[zdlupikkkdi]

Vacuum inside the balloon would create even more lift than helium because it is lighter (no mass?), but then there is no pressure inside the balloon to support the balloon structure against air pressure and the balloon would crush. It would require stronger and heavier balloon material to keep the vacuum and then air pressure wouldn't be high enough against the balloon to win the gravity force.

I don't exactly understand why air pressure focus to create force the way that the balloon raise (opposite direction from the gravity) and not just float more or less randomly in the air even without wind? Does gravity force gas to behave that way when gas pressure suddenly increases?

The path of least resistance.

The air around the balloon is applying pressure, the lateral forces are equal so they cancel out but the pressure underneath the balloon is greater than the pressure above the balloon. The net force is in the direction opposite of gravity.

It is identical to a buoyant object at the bottom of a large body of water.

[prmwsinfo]

The path of least resistance.

The air around the balloon is applying pressure, the lateral forces are equal so they cancel out but the pressure underneath the balloon is greater than the pressure above the balloon. It is identical to a buoyant object at the bottom of a large body of water.

Yes, but....if a plane was on a giant conveyor belt, and the conveyor belt matched the plane's speed as the plane accelerated, could the plane take off?

[zdlupikkkdi]

Yes, but....if a plane was on a giant conveyor belt, and the conveyor belt matched the plane's speed as the plane accelerated, could the plane take off?

Only if the pilot was drunk.

[metrocartockasur]

The path of least resistance.

The air around the balloon is applying pressure, the lateral forces are equal so they cancel out but the pressure underneath the balloon is greater than the pressure above the balloon. The net force is in the direction opposite of gravity.

It is identical to a buoyant object at the bottom of a large body of water.

Uh, i would say that's not quite correct - have you considered the pressure gradient and area is negligible compared to the lifting force.

Consider instead the mass of the gas inside the balloon and the difference between that and the air displaced, which is 0.1664 kg for helium* Vs 1.205 kg for air* , a difference of 1.0386 kg per cubic metre at STP.
Then multiply by the gravitational constant, 9.81N/kg at ground level, and you have a nominal lifting force of around 10.2N/cubic metre. this lifting force is because the ligher gas is enclosed and so the force is applied to the enclosure and any part thereof.
That is around 0.065lbs/cubic foot.
I think that's right, been a while - I'm sure one of you'll correct me if it's not, though


*source - http://www.engineeringtoolbox.com/ga...ity-d_158.html

[refsherne]

All this technical discussion. But no one has actually pointed out he isn't going to break the sound barrier.

Still a huge achievement and the guy has got balls.

[zdlupikkkdi]

Uh, i would say that's not quite correct - have you considered the pressure gradient and area is negligible compared to the lifting force.

Consider instead the mass of the gas inside the balloon and the difference between that and the air displaced, which is 0.1664 kg for helium* Vs 1.205 kg for air* , a difference of 1.0386 kg per cubic metre at STP.
Then multiply by the gravitational constant, 9.81N/kg at ground level, and you have a nominal lifting force of around 10.2N/cubic metre. this lifting force is because the ligher gas is enclosed and so the force is applied to the enclosure and any part thereof.
That is around 0.065lbs/cubic foot.
I think that's right, been a while - I'm sure one of you'll correct me if it's not, though


*source - http://www.engineeringtoolbox.com/ga...ity-d_158.html

Ah, there he is.

Hmmm... Where does the "lifting force" come from, if not from the surrounding air pressure? Consider the same experiment on the moon. What would happen to all of the planes in the sky if the atmosphere disappeared?

The density of air that you provided is the density of air near the surface of the earth. Gravitational force compresses all mass to the surface. Including air.

The difference in pressures is the key to all of this. The balloon rapidly accelerates as it tries to approach equilibrium.

[metrocartockasur]

I used STP, as it's a representative figure and illustrates the difference in mass that might be found.
The pressure is immaterial - hold the balloon by a string and you'll feel a lifting force, but the skin of the balloon has the same pressure inside and outside for every point on it's surface (a little more inside, to inflate it against the tension of the balloon membrane trying to shrink down to size). The whole point of flexible membranes, as found in balloons, is that they will flex to equalise the pressure on both sides of the membrane at any point.
If you had an extremely accurate manometer, sure, there would be a very slight difference at the top and bottom, but that difference would be the same inside and outside the balloon.
What i suspect may be throwing you is the way the balloon is only partially inflated with helium at ground level, yet at high altitudes it's full. This is because the air pressure at altitude is lower (much lower at high altitudes, such as those planned for the jump) and the gas in the balloon will expand because of the low pressure - remember the old PVT thing for gases - but the gas is trapped in the balloon and can only expand to the limit of the balloons ability to expand to equalize the internal to external pressure without rupturing. the air temperature at such altitudes is also a lot lower than at ground level - well below -40C, I believe - which would reduce some of the potential expansion* but not enough. Temperature aside, I heard mention of 2psi pressures for the altitude they're aiming for, which would mean the balloon would have to hold approximately seven times the volume of gas that it did at the ground level.

Strange as it may seem, the lifting force the balloon can apply should be approximately constant throughout.


CBF checking the figures at the moment, but doesn't really matter in context.

[Unhappu]

Only if the pilot was drunk.

I thought everyone knew by now that the plane takes off.

[zdlupikkkdi]

I thought everyone knew by now that the plane takes off.

The only thing that takes off is your mother's blouse when I offer her draconian forms of foot play.

--- Post Update ---

Gordo,

The "lifting force" that you speak about is the same as the resultant force in my post. Really there is no "lifting force", as it is the result of the pressures - meaning, it is not an independent force that acts on the balloon, it is the resultant of the pressures of the atmosphere.

[Unhappu]

I don't think someone who believes that a conveyor belt can stop an airplane from taking off is qualified to speak on the concept of lift.

[zdlupikkkdi]

I don't think someone who believes that a conveyor belt can stop an airplane from taking off is qualified to speak on the concept of lift.

I was on the "conveyer belt does not matter" side of that argument, homey. Wheel-to-ground friction is unnecessary for lift off.

[Unhappu]

A likely story!

[zdlupikkkdi]

A likely story!

Your mother was in a movie with a similar title: A Licky Story.

I am bad at mom jokes.

[Zavdpacq]

Gordo, master of all.

[ProomoSam]

It's been a few years since Gordo did anything. Almost always the same verse in his posts

[CIAFreeAgent]

I am bad at mom jokes.

It's ok, everyone else is good with your mom.

[metrocartockasur]

Sod hayfever!
Made a mistake earlier in the force bit.
All the gas stuff still applies, but there is an internal pressure from the light gas to the underside of the opper surface of the balloon. This force, or pressure, is balanced by the outside pressure and the tension in the balloon fabric, this force being passed to the load as a lifting force.
Consider the Chinese lanterns where a small fire is lit underneath - consider also the large lifting balloons that sometimes use a net over the upper surface that is used to contain the lifting balloon with ropes or hawsers from that to the load, to reduce or prevent the load being through the balloon material.

Anyway, 3am local time, so probably not explaining it too well and it's time for bed! Should give you something to think on, anyway.

Night all.

[zdlupikkkdi]

Sod hayfever!
Made a mistake earlier in the force bit.
All the gas stuff still applies, but there is an internal pressure from the light gas to the underside of the opper surface of the balloon. This force, or pressure, is balanced by the outside pressure and the tension in the balloon fabric, this force being passed to the load as a lifting force.
Consider the Chinese lanterns where a small fire is lit underneath - consider also the large lifting balloons that sometimes use a net over the upper surface that is used to contain the lifting balloon with ropes or hawsers from that to the load, to reduce or prevent the load being through the balloon material.

Anyway, 3am local time, so probably not explaining it too well and it's time for bed! Should give you something to think on, anyway.

Night all.

I can't tell if you are agreeing with me or trying to explain to me what I have been trying to explain to you!

--- Post Update ---

It's ok, everyone else is good with your mom.

Awww... It was her 59th birthday this weekend. I hope you all treated her well.

[CIAFreeAgent]

Awww... It was her 59th birthday this weekend. I hope you all treated her well.

...like a rented mule in mexico.

[esdfsdflast]

Awww... It was her 59th birthday this weekend. I hope you all treated her well.

http://www.urbandictionary.com/defin...ama+hot+pocket

Her present from me...