A strange but good question....
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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.
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.
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..... good grief...
"here endeth the lesson"???? 
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...!!
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.
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...!!
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.
Last edited by itchybum; 17th Aug 2004 at 15:51.
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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
G
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
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.
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.
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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.
In a sealed environment Conservation of Energy will take place.
For every action there is an equal but opposite re-action.
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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.
Looks like smooth sailing for those up the front of Cockatoo flight 222.
Soulman.
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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
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
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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.
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.
itchybum
Sorry you beat me to it.
When the birds take off, aren't they now supporting their own weight (i.e. no longer exerting force on the airframe)?
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.
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?
itchybum
Sorry you beat me to it.
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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?
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?
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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
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
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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.
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.
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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.
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.
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Im still not convinced either way.
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
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!
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
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
Regards
Mark
Regards
Mark