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Helium vs Hot Air Lift

02/20/2017 6:28 PM

Hi, my son asked me this question and after delving into the rabbit hole of google, I still have no idea. Which type of balloon provides more lift: helium or hot air? I understand that helium is lighter than air but is that enough to generate more lift? thanks a lot, it is tricky having a 13 year old interested in physics and physics is not my area.

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#1

Re: Helium v hot air lift

02/20/2017 7:15 PM

Helium.

Hot Air Balloon Physics — Analysis
Let's examine the physics of a hot air balloon using a sample calculation.
The heated air inside the envelope is at roughly the same pressure as the outside air. With this in mind we can calculate the density of the heated air at a given temperature, using the Ideal gas law, as follows:
P = ρRT
Where:
P is the absolute pressure of the gas, in Pa
ρ is the density of the gas, in kg/m3
R is the gas constant, in Joules/kg.K
T is the absolute temperature of the gas, in Kelvins (K)
Now,
Normal atmospheric pressure is approximately 101,300 Pa
The gas constant for dry air is 287 Joules/kg.K
The air inside the envelope is typically heated to an average temperature of about 100 degrees Celsius, which is 373 K
Substituting the above three values into the Ideal gas law equation and solving for ρ we get ρ = 0.946 kg/m3. This is the density of the heated air inside the envelope. Compare this to normal (ambient) air density which is approximately 1.2 kg/m3.

http://www.real-world-physics-problems.com/hot-air-balloon-physics.html

Density of helium = 0.164 kg/m3 which is less than 0.946 kg/m3

http://physics.info/density/

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#3
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Re: Helium v hot air lift

02/20/2017 7:27 PM

If the air in a hot-air balloon is heated too much, it will melt (or at least weaken) the fabric of the balloon. This limits the reduction of air density to around 75-80% of ambient air density.

Helium has an atomic weight of ~2, compared to the average molecular weight of air of 29; hence is about 7% as dense as air. Thus much better lift for the same volume of helium.

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#4
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Re: Helium v hot air lift

02/20/2017 8:37 PM

They have similar lifting force.....same volume = same lifting force

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#6
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Re: Helium v hot air lift

02/20/2017 8:58 PM

....of course the air would have to be heated to a point where the density was equal to the helium........which is probably not practical...

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#10
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Re: Helium v hot air lift

02/21/2017 9:41 AM

No, ordinary helium He has atomic (and molecular, as it's monotomic) weight ∼4. Hydrogen H2 has molecular weight ∼2. Deuterium D2 has molecular weight ∼4.

This was discussed in a post a year or so back.

Also worth pointing out to the OP that it's the difference in density between the stuff in the balloon and the outside air that determines the lift.

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#17
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Re: Helium v hot air lift

02/21/2017 11:51 PM

Good catch. I must have spaced out and thought atomic number rather than weight.

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#21
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Re: Helium v hot air lift

02/22/2017 9:14 AM

No problem

Previous discussion was called helium-3 for blimps, started 10 June 16, in case link below doesn't work.

http://cr4.globalspec.com/comment/1170734

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#2

Re: Helium v hot air lift

02/20/2017 7:22 PM

Hydrogen has rather greater lift than helium and air, but dangerous.

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#5
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Re: Helium v hot air lift

02/20/2017 8:42 PM

Helium weighs twice as much as hydrogen, but both are much lighter than air. Since the buoyancy is due to the difference in density between the gas and air, the lifting power of hydrogen is only about 8 percent greater than helium.

https://en.wikipedia.org/wiki/Lifting_gas

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#7

Re: Helium vs Hot Air Lift

02/20/2017 9:05 PM

now if you were to put helium balloons inside of your hot air balloon they would float, expand and then burst. ..yes in that order.

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#8

Re: Helium vs Hot Air Lift

02/20/2017 9:31 PM

Helium is more expensive than hot air, but hot air balloons require constant energy input.

They both provide similar lift, when used as designed.

You are blessed to have such a son.....congrats.

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#9

Re: Helium vs Hot Air Lift

02/21/2017 6:04 AM

"I understand that helium is lighter than air but is that enough to generate more lift?"

Yes.

Floating things in air is exactly like floating things in water. The lift provided by displaced water is equal to the weight of the displaced water (remember Archimedes?).

Imagine that you have a 10 cm cube (1 liter) of stuff anchored to the bottom of a fish tank with a force sensor in the anchor. 1 liter of water weighs 1 kilogram so the displaced water gives 1 kilogram force of lift. Using numbers in the same ratio as Rixter's calculations above the: the hot air balloon is equivalent to a cube of weight 0.788 Kg, so the force sensor measures 0.212 Kg force; the helium balloon is equivalent to a cube of weight 0.137 Kg, so the force sensor measures 0.863 Kg force.

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#11
In reply to #9

Re: Helium vs Hot Air Lift

02/21/2017 9:56 AM

NO! NO! NO! kg is not a force....even if you put the word force after it!!!!!!

Sorry to nitpick but you struck a nerve with me. A liter of water does not weigh 1 kilogram. It may have 1 kg of mass, but it's weight is not measured in kg, but in a force unit such as Newtons.

As engineers and scientists we are supposed to be rigorous with our terms and units. One of the selling points of the metric systems is that the English system of units was messy and that dealing with mass and force having the same unit designation (pound) was confusing, as one had to you the gravitational conversion factor, gc, when analyzing problems with the English system of units (actually, it's needed for SI units too, but it's value is 1.0 so if it's forgotten then the results are not the wrong value unlike if it's forgotten in the English system). Unfortunately, that selling point no longer has the same weight (pardon the pun) as more and more people in the engineering community seem to be adopting 'kg force' to express the weight of a kg.

My dynamics book (yes, i still have it after 30+ years) states "The kilogram is to be used only as a unit of mass and never as a unit of force".

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#12
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Re: Helium vs Hot Air Lift

02/21/2017 12:34 PM
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#13
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Re: Helium vs Hot Air Lift

02/21/2017 2:33 PM

Yeah, I know. It's not that I don't know what it represents. It just irritates me personally.

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#14
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Re: Helium vs Hot Air Lift

02/21/2017 2:51 PM

And if I'd introduced Newtons into the explanation: would that have helped the OP more or less?

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#15
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Re: Helium vs Hot Air Lift

02/21/2017 3:11 PM

Since I'm not the OP, I can't answer that. If you had introduced Newtons the explanation would have been in line with SI units and I would not have been irked.

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#16
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Re: Helium vs Hot Air Lift

02/21/2017 11:36 PM

And my bug bear is that the kilo prefix has a small k!

;-)

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#19
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Re: Helium vs Hot Air Lift

02/22/2017 6:12 AM

Thanks for that. That was something I didn't get wrong deliberately.

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#18

Re: Helium vs Hot Air Lift

02/21/2017 11:53 PM

All of the math, formulas and other information presented here is great, if your son wants to work out theoretical's on paper,,

But you could buy him or help him construct physical working models ( small scale ) of balloons, then you and him as a father and son team, could actually experiment with helium and hot air :)

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#20

Re: Helium vs Hot Air Lift

02/22/2017 9:05 AM

Has anyone ever tried a hot helium balloon?

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#22
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Re: Helium vs Hot Air Lift

02/22/2017 1:02 PM

The hot air balloon has an opening through which the heat source directs its heat. The helium balloon is sealed to prevent the loss of the very expensive helium. If you want to heat the helium balloon envelope, that's up to you, but I would not recommend a propane burner. An internal heater would have to bring its own oxygen or run on electricity.

That should be sufficient in reasons for not heating a helium balloon.

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