4 ms·

The physics of this are insane. Then again, by my calculation the A380 displaces roughly 11 million cubic feet of air per second at the max landing weight.

by jseip 10y ago

rjurney 10y ago
Why does the landing weight effect the air it displaces?
- thrill 10y ago
  Not displacement (volume), but pressure from velocity: lift. The amount of lift by definition has to equal the weight of the aircraft at 1G.
  - vpribish 10y ago
    ... and the force of that lift on the plane has an equal and opposite on the air. A Heavier plane disturbs the air more.
    - deleted 10y ago
      [deleted]
- ars 10y ago
  Weight of airplane = mass of air displaced per second * velocity of air. The weight of the airplane is a force downward, the air must match that force, either by throwing a lot of air with little velocity, or a less air, faster.
Dylan16807 10y ago
Though note that a box around a cruising A380 at 827 feet per second * 79 feet tall * 262 feet wide is itself 17 million cubic feet.
- bb611 10y ago
  That's a pretty poor approximation, and I think the result discounts the meaning of the original point. The A380 is a lot more like 3 boxes (based on A380-800: http://www.airbus.com/fileadmin/media_gallery/files/tech_data/AC/Airbus-AC-A380-Jan16.pdf http://www.airbus.com/fileadmin/media_gallery/files/tech_dat...): 1) A fueselage box with a cross section of 2067 ft^2 2) A wing box of ~1200 ft^2 (a very broad approximation because of engines, the actual wing box - the part of the plane where the wings meet the fueselage - and the curve of the wings), 3) A tail box with a cross section of 144 ft^2 So a total cross section of ~3411 ft^3 * 827 fps = ~3 million ft^3 Point being, the plane isn't moving 11 million cubic feet just to move forward, it pushes on a little more than a quarter of that directly. These wake effects happen because it's disrupting so much air outside of the zone it actually travels in, mostly below and behind it.
  - Dylan16807 10y ago
    > That's a pretty poor approximation It's not an approximation. I'm giving a sense of scale, not saying how much volume the plane has. The point is that while 11 million cubic feet sounds like a lot, if you put it in a big box next to the plane then they would be on the same scale. > the plane isn't moving 11 million cubic feet just to move forward, it pushes on a little more than a quarter of that directly That's jseip's post's problem, not mine.
jseip 10y ago
EDIT: this is the net downward displacement of air sufficient to equal the A380 850,984lb max landing weight assuming a ballpark air density of ~0.075 lb/ft^3. The aircraft displaces much more air via its engines, fuselage and secondary effects, but the above number seems most relevant to the Challenger 604 ~1,000 feet beneath the aircraft :).

The physics of this are insane. Then again, by my calculation the A380 displaces roughly 11 million cubic feet of air per second at the max landing weight.

by jseip 10y ago

The physics of this are insane. Then again, by my calculation the A380 displaces roughly 11 million cubic feet of air per second at the max landing weight.

rjurney 10y ago

Why does the landing weight effect the air it displaces?

thrill 10y ago
Not displacement (volume), but pressure from velocity: lift. The amount of lift by definition has to equal the weight of the aircraft at 1G.
- vpribish 10y ago
  ... and the force of that lift on the plane has an equal and opposite on the air. A Heavier plane disturbs the air more.
  - deleted 10y ago
    [deleted]
ars 10y ago
Weight of airplane = mass of air displaced per second * velocity of air. The weight of the airplane is a force downward, the air must match that force, either by throwing a lot of air with little velocity, or a less air, faster.

Dylan16807 10y ago

Though note that a box around a cruising A380 at 827 feet per second * 79 feet tall * 262 feet wide is itself 17 million cubic feet.

bb611 10y ago
That's a pretty poor approximation, and I think the result discounts the meaning of the original point. The A380 is a lot more like 3 boxes (based on A380-800: http://www.airbus.com/fileadmin/media_gallery/files/tech_data/AC/Airbus-AC-A380-Jan16.pdf http://www.airbus.com/fileadmin/media_gallery/files/tech_dat...): 1) A fueselage box with a cross section of 2067 ft^2 2) A wing box of ~1200 ft^2 (a very broad approximation because of engines, the actual wing box - the part of the plane where the wings meet the fueselage - and the curve of the wings), 3) A tail box with a cross section of 144 ft^2 So a total cross section of ~3411 ft^3 * 827 fps = ~3 million ft^3 Point being, the plane isn't moving 11 million cubic feet just to move forward, it pushes on a little more than a quarter of that directly. These wake effects happen because it's disrupting so much air outside of the zone it actually travels in, mostly below and behind it.
- Dylan16807 10y ago
  > That's a pretty poor approximation It's not an approximation. I'm giving a sense of scale, not saying how much volume the plane has. The point is that while 11 million cubic feet sounds like a lot, if you put it in a big box next to the plane then they would be on the same scale. > the plane isn't moving 11 million cubic feet just to move forward, it pushes on a little more than a quarter of that directly That's jseip's post's problem, not mine.

jseip 10y ago

EDIT: this is the net downward displacement of air sufficient to equal the A380 850,984lb max landing weight assuming a ballpark air density of ~0.075 lb/ft^3. The aircraft displaces much more air via its engines, fuselage and secondary effects, but the above number seems most relevant to the Challenger 604 ~1,000 feet beneath the aircraft :).