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"Gravity Powered" Airplane - Will This Work?

04/11/2006 12:25 PM

There's an article over on Damn Interesting about a physicist named Robert Hunt who's proposing a "gravity powered" airplane. He's formed a company, Hunt Aviation to pursue it.

The basic concept is that the plane has two nacelles containing helium bladders. The bladders can be inflated or deflated at will. The plane starts on the ground and the bladders are inflated, making it bouyant. It rises to the desired altitude and then the bladders are deflated and the plane glides to a lower altitude on its wings. Once at lower altitude, the helium bladders are re-inflated and the process repeats.

I see a couple of problems with this idea. The first is that, in order to carry any appreciable amount of payload, the helium bladders and their associated nacelles would have to be huge. The wings and associated structure of this vehicle make for a lot of added weight that conventional lighter-than-air ships do not have, and those ships already need an immense volume of helium to carry any appreciable payload. It would be interesting to run the math.

The second problem I forsee is that the constantly changing altitude would cause pressurization problems. The pressurization of the cabin space of the vehicle would need to be changing constantly, and would need to be designed to withstand a large number of pressurization cycles.

Am I missing something?

(Hunt also has a design for a vertical axis wind turbine on the Hunt Aviation web site. It uses "blades" that expand when they're positioned to provide power and collapse when they're not. Interesting but very complex mechanically, which is not a recipe for long-term reliability).

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The Feature Creep

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#1

Perpetual Motion?

04/11/2006 3:31 PM

Looks like he is trying to create a flying perpetual motion machine? Who is the pilot, Maxwell's demon?

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Anonymous Poster
#2

You've got to be kidding me....

04/11/2006 6:51 PM

Ok, I must be missing something. You have an airplane with a fixed amount of helium that will be moved from an (assumed) pressurized container to a bladder and back in order to change altitude. The premise here I assume is that the helium in the bladder will make the net weight of the plane less, and helium in the pressurized tank will make net weight more. Helllloooo...the effect of the helium is going to be the same regardless of where it is located. If someone created a company to research this idea, then some engineer somewhere needs to have his noggin whacked for not telling this entrepreneur he's chasing a helium-filled goose!

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#3
In reply to #2

Re:You've got to be kidding me....

04/12/2006 6:24 AM

It's not simply the weight of the Helium that counts; the lift in an aerostat comes from the weight of air displaced by the Helium, so in theory this could work. My problem is with the considerable amount of energy required to compress (or to be more realistic, liquefy, the helium). It takes more than 1Mwh to produce 200 litres of liquid Helium a day from the gas at room temperature. You're not going to get this kind of power from a trailing windmill... The idea is not "Perpetual Motion". I've studied thermodynamics long enough to realise that such a thing is impossible - there is no such thing as a free lunch in Physics. Even if you could build a frictionless machine that would spin forever, it would grind to a halt as soon as you hooked up the dynamo... Heat Pumps are often seen as "Something for Nothing" machines, even by engineers who should know that a block of ice is not cold, but 273 K degrees hot. This attitude has hampered development of a very promising technology. I would welcome a thread on heat pumping...

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#5
In reply to #3

Re:You've got to be kidding me....

04/12/2006 9:26 AM

The first thought that came to my mind was, "There ain't no such thing as a free lunch," but I like the way you said it better. The guy's not claiming to do such a thing for free, but I doubt seriously that it will work. For one thing, helium bags need to be able to expand at high altitudes so that increasing displacement of rarified air still provides lift to a terminal altitude. Dirigibles with rigid air frames simply carry such a large volume of helium (or hydrogen in the case of Germany's 1930's versions) that they could carry a decent payload, but not at ten mile altitudes. The ludicrous idea that wind turbines can be used to pump gaseous helium back into high pressure tanks would make the glider into a brick, if they produced enough power to do so. I could go on and on, but I should do some calculations first. Wouldn't want to say something bad about the idea that turns out to be wr-wr-wr- . . . not right, now would I?

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#8
In reply to #3

Re:You've got to be kidding me....

04/12/2006 10:51 AM

OK, It seems you are right here about the quantity of helium required to make this work. Using the known facts about airships, it seems that the volume required to get to any appreciable altitude would be overwhelming. Just a reminder: air density decreases with altitude. A balloon pilot friend of mine told me that at sea level, roughly 5,000 liters of Helium is required for only 5.2 kg of lift. That is a LOT of gas for not much lift. Stephan

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#4

helium lifted plane

04/12/2006 8:17 AM

I'm with this to the point of re-inflating with helium. If the helium wasn't already on board, where is it coming from???

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Power-User

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#6

Hunt Plane

04/12/2006 10:10 AM

Years ago Popular Mechanics or Science had an article on a submersable propelled by this technique (using compressed air) -- a much more workable concept.

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#7

I think the principal is sound

04/12/2006 10:48 AM

To clarify for some of the posters above:

This guy is not dumping helium overboard when he deflates the bladders. He's pressurizing it into tanks and replacing its volume with air - which decreases bouyancy. Nothing wrong there. Then when he gets to lower altitude, he re-inflates the bladders from the pressurized tanks to increase bouyancy. The only power he needs to provide is enough to run a compressor whose power rating will be determined by how fast he wants to re-compress the helium.

The principal works (I think). My issues were with the payload and the constant pressure changes.

Bill correctly points out above that, since the bladders can't expand much at high altitude (they're in a rigid frame) the payload capacity is even further reduced from what it might have been.

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#9

Helium Storage

04/12/2006 11:24 AM

At least two fairly significant advancements must be made to make this viable:

First, you need a much more efficient way of compressing gases. Compressing gas is one of the most inefficient and costly things in industry today. This is further complicated by using wind turbines to generate the power.

Second, you need a way lighter storage mechanism for these compressed gases. Current pressure vessels are heavy, even those utilizing composite construction. The hydrogen economy faces a similiar problem, which may be solved using carbon nanotubes.

I'm sure there are more, but these are at least two issues that need to be addressed before this thing ever leaves the ground.

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Anonymous Poster
#10
In reply to #9

Re:Helium Storage

04/13/2006 8:11 AM

I think the problem of compression can be solved by using the atmospheric temperature. The higher you go up in the troposphere (usually) the colder the temperature gets. Using the volume decreasing powers of temperature it would be easier to pressurise it in tanks. The bpt of helium i guess is around -200 if the atmospheric temp is -90 its 'only' another 110 degrees left. Using this technique would not solve the issue completely but it will surely helps!!

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Commentator

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#11

Re: "Gravity Powered" Airplane - Will This Work?

08/20/2009 5:09 PM

There are gravity powered aircraft amongst us today they are called sailplanes just had to say it . It does seem a good idea though

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