carrying 10 times its own weight. There is a practical limit on the size of an aircraft, before the structure gets too heavy and the carrying capacity is zero.
For things to scale properly the strength and stiffness of the materials would also have to increase.
The design codes for structures specify two conditions, ie max stress in material and maximum deflection of structure.
 
basspaul
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Re: When is too much wing flex bad?

Wed Jan 08, 2020 5:09 pm

In the helicopter world, if the wing (the main rotor) flexes too much it hits the tailboom. There are some ground situations that can cause tail boom strikes, but if it happens in flight, your flight usually ends then and there.

https://blog.aopa.org/aopa/2010/11/04/tail-boom-strikes/
 
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Re: When is too much wing flex bad?

Thu Jan 09, 2020 1:17 am

basspaul wrote:
In the helicopter world, if the wing (the main rotor) flexes too much it hits the tailboom. There are some ground situations that can cause tail boom strikes, but if it happens in flight, your flight usually ends then and there.

https://blog.aopa.org/aopa/2010/11/04/tail-boom-strikes/


The refueling probe also lives in the danger zone, apparently... :shock:

https://www.youtube.com/watch?v=VAdpKpppZiA
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