Dig up and old question....

[disco_air]

Some of you may remember the previous post on the question I asked over a year ago... here (http://www.pprune.org/forums/showthread.php?s=&threadid=141348)

Now... what happens to the aircraft's balance situation if the birds fly forward or aft in the cabin (without landing and touching the airframe)? Does it change?

And if so, through what medium is their weight force transferred to the relevant loading station of the aircraft?

And if it doesnt, where is the bird's weight transferred to giving the aircraft's CofG position?

muahahahaha


...Disco

Sorry, but I had to ask.

[pall]

If the bird does not touch any part of the aircaft interior then it's weight or position must have no bearing on the aircraft weight and balance.

If it lands on any part of the interior, at that time it becomes an influencing factor on weight and balance.

I think this is like the philosophical question: If a leaf falls in the forrest and know one is there to hear it, will it still make a sound?

[Ejector]

Is this a bit like, a aircraft is heavier pressurized than depressurized as there is more air onboard !!!!! so to save cash, give the pax a little sleep,,oh don't tell Dixon

[404 Titan]

disco_air

You have already asked this question. Maybe you should re-read the thread or better still I will reproduce it for you.

You wrote:
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 wrote:
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.

[pall]

disco_air I have just read the postings from last year. I would have to say this matter recieved a fair run! Not to say it can't be rediscussed but one wonders what will change with further discussion.

404 Titan, I tend to lean toward your view.

Will give this some more thought.

The question remains, do you consider the interior of the aircraft as a discreet closed environment. If so any enegy expended by birds to support their own wt must altimately be transmitted to the aircraft via the internal air.

[Lodown]

And here's another you can do with your kids in the car...

Take a balloon that is lighter than air and leave it float against the roof of the car.

Which direction does the balloon move when you accelerate and decelerate? Why?

[disco_air]

404:

Still doesnt answer my question. Through what station is this "consolidated" weight of air and birds transferring its weight to the airframe?

It must be acting through a Centre of Gravity of its own as well. So where would this be calculated?


...Disco

[swh]

Through what station is this "consolidated" weight of air and birds transferring its weight to the airframe?

You would have to calculate a sum of moments and devide it by the total mass to answer that, same way you do for passengers.

[Lodown]

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.

How?

First, the bird in flight is in equilibrium, meaning its lift is equal to its weight.

Second, air is a gas, which means that the pressure will be evenly distributed throughout the entire aircraft. So how does the weight get distributed solely to the floor? Extending your argument, increased pressure (?) on the floor wil be accompanied by a decrease in pressure on the ceiling. When an aircraft takes off from an aircraft carrier, does the weight of the airborne aircraft transfer to the deck prior to going over the bow? Then what happens?

Cheers

[swh]

Second, air is a gas, which means that the pressure will be evenly distributed throughout the entire aircraft.

Strictly speaking, air is a fluid. It has mass, density, and volume and cannot resist shear.

Depending on the composition of air, the density changes, this happens are you go further away from the surface.

Depending on the local air pressure, the density also changes.

Density is mass/volume, the weight of that air (mass x g) exerts a slightly higher pressure towards earth to transfer the mass of the air over the area of the surface (maybe 8.0000000000001 psi) than it does on the walls (say 8 psi).

A better example is a high pressure gas cylinder, it exerts pressure evenly on all sides of the container, however a full gas cylinder will have a higher mass than an empty one.

[Richo]

Hay Disco

Sounds like a Question for MYTHBUSTERS. Thisr website is available under that name. they are always looking for MYTHS to bust (though they rearly get the FACTS completly correct.

But I bet you average Joe would think it worthy of testing.

HAve a go, send them a email and see if you can make the TV, either SBS or see it repeated some weeks later on 7.

Good luck

Richo

PS I have NO Fuc^%$ing idea, all to complicated for me.

[404 Titan]

Lodown

Please don’t get confused by the pressure exerted on the airframe by a pressurised aircraft. We are not talking about that. We are talking about the weight of the air because of its mass x Gravity. Let’s look at a non pressurized aircraft if it helps to show what I am talking about. The total weight of that aircraft in flight is the BEW + crew + fuel + payload (pax and freight) + air in the cabin. Because the air is always in the cabin its weight is actually taken into account in the BEW so as pilots we don’t see or think about it. If some of the freight are birds in a large cage what do you think happens to their weight if they all become airborne? Their weight is now being supported by the air in the cabin instead of their perches which in turn is being supported by the floor of the aircraft. The weight of those birds is now evenly distributed across the floor of the aircraft just as the weight of a Jumbo Jet is evenly distributed across the surface of the entire earth when it is in flight. If this wasn’t the case, every time one flew over we would hear of the hoards of people being crushed to death by their enormous weight.

Another way to look at it is a swimming pool. The weight of the pool consists of the pool + water + swimmers. The weight of the swimmers is evenly distributed across the bottom of the pool not just below the spot they are swimming.

disco_air

Just like any object in the aircraft they will have their own centre of gravity. In the case of the airborne birds in the cabin their C of G will be roughly the centre of the cabin.

[Oktas8]

Perhaps considering a helicopter might be helpful. A helicopter flies using the same principles as a fixed wing aircraft or a bird - air flowing over an aerofoil causing a reduction in pressure.

When a helicopter hovers over the sea the water is flattened out a little (excuse the bad terminology!), and small waves tend to be blown out towards the edges by the air circulation.

Isn't it true that this particular helicopter is supported by air being accelerated downwards, and the air is brought to rest by the ground (water)? So by F = m.a, there is a force on the surface of the earth.

If the helicopter was hovering inside a C5, the floor of the C5 would have to support the deceleration of the downwash air. Ditto for birds I suppose.

On a more techy note - 404 Titan - I'm not sure that the weight of aircraft is quite spread evenly over the entire earth's surface. Although the swimmers in your pool are supported by the water, which transfers their weight to all parts of the pool's floor evenly, I'm not sure it's the same for aircraft. Swimmers are floating, they displace a weight of water equal to their own weight, the displaced water spreads itself out evenly over the rest of the pool. However, heavier-than-air aircraft don't float in air, as they are supported by constantly accelerating air downwards. They don't displace a weight of air equal to their own weight, so there will be a pressure pulse under an aircraft. Stand under a 747 flying at 20' and you'll detect it, but perhaps the pressure pulse under aircraft flying at 500'+ is too spread out to be easily detected.

Cheers to all,
O8 :)

[404 Titan]

Oktas8

Don’t get confused by the downward force of air from a helicopters rotors or the downward air rush from the birds wings. It isn’t relevant and has no effect on the weight or the CofG of the aircraft. Let’s take the same pool that I used in my previous thread. That same swimmer now has a set of fins on and uses them to try and propel him/herself vertically straight up out of the water. Of course this swimmer can only get to the surface of the water and then basically becomes stationary but is still exerting considerable energy. There is a large downwash of water from the fins that hits the bottom of the pool. What do you think happens to the weight of the pool and its CofG? Simple, nothing. It’s an equal and opposite reaction in a closed environment. The Earth is also a closed environment.
They don't displace a weight of air equal to their own weight, so there will be a pressure pulse under an aircraft. Stand under a 747 flying at 20' and you'll detect it, but perhaps the pressure pulse under aircraft flying at 500'+ is too spread out to be easily detected.
The force produced by the wing of an aircraft or the rotor of a helicopter (lift) in level flight is equal to or very close to the weight of that aircraft. The downward force of the air and increase in the air pressure just under this aircraft again isn’t relevant. What one must ask is does the weight of the earth, if it could be measured change? Again the answer is no.

[Lodown]

I hope I'm not coming across as bombastic, but I can't seem to make sense of this thing with a bird in flight in an aircraft being supported by the pressure on the floor. I ask these questions to explain this as much to myself as anyone else and enjoy this written conversation. Please excuse my ignorance in advance and I look forward to being convinced one way or the other.

404, your last post seems to support a rebuttal of your previous arguments.

Combine some sentences from the last posts...
The force produced by the wing of an aircraft or the rotor of a helicopter (lift) in level flight is equal to or very close to the weight of that aircraft. The downward force of the air and increase in the air pressure just under this aircraft again isn’t relevant.
Substitute "bird" for "an aircraft or the rotor of a helicopter" and "aircraft" and read again.

Second, Oktas' quote:
Isn't it true that this particular helicopter is supported by air being accelerated...
Stop! Don't include "downwards", or "brought to rest by the ground or water."
Lift is produced by air being accelerated. The previously accelerated air is brought to rest or equilibrium by a variety of means, but most commonly with collisions with other molecules in the air, but it really doesn't matter how it is brought to rest for the sake of this conversation.

404, as Oktas pointed out, the swimming pool is not a good analogy. You've pointed out a very good example of bouyancy and not aerodynamic lift. That will work well for balloons in aircraft, but doesn't seem applicable here.

You've indicated earlier that when the bird takes off in the aircraft, its weight is still supported by the aircraft. On a vector diagram I can indicate this with a vector downward indicating aircraft weight and a vector downward representing bird weight. On the upper side of the diagram, I can indicate aircraft lift. The bird takes flight in the cabin. What do I do to indicate the Lift vector from the bird? How do I indicate it? Do I combine it with the Lift vector of the aircraft, or do I ignore it?

If some of the freight are birds in a large cage what do you think happens to their weight if they all become airborne? Their weight is now being supported by the air in the cabin instead of their perches which in turn is being supported by the floor of the aircraft.

I think this is slightly incorrrect. Please correct me if I am wrong, but shouldn't this read:
"Their weight is now supported by the birds accelerating the air in the cabin (albeit towards the floor via their wings) instead of their perches..." Even if there is a slight pressure buildup on the floor, it is going to be balanced by a slight pressure reduction elsewhere in the cabin.

BTW with the balloon in the car, accelrating the car will see the balloon move forward. Braking will move the balloon to the back. Why? The balloon is lighter then air. The air has momemetum and during acceleration will want to move towards the rear of the car. The balloon with less momentum will be forced forward and vice versa during braking. Fun to do with kids in the car.

[404 Titan]

Lodown, Oktas8, & disco_air

This is straight out of a one of my very old Uni physics texts.

The Problem

A container full of ducks sits on scales being weighed. If you were to scare the ducks so they took to the air and flapped about, would this reduce the overall weight?

Think about it.

The solution:

The weight, or mass of the container would remain exactly the same as long as the flapping ducks remained within the container. The lift from the ducks' wings would exactly balance their weight. This relates to Newton's third law which states that action and reaction are equal and opposite.

By the way, it is irrelevant whether we are talking about buoyancy or lift. The same principal still applies because we are dealing with mass and weight caused by gravity and Newton's Third Law. Just because one is being supported because it is lighter than its surrounding and the other is producing lift from its wings is a red herring. It makes absolutely no difference to the point I am trying to make. This question has been around since man first learnt how to fly. Since then some very very intelligent people, more intelligent than you or I, have said that if the birds become airborne in the closed environment the total weight of that environment won’t change.

[Lodown]

Okay, thanks for the detail in your post. It is much appreciated.

[Oktas8]

Just to clarify my post above, I agree with 404 Titan that the overall weight of an aircraft will not change, whether the freight is airborne or not. In my post I was trying to paint a word-picture of why I believe this to be so, without referring to Newton. But, I appear to have been totally unclear. :( [New Year's Resolution - give up posting on PPRuNe...]

But there is one thing I'd like to ask - the weight of an aircraft does not change if the birds get airborne. Agreed. But what mechanism transfers the weight of the flapping birds, to the aircraft (which still has to support their weight)? Two of you have suggested that the downwash is irrelevant. However, please consider this chain of events:

The birds generate lift by accelerating air downwards when flapping. Accelerated air is brought to rest by friction with other air below the birds, so transferring momentum to air below the birds. Since rate of change of momentum = force, and weight is the relevant force here, can't we say that the layer of air under the birds is supporting the weight of the birds? The layer of air under the birds is itself supported by the aircraft, hence the aircraft supports the weight of the birds.

Regarding CG changes with moving birds inside cabin -

As far as lift and weight go, buoyancy makes a good example of what 404 is talking about. However, (and only because someone mentioned CG changes), buoyancy must not be confused with heavier-than-air lift in this discussion. Floating swimmers go to one end of the pool, or to the other, but the CG of the pool does not change, because the average density of every part of the pool remains unchanged.

However, the average density of different spaces inside an aircraft do change if birds move around (weight of air in forward cabin < weight of flapping birds in aft cabin), so would there not be some effect on CG if the airborne birds flew from one end of the cabin to the other?

Hoping I've explained myself better this time,
O8

[AlbyMangels]

You lads need to get a life. Go get some pork on your fork instead of living and breathing aircraft all your life! :yuk:

[Centaurus]

In another lifetime I flew single seat Vampires and did air-to-air cine camera gunnery. One Vamp was the stooge, the other the attacker. After pressing the firing button for the camera shot, the attacker would bunt and pass directly under the stooge missing it by maybe 50 feet then break away. The stooge would sometimes complain of a jolt as the other Vampire passed close and directly underneath. The closer the miss, the more violent was the jolt, usually causing a "Hey wotchit!" from the stooge. I often exactly what caused the jolt. Any thoughts?

[404 Titan]

Oktas8
But what mechanism transfers the weight of the flapping birds, to the aircraft (which still has to support their weight)?
The air in the cabin is supporting the weight of the birds which is in turn transferred to the floor of the cabin the same way that a water skier’s weight is supported by the water under the ski which in turn is transferred evenly to the ocean floor.
so would there not be some effect on CG if the airborne birds flew from one end of the cabin to the other?
It all depends on how big the birds are, how many there are and how high they fly in the cabin. Birds flying near the floor would have more of an effect on the CofG than birds flying near the ceiling. As the birds get higher their weight will be more evenly distributed over the cabin as their weight is more evenly distributed across the volume of air in that cabin. If we are only talking about a small number of birds I would say that their weight would be fairly evenly distributed once out of ground effect.

Centaurus

I know the Vampire was subsonic but there still may have been some shock wave on the aircraft, especially since it was over taking you. That would certainly cause an upset if you were close.