(2000lb) Birds on a Plane?
November 8, 2020 1:51 PM   Subscribe

From the perspective of an external observer watching a cargo plane with a 2000lb bird(s) flying around inside, what happens to the bird and plane when the cargo plane opens its cargo-bay doors?

There's a supposed "(flying) birds in a truck" riddle - how much does the truck/birds weigh? - that seems like not much of a riddle at all, even before being addressed by MythBusters et al. (hint: flight exerts a downward force). This AskMeta? is more than that peanut of a gedankenexperiment.

When I was young, a retired-hotshot Air Force-cum-United Airlines pilot of an uncle would ask me about a 2000lb+ bird flying around in an airplane. He'd bring up the "birds in a truck" riddle, and he would point out that when you dropped cargo out of an airplane then the airplane would "jump" up, then he would ask in order:
1) "What happens when a 2000lb bird, inside a cargo plane, starts flying?" I think the answer is obvious on this: the plane would do nothing new.
2) "What happens when the cargo plane opens its doors?"
...

The question to me comes down to how relative reference frames and forces work in flight; and where and exactly how forces are distrubted from lift.

I think the lift would be only under the bird in question, as it flapped its wings. Therefore it would be like pushing any other cargo (albeit with an air cushion underneath) out of the plane. The plane would lurch up, and the bird would continue to exert force downward on the air particles beneath where the plane is and continue to maintain the same vertical elevation. Persuming the speeds of the airplane were survivable to the bird, the horizontal speed would remain the same as upon exit, then decline with drag or require exertion by the bird.
Therefore, to an external observer the bird would appear to stay in place, and the plane would lurch upwards. The bird would appear to an internal observer to fall out of the plane. And the plane would appear to shoot upwards to the bird.

However, I'm mildly uncertain about the solution and would like challenges to it.
posted by rubatan to Science & Nature (10 answers total) 1 user marked this as a favorite
 
Response by poster: If you want to speculate further, I started thinking about this gedankenexperiment because I was thinking about the idea of toroid space stations. In true zero-g (not microgravity of earth's orbit), why is there any reason to think you're in the inertial reference frame that would result in a centripedal acceleration to create false gravity?
(I can provide answers but would rather leave it to the reader).

The question then is: what if you jumped up in the air?
posted by rubatan at 1:53 PM on November 8, 2020


Best answer: For your second question, the answer is that if you jump, you stop being accelerated by the station floor, and continue on your lateral trajectory until you intersect the station floor again--at roughly the same spot you jumped from, because the station has spun under you. It looks just like jumping in gravity.

For the first question, you are mostly right. It is simpler for me to visualize if I imagine a quad-copter drone instead of a bird, because then I can picture the downdraft more easily from the propellers. My one tweak is that as the drone drifts backward out the cargo door, the loading in the plane shifts, so the plane noses up until the drone is all the way out (unless the pilot compensates with the elevator).
posted by agentofselection at 3:21 PM on November 8, 2020


"What happens when a 2000lb bird, inside a cargo plane, starts flying?" I think the answer is obvious on this: the plane would do nothing new.

I think you're wrong on this. The reason the truck-full-of-birds question is counter intuitive is because you're really measuring the force of the truck+bird system that is being exerted on the ground, which won't change. If you look at a plane+bird system, you're dealing with two things that can each move in different directions (as opposed to the truck which is fixed to the earth). What you would see in your plane situation would also depend where the bird is physically located inside the fuselage; the force of the bird's motion would act as a fulcrum relative to the center of gravity of the plane, so it would cause either a nose up or down motion depending on its location.

To back up this assertion, I will point out that I can cause my (small) airplane to move by bouncing around in my seat or leaning over. The effect is very small, but noticeable.

"What happens when the cargo plane opens its doors?"

If the bird is actively flying? Nothing. If the bird is not flying? Again, depends on the configuration of the plane: aft cargo doors (like on a C-130) or large side doors (like you see on most civilian freighters) would have no effect at all. (The side doors would get ripped off in flight, but that's a different question.) Bomb bay doors like on a B-52? It would have the exact same effect as if any other 2000 lb weight suddenly failed to have something supporting it underneath. The plane would jump up a bit and the bird would start plummeting to the earth. Whether it starts flying or not is immaterial to the question, since it is now no longer part of the plane system.
posted by backseatpilot at 4:23 PM on November 8, 2020 [2 favorites]


What happens when a 2000lb bird, inside a cargo plane, starts flying?

One of the reasons that it's hard to mentally picture the physics of what would happen in this scenario, is that it really doesn't make sense in the first place.

The heaviest bird in existence today is the ostrich, at up to about 300lb, and it's completely incapable of flight. The heaviest flying bird is the turkey, weighing only up to about 90lb.

If you go back 25 million years, the heaviest flying bird ever known to have existed was Argentavis magnificens. It still weighed only about 240lb max, and had a wingspan of 27ft.

Even if you go back another 40 million years to Quetzalcoatlus, the heaviest flying creatures in the history of the Earth, that still only weighed up to about 550lb. It was as tall as a giraffe. And had a wingspan of up to 43ft.

You may start to see where I'm going with this.

A 2000lb flying bird, if even biologically possible, would be colossal. Extrapolating from the above examples, would need a wingspan of at least 100ft, probably more. It would be comparable in size to a cargo plane on its own.

Even the cargo bay on an Airbus Beluga is only about 22ft wide.

If your 2000lb bird's neck and tail were short enough, it might just about be able to stretch itself out in that Beluga - which is about 150ft long - if it were facing sideways.

But it's not going to be able to take off. Big birds can't just leap into the air with a big flap. They need run to pick up speed. And this thing weighs as much as a small elephant. It needs a full-on runway!

So, back to your questions:

What happens when a 2000lb bird, inside a cargo plane, starts flying?

Trick question. It can't! It can barely fit inside!

What happens when the cargo plane opens its doors?

The bird gets out, spreads its wings, and finally has space to even think about flying.
posted by automatronic at 4:52 PM on November 8, 2020 [5 favorites]


I love this question. (I'm definitely not a pilot, but I am a physicist. One who has only very briefly considered this and might be missing something.)

I have no objection to backseatpilot's answer. In general, I think the answer is, "it's complicated." The 2000lb bird is flapping around, which probably creates significant pressure differences in different parts of the aircraft. The pressure pushes on different parts of the plane and could change the plane's trajectory. Not because the physics is any different, but because the bird is literally pushing (through the air) on either the front or back of the plane. (Pressure is force per unit area. If you blow on a wall, you are applying force to the wall. If you blow on a commercial plane's window, you're applying force to the plane and changing its course very slightly. If you do it from two seats over, it has less impact.)

If, instead of a plane, you had a mile-sized dirigible or a space station, where there's plenty of room for any pressure and density changes to reach equilibrium before they get to the walls, you'd get the same answer as the train. On the plane, if the bird was positions in just such a way that it's wing flapping pressed down in the same place as when it was standing, nothing would change. Energy and momentum conservation still hold. The plane has the same weight at all times. It's only a question of which part of the plane is heavier with respect to the location of the wings, which is important for planes.

What happens when the doors open is even more complicated, I suspect that if the bird is near the edges of the plane, there still might be a pressure differential that matters, but also some of that will be mediated by air coming and going into the outside air. If you beat a drum in small room, it's a lot louder than when you beat a drum in a park. It's more or less the same physics, but added up over the entire inside surface of the plane instead of your ear drums. (My experience on LC-130s suggests that everything turns into howling chaos when the doors open, even when the plane is just taxying. Turbulance will make taking data challenging and probably scare the hell out of the 2000lb bird.) If some of the pressure caused by the birds wings causes air to come or go out of the door on a timescale that's similar to bird wing flapping, then the plane will weigh less than it would if the bird was sitting on the floor. The whole system that includes the planet, though, still conserves energy and knows there's a 2000lb bird in it.
posted by eotvos at 7:15 AM on November 9, 2020 [1 favorite]


(To be clear, and perhaps needlessly pedantic, when you blow on a plane window, you are also being blown back, which pushes on your seat with the same force. But, that's happening in a different place with respect to the shape of the plane and the location of its wings.)
posted by eotvos at 7:29 AM on November 9, 2020


Sorry, my answers above assume the bird stays inside the plane when the door is opened, which is not actually the question that was asked. I'm not entirely sure what "stays in place" means in this context.
posted by eotvos at 7:59 AM on November 9, 2020


Best answer: A 2000 bomb is rigidly attached to the aircraft and that attachment is removed in one very short moment, meaning that the transition from attached to not attached happens very quickly. Thus rather dramatic results are visible, the aircraft instantly rebounds upwards noticeably etc.

Whereas, a bird flying around the aircraft is still being supported by the aircraft but the connection is far more diffuse. The support "point" isn't a point at all--as it is with the bomb. It is, rather, a diffuse area spread over a large area of the interior of the aircraft. And the connection is rather 'springy' rather than being rigid.

So (keeping things a bit more realistic per automatronic's comment above) let's say you dropped a 20 pound bomb out of the cargo bay. The moment you released it the aircraft would feel an additional 20 pound upwards force.

Let's say the rear cargo door is open and a 20-pound bird flew from the front of the aircraft to the rear and then out the rear cargo bay.

You would notice the same 20-pound upwards force on the aircraft, but instead of the upwards force going from 0-20 pounds in say 0.05 seconds it would happen over say 1-2-3 seconds as the downwards force from the air pushed down (and sideways and upwards and other directions) by the bird's wings and body gradually transitions from hitting the interior of the aircraft to outside air.

End result is, you would notice the same general effect as dropping a dead weight of the same mass but the it would happen over a much longer time period. Thus the upwards acceleration experienced by the aircraft would be noticeably less with the bird, though the acceleration would be extended over a far longer time period--likely seconds rather than 100s of a second.

Also, for any reasonable sized bird, the buffeting caused by opening the cargo bay door may very well massively overwhelm the effect of the bird leaving the aircraft. A 2000-pound bomb has a large an immediate effect on the aircraft because it is heavy relative to the aircraft and the release occurs instantly. As pointed out above, how exactly a 2000-pound bird is going to fly within a cargo plane isn't so obvious, so common sense has a hard time figuring out the results.

But take for example a more realistic analog: Suppose a 2000-pound load is suspended on a hover-platform. Now the crew turns on the hover platform, suspending the load on a cushion of air, and then pushes it across the cargo bay floor and out the rear cargo bay doors.

Now what happens to the aircraft & the 2000-pound load seems a lot more obvious; the fact that it happens to be suspended on a cushion of air while it's within the cargo bay seems to make little difference. In fact the end result would be nearly the same as dropping any other 2000-pound load out the end of the cargo bay. Any difference would be due to the fact that it might take certain period of time for the load to transition from being fully supported by the aircraft to partially supported to not supported at all. Which is different from a bomb, which has as mechanism allowing it to be released from the aircraft pretty much instantaneously.

TLDR: The net amount of mass lost and thus the change in downward force on the aircraft before/after will be the same in a bomb drop vs flying bird drop. However the transition time load-to-no-load will be greatly different, resulting in a much different amount of (upward) acceleration being experienced by the aircraft, simply because the change in downward force happens over a very much longer period of time for the bird drop.

The bird drop scenario will result in a lower acceleration applied for a longer period of time than the bomb drop.
posted by flug at 12:06 PM on November 9, 2020 [1 favorite]


Best answer: If we model the airplane as a hollow cylinder... as it accelerates up to speed the air and everything inside is pushed backwards. The things on the floor are held in place by static friction, the molucules of air inside fly back and press on the back wall. As you hit a cruising speed and stable velocity things have balanced out, there is no more acceleration and the inside is at equilibrium.

Now if it's going really fast when it opens the back doors... the lower pressure behind the moving cylinder will attempt to suck the air out and there's no wall to stop it. Drag is both the thing pushing from the front impeding your progress and the pulling from the back doing the same. If the cylinder is flying fast enough the air pressure will drop below the pressure needed for a bird to fly for one, then if the bird while falling to the floor is still in the air as it gets sucked out it's gone. If the wind from the air rushing out is great enough to overcome the static friction of things on the deck they'll be gone as well. The plane will lighten as the air is removed and stuff not tied down or holding on is sucked out the back.

Now the force of the flapping wings (if it were possible) is just molecules of air that might never even reach the floor to impart downward momentum to the plane in the first place.

Now I'd imagine that planes don't open the doors going really really fast. Eventually the reduced pressure inside starts sucking in some turbulent flow around the edges of the door. Or there are drafts coming from somewhere else. Otherwise we'd all suffocate when driving with the window down.

This is not as wild as it sounds. This is how airbrushes, perfume atomizers, water-faucet powered vacuum sources work. Some bit of the Venturi effect.

The plane would go up regardless as some air is leaving and its lighter just from that, but it would also probably go down from increased drag, or it might get a momentary boost by reducing the vacuum drag pulling it back by filling it with that lost air. It's a crapshoot. :)
posted by zengargoyle at 2:45 PM on November 9, 2020


Response by poster: Ultimately I don't so much care about all the "details" (the quotations might only be obviously humorous to those who know I was trained in my undergrad as a physicist & mathematician rather than an engineer doing applied work (I did the least of all "applied" work) at least for those theses). The teetering of the plane etc is a legitimate point, as might be "biology", but I divorce myself from such realities. Whatever the idealized system might be from quadcopters to levitation tables, and nearly infinite, low-mass dirigibles lets go with those: some ideal flapping body, in a proportionally larger body with doors flying through air, whose doors then open. But I'll accept that as my failure in description in hopes to make it entertaining enough to answer.

The real pragmatics I was wondering about is at what point all the turbulence from the wing flapping, particles bouncing off of other particles, is the "birds" downward force outside the plane or does it spread enough outwards?Is it all down or how rapidly does it diffuse toward the walls of the plane? If its downwards then I would think the plane would lift and the bird would stay stationary.
A bird flapping in open air 100s of feet above the ground doesn't actually exert a force straight down (at the speed of sound) to keep it up, that would be to slow, the force is only on the enough air particles around it that are needed to bounce off of other air particles, then those bounce off of others, etc distributing the force. Is this the right way to see it?

I'm impressed with the (venturri) vaccuum consideration. I hadn't considered it and should have. In the end, cargo, bomb, or bird they're also being physically sucked out. Is it reasonable then to think of this as speeding up the process of the force vectors pointing out of the plane? Its just enough to make my headspin of suction forces behind the "bird" to end with zengargoyle's " It's a crapshoot."
posted by rubatan at 11:47 AM on November 10, 2020


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