A strange but good question....
Disco,
Please please don’t tell me your believing half this stuff...and this is a wind up…
Easiest simplest example I can give you....
Remember those little snow man things people bring back from cold places, which has those little white specks of plastic in it and a picture of a city in it, you shake it up, and you see the city in a snow storm.
Now with all the little pieces of plastic are resting on the base, or when you shake the thing up and them all flying about the little ornament, it will still weigh the same, try it at home.
If you move all the little pieces of plastic to one side, and put a scale on either side, record the reading from each scale, then shake it up and let the little pieces of plastic fall evenly across the base, record the reading from each scale, you will see a small change in the distribution of scale readings, which equates to a CofG change, but the total weight will not have changed.
Please remember when you measure your “weight”, you actually have a calibrated mass figure given to you, in g, kg, or t.
An aircraft is pretty well much a sealed environment, if it was not sealed, you could not maintain a pressure differential, sure your letting air out the outflow, but your also replacing it with fresh bleed, no nett change, constant pressure differential.
Yes the aircrafts weight (which is its mass*acceleration of gravity) does include the air in it, much like the oxy cylinder example I gave before.
Please please don’t tell me your believing half this stuff...and this is a wind up…
Easiest simplest example I can give you....
Remember those little snow man things people bring back from cold places, which has those little white specks of plastic in it and a picture of a city in it, you shake it up, and you see the city in a snow storm.
Now with all the little pieces of plastic are resting on the base, or when you shake the thing up and them all flying about the little ornament, it will still weigh the same, try it at home.
If you move all the little pieces of plastic to one side, and put a scale on either side, record the reading from each scale, then shake it up and let the little pieces of plastic fall evenly across the base, record the reading from each scale, you will see a small change in the distribution of scale readings, which equates to a CofG change, but the total weight will not have changed.
Please remember when you measure your “weight”, you actually have a calibrated mass figure given to you, in g, kg, or t.
An aircraft is pretty well much a sealed environment, if it was not sealed, you could not maintain a pressure differential, sure your letting air out the outflow, but your also replacing it with fresh bleed, no nett change, constant pressure differential.
Yes the aircrafts weight (which is its mass*acceleration of gravity) does include the air in it, much like the oxy cylinder example I gave before.
disco_air
Yes the CofG will change as the birds become airborne compared to when they were perched but the weight will remain the same. That is the question that was originally asked. As I have previously alluded to the weight of the birds when airborne will be evenly distributed across the floor of the aircraft by the air. Therefore the CofG won’t change if the birds fly from the front of the aircraft to the rear as long as they don’t become grounded. If they walked to the front or rear then this is a different story.
Soulman
No the question asked was what happens to the weight not the CofG. As has been shown weight doesn't change. CofG may change when the birds become airborne but the weight doesn't change. I might point out that even though the lift of the wing may change because the CofG changes the gross lifting force from the wing and tail plane would remain the same in steady level flight because of the varying lift forces produced by the tail plane to counteract the various CofG positions.
By your theory, 404 Titan, if the birds all flew towards the back of the aircraft (lets assume they stay airborne), since the weight hasn't disappeared, does that mean the aircraft's CofG will move rearwards in flight, causing it to pitch up with all else being equal?
Soulman
This contradicts what most of us have already concluded - since the aircraft and it's occupants are considered to be one system, the net force is zero.
1. Yes. The air within the cockpit, although turbulent, will remain relatively stationary with reference to the aircraft.
2. Nothing can travel at the speed of light, therefore the question is meaningless. The theory that governs what light does relative to an observer (the train) is the same theory that dictates that the train could not achieve the speed of light. If you discard it to allow for the train travelling that fast, then you can't use it to describe anything else in the scenario.
3. Technically you could I suppose. However the weight of the aircraft would still be the same.
2. Nothing can travel at the speed of light, therefore the question is meaningless. The theory that governs what light does relative to an observer (the train) is the same theory that dictates that the train could not achieve the speed of light. If you discard it to allow for the train travelling that fast, then you can't use it to describe anything else in the scenario.
3. Technically you could I suppose. However the weight of the aircraft would still be the same.
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I'll start with Silberfuchs, because I like those questions.
1. No. If all the airflow in your face is supersonic, then you can't get a sound wave to move forward. You probably can't even make a sound because the dynamic pressure on your face from supersonic flow right at it would be quite painful. That's theory, now for truth. There is probably air that is not moving supersonic relative to you stuck in the cockpit, you should be able to get the sound to move forwards in that. Also, sound travels in metal so you could try to send a signal through the canopy frame.
2. Yes. Not as effective since it wouldn't travel forward from the train, but since you can see the lights from the sides, that will still work although it would appear as a point flash of light from "nowhere".
3. That would work, although to be truly honest to the aircraft limitations, you'd have to invite the passengers to use another of the conveniences of modern airliners. Easier would be to get the trolly dollies to leave their makeup bags behind.
Birds flying in Airplane
Final answer is the airplane won't climb.
The birds stay aloft by pushing air downwards. This is the same as airplane's although if you follow the Bernoulli argument, you have to dig deep into the physics to see that air needs to be pushed downwards. Trust me.
The downwards moving air hits the floor of the cabin and creates a force equal to the weight of the pigeons (as long as the pigeons aren't climbing or accelerating).
The effort exerted by the pigeons shows up as kinetic energy of the birds, and kinetic energy of the air inside the cabin. Eventually some of the air will be slowed, and that will create a small amount of heat (friction).
The vortices appear directly underneath, but the aircraft/bird will move forward, making some think that vortices appear behind.
If the downflowing air doesn't hit the cabin floor, then the entire weight of the birds isn't felt by the aircraft anymore (extend this to C5 with open doors, mesh cages, etc.)
There are lots of ifs ands and buts that haven't been thrown in here yet, but in the end, For every action there is an equal and opposite reaction. To get a bird up in a mass of air, some of that air must come down. QED
Matthew.
1. No. If all the airflow in your face is supersonic, then you can't get a sound wave to move forward. You probably can't even make a sound because the dynamic pressure on your face from supersonic flow right at it would be quite painful. That's theory, now for truth. There is probably air that is not moving supersonic relative to you stuck in the cockpit, you should be able to get the sound to move forwards in that. Also, sound travels in metal so you could try to send a signal through the canopy frame.
2. Yes. Not as effective since it wouldn't travel forward from the train, but since you can see the lights from the sides, that will still work although it would appear as a point flash of light from "nowhere".
3. That would work, although to be truly honest to the aircraft limitations, you'd have to invite the passengers to use another of the conveniences of modern airliners. Easier would be to get the trolly dollies to leave their makeup bags behind.
Birds flying in Airplane
Final answer is the airplane won't climb.
The birds stay aloft by pushing air downwards. This is the same as airplane's although if you follow the Bernoulli argument, you have to dig deep into the physics to see that air needs to be pushed downwards. Trust me.
The downwards moving air hits the floor of the cabin and creates a force equal to the weight of the pigeons (as long as the pigeons aren't climbing or accelerating).
The effort exerted by the pigeons shows up as kinetic energy of the birds, and kinetic energy of the air inside the cabin. Eventually some of the air will be slowed, and that will create a small amount of heat (friction).
The vortices appear directly underneath, but the aircraft/bird will move forward, making some think that vortices appear behind.
If the downflowing air doesn't hit the cabin floor, then the entire weight of the birds isn't felt by the aircraft anymore (extend this to C5 with open doors, mesh cages, etc.)
There are lots of ifs ands and buts that haven't been thrown in here yet, but in the end, For every action there is an equal and opposite reaction. To get a bird up in a mass of air, some of that air must come down. QED
Matthew.
The longer post of Itchybum explains it all with one slight remark... Compairing two different situations: one before takeoff of the birds, one after. For the birds to be able to takeoff they need energy. So they will have consumed energy which they got from... fat ie 
. But also, the 747 will have lost some fat too 
I have been looking for the "visual" proof, but can't find it. I guess the tv company removed it from their website. About a year and a half ago there was a very popular (despite being scientific) tv program on Belgian television where scientific questions were answered. One of them was: put a heli in a glass container. Seal it, then let the heli fly. Does the wait of the container (with air and heli inside) change when the heli starts flying? You should know the answer by now...
(ps: if you wondered, they used a small RC heli)
. But also, the 747 will have lost some fat too I have been looking for the "visual" proof, but can't find it. I guess the tv company removed it from their website. About a year and a half ago there was a very popular (despite being scientific) tv program on Belgian television where scientific questions were answered. One of them was: put a heli in a glass container. Seal it, then let the heli fly. Does the wait of the container (with air and heli inside) change when the heli starts flying? You should know the answer by now...
(ps: if you wondered, they used a small RC heli)
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Ok I'm now convinced.
In a perfectly sealed environment, the air displaced to create lift for the birds is being supported by the cabin structure and thus the airframe. Therefore will have no weight change.
For all intensive purposes, an aircraft cabin is a sealed environment. The air that does flow has negligible effect.
The mesh aviary example given by GW_04 however would have a weight change since the air displaced to create lift is free to flow out of the cage and is thus not still supported by the structure.
Thanks for all your help! I am now a wiser man
Oh and the other questions raised...
1. Yes, the air between the two pilots will still have zero velocity
2. If the train could hypothetically reach the speed of light, the light emitted from the lamp at the front will still appear to travel at the speed of light towards an observer in front of the train. It is time that becomes the variable as c is a constant from any frame of reference. See time dilation....
3. Simply a question of legality rather than physics
Thanks again, I will surely spring another 'thought experiment' on you when I think of one!
...Disco
In a perfectly sealed environment, the air displaced to create lift for the birds is being supported by the cabin structure and thus the airframe. Therefore will have no weight change.
For all intensive purposes, an aircraft cabin is a sealed environment. The air that does flow has negligible effect.
The mesh aviary example given by GW_04 however would have a weight change since the air displaced to create lift is free to flow out of the cage and is thus not still supported by the structure.
Thanks for all your help! I am now a wiser man
Oh and the other questions raised...
1. Yes, the air between the two pilots will still have zero velocity
2. If the train could hypothetically reach the speed of light, the light emitted from the lamp at the front will still appear to travel at the speed of light towards an observer in front of the train. It is time that becomes the variable as c is a constant from any frame of reference. See time dilation....
3. Simply a question of legality rather than physics
Thanks again, I will surely spring another 'thought experiment' on you when I think of one!
...Disco
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Okay, 2 more questions...
1. What if the birds were in a huge helicopter, flying with its doors open (assuming the birds don't get sucked out or fly away)?
2. One that's also created a few discussions, An aircraft has lost its tail and is a vertical dive (terminal velocity?) and the pilot is climbing up the side of the fuselage to bail out through the tail. Is the effort required by the pilot to climb up the same as it would be if the aircraft was simply on the ground, standing on its nose? Run Amock...
1. What if the birds were in a huge helicopter, flying with its doors open (assuming the birds don't get sucked out or fly away)?
2. One that's also created a few discussions, An aircraft has lost its tail and is a vertical dive (terminal velocity?) and the pilot is climbing up the side of the fuselage to bail out through the tail. Is the effort required by the pilot to climb up the same as it would be if the aircraft was simply on the ground, standing on its nose? Run Amock...
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birds in aircraft cont.
imagine a yacht sailing in the sea.a few birds flying along decide to go and fly into the yacht's cabin but remain in flight. will the total weight of the yacht increase if they dont land inside the cabin....
imagine a yacht sailing in the sea.a few birds flying along decide to go and fly into the yacht's cabin but remain in flight. will the total weight of the yacht increase if they dont land inside the cabin....
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As for the birds.... flying might be one thing, but what if they jump (no flapping, so no transfer of force from legs to air movement)? Like the group of footballers sitting on the back of a bus all jumping as it goes over a speed bump. I can testify that the bus does indeed behave as if it's half a ton lighter all of a sudden!
As for the other questions:
1) In terms of air velocity, yes... the local velocity will be a long way under supersonic. However, most of the air will be sucked into the slipstream, and the engines tend to be a little loud
2) Yes. But it's all relative
3) No. The load allowance for catering is considered part of the ZFW regardless of how much has been eaten. Anything that is excreted ends up sitting in the tanks anyway... So, not possible scientifically, or legally.
I think
Lancer
As for the other questions:
1) In terms of air velocity, yes... the local velocity will be a long way under supersonic. However, most of the air will be sucked into the slipstream, and the engines tend to be a little loud
2) Yes. But it's all relative
3) No. The load allowance for catering is considered part of the ZFW regardless of how much has been eaten. Anything that is excreted ends up sitting in the tanks anyway... So, not possible scientifically, or legally.
I think
Lancer
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Quick calculation:
For a 737 cabin (about 130 cubic metres), and air at 20 degrees weighing about 1.204 milligrams per cubic centremetres, the weight of the air is about 160kgs.
I don't think you'd notice the change in weight in your emergency descent...
For a 737 cabin (about 130 cubic metres), and air at 20 degrees weighing about 1.204 milligrams per cubic centremetres, the weight of the air is about 160kgs.
I don't think you'd notice the change in weight in your emergency descent...
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I would have thought you would have an INCREASE in weight during the depressurisation as the birds would eventually fall to the floor. Though....i suppose you wouldnt notice it all that much as inevitably some would be younger or more healthy than others so would last a little longer, so it would be a gradual change.
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Roger's questions
1) Birds in helicopter are supported by air, which is supported by helicopter. Or, if the birds are near the doors, their downwash might be outside the helicopter, and helicopter does not "feel" their weight. See previous posts.
2) Is the aircraft at terminal velocity? Speed is irrelevant, acceleration is not. Aircraft accelerating? Pilot feels weightless, or at least lighter than normal. Aircraft constant speed? It may as well be stationary for all the pilot knows. Think of an aircraft in a steep high speed descent - are you lighter? See previous posts on geostationary satellites, men on their way to moon etc.
O8
1) Birds in helicopter are supported by air, which is supported by helicopter. Or, if the birds are near the doors, their downwash might be outside the helicopter, and helicopter does not "feel" their weight. See previous posts.
2) Is the aircraft at terminal velocity? Speed is irrelevant, acceleration is not. Aircraft accelerating? Pilot feels weightless, or at least lighter than normal. Aircraft constant speed? It may as well be stationary for all the pilot knows. Think of an aircraft in a steep high speed descent - are you lighter? See previous posts on geostationary satellites, men on their way to moon etc.
O8
Down wash from the flapping wings is misleading and actually incorrect. It isn’t relevant to the weight question. Think of the birds flying in the cabin like a rubber ducky in a kid’s bathtub. The weight of the bathtub consists of the tub, the water, the kid in it and the rubber ducky on the surface. The birds on the air is the same as the duck on the water.
But titan 404 the mass of the birds displacing the air whether stationary or flying has already been taken into account so it wouldn't cause a proportional change in the air pressure inside. If a ball is float 1 cm from the top of a glass or at the bottom of the glass it still takes up the same volume...... it has to be the downwards force produced through the lift generated.. not the weight of the birds.... put the birds and the air around the birds into a seperate systems within the cain system
Since we are talking about an aircraft as a enclosed environment.... lets put the bloody thing on the ground for ease of thinking.... why.. because it doesn't matter.
To cabin is supporting the mass of the air which is being disturbed by the the bird generating lift therefore producing the downwards force to produce their lift and applying a force on the bottom of the cabin to generate an increase in weight (or no change is should say)... if the undercarriage was put on a scale.
As for bernoullis principle...... well if birds are flapping their wing to flying arn't they using air deflection?.... but either way.... in this example both create a downwards force.
now i have a question........ what if the birds were inverted ????
Since we are talking about an aircraft as a enclosed environment.... lets put the bloody thing on the ground for ease of thinking.... why.. because it doesn't matter.
To cabin is supporting the mass of the air which is being disturbed by the the bird generating lift therefore producing the downwards force to produce their lift and applying a force on the bottom of the cabin to generate an increase in weight (or no change is should say)... if the undercarriage was put on a scale.
As for bernoullis principle...... well if birds are flapping their wing to flying arn't they using air deflection?.... but either way.... in this example both create a downwards force.
now i have a question........ what if the birds were inverted ????
Bula
Wrong Wrong Wrong. The downward force of the air has no effect on the weight. Let me use the water model again to illustrate. This time we will use a deep pool with a diver wearing fins. The diver is weighted so as to give him negative buoyancy. The weight of the pool consists of the pool, the water and the diver and all his gear. Now for the diver to remain on the surface he has to expel energy by kicking with his fins. This causes a downward flow of the water beneath him, which hits the bottom of the pool. Does the weight of the pool and its contents change because of this? No, of course it doesn’t. To be clear what is happening here, the downward force is cancelled by the upward force of the diver in a sealed environment, i.e. equal and opposite forces. Basic rule of physics. This is exactly the same as the birds in flight. As for the birds being inverted? No difference. Equal and opposite forces in a closed environment.
Wrong Wrong Wrong. The downward force of the air has no effect on the weight. Let me use the water model again to illustrate. This time we will use a deep pool with a diver wearing fins. The diver is weighted so as to give him negative buoyancy. The weight of the pool consists of the pool, the water and the diver and all his gear. Now for the diver to remain on the surface he has to expel energy by kicking with his fins. This causes a downward flow of the water beneath him, which hits the bottom of the pool. Does the weight of the pool and its contents change because of this? No, of course it doesn’t. To be clear what is happening here, the downward force is cancelled by the upward force of the diver in a sealed environment, i.e. equal and opposite forces. Basic rule of physics. This is exactly the same as the birds in flight. As for the birds being inverted? No difference. Equal and opposite forces in a closed environment.
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Birds inverted, no different, as the air displacement is still supported by the cabin (in the perfectly sealed environment)
Birds in an open helicopter, same deal as the mesh aviary as opposed to a sealed one - a change in weight if the birds landed because it is now supporting weight it wasnt before (due to the air inside not being supported by the helicopter - read: non-sealed environment)
Think about a flying elephant landing on a small boat. It will certainly experience weight it wasnt supporting before because of no sealed environment.
Plane in the terminal velocity dive - It is at a constant velocity, so acceleration is zero (F=ma). Therefore zero net force - the air resistance force is equal and opposite to the force due to gravity.
But the pilot will still be experiencing acceleration due to gravity while climbing up to the tail so will experience the same forces as if climbing on the ground.
Thats what I think anyway. Thanks to this discussion, i understand the sealed factor a bit more.
...Disco
You beat me to it 404! Thanks for your help mate
Thanks also to swh's great analogy!
Birds in an open helicopter, same deal as the mesh aviary as opposed to a sealed one - a change in weight if the birds landed because it is now supporting weight it wasnt before (due to the air inside not being supported by the helicopter - read: non-sealed environment)
Think about a flying elephant landing on a small boat. It will certainly experience weight it wasnt supporting before because of no sealed environment.
Plane in the terminal velocity dive - It is at a constant velocity, so acceleration is zero (F=ma). Therefore zero net force - the air resistance force is equal and opposite to the force due to gravity.
But the pilot will still be experiencing acceleration due to gravity while climbing up to the tail so will experience the same forces as if climbing on the ground.
Thats what I think anyway. Thanks to this discussion, i understand the sealed factor a bit more.
...Disco
You beat me to it 404! Thanks for your help mate
Thanks also to swh's great analogy!
Last edited by disco_air; 21st Aug 2004 at 15:33.
