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Why Weights On Wings Without Engines?  
User currently offlineA380900 From France, joined Dec 2003, 1106 posts, RR: 1
Posted (3 years 6 months 1 week 6 days 19 hours ago) and read 4470 times:

I guess the weights are there so that the wings stay in shape but can someone elaborate a bit? Is it a problem if a wing remains without weights for 2 hours, for 2 days or for two months for instance? Are the weight quite elaborat in their construction to emulate how an engine is attached or is it cruder?

What happens if the wings remain without weights for too long? Is the wing lost?

Also are these planes kept with some fuel in them in addition to the weights to keep the wing bent?

[Edited 2011-01-20 10:51:52]

15 replies: All unread, jump to last
 
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 1, posted (3 years 6 months 1 week 6 days 19 hours ago) and read 4460 times:

To maintain a forwards enough C of G so that it doesn't sit on it's tail during work on it that might shift weight about. 

[Edited 2011-01-20 11:01:07]


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User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 2, posted (3 years 6 months 1 week 6 days 19 hours ago) and read 4448 times:

Quoting A380900 (Thread starter):
What happens if the wings remain without weights for too long? Is the wing lost?


The weights are installed to keep the center of gravity correct. Without the weights the plane can tip on its tail.

Quoting A380900 (Thread starter):
Also are these planes kept with some fuel in them in addition to the weights to keep the wing bent?


While there may be some un-used fuel in the tanks, none is added to induce bending.

When major work is required on the wings they are defueled and then jacked into a neutral position. Having them bend down is un-natural.


User currently offlineCitationJet From United States of America, joined Mar 2003, 2425 posts, RR: 3
Reply 3, posted (3 years 6 months 1 week 6 days 17 hours ago) and read 4317 times:

Quoting 474218 (Reply 2):
The weights are installed to keep the center of gravity correct. Without the weights the plane can tip on its tail.

Correct. That is why weights are used on planes with wing mounted engines.

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And not used on planes with rear mounted engines.

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User currently offlineA380900 From France, joined Dec 2003, 1106 posts, RR: 1
Reply 4, posted (3 years 6 months 1 week 6 days 16 hours ago) and read 4261 times:

I thought it was to maintained the right shape for the wing... I had it completely wrong. It makes more sense though. Thanks.

User currently onlinePihero From France, joined Jan 2005, 4392 posts, RR: 76
Reply 5, posted (3 years 6 months 1 week 6 days 14 hours ago) and read 4179 times:
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Quoting CitationJet (Reply 3):
Quoting 474218 (Reply 2):
The weights are installed to keep the center of gravity correct. Without the weights the plane can tip on its tail.

Correct. That is why weights are used on planes with wing mounted engines
And not used on planes with rear mounted engines

Not at all. What you're seeing is an illustration of the *zero fuel weight* concept and the wing bending relief brought by the engines'weight.
Without those weights, the whole weight of the aircraft goes on the wing box as compression (on the top of the wing) or tensile (the bottom of the wing) loads.
the weights, placed outside the landing gear, provide a downward traction that relieves those loads on the wing box.
If it was a CofG problem, it would have been simpler to support the rear fuselage, with a cane / pole etc...

As for rear mounted engines, obviously they can't play any role in our scheme.


Quoting A380900 (Thread starter):

What happens if the wings remain without weights for too long? Is the wing lost?

No, but there could be some deformation.

Quoting A380900 (Thread starter):

Also are these planes kept with some fuel in them in addition to the weights to keep the wing bent?

Depends how long the airplane is going to stay that way. Some fuel could help, but rather quickly, water will be absorbed, which is never a good thing



Contrail designer
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 6, posted (3 years 6 months 1 week 6 days 13 hours ago) and read 4129 times:

Quoting Pihero (Reply 5):
Without those weights, the whole weight of the aircraft goes on the wing box as compression (on the top of the wing) or tensile (the bottom of the wing) loads.

Those are flight loads. On the ground, top of the wing is in tension, bottom is in compression, and the wing is only supporting its own weight (the wing can't "see" the fuselage).

Adding weight to the wing on the ground just increases the weight that the wing has to support and makes the loads worse. You only get bending relief in flight. And nobody is flying with weights instead of engines.

Quoting Pihero (Reply 5):
the weights, placed outside the landing gear, provide a downward traction that relieves those loads on the wing box.

No, it aggravates ground loads. The wing was already in tension on the top on the ground...adding weight to the wing just makes the loading worse between the weights.

Quoting Pihero (Reply 5):
If it was a CofG problem, it would have been simpler to support the rear fuselage, with a cane / pole etc...

Except then you can't move the airplane. Weights are much simpler.

Quoting Pihero (Reply 5):
Quoting A380900 (Thread starter):
What happens if the wings remain without weights for too long? Is the wing lost?

No, but there could be some deformation.

Not if it's designed properly. You can only get deformation by going above the elastic limit. That should only happen in the region between limit and ultimate load. There is no way an aircraft sitting on the ground unloaded can get anywhere close to limit load, let alone ultimate.

Tom.


User currently offlineCCA From Hong Kong, joined Oct 2002, 830 posts, RR: 14
Reply 7, posted (3 years 6 months 1 week 6 days 10 hours ago) and read 4080 times:

Quoting Pihero (Reply 5):
Quoting CitationJet (Reply 3):
Quoting 474218 (Reply 2):

Not at all. What you're seeing is an illustration of the *zero fuel weight* concept and the wing bending relief brought by the engines'weight.
Without those weights, the whole weight of the aircraft goes on the wing box as compression (on the top of the wing) or tensile (the bottom of the wing) loads.
the weights, placed outside the landing gear, provide a downward traction that relieves those loads on the wing box.
If it was a CofG problem, it would have been simpler to support the rear fuselage, with a cane / pole etc...


Sorry you are completely incorrect, it's purely C of G the loads on the wing box while on the ground are nothing compared to while in flight. Having towed 747s in a previous life without engines we placed ballast in the fwd cargo as we didn't have weights to hang off the pylons. A/C without pylons installed also use this method.

Plus it's difficult to tow an A/C with a tail stand or crane attached!



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User currently onlinePihero From France, joined Jan 2005, 4392 posts, RR: 76
Reply 8, posted (3 years 6 months 1 week 6 days 3 hours ago) and read 3918 times:
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Quoting tdscanuck (Reply 6):

Those are flight loads. On the ground, top of the wing is in tension, bottom is in compression, and the wing is only supporting its own weight

I certainly beg to differ : you seem to be forgetting the landing gear ; the attachment to the wing becomes the axis of a fucrum and the weight outside the gear reduces the loads on the wing attachment to the fuselage.
Take a plastic ruler, just held by the tips ; place a bottle on top of the middle : you'll have a nice bend , and the conditions -extreme, I agree- of an in-flight aircraft. OK.
Now, put that contraption on top of two books, placed some 2 to 3 inches from the centre - and the bottle- to simulate the landing gear... nothing changes;;;until you place some weights outside the books and the whole picture changes.
That's what the weights do.
The CofG moving forward is a by-product.



Quoting tdscanuck (Reply 6):

Quoting Pihero (Reply 5):
If it was a CofG problem, it would have been simpler to support the rear fuselage, with a cane / pole etc...

Except then you can't move the airplane. Weights are much simpler.
Quoting CCA (Reply 7):
Having towed 747s in a previous life without engines we placed ballast in the fwd cargo as we didn't have weights to hang off the pylons. A/C without pylons installed also use this method.

That's exactly the point : It's much easier to ballast the damn thing with a weight in the forward hold than it is to hang those thingies on the pylons.

Quoting CCA (Reply 7):

Plus it's difficult to tow an A/C with a tail stand or crane attached!

  

Quoting CCA (Reply 7):

it's purely C of G the loads on the wing box while on the ground are nothing compared to while in flight.

I thought that weight = lift. On the ground, the only difference is the weight supported by the nose gear, and that is CofG critical, and the position of the lift centre.



Contrail designer
User currently offlineCCA From Hong Kong, joined Oct 2002, 830 posts, RR: 14
Reply 9, posted (3 years 6 months 1 week 6 days 2 hours ago) and read 3881 times:

The landing gear supports the weight of the A/C on the ground the wings support it in flight, the wings only support the weight of fuel in the wings on the ground.

How could loading several pallets of ballast in the fwd cargo be easier than hanging simple blocks off the pylons, plus they don't interfere when working in the fwd cargo where easy access to the avionics and oxygen systems can be done.



C152 G115 TB10 CAP10 SR-22 Be76 PA-34 NDN-1T C500 A330-300 A340-300 -600 B747-200F -200SF -400 -400F -400BCF -400ERF -8F
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (3 years 6 months 1 week 6 days ago) and read 3834 times:

Quoting Pihero (Reply 8):
I certainly beg to differ : you seem to be forgetting the landing gear ; the attachment to the wing becomes the axis of a fucrum and the weight outside the gear reduces the loads on the wing attachment to the fuselage.

I'm not forgetting the landing gear. On the ground, the landing gear carries the whole load and the wing is a cantilever beam that's carrying it's own weight (and any fuel that's in it) and generating no lift. The wing bends *down* from the gear attachment point as you go outboard. Adding weight outboard of the gear increases the bending moment, increasing vertical force on the gear (reaction to the weight) and increasing bending moment in the entire wing (including that portion between the gear) between the weights.

Quoting Pihero (Reply 8):
Take a plastic ruler, just held by the tips ; place a bottle on top of the middle : you'll have a nice bend , and the conditions -extreme, I agree- of an in-flight aircraft. OK.
Now, put that contraption on top of two books, placed some 2 to 3 inches from the centre - and the bottle- to simulate the landing gear... nothing changes

I agree with your approximation of the in-flight aircraft. What you seem to be missing is that, on the ground, you're *not* supporting the ruler by the tips. There is no lift on the ground. The only support for the wing on the ground is the landing gear attachment points. In addition, the weight of the bottle (fuselage) on the ground is held directly by the landing gear, not by the wing.

The appropriate ground model would be to put the two books 2-3" apart, sit the bottle and the ruler across the books. The ruler will sag a little under its own weight. Now strap weights to the ruler outboard of the books...the ruler bends more.

Quoting Pihero (Reply 8):
On the ground, the only difference is the weight supported by the nose gear, and that is CofG critical, and the position of the lift centre.

In flight, none of the gear carry any load. On the ground, they carry all of it (and the wing experiences no lift). This makes a huge difference.

Tom.


User currently onlinePihero From France, joined Jan 2005, 4392 posts, RR: 76
Reply 11, posted (3 years 6 months 1 week 6 days ago) and read 3828 times:
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Quoting CCA (Reply 9):

How could loading several pallets of ballast in the fwd cargo be easier

Answer : You'll need an awful less weight there.

Quoting CCA (Reply 9):
The landing gear supports the weight of the A/C

I thought the main landing gear, which bears most of the weight of the aircraft is under the wing, and the wing is attached to the fuselage, thus creating quite a set of loads on the wing attachment to the fuselage.



Contrail designer
User currently offlineDocLightning From United States of America, joined Nov 2005, 19417 posts, RR: 58
Reply 12, posted (3 years 6 months 1 week 5 days 10 hours ago) and read 3624 times:

Quoting tdscanuck (Reply 6):
And nobody is flying with weights instead of engines.

I'm sure FR will find a way to charge an extra fee for engines.  
Quoting Pihero (Reply 8):

That's exactly the point : It's much easier to ballast the damn thing with a weight in the forward hold than it is to hang those thingies on the pylons.

So why do they hang them, then?


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 13, posted (3 years 6 months 1 week 5 days 9 hours ago) and read 3620 times:

Quoting Pihero (Reply 11):
I thought the main landing gear, which bears most of the weight of the aircraft is under the wing, and the wing is attached to the fuselage, thus creating quite a set of loads on the wing attachment to the fuselage.

It's actually behind the wing on most designs. In a typical arrangement, the forward landing gear bearing is on the wing's rear spar and the aft bearing is on the landing gear beam. One end of the landing gear beam attaches to the rear spar farther outboard, the other attaches directly the fuselage. Some load goes directly from the gear to the fuselage via the inboard end of the landing gear beam, the rest connects in to the rear spar and then into the fuselage.

For planes with more than two main gear (A340, 747, A380, DC-10, etc.) the load from the body gear goes straight into the fuselage and doesn't have much to do with the wing.

So there is indeed a moment between the gear attachment on the wing and the side-of-body joint where the fuselage loads enter. However, this moment isn't appreciably altered for most geometries by hanging weights off the pylons because, although the moment from the weights does act in the opposite sense, the weights also increase the force on the gear and hence increase the moment from the gear (the exact change depends, among other things, on the distances between the gear bearing to fuselage, beam attachment point to fuselage, and pylon to fuselage).

This is very very different from the flight case, where the whole wing is under the same bending moment due to lift and the reaction to the increased weight on the pylons is distributed over the entire wing, rather than as a point load at the landing gear attachment.

Tom.

[Edited 2011-01-21 20:57:00]

User currently onlinePihero From France, joined Jan 2005, 4392 posts, RR: 76
Reply 14, posted (3 years 6 months 1 week 5 days 1 hour ago) and read 3555 times:
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Quoting DocLightning (Reply 12):
Quoting Pihero (Reply 8):

That's exactly the point : It's much easier to ballast the damn thing with a weight in the forward hold than it is to hang those thingies on the pylons.

So why do they hang them, then?

That's exactly my question . They could move the CoG forward with a much smaller weight in the forward cargo hold.
A simple quick look at a three way drawing of any twin will show that the engines are not very far in front of the main gear, which means that their influence on the CoG position relative to that gear is minimal.

Quoting tdscanuck (Reply 13):
This is very very different from the flight case, where the whole wing is under the same bending moment due to lift and the reaction to the increased weight on the pylons is distributed over the entire wing, rather than as a point load at the landing gear attachment.

That's bull and you know it.
Statically - and the MZFW concept is about static - we could break down all the forces affecting a complete airframe into a finite number of forces, each one with its own center of application. Therefore, considering that the engines'weight is part of the overall weight of the airplane, that applies from the CofG completely defeats your original assumption...therefore, you'll have to consider each force on its own in the general static situation : the weights cause a moment relative to the axis of a fulcrum which is the landing gear attachment *centre*, exactly as you consider a theoretical *center of lift* that moves with AoA and Mach number etc...
This concept is not, per se, about weight and CoG, but a moment relative to the point at wxhich wing and fuselage join.



Contrail designer
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 15, posted (3 years 6 months 1 week 4 days 19 hours ago) and read 3470 times:

Quoting Pihero (Reply 14):
That's bull and you know it.
Statically - and the MZFW concept is about static - we could break down all the forces affecting a complete airframe into a finite number of forces, each one with its own center of application

There is a fundamental difference between point loads and distributed loads...the shear and moment curves are totally different and the resulting stresses throughout the beam (wing) are totally different. Lift is a distributed force, landing gear support is a point load.

It's not bull, it's basic structural mechanics.

Tom.


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