6 ms·

Why Airplane Wings Oscillate in Turbulence

21 points by miralabs 11y ago

vacri 11y ago
On a tangent, one thing that slightly freaks me out a bit is that on the ground, the wings hang from the plane (wings bend down); whereas in the air, the plane hangs from the wings (wings bend up).
- joosters 11y ago
  Well, the wings of the plane are the only bit that actually flies, so you can think of the fuselage as just hanging on to the wings for dear life...
  - WalterBright 11y ago
    That's actually incorrect. What keeps the airplane flying is gripping the armrests. The proof is simple - when the airplane dips, grip the armrests harder. The plane will recover.
vlehto 11y ago
I've been wondering why passenger planes and military cargo planes have so different layout? So far my best guess is that it's related to landing gear. Landing gear makes significant portion of the total weight, you want to keep it short and compact. Engines are the heaviest things in passenger planes, so landing gear is close to engines. http://i.telegraph.co.uk/multimedia/archive/03137/plane_3137447b.jpg http://i.telegraph.co.uk/multimedia/archive/03137/plane_3137... With cargo plane the heaviest part of the plane is the cargo. http://cdn23.us2.fansshare.com/photos/antonovan124/antonov-an-low-level-flight-cargo-plane-wallpapers-wallpaper-256058768.jpg http://cdn23.us2.fansshare.com/photos/antonovan124/antonov-a... It seems bit weird that one of the most dictating thing to airplane layout is not really related to flying itself. But I could be wrong on this one. Does anybody know better?
- Gravityloss 11y ago
  Rough fields - high engines.
- lotsofactors 11y ago
  Mostly, the high-wing and landing gear configuration on many cargo aircraft designs is intended to facilitate a low loading floor and the ability to load and unload the aircraft on rough fields without special ground equipment (e.g. by means of a ramp integrated with the the rear doors of the aircraft like a C-130 or with a deployable forward ramp and kneeling front gear like the An-124.) Another benefit is providing extra ground clearance for the engines to reduce foreign object ingestion. Everything else flows from this. The wing spar and structural supports needed in a cantilever monoplane (i.e. not a biplane with a truss structure) take up a fair amount of room. By moving these structural elements to the top of the aircraft, it's possible to get a low floor and still hang engines from pylons beneath the wings. The wings themselves will be angled downward from root to tip (known as anhedral) to partly counteract the pendulum stability caused by having a center of gravity so far below the center of pressure. Passenger aircraft do not usually use this configuration because it is heavier - the sides of the fuselage must be built stronger to support the load, compared to a low-wing configuration. Weight costs performance costs fuel costs money, so you don't build a large aircraft like this unless it has to operate at fields without ground equipment.
  - vlehto 11y ago
    There are lot of bombers with high wing (B52, almost anything several engines from WWII). About half of all propeller driven passenger planes have high wing. >Unlike the An-124, the An-225 was not intended for tactical airlifting and is not designed for short-field operation It still has high wing. And U2 has high wing. It's probably the plane with most carefully calculated weight ever. If it was inherently heavier, this would make no sense.
    - lotsofactors 11y ago
      > There are lot of bombers with high wing (B52, almost anything several engines from WWII). Most bombers dispense ordnance from their undersides, ideally near the center of gravity; limiting the structural members needed in those parts simplifies things. > About half of all propeller driven passenger planes have high wing. These are designed not to require ground vehicles for loading and unloading. Propellor ground clearance is also an issue for the larger types. > Unlike the An-124, the An-225 was not intended for tactical airlifting and is not designed for short-field operation > It still has high wing. The An-225 carries cargoes of unusual bulk to unusual places. Being able to get these cargoes on and off the aircraft without some sort of lift has advantages. Low-wing freighters, which are all variants of passenger types these days, almost always carry palletized cargo of standard dimensional units (See https://en.wikipedia.org/wiki/Unit_load_device https://en.wikipedia.org/wiki/Unit_load_device) - this allows for standardized ground handling equipment for most loads. > And U2 has high wing. It's probably the plane with most carefully calculated weight ever. > If it was inherently heavier, this would make no sense. The U2 is not a high-wing aircraft, it is a mid-wing aircraft with its spar roughly running through its center of gravity. This is actually the most efficient and aerodynamically cleanest configuration, and many high-performance aircraft use this arrangement. The main problem with it for passenger or cargo use is that the wing structure ends up interfering with the middle of the fuselage volume near the center of gravity, which tends to be valuable space in a transport aircraft.
LoSboccacc 11y ago
On a tangential note plane wins are quite strong themselves Here's Boeing wing being stress tested: https://m.youtube.com/watch?v=sA9Kato1CxA https://m.youtube.com/watch?v=sA9Kato1CxA
Gravityloss 11y ago
The lift varies since the aircraft flies fast through regions where the air has different velocity. Think about a column of ascending air, and the plane flying quickly through that. All else being equal, the plane will accelerate vertically until it is traveling vertically at the same speed as the air column (if the column is large enough). That's why flying low and fast is extremely bumpy and taxing for aircraft. Lots of air speed variations.
asmithmd1 11y ago
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).
zwiteof 11y ago
The article's explanation is a bit simplified. It sounds like his explanation is more akin to a body freedom flutter response where the aircraft's short-period mode is coupled to the structural response. What he observed is probably closer to a more classical flutter response with the wing's bending and torsion coupled. A gust hits the wing, which increases the load, this increases the bending deformation which can also induce torsion causing a twist in the wing. At too high of a speed (the flutter speed), the response is unstable and can quickly become catastrophic. At normal speeds, the oscillation will tend to die out as the restoring force of the structure and the damping from the structure, aerodynamic loads and controls causes the response to die out. Interestingly, you can go past the first flutter speed of an aircraft with a properly designed control system (aeroservoelasticity) meaning you can get away with a lighter (read, more flexible) wing structure.

21 points by miralabs 11y ago

vacri 11y ago

On a tangent, one thing that slightly freaks me out a bit is that on the ground, the wings hang from the plane (wings bend down); whereas in the air, the plane hangs from the wings (wings bend up).

joosters 11y ago
Well, the wings of the plane are the only bit that actually flies, so you can think of the fuselage as just hanging on to the wings for dear life...
- WalterBright 11y ago
  That's actually incorrect. What keeps the airplane flying is gripping the armrests. The proof is simple - when the airplane dips, grip the armrests harder. The plane will recover.

vlehto 11y ago

I've been wondering why passenger planes and military cargo planes have so different layout? So far my best guess is that it's related to landing gear. Landing gear makes significant portion of the total weight, you want to keep it short and compact. Engines are the heaviest things in passenger planes, so landing gear is close to engines. http://i.telegraph.co.uk/multimedia/archive/03137/plane_3137447b.jpg http://i.telegraph.co.uk/multimedia/archive/03137/plane_3137... With cargo plane the heaviest part of the plane is the cargo. http://cdn23.us2.fansshare.com/photos/antonovan124/antonov-an-low-level-flight-cargo-plane-wallpapers-wallpaper-256058768.jpg http://cdn23.us2.fansshare.com/photos/antonovan124/antonov-a... It seems bit weird that one of the most dictating thing to airplane layout is not really related to flying itself. But I could be wrong on this one. Does anybody know better?

Gravityloss 11y ago
Rough fields - high engines.
lotsofactors 11y ago
Mostly, the high-wing and landing gear configuration on many cargo aircraft designs is intended to facilitate a low loading floor and the ability to load and unload the aircraft on rough fields without special ground equipment (e.g. by means of a ramp integrated with the the rear doors of the aircraft like a C-130 or with a deployable forward ramp and kneeling front gear like the An-124.) Another benefit is providing extra ground clearance for the engines to reduce foreign object ingestion. Everything else flows from this. The wing spar and structural supports needed in a cantilever monoplane (i.e. not a biplane with a truss structure) take up a fair amount of room. By moving these structural elements to the top of the aircraft, it's possible to get a low floor and still hang engines from pylons beneath the wings. The wings themselves will be angled downward from root to tip (known as anhedral) to partly counteract the pendulum stability caused by having a center of gravity so far below the center of pressure. Passenger aircraft do not usually use this configuration because it is heavier - the sides of the fuselage must be built stronger to support the load, compared to a low-wing configuration. Weight costs performance costs fuel costs money, so you don't build a large aircraft like this unless it has to operate at fields without ground equipment.
- vlehto 11y ago
  There are lot of bombers with high wing (B52, almost anything several engines from WWII). About half of all propeller driven passenger planes have high wing. >Unlike the An-124, the An-225 was not intended for tactical airlifting and is not designed for short-field operation It still has high wing. And U2 has high wing. It's probably the plane with most carefully calculated weight ever. If it was inherently heavier, this would make no sense.
  - lotsofactors 11y ago
    > There are lot of bombers with high wing (B52, almost anything several engines from WWII). Most bombers dispense ordnance from their undersides, ideally near the center of gravity; limiting the structural members needed in those parts simplifies things. > About half of all propeller driven passenger planes have high wing. These are designed not to require ground vehicles for loading and unloading. Propellor ground clearance is also an issue for the larger types. > Unlike the An-124, the An-225 was not intended for tactical airlifting and is not designed for short-field operation > It still has high wing. The An-225 carries cargoes of unusual bulk to unusual places. Being able to get these cargoes on and off the aircraft without some sort of lift has advantages. Low-wing freighters, which are all variants of passenger types these days, almost always carry palletized cargo of standard dimensional units (See https://en.wikipedia.org/wiki/Unit_load_device https://en.wikipedia.org/wiki/Unit_load_device) - this allows for standardized ground handling equipment for most loads. > And U2 has high wing. It's probably the plane with most carefully calculated weight ever. > If it was inherently heavier, this would make no sense. The U2 is not a high-wing aircraft, it is a mid-wing aircraft with its spar roughly running through its center of gravity. This is actually the most efficient and aerodynamically cleanest configuration, and many high-performance aircraft use this arrangement. The main problem with it for passenger or cargo use is that the wing structure ends up interfering with the middle of the fuselage volume near the center of gravity, which tends to be valuable space in a transport aircraft.

LoSboccacc 11y ago

On a tangential note plane wins are quite strong themselves Here's Boeing wing being stress tested: https://m.youtube.com/watch?v=sA9Kato1CxA https://m.youtube.com/watch?v=sA9Kato1CxA

Gravityloss 11y ago

The lift varies since the aircraft flies fast through regions where the air has different velocity. Think about a column of ascending air, and the plane flying quickly through that. All else being equal, the plane will accelerate vertically until it is traveling vertically at the same speed as the air column (if the column is large enough). That's why flying low and fast is extremely bumpy and taxing for aircraft. Lots of air speed variations.

asmithmd1 11y ago

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).

zwiteof 11y ago

The article's explanation is a bit simplified. It sounds like his explanation is more akin to a body freedom flutter response where the aircraft's short-period mode is coupled to the structural response. What he observed is probably closer to a more classical flutter response with the wing's bending and torsion coupled. A gust hits the wing, which increases the load, this increases the bending deformation which can also induce torsion causing a twist in the wing. At too high of a speed (the flutter speed), the response is unstable and can quickly become catastrophic. At normal speeds, the oscillation will tend to die out as the restoring force of the structure and the damping from the structure, aerodynamic loads and controls causes the response to die out. Interestingly, you can go past the first flutter speed of an aircraft with a properly designed control system (aeroservoelasticity) meaning you can get away with a lighter (read, more flexible) wing structure.