A strange but good question....
 

Here's a rather silly one which I still could not give a good answer to!

Say a large aircraft is flying along, full of many, many pigeons (enough to make significant weight) and they are all happily perched on whatever inside the cabin.

Now, the leader of this flock of pigeons (or parrots, magpies, ducks, toucans - whatever you want them to be) says "alright, lets all go flying around the cabin" and they do.

Does the aircraft's weight decrease as they all fly off their perches? Will it therefore climb (all other things being equal)? What if the aircraft is pressurised?

This sparked a rather interesting conversation at work!

Difficult to answer without more details. Should we also take account of the weight of bird feed and excreta, or is this flight a "no frills" service? :D

well at a guess, i reakon the downward force produced by the beating wings would impact on the floor. this force would equal the pigeons weight.

Suppose its similar to riding in a train, when u jump u dont get left behind, u have mass hence inertia

back to the pigeons, once airborne in the cabinu dont think that they will get smacked up against the rear of the aircraft once they are airborne do u?

The mass of the aircraft will remain the same regardless.

disco_air

Very simply it has no effect on the weight of the aircraft. You have to ask yourself:

1. What is supporting the birds while they are flying around the cabin of the aircraft? Answer: The air in the cabin.
2. What is supporting the air in the cabin?
Answer: The aircraft.

If the aircraft is supporting the air and the air is supporting the flying birds the net effect or difference is zero compared to if they were perched somewhere in the cabin.

I would have expected better from you folks.

The air is not "supporting" the pigeons.

The pigeons are deriving lift from it.

The only downward force applied to the floor will be from those pigeons in "ground effect".

Of course the total mass will not change. Nor would it in zero gravity. Weight will change, mass will not.

Anyway get back to work disco_air you slacker.:E

currawong

Have you forgotten everything you learnt in BAK, Lift = weight, Thrust = Drag etc etc. Yes the pigeon does derive live from the air but also exerts a downward force in the form of weight because of its mass and gravity. This extra weight is displaced amongst the air mass in the cabin of the aircraft and therefore the weight of the aircraft doesn’t change. If you don’t believe me ask yourself the simple question. Where does the weight of the bird go? While it is still in the Earths gravity it will have weight because of its mass.
Yes very true but gravity hasn't changed therefore neither has the weight.

W = mg
Where:
o W = Weight: The force of gravity pulling an object downward at the surface of the earth
o m = mass: measured in kg
o g = gravity: measured at 9.81 m/s/s

Therefore while a bird with a mass of 1.5kg has a force of gravity acting on it, it has weight. Simple law of physics.
:8

I assume the aircraft is squawking?

Sorry, couldn't resist:yuk:

I think some of you guys have forgotten the basic principals of flight. The upwards force of lift is generated mainly from the low pressure region above the wing (Bernoulli's Theorem). This pressure difference creates induced drag which is overcome by the power of the engine(which gets its energy from fuel)so we can fly around and enjoy ourselves.
This "downforce" you talk off only has effect on the earth (or aircraft floor in this case) when close enough to ground effect pressure wave. ie- you will not get crushed by a 400 ton 747 admiring it from below on short final.
Some may comment but I think ground effect height is a function of wingspan yes??? Not sure.

Ask yourselves this then....
If all energies must remain constant (the very principal of Bernoulli's Theorem), and the birds are exerting energy by flapping there wings, then where does this energy go?
Does it simply disolve as sound from the wings?
I think that the birds flying around in the middle of the cabin having no ground effect on the cabin are flying in space totally seperate from the aircraft using there own energy.
Therefore weight will decrease and the aircraft remaining in its current configuration will climb. And the birds will have to as well of course to remain in the centre of the cabin.
As for pressurisation, it will allow the birds to flap there wings slower to get the same lift to remain airbourne.

GW :)

Disco,

I think the aircraft being airborne is just a distraction getting in the way of a good story...

If you draw a control volume around the aircraft, the aircraft and everything in it could be represented as a single mass at the CofG, gravity applied to the mass gives the weight.

If they all start flying, the birds are still in the control volume, the total mass, hence weight would not change (no fuel/oil burn change assumption made).

The only change would be a very very minor change as the CofG which would move very so slightly away from the earths CofG, and gravity reduces in proportion to 1/r^2, where r is the distance from earths CofG, so yes the weight will reduce, but very very very small amount, and yes if airborne you will have excess lift so an upward acceleration of some very very very small magnitude would be expected.

A pressurised aircraft would weigh more than a non-pressurised aircraft, just due to the higher mass of air inside, i.e. a fully oxy cylinder weighs more than an empty one, higher mass of oxy in it.


:ok:

Edit : should have put an upwards had a upwards

GW_04
Whaaaaaat! Wrong Wrong Wrong. Gravity doesn’t decrease simply because you are not touching the ground. If this was true the moon would fling off into space and the Earth wouldn’t experience tides. Infact the Universe wouldn’t exist as we know it. I think you need to get yourself a good book on physics or have a search of the Internet and have a look at what gravity is and its relationship to two masses and the distance between them.

As for your argument why don’t we get crushed under a B747 when it flies over? Well it is because the weight of it is distributed over the entire Earth by the atmosphere, not just the parcel of air underneath it.

We all know about how lift is generated, blah blah blah. What you forget is that in this Universe that we live in, for every force there is an equal and opposite force. In this case gravity acting on the mass. The small distance the bird is from the ground would have little effect on the gravitational force, therefore little effect on the weight. The next thing you will be telling us is that there is no such thing as gravity, the Earth sucks.
;)

it is all about the frame of reference, this being the aircraft with the birds in it. Therefore the mass of the aircraft does not change and the frame of reference (aircraft) behaves the same in its frame of reference (the earth). Even thought the earth is spinning and flying through space orbiting the sun!:ok:

Basic Physics 101.:cool:

t_cas & swh

You got it. Agree totally.

Yeah, but what if the aircraft is C-5 or similar? Takes off with the pigeons all strapped in, and once airborne, the birds all take off. Previous answers would say the weight doesn't change.

Then they open the front cargo door (ignoring the real facts like "impossible" and "pilot can't see") and the rear door too. The birds are flying, with air passing through the cabin. If one flies out the back door, has the weight changed?

If a million pigeons in a tight flock now flies through the cabin from one door to the other, does the plane suddenly drop from the sky due to the extra weight?? And then zoom back up when they flap out the back door?

How did Superman fly without flapping his arms or otherwise displacing air downwards?:8

?????????????
 

Maybe its like saying, 'what if you had 10 monkeys and 200 kilos of bannanas and the monkeys ate all the bannanas'. Does the weight then change?

I dont think so!!!

AC, now your talking about something different all together. To put it in simple terms. If you stand outside in your back yard on a set of scales, holding a brick, then throw it up in the air, your weight will change. Would it not? Now, if you were in an unpressurised aircraft standing in the cabin with the same brick, and throw it...what is the difference?? The atmosphere would be supporting the brick, and not the contained air within a pressurised cabin.

Straight up whack.

Downwash is proportional to weight.

Not equal to it.:ok:

Let us not confuse "weight" with "mass".

Here endeth the lesson.

Woomera

Gravity does change "just because we are not touching the ground"

The further we are from touching the ground, the weaker it's effect is. Hence "weightlessness" in orbit. It still has an effect, proportional to the mass of the two bodies concerned and the distance between them.




:E

Like Woomera said - just to clear up the area of weight and mass.

Weight is the gravitational force which acts on an object. The weight of an object changes as the gravitational field strength changes, since W=mg

Mass is a measure of resistance of a body to acceleration.

Currawong - are you referring to F=GMM/R^2 where F=mg?

Soulman,

Not sure old mate. Still trying to figure out if disco_air is the evil reincarnation of my sadistic old year 10 physics teacher....

Yes, but by such a small amount (within earth's atmosphere) that it's not worth talking about.

Nope. Weightlessness in orbit is caused by the fact that you are zipping around the earth at just the right speed to counteract gravity. If you stopped, you'd find that you are still quite heavy.