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.

AerocatS2A,

How then do geostationary satelites maintain station?

Why then do astronauts float around when not in orbit? As in on their way to the Moon?

Why are we lighter on the Moon? Smaller mass, less attraction of course.

:E

..... good grief...
 

"here endeth the lesson"???? :confused:

How did the topic get left so far behind?

currawong:
A geo-synchronous orbit allows a satellite to maintain a position over the earths surface because its radial speed matches the rotation of the earth. Ie one orbit every 24hrs. The satellite has no idea what the earth is doing underneath it, all it knows is the centrifugal force due to its speed is almostly perfectly equal to its weight.

On the way to the moon, there is a process of acceleration underway the whole time. For the first phase, the ship is under the influence of earths gravity and is slowing down as soon as the rocket engines cease to fire. Eventually, it enters an area where the influence of the moons gravity is predominant. Now it is increasing velocity again. The whole package is always subject to acceleration due to the sum total of the gravitational influences on it and therefore the men inside experience "weightlessness".

Are you serious with the moon mass remark? Of course not!! You know perfectly well the moon, having less mass of its own, exerts a lesser gravitational force upon the mass (unchanged) of the astronaut, who weighs less as a result...!! ;) :ok:

Soulman to futher define what you said, Mass is predominantly a measurement of the amount of matter of a body; Inertial Mass (IE inertia) is a measure of a bodys resistance to changes in velocity.

In a closed environmment (ie the airliner) the downward force (lift) produced by the birds will offset the reduction of weight on the floor by the birds being "in flight". This is in keeping with a High School physics principle called "Conservation of Energy".

Try picturing a helicopter inside the airliner (I know it's unreal but then so's some of the physics being discussed here) instead if you don't agree. There is no net change in energy besides the "fuel" being used by the birds or whatever being converted to waste. How could there be?

Also, don't forget, it's not just the "downwash" but the "suction" effect on the ceiling of the aircraft of the air being pulled downward.

If there was, then we would have massive savings in fuel costs being enjoyed by the airlines... all the passengers seats being suspended in the cabin by helium balloons like that Larry guy in LA with the Darwin Award. Trolleys "floating" down the aisle, etc, etc.

Jumping up and down in a subway train and not flying off the back of the train is a different effect in principle.

Birds flying through a C-5 from one end to the other are a totally different effect to the original question.





As for Superman\'s abililty to fly, this is a totally different prospect.

You see, in the early 20th century, he couldn\'t actually fly, he could only "leap one-eighth of a mile; hurdle a twenty-Story building . . . raise tremendous weights . . . run faster than an express train.”

By the \'40s or so, with comic-book competition heating up, Superman\'s strength and powers began to change for the better.

Initially his abilities were put down to having been born to a race that lived on a planet with a gravity well stronger than ours. This seemed to keep the punters happy.

Years later, someone noticed that he seemed to have no difficulty in changing direction in mid-leap. The yellow sun/red sun theory came into play and soon Superman could fly, rather than just jump a long way.

No one ever bothered to explain the physics involved in dealing with the force of gravity needed to allow such proportionally more powerful acts of strength or, indeed, in creating a starship that could escape such gravity.

404 titan

yes you are correct in the fact I need to brush up on the physics!
Its been a long time since I was calculating Newtons laws and gravity/acceleration type probs. Will get out some old text books I think and have a refresh since reading some of the formula here....but,
I never said gravity decreases because your not near the ground.
I am in no way disputing the gravity laws here. I still have not seen here in this topic someone to explain the energy the birds are exerting from flaping there wings and burning there own fuel!
As Woomera says dont confuse weight/mass. Its basic to know weight is a function of gravity and mass a chemical property of density. I dont see how this comes into this one.
Please explain the energy from the birds! What happens to this energy?
t_cas...you talk frame of ref with the birds. Correct! But you dont take into account the birds all start there motors and burn fuel...thus relieving work from the aircraft! Hmmmm.
Still not convinced with others yet :rolleyes:

G

GW_04

The question originally asked by disco_air was what happens to the weight of the aircraft, not what happens to the energy exerted by the birds. Lets look at your argument for a moment to point out what I am trying to say. If we had a planeload of passengers seated in the cabin. They aren’t exerting much energy because they are seated. Now imagine all these same passengers now suspended from the ceiling of the aircraft by individual ropes (not around their necks). The only thing keeping them attached to the ropes is the considerable energy they are now exerting holding onto these ropes. The weight of the aircraft hasn’t changed but there is a hell of a lot of calories being burnt by everyone on board compared to when they were all seated. As you should be able to see the energy being exerted by the birds isn’t relevant to this question, as it doesn’t affect the weight. :ok:

Someone introduced a red herring when he mentioned "Mass" on Pg1. Mass has nothing to do with it beyond being responsible for the CONSTANT value of weight for the birds and aircraft. Neither does inertia have anything to do with it.

In a sealed environment Conservation of Energy will take place.

For every action there is an equal but opposite re-action.

All answers given above are a bit confusing for me. Can somebody please give me the correct answer in simple pigeon (sic) english?

I seem to recall this as a examination question in the good 'ol DCA days.:{

Speaking with my Yr 12 Physics teacher today at lunchtime and asked him this very question. He was of the opinion that it would have no net effect since you can regard the plane and it's occupants (the birds) as one system - hence there is no change in the net force.

Looks like smooth sailing for those up the front of Cockatoo flight 222.


Soulman.

WTF
 

Sorry guys but I'm still none the wiser here! 404 Titan and GW_04 have really good points I cant dispute!

But....

When the birds take off, aren't they now supporting their own weight (i.e. no longer exerting force on the airframe)?

404, your example of hanging pax from the ceiling has the force still exerted on the airframe thus contributing to aircraft 'weight'.

I don't think frames of reference have any bearing on the matter, since we are not talking about velocity. Thus the jumping up in the train and not falling back does not apply.

If you think of standing on a set of scales and jumping up. What happens? The weight (the force you were exerting on the scales) is removed isnt it?

Or perhaps mid air refuelling as an example - the fuel taken onboard becomes the aircraft's weight when it is no longer being supported by the wings of the aircraft that is refuelling it.... and vice versa!

Anyone up for a practical experiment???? :}

...Disco

Scales are a different case too. You are only weightless for a brief period. When you jump you impose an additional force upon the scales before you leave them. When you land there is a force additional to your weight momentarily imposed also.

If you jump up and down in an airplane it will momentarily enjoy the benefit of the absence of your weight, then when you land (and jump) each time, it will pay a penalty in additional "weight" for your jumping. The net effect will be zero.

disco_air
No. The total weight of the aircraft consists of:

Aircraft + fuel + cabin air + occupants = Total Weight

As has been stated before it is a sealed environment. Just because the birds are flying around the cabin doesn’t mean the weight has disappeared off into the outside environment. It hasn’t because it can’t. Instead of the weight of the birds being transferred to the weight of the aircraft via their perches, it is now being transferred to the aircraft via the air mass in the aircraft. Remember it is a sealed environment.
In reference to the scales, yes the weight is removed but you have to look at the scales over the entire process. As you push off the scales your weight will appear to increase. This is because you are applying a force to jump into the air. As you leave the scales they will go to zero. When you land back down on them the force of your landing will momentarily cause the weight to be more than your actual weight. If you compared your weight on the scales over one minute you will find the average weight of you jumping up and down on the scales to your real weight would be the same. If you don’t believe me try it. Mind you if you break the scales I take no responsibility.:uhoh::hmm:

itchybum
Sorry you beat me to it.:ok:

righto, so what if the plane doors were open?

Surprised that no one has yet put forth the proposition that the weight of the earth is reduced by the weight of all the aircraft flying, or by the weight of a ship when it launches down a slipway into the sea.

Certainly the total air pressure on the surface of the earth is increased a little bit by the weight of the aircraft flying. Same for the birds flying within the aircraft; so in turn the weight of the birds is still transmitted to earth.

And which way does the smoke go from the cigar smoking aerobatic pilot in an enclosed cockpit when on the top of a loop?

Has anyone ever managed to measure the overpressure at ground level directly beneath the centre of a heavy at rotate?

404 titan

what if the birds were in two separate averies on the ground with the same scenario. One avery had mesh sides and one had airtight sides.
Are you saying the airtight one would have no weight change and the mesh side one would?
This takes some getting the head around.

G :)

yeh yeh and what if the mesh aviary door was open??

Ahem... the only thing that would change is the centre of gravity:cool:

Regards

Mark

Mark - if it were to change the centre of gravity, the aircraft would change its current position. We are assuming that the aircraft is in level flight at constant velocity - so changing the centre of gravity would cause the aircraft to climb/descend.

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.

Soulman.

Milt,

You will not find downwash directly beneath a moving aircraft.

You will find it a little way behind, shaped like a long teardrop.

Proportional but not equal to the aircrafts weight.

About a third of potential downwash is lost as wingtip vorticies.

Which is unfortunate because complete span downwash would be useful in some applications (pun intended)

If you have the chance you will see this "footprint" when the aircraft is flown close to some visible media, such as smoke, water or a crop such as wheat.

Half a wingspan or less close, that is, or you will see nothing.

:ok:

Im still not convinced either way. :confused:

Your 'sealed environment' model sorta makes sense, but aircraft are not sealed completely anyway.

Also would that mean that in an aircraft's total weight, you would include the weight of the air in the cabin?

Given the ISA MSL density is 1.2256kg/m³ and g = 9.81

and W (Force) = mg

.....then a 100 cubic metres of air will exert 1200 Newtons towards the centre of the earth (and so onto any medium that is supporting it, with an equal and opposite force)

Scales... bad example of mine :hmm:

Good point, OZBusDriver....

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?

I apologise in advance for all those who go nutty over this... If you havent already! :}

soulman Exactly. My CFI related this self same thing to me in 1976. As the birds fly up or back or forward the aircraft would suffer a CG shift. If the birds flew up the CG would rise and the aircraft would change level down by the exact same amount. So if you were observing from outside th aircraft looking at the fixed CG .As the birds moved about, Your frame of ref relative to the CG would not change but the aircraft would move up or down in relation to that CG. In the frame of ref of within the aircraft the position of the CG would move. Thanks very much to the most amazing Mr John Young THE best pilot I have ever flown with and also proof that a nucklehead can remember something of his training after 28 years:D

Regards

Mark