Another problem like the conveyor

[Keith Lane]

There you sit on the line, engines roaring, brakes locked. You've got to get this massive load of carrier pigeons (oh, say about a ton of them) to the troops. Problem is, you're WAAAY over gross. As you start your takeoff roll, the birds get spooked and fly up off the floor (hey they aren't caged, they are "free range" birds).

Question: Does the plane suddenly lose enough "weight" to takeoff.

If it does, you better send the SIC back there to keep 'em stirred up till you land.
Curious minds want to know. ????????

 

[jesse]

There you sit on the line, engines roaring, brakes locked. You've got to get this massive load of carrier pigeons (oh, say about a ton of them) to the troops. Problem is, you're WAAAY over gross. As you start your takeoff roll, the birds get spooked and fly up off the floor (hey they aren't caged, they are "free range" birds).

Question: Does the plane suddenly lose enough "weight" to takeoff.

If it does, you better send the SIC back there to keep 'em stirred up till you land.
Curious minds want to know. ????????

First off let's start off with: What is weight?

Weight = Mass x graviational magnitude

So. When a bird goes airborne, It is generating lift to cancel out the weight.
Remember the classic:

-----------Lift-----------
Drag--- (airplane) -----thrust
---------weight-----

you get enough lift to cancel out the weight.

So yes. If you had a ton of birds in your plane and they all tookoff at once..and you were generating enough lift to takeoff with your new weight. You are set.

Birds land in airplane again. Weight increases. If weight exceeds lift, down you go.


Another way to look at this is:

Put your car on a scale.

Get inside your car.

Jump up inside the car, The weight on the scale would obviously decrease until you landed.

...but then again..what do I know, I never made it past the 10th grade.

 

[Skip Miller]

Question: Does the plane suddenly lose enough "weight" to takeoff.

The plane does not lose weight if the birds are flying. the birds generate lift the same way an airplane's wing does: by accelerating air downwards. That downward moving air is going to contact the plane's floor and transfer the birds' weight to the plane. ergo no loss of weight.

Now ask the same question if the birds are in a B-24. Get the birds flying in the bomb bay and open the doors. Assume the birds don't fly out, but instead fly "in formation" with the plane, staying in the bomb bay. Now the plane is lighter!

The first plane, even though not pressurized, can be considered a closed system for this example. The second plane is an open system.

-Skip

 

[jesse]

The plane does not lose weight if the birds are flying. the birds generate lift the same way an airplane's wing does: by accelerating air downwards. That downward moving air is going to contact the plane's floor and transfer the birds' weight to the plane. ergo no loss of weight.

Now ask the same question if the birds are in a B-24. Get the birds flying in the bomb bay and open the doors. Assume the birds don't fly out, but instead fly "in formation" with the plane, staying in the bomb bay. Now the plane is lighter!

The first plane, even though not pressurized, can be considered a closed system for this example. The second plane is an open system.

-Skip

I thought about that. Then I realised most small planes do not hold pressure i mean at ALL.

Get a cold. Change altitude by even 50 feet quickly. It'll hurt like hell.

So I think to some extent you would get lighter?

 

[Skip Miller]

I thought about that. Then I realised most small planes do not hold pressure i mean at ALL.

Right. Now hold your hand out flat. Blow on it. Feel the air impact your hand? Now your hand is holding less pressure than the interior of a GA airplane, but it still feels the moving air impact it.

-Skip

 

[jesse]

I see.

 

[Skip Miller]

I see.

Good! Now go wash your hand.....

-Skip

 

[fgcason]

So yes. If you had a ton of birds in your plane and they all tookoff at once..and you were generating enough lift to takeoff with your new weight. You are set.

Birds land in airplane again. Weight increases. If weight exceeds lift, down you go.

Interesting theory. One teeny problem though: Conservation of energy in a closed system.

What's holding the bird up while flitting around inside the plane? It's suddenly back to pulling on the rope while rolling on the conveyor belt runway. Lift cancels out weight for the bird itself however aerodynamics are involved and that energy must account for. Downwash from the wings goes somewhere...like against the floor. The further the lift is generated from the floor, the more the energy is distributed over a larger surface area below due to air flow patterns. 2lbs of force applied over 9 sq meters feels like nothing while laying under it but the cumulative force is still 2lbs.

Think of it like this: Does a hovercraft weigh anything while hovering at 3" AGL? Can you load 50 tons of hovercraft in a 25 ton useful load plane and takeoff 25 tons overgross just because it's not touching the floor? Same concept.

Another way to look at this is:

Put your car on a scale.

Get inside your car.

Jump up inside the car, The weight on the scale would obviously decrease until you landed.

Correct but that's changing the conditions of the experiment to some extent.
Jumping up in the car, or a plane, creates an increased down force during vertical acceleration then no additional down force during ballistic motion, then down force during landing. Net closed system force difference = 0 even though there is a momentary reduction in weight while on a ballistic trajectory before hitting the floor again.


These are fun questions aren't they???

 

[jesse]

These are fun questions aren't they???

\

Yes they are.

That is one thing hta twas bothering me. Is like you said you have to account for the energy.

 

[fgcason]

\

Yes they are.

That is one thing hta twas bothering me. Is like you said you have to account for the energy.

Now, what happens if there is a 1000lb helium balloon floating around inside the cargo bay instead of a bird flapping around in there? (to simplify the experiment: assume a sealed constant pressure cabin and the balloon doesn't leak or bump into the sides of the cargo bay)

 

[EdFred]

Now, if the plane was filled with helium balloons....

 

[AirBaker]

Now, if the plane was filled with helium balloons....

*In a high pitched voice* 2 2 1 2 Romeo ready for takeoff...

 

[gismo]

Now, what happens if there is a 1000lb helium balloon floating around inside the cargo bay instead of a bird flapping around in there? (to simplify the experiment: assume a sealed constant pressure cabin and the balloon doesn't leak or bump into the sides of the cargo bay)

Not sure what you mean by a thousand pound helium ballon. Is that 1000lb of helium in a balloon? If so how much does the envelope weigh? I assume that the envelope would weigh considerably less than the "lift" (difference in weight between the helium and the air it displaces) generated by 1000 lb of helium.That does make the plane lighter. The reason is that the helium takes up space that would otherwise be occupied by air (which weighs considerably more).

 

[fgcason]

Not sure what you mean by a thousand pound helium ballon. Is that 1000lb of helium in a balloon? If so how much does the envelope weigh? I assume that the envelope would weigh considerably less than the "lift" (difference in weight between the helium and the air it displaces) generated by 1000 lb of helium.That does make the plane lighter. The reason is that the helium takes up space that would otherwise be occupied by air (which weighs considerably more).

The intent was 1000lbs of balloon contraption when sitting on a scale with the helium in a cylinder and the envelope wadded up in the basket. Then inflate and put in the plane.

 

[HPNPilot1200]

If anyone finds more time to read about this topic/theme, you ought to visit:

http://www.avweb.com/news/columns/191034-1.html

(Note: I haven't read it yet, but found the link for it.)

 

[spiderweb]

The weight doesn't change.

 

[Nav8tor]

Now, what happens if there is a 1000lb helium balloon floating around inside the cargo bay instead of a bird flapping around in there? (to simplify the experiment: assume a sealed constant pressure cabin and the balloon doesn't leak or bump into the sides of the cargo bay)

If the balloon is floating without touching the ceiling then the lift of the helium would equal the weight of the balloon so the net result would be no effect on aircraft weight.

And that would be one big balloon.

 

[Henning]

yes. If you had a ton of birds in your plane and they all tookoff at once..and you were generating enough lift to takeoff with your new weight. You are set.

Birds land in airplane again. Weight increases. If weight exceeds lift, down you go.


Another way to look at this is:

Put your car on a scale.

Get inside your car.

Jump up inside the car, The weight on the scale would obviously decrease until you landed.

...but then again..what do I know, I never made it past the 10th grade.

Need a course on Newton and aerodynamics (see if Jack Thelander is still instructing in Long Beach). The lift has an equal and opposite down force on the aircraft it is flying in. It's weight is still in the aircraft, the force is being relayed through the air rather than through its feet, but that same "weight" is still acting upon the aircraft at all times.

Neat experiment if you want to do it: take a bottle 2/3 full of colored water, preferably a glass bottle and a bowl of water to your nearest airport where they land heavies and stand under the approach end. Take the bottle and turn it upside down into the bowl of water so you have a nice standing column. Watch the water level inside the bottle as a heavy goes over.

 

[RotaryWingBob]

Barry Schiff has an interesting discusion about this in his Profiient Pilot books (Vol 1, I think, but it could be Vol 2 -- I'm too lazy to look it up). In any case, the weight doesn't change when the birds start flying.

Supposedly the question was first posed by Lindburgh on his famous flight to Europe when he noticed he had a fly aboard.

 

[Skip Miller]

Supposedly the question was first posed by Lindburgh on his famous flight to Europe when he noticed he had a fly aboard.

...and lots of time on his hands.

-Skip

 

[Dan Smith]

The weight doesn't change.

Right - the total mass in the closed system is the same, whether it's birds on the floor, birds in the air, helium balloons, whatever. Whatever the state, for the airplane to climb it has to accelerate that total mass in an upward direction. Too much weight = insufficent lifting force to move it = no acceleration = no climb. However, if the plane with the birds was placed on an idealized conveyer belt moving in the opposite direction...

Dan

 

[Len Lanetti]

Question: Does the plane suddenly lose enough "weight" to takeoff.

No. Either take my word for it or see Newton's 3rd law.

Len

 

[gismo]

The intent was 1000lbs of balloon contraption when sitting on a scale with the helium in a cylinder and the envelope wadded up in the basket. Then inflate and put in the plane.

OK, but the key issue is whether the inflated balloon had positive, buoyancy or not. If it does then the plane is being "lifted" but the balloon, but if the balloon just hangs there without rising or falling (that will change if the temperature does BTW) then there's no effect on the airplane when you inflate the balloon, assuming that the airplane isn't hermetically sealed.

 

[fgcason]

OK, but the key issue is whether the inflated balloon had positive, buoyancy or not. If it does then the plane is being "lifted" but the balloon, but if the balloon just hangs there without rising or falling (that will change if the temperature does BTW) then there's no effect on the airplane when you inflate the balloon, assuming that the airplane isn't hermetically sealed.

Conditional modifiers. Glack! It's a tuff crowd around here sometimes.

My thought was the inflated balloon was just hanging there inside the plane with netural bouyancy not touching anything. Float it in and close the hermetically sealed door. Point being about the total mass and weight involved when displaced heavier air is replaced with enough helium to float it off the floor. IOW: Close the plane door, seal the plane up in NASA's vacuum chamber on a weighing scale with the pressurized cargo bay holding the balloon and what are the weights when (1) the balloon uninflated and laying on the floor) and (2) when it's floating free in the plane. Start the conditions of the weighing the same (doors open in a pressurized room, adjust balloon accordingly, seal it up, suck the room empty and measure)

On second thought, maybe the balloon in the plane isn't the best sort of mind game example but it does make one think a bit especially in light of the general confusion that the birds flitting around the cabin making you go overgross causes.

 

[gismo]

Conditional modifiers. Glack! It's a tuff crowd around here sometimes.

My thought was the inflated balloon was just hanging there inside the plane with netural bouyancy not touching anything. Float it in and close the hermetically sealed door. Point being about the total mass and weight involved when displaced heavier air is replaced with enough helium to float it off the floor. IOW: Close the plane door, seal the plane up in NASA's vacuum chamber on a weighing scale with the pressurized cargo bay holding the balloon and what are the weights when (1) the balloon uninflated and laying on the floor) and (2) when it's floating free in the plane. Start the conditions of the weighing the same (doors open in a pressurized room, adjust balloon accordingly, seal it up, suck the room empty and measure)

Well, sealing the plane is a key factor, because when you release the He from the high pressure bottle, there's no change in the weight of the plane's content, but if you leave the door open, the helium displaces a bunch of air, and that air had mass and weight. With the air gone, weighing the plane would show a decrease equal to the weight of the lost air. You could get a similar effect by sealing the cabin and pumping out the air assuming the seals would hold against the reverse pressure.

On second thought, maybe the balloon in the plane isn't the best sort of mind game example but it does make one think a bit especially in light of the general confusion that the birds flitting around the cabin making you go overgross causes.

 

[Bill]

inflate the balloon, assuming that the airplane isn't hermetically sealed.

And if the balloons are filled with He, then you could do mil spec leak testing!

Test test test