5 ms·

In my mind he didn't address the fundamental reason why the wing visibly oscillates while most other structures that you trust your life to like buildings, brid

In my mind he didn't address the fundamental reason why the wing visibly oscillates while most other structures that you trust your life to like buildings, bridges, and cars do not. Airplanes are designed with a 1.5 factor of safety A factor of safety is applied to a design after every load the structure will be subjected to is calculated, they multiply by 1.5 to be sure the structure will be safe. 1.5 may sound conservative but it is the smallest factor of safety someone normally encounters, spacecraft and fighter jets might use 1.2 while cars often use 3; buildings and bridges use 5. Airplanes are used much closer to the strength limit of their materials so we see them flexing. Of course everything deflects under load - my strength of materials instructor illustrated this by analyzing how much an anvil compresses when a fly lands on it; answer: less than can be measured.

Asbostos 11y ago
Being close to failure isn't a requirement for visibly bending. For example, see wire rope or cloth. Anything very long and thin can flex a long way without much strain on any individual part of it. Low FoS yes, but even if you doubled it to 3, you'd still see the wing tip moving - only half as much. Truck trailers are also very soft in torsion. See an empty one go over a big bump and it'll visibly twist. Not close to failure, just not very stiff.
- dalke 11y ago
  Or a ship in heavy waves - https://youtu.be/oiqofDBsFL0?t=60 https://youtu.be/oiqofDBsFL0?t=60 . But not close to failure.
Dwolb 11y ago
My question would be, "What's 1?". I don't know if 1 is the mean, 1 standard deviation, 2 standard deviations, etc. I'd be really worried if 1 was "A fully loaded plane of 180lbs adults with two checked bags of 50lbs each flying in clear blue skies with no wind."
- chiph 11y ago
  "1" is what the plane would be expected to experience during normal operation, where that includes flying through a storm, crosswinds, etc. How did they determine this figure? By years and years of analyzing successful designs and crashes (not-so-successful designs).
  - maxerickson 11y ago
    Just to add a word, it would be the maximum expected there, so would be for a plane loaded to the maximum takeoff weight doing the most aggressive maneuver in the worst weather (or really, most adverse external conditions).
    - Dwolb 11y ago
      Is this 'the maximum ever observed in a real setting'? Could it be 'the maximum inputs ever recorded on earth and then used in my calculation'?
      - drewkett 11y ago
        It's the maximum load you would expect to see in the life of the aircraft
        Dwolb 11y ago
        Sorry about this, I'm struggling a bit. Isn't the "load observed over the life of an aircraft" just a statistical distribution? Therefore a maximum doesn't make a lot of sense. It would make more sense to talk about standard deviations away from the mean unless the maximum was "maximum ever recorded in our dataset"
        maxerickson 11y ago
        First, I'm not doing structural design, so maybe someone will wander by with a better answer. Anyway, it's probably overly simplistic, but passing the load test would involve surviving the maximum expected load with zero damage. Implying that lesser loads would also never do damage. In reality, cyclic fatigue is also considered. This seems like a nice discussion that shows the complexity: http://www.coe.montana.edu/me/faculty/cairns/Introduction%20to%20Aerospace-Web/04-DesignPhilosophy.pdf http://www.coe.montana.edu/me/faculty/cairns/Introduction%20...
qbrass 11y ago
If the wings were stiff, the forces that would normally deflect the wings would be transferred into the cabin.
- zwiteof 11y ago
  No, the forces are transferred to the fuselage in any case. If they weren't, the wing wouldn't be bending relative to the cabin, it would just experience rigid body motion instead.
  - cmsmith 11y ago
    Not quite true. As long as the frequency of the dynamic load isn't too slow relative to the natural period of the wing, then not all of that force has to be resisted by the wing connection. That's why wings are flexible, why we put shock absorbers in cars, and why flexible buildings can be designed for lower seismic loads.
codingdave 11y ago
But buildings and bridges do oscillate. Architects just try to keep it below the level of human perception because they know people would freak out. The fact that we do not often see them oscillate has more to do with design and psychology than physics.
cmsmith 11y ago
Structural engineer here: Buildings and bridges do not use 5, give us a little credit We mostly assume that materials are 90% as strong as they say they are, that the building weighs 1.2 times as much as it really does, and that stuff in the building weighs 1.6 times as much as it really does. The total factor of safety is something like 1.5. The reason that buildings don't vibrate (noticably under service loads) is that people would get queasy, so we design things for stiffness as well as strength. Aircraft are more space and weight limited, and no one is sitting on the end of the wing, so the engineers mostly just care that everything will stay attached.
WalterBright 11y ago
If a wing is too stiff, you get one kind of flutter, too soft, and you get another. The idea is to be midway between those two cases. Flutter (dynamic instability) will rip the wings off. An awful lot of design effort goes into avoiding flutter. Blow on a stretched rubber band, and it'll vibrate, too.

Asbostos 11y ago

Being close to failure isn't a requirement for visibly bending. For example, see wire rope or cloth. Anything very long and thin can flex a long way without much strain on any individual part of it. Low FoS yes, but even if you doubled it to 3, you'd still see the wing tip moving - only half as much. Truck trailers are also very soft in torsion. See an empty one go over a big bump and it'll visibly twist. Not close to failure, just not very stiff.

dalke 11y ago
Or a ship in heavy waves - https://youtu.be/oiqofDBsFL0?t=60 https://youtu.be/oiqofDBsFL0?t=60 . But not close to failure.

Dwolb 11y ago

My question would be, "What's 1?". I don't know if 1 is the mean, 1 standard deviation, 2 standard deviations, etc. I'd be really worried if 1 was "A fully loaded plane of 180lbs adults with two checked bags of 50lbs each flying in clear blue skies with no wind."

chiph 11y ago
"1" is what the plane would be expected to experience during normal operation, where that includes flying through a storm, crosswinds, etc. How did they determine this figure? By years and years of analyzing successful designs and crashes (not-so-successful designs).
- maxerickson 11y ago
  Just to add a word, it would be the maximum expected there, so would be for a plane loaded to the maximum takeoff weight doing the most aggressive maneuver in the worst weather (or really, most adverse external conditions).
  - Dwolb 11y ago
    Is this 'the maximum ever observed in a real setting'? Could it be 'the maximum inputs ever recorded on earth and then used in my calculation'?
    - drewkett 11y ago
      It's the maximum load you would expect to see in the life of the aircraft
      - Dwolb 11y ago
        Sorry about this, I'm struggling a bit. Isn't the "load observed over the life of an aircraft" just a statistical distribution? Therefore a maximum doesn't make a lot of sense. It would make more sense to talk about standard deviations away from the mean unless the maximum was "maximum ever recorded in our dataset"
        maxerickson 11y ago
        First, I'm not doing structural design, so maybe someone will wander by with a better answer. Anyway, it's probably overly simplistic, but passing the load test would involve surviving the maximum expected load with zero damage. Implying that lesser loads would also never do damage. In reality, cyclic fatigue is also considered. This seems like a nice discussion that shows the complexity: http://www.coe.montana.edu/me/faculty/cairns/Introduction%20to%20Aerospace-Web/04-DesignPhilosophy.pdf http://www.coe.montana.edu/me/faculty/cairns/Introduction%20...

qbrass 11y ago

If the wings were stiff, the forces that would normally deflect the wings would be transferred into the cabin.

zwiteof 11y ago
No, the forces are transferred to the fuselage in any case. If they weren't, the wing wouldn't be bending relative to the cabin, it would just experience rigid body motion instead.
- cmsmith 11y ago
  Not quite true. As long as the frequency of the dynamic load isn't too slow relative to the natural period of the wing, then not all of that force has to be resisted by the wing connection. That's why wings are flexible, why we put shock absorbers in cars, and why flexible buildings can be designed for lower seismic loads.

codingdave 11y ago

But buildings and bridges do oscillate. Architects just try to keep it below the level of human perception because they know people would freak out. The fact that we do not often see them oscillate has more to do with design and psychology than physics.

cmsmith 11y ago

Structural engineer here: Buildings and bridges do not use 5, give us a little credit We mostly assume that materials are 90% as strong as they say they are, that the building weighs 1.2 times as much as it really does, and that stuff in the building weighs 1.6 times as much as it really does. The total factor of safety is something like 1.5. The reason that buildings don't vibrate (noticably under service loads) is that people would get queasy, so we design things for stiffness as well as strength. Aircraft are more space and weight limited, and no one is sitting on the end of the wing, so the engineers mostly just care that everything will stay attached.

WalterBright 11y ago

If a wing is too stiff, you get one kind of flutter, too soft, and you get another. The idea is to be midway between those two cases. Flutter (dynamic instability) will rip the wings off. An awful lot of design effort goes into avoiding flutter. Blow on a stretched rubber band, and it'll vibrate, too.