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Can wings snap off if empty?

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Can wings snap off if empty?

Old 20th Mar 2019, 19:04
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Can wings snap off if empty?

I was watching a documentary on YouTube about a company called "Omega" (I think) that provides refuelling services to USAF (I think). One phrase caught my attention. The narrator said that if there was no fuel in the wings, the loaded plane would simply snap them off with its own weight. The channel in question used to describe the process of starting a 777 engine as "the compressed air pushes against the piston making the crankshaft rotate", so I'm not sure whether I should believe them.

So, is that true? Is that true for just KC-135 or for other planes, too? For example, can a Boeing 777-200F loaded with 110 tons of cargo take off with nearly empty wing tanks?
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Old 20th Mar 2019, 23:48
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Probably too simplistic. If a structure is loaded to gross excess, it will fail and certainly can break. The reference to (wing) fuel is that the presence of fuel (ie mass or weight) in the wing, acts to reduce the bending loads on the wing in flight and, so, can increase the margin between current structural loads and the loads at which failure might be a concern. The OEM prescribes limitations in operating the aircraft to keep loads in check so that failure is not a likely event.

The starting description probably is indicative of the high standard of research in the reporting organisation ?
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Old 20th Mar 2019, 23:49
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You can't increase useful load beyond a certain point no matter how little fuel you take with you:
https://en.wikipedia.org/wiki/Zero-f...aft_operations

To put it very simple: Weight in the wings isn't a problem for the wing root, weight in the fuselage is.
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Old 21st Mar 2019, 00:17
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SEE 777 WING TEST

Originally Posted by ProPax View Post
I was watching a documentary on YouTube about a company called "Omega" (I think) that provides refuelling services to USAF (I think). One phrase caught my attention. The narrator said that if there was no fuel in the wings, the loaded plane would simply snap them off with its own weight. The channel in question used to describe the process of starting a 777 engine as "the compressed air pushes against the piston making the crankshaft rotate", so I'm not sure whether I should believe them.

So, is that true? Is that true for just KC-135 or for other planes, too? For example, can a Boeing 777-200F loaded with 110 tons of cargo take off with nearly empty wing tanks?

SEARCH on 777 wing test wingns were empty
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Old 21st Mar 2019, 00:34
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the presence of fuel (ie mass or weight) in the wing, acts to reduce the bending loads on the wing in flight
At one stage the C-130 was having wing fatigue issues, in order to reduce the bending moment, while investigations took place, an instruction was issued to keep a certain substantial minimum amount of fuel on board (in the wings).
I was watching a documentary on YouTube
Recently watched a few aviation shows on TV and have had an interesting education. Learnt that a Cessna 310 uses kerosine as a fuel, the proper name for an engine is a "thruster", and a A320 and 737 are one in the same aircraft. Pity that shows dedicated to a subject are so poorly researched, but then, it's just entertainment to an ignorant audience.
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Old 21st Mar 2019, 22:23
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Surely a simple analysis of this is sufficient: if the wings snap off if there's no fuel in the tank, you can't use all the fuel.

There's no point having fuel you can't use because it's actually structurally required ballast.
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Old 21st Mar 2019, 22:39
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Originally Posted by dtaylor1984 View Post
There's no point having fuel you can't use because it's actually structurally required ballast.
But that is the point of having it. Better than filling the tanks with water as ballast.

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Old 21st Mar 2019, 23:40
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Well there is the case of center tanks.
So with empty tanks you couldn't fill up the center tanks, but with full wing tanks it is fine.

I guess with military tanker aircraft that is especially true if they have large center tanks.

Does anyone have actual numbers for military tankers on how much more weight they are allowed to have inside the fuselage?
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Old 22nd Mar 2019, 08:10
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Originally Posted by ProPax View Post
I was watching a documentary on YouTube about a company called "Omega" (I think) that provides refuelling services to USAF (I think). One phrase caught my attention. The narrator said that if there was no fuel in the wings, the loaded plane would simply snap them off with its own weight. The channel in question used to describe the process of starting a 777 engine as "the compressed air pushes against the piston making the crankshaft rotate", so I'm not sure whether I should believe them.

So, is that true? Is that true for just KC-135 or for other planes, too? For example, can a Boeing 777-200F loaded with 110 tons of cargo take off with nearly empty wing tanks?
This sounds like pseudo engineering fiction. Every bit of weight that you add to an airplane will impact the loading of the structure. The manner in which weight added impacts loading depends on where that weight is added and how the airplane is being supported. You can imagine that the distribution of loads is quite different when sitting on the gear on the ground vs. flying with lift being generated by aerodynamics. The main reason that main landing gear are mounted near the wing root on most configurations is to keep the difference in the distribution of loads between in air and on ground as small as possible. As I hope you can imagine, some of the airplane structure sees maximum loading when on ground while other parts of the structure experience maximum loading when in air.

To speak directly to the question posed, any weight added to the airplane at any point within the wings or in the fuselage will cause loads within the structure to go up. The distribution of those increased loads will depend on where that weight is added. Similarly, taking weight out of the airplane will lower loads. It is pure fancy to think that structure will be at greater risk from a static loading perspective with less weight than it will be with more weight. There are also balance considerations with regard to cg location and structural flutter, but those points will have to wait for another lesson.

In general, it is easiest to design structure if the payload is located near where the supporting lift is generated. For that reason it makes sense to put as much of the fuel as you can in the wings as carrying the same amount of weight in the fuselage would be much harder on the structure in flight - particularly during an elevated g maneuver. Many airplanes are volume constrained within the wing cross section and thus cannot fit all of the fuel they need to carry in the wings so they also have fuselage tanks. Usually the fuselage tank is the last one that you would fill only using it if the wings are already full and you need more fuel for the mission. Similarly, the fuselage fuel is usually the first to be burned. One of the vicious design cycles that you can find yourself in is that if the range of the airplane is not quite what you had for a target you will have to add fuel and likely will have to add that fuel by adding capacity to the fuselage tank. The penalty that you pay is increased structural loads as that fuel does not sit near the source of the lift that must carry it. The wing root loads go up, add more structure, that increases weight, fuel burn goes up, and now you need to add even more fuel. As you can imagine, most new airplane designs go through some serious weight reduction efforts to get the design to close meeting the payload / range targets they are after.

One of the interesting optimizations that takes place on modern commercial transports that are simultaneously optimized for fuel efficient cruise and minimum structural weight is management of lift during flight. For minimum drag the optimum spanwise wing loading is essentially elliptical with a good deal of lift generated on the outer portion of the wing. This lift distribution, however, generates very large bending moments at the wing/body joint - particularly when maneuvering at elevated load factors. These high bending moments would require heavy structure in that portion of the airframe. For lower wing bending moments and thus to enable lighter weight structure it would be desirable to have wing spanwise loading the has more of the lift on the inboard wing and less out near the wing tips. Starting with 787 Boeing has incorporated wing maneuver load alleviation into its control systems whereby the spanwise lift distribution is managed to be that for efficient, low drag operation when at 1g cruise and more inboard during elevated g maneuvers.

Time for the end of this lecture - FCeng84 signs out (gums is rubbing off on me!)
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Old 22nd Mar 2019, 10:07
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Just an aside. When Boeing produced the B727 they tested one to destruction. The aircraft was in a hangar and very securely attached to the hangar floor, the tips of the wings were secured, by cables, to a winch in the roof and then slowly cranked up until the wings gave way, Just before the wings broke they were at a ridiculously high angle, well above the level of the fuselage, the bang, when they went, was heard all over the airfield!
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Old 22nd Mar 2019, 10:28
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Couldn't find the 727 test, but this one of the 777 gives you an idea of how things go if you push it .. even if they do play the bang over and over and over ...

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Old 22nd Mar 2019, 10:39
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Originally Posted by FCeng84 View Post
Starting with 787 Boeing has incorporated wing maneuver load alleviation into its control systems whereby the spanwise lift distribution is managed to be that for efficient, low drag operation when at 1g cruise and more inboard during elevated g maneuvers.
This idea of adjustable span-wise distribution of lift has been around longer than many will think. The Vickers VC-10 had a manually operated switch for an 'Aileron Upset' system, that moved the ailerons symetrically upwards by a couple of degrees during certain phases of flight. This moved the centre of lift on the wings inboard, thereby reducing the wing bending moment, and was mainly used at lower altitudes and heavier weights, when there was more likely to be greater amounts of centreline (fuselage) fuel on board.

In general, once that weight was burned off, the Aileron Upset was switched off. Having Aileron upset applied did, of course, have it's own limiting effect on the aircraft's performance envelope, and when switched off the aircraft's altitude capability was improved. This was due to the extra drag created by having those ailerons a couple of degrees upwards from a more normal free-trail position.

A similar idea was employed on the Handley Page Victor K2. At conversion from bomber to tanker the ailerons were permanently rigged upwards by a couple of degrees, to shift the centre of lift inboard slightly and relieve wing bending moments. Didn't prevent the ol' beastie from having an Mno of 0.93, or being capable of reaching FL490 (the latter limit was imposed because of oxygen regulators, rather than aerodynamic capability, but that is another story entirely ...).
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Old 22nd Mar 2019, 12:25
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It's always interesting to notice two long-haul aircraft at adjacent gates, one just arrived, the other fuelled for departure, and the different wingtip heights. But actually, it's not a huge difference. The structural loads imposed by g-force in manoeuvering and turbulence, or assymetric landing where the main gear is attached to the wings, are greater.

And the propensity to "snap" is governed more than anything else by the wing design, simplistically how strong their structure is. Some aircraft have more robust designs than others. Did I say MD-11 ???

There's this unfortunate video of a total wings failure on a Hercules doing a fire retardant drop, at the point where there's a combination of sudden loading change and significant manoeuvring.

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Old 22nd Mar 2019, 14:03
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Originally Posted by FCeng84 View Post
Time for the end of this lecture - FCeng84 signs out (gums is rubbing off on me!)
I had to re-read this a few times and search a few (dozens) articles on Wikipedia (mea culpa!!!), but it was worthwhile. I like written lectures and I do that myself when I know something about the subject. So, when in doubt, babble! :-D Know that you always have at least one grateful reader here. Thank you!

PS One thing I still can't understand. How do you manipulate controls to relieve load on the wing root? I realize that various wing designs give a different wing and wing root loads, but how can control of the already made wing impact it's load factor?
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Old 22nd Mar 2019, 17:42
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Originally Posted by ProPax View Post
I had to re-read this a few times and search a few (dozens) articles on Wikipedia (mea culpa!!!), but it was worthwhile. I like written lectures and I do that myself when I know something about the subject. So, when in doubt, babble! :-D Know that you always have at least one grateful reader here. Thank you!

PS One thing I still can't understand. How do you manipulate controls to relieve load on the wing root? I realize that various wing designs give a different wing and wing root loads, but how can control of the already made wing impact it's load factor?
Thanks for the kind words.

As load factor increases, ailerons and outboard spoilers trailing edge up (dumps some outboard lift) and at the same time inboard spoilers down (if, up - preserves inboard lift). The challenge is what this does to longitudinal center of lift and pitching moment on a swept wing airplane and how to manage to preserve acceptable maneuver response from the pilot's point of view.

Last edited by FCeng84; 23rd Mar 2019 at 10:36. Reason: Typo
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Old 22nd Mar 2019, 18:19
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If it ever got to the stage where fuel would have to be carried for ballast, then you would simply beef up the wing structure at the design stage, surely.
The extra weight would help reduce the bending moment.
Why go to the trouble of designing a lightweight structure, and then adding weight?
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Old 22nd Mar 2019, 20:49
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Originally Posted by 16024 View Post
If it ever got to the stage where fuel would have to be carried for ballast, then you would simply beef up the wing structure at the design stage, surely.
The extra weight would help reduce the bending moment.
Why go to the trouble of designing a lightweight structure, and then adding weight?
Extra weight will not reduce bending moment. It will increase the amount of moment that the structure can tolerate
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Old 22nd Mar 2019, 22:20
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If lack of fuel in the wings causes them to break off, how did the Gimli glider or the Air Transat planes that ran out of gas in the air land successfully? And does this mean that Avianca at JFK really crashed from the wings snapping off and not from fuel exhaustion?
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Old 23rd Mar 2019, 00:02
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Originally Posted by 16024 View Post
If it ever got to the stage where fuel would have to be carried for ballast, then you would simply beef up the wing structure at the design stage, surely.
The extra weight would help reduce the bending moment.
Why go to the trouble of designing a lightweight structure, and then adding weight?
some aircraft such as the herc have a ZFW that changes due to airdrop. Once the load is used, the excess fuel can then be burned as recovery fuel. Often on result missions, you have to plan on rtb without completing the drop so then the fuel becomes unusable.
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Old 23rd Mar 2019, 05:13
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Originally Posted by SeenItAll View Post
If lack of fuel in the wings causes them to break off, how did the Gimli glider or the Air Transat planes that ran out of gas in the air land successfully? And does this mean that Avianca at JFK really crashed from the wings snapping off and not from fuel exhaustion?
Well, by definition, those aircraft, with complete fuel exhaustion, had no fuel in the center (fuselage) tank either. It is not so much a lack of fuel in the wings, but a lack of fuel in the wings while still having substantial fuel weight in the fuselage, that puts one in more danger.

There are aircraft that carry no fuel in the wings, ever (not enough room - F-104, B-47, Short 330/360, and of course any helicopter ever built )

Plus, there is the concept of "margins." We may have less margin for high-G maneuvers/events without fuel in the wings. That is not the same as zero margin at all times.

Scenario - a flight over the Atlantic/Pacific, and for some ungodly reason all the pumps in the center tank fail before it empties. Choices - ditch, or burn off wing fuel to divert. The aircraft won't immediately fall out of the sky, but the pilots would (or should) know they would have progressively less margin for turbulence or manuevering stress, and treat the aircraft tenderly until safely down.

It is good policy to fill the wing tanks first and empty them last. It keeps down the immediate stresses and reduces fatigue over the life of the plane. In some cases (heavy lifters, always loaded to MTOW or MZFW) it will be more critical than in others.
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