Hyett6420

Can aeroplanes be like ships?

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13 minutes ago, Hyett6420 said:

So in theory you could take a lightweight bag "big enough" to displace the correct weight of air at the surface of our atmosphere, (whatever that is) and use helium to inflate struts in it, like these new fangled blow up tent things.  The inflated poles would create a structure to the light weight bag.  So if the bag weighed 2000g plus inflated poles of 200g you would need to make it big enough to create enough space to have internally a vacuum of greate than 2.2kg in weight?

Just ignore @1964-M20E, we can continue to talk in geek :D

That would be (almost) true if you could make a bag that could withstand external pressure.  Remember, though the helium in the inflated structure would produce buoyancy itself as well.  The structure would be neutrally buoyant at some fixed altitude.  I suppose you could then add or subtract air from the vacuum chamber to alter your altitude.

Realistically, of course, there's no way you could make a pressure vessel that light.

@N201MKTurbo makes a good point--elastic balloons are not neutrally buoyant because they are able to expand, so you could make an airship with balloons that is slightly negatively buoyant, and it will stay that way more or less.  You'd just use small motors or lift devices to move the ship up and down.

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On 9/25/2018 at 7:03 PM, jaylw314 said:

Just ignore @1964-M20E, we can continue to talk in geek :D

That would be (almost) true if you could make a bag that could withstand external pressure.  Remember, though the helium in the inflated structure would produce buoyancy itself as well.  The structure would be neutrally buoyant at some fixed altitude.  I suppose you could then add or subtract air from the vacuum chamber to alter your altitude.

Realistically, of course, there's no way you could make a pressure vessel that light.

@N201MKTurbo makes a good point--elastic balloons are not neutrally buoyant because they are able to expand, so you could make an airship with balloons that is slightly negatively buoyant, and it will stay that way more or less.  You'd just use small motors or lift devices to move the ship up and down.

Sorry ive just come back to this.  So steel is not light, so im not sure you would need to build it light (except to get it up there lf course), but if you built a steel hulled EXTREMELY tall and wide ship in theory it should "float" on the top of the upper atmosphere.  

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1 hour ago, Hyett6420 said:

Sorry ive just come back to this.  So steel is not light, so im not sure you would need to build it light (except to get it up there lf course), but if you built a steel hulled EXTREMELY tall and wide ship in theory it should "float" on the top of the upper atmosphere.  

i'm...not sure about that.  If you had a ship-like "hull", you'd still have to enclose the top, since (unlike water), there's no actual discrete "surface".  If there were openings in the "deck," air would eventually flow in over the sides into the hull and it would sink.  If you made the hull airtight, there would also be the exact same pressure problem, with vacuum inside the hull and air outside.

Of course, once you seal the deck to keep air out, you've just gone and made another pressure vessel, so the shape becomes irrelevant anyway.  You might as well make it spherical or cylindrical to save weight and maximize strength (basically a submarine).

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Why using vacuum (space) on the inside does not work unless you have a very strong structure:

 

 

 

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1 hour ago, jaylw314 said:

i'm...not sure about that.  If you had a ship-like "hull", you'd still have to enclose the top, since (unlike water), there's no actual discrete "surface".  If there were openings in the "deck," air would eventually flow in over the sides into the hull and it would sink.  If you made the hull airtight, there would also be the exact same pressure problem, with vacuum inside the hull and air outside.

Of course, once you seal the deck to keep air out, you've just gone and made another pressure vessel, so the shape becomes irrelevant anyway.  You might as well make it spherical or cylindrical to save weight and maximize strength (basically a submarine).

But if the freeboard is high enough the air could not flow in. Same principle as wooden dingies or even steel yachts. 

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55 minutes ago, TTaylor said:

Why using vacuum (space) on the inside does not work unless you have a very strong structure:

 

 

 

Youve missed the point about the hull strength being sufficient to reflect that pressure. Same as any boat

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4 minutes ago, Hyett6420 said:

But if the freeboard is high enough the air could not flow in. Same principle as wooden dingies or even steel yachts. 

I suppooooooose....  If it was 500 miles tall, it would take a long time for air to flow in, but there is still a little gas at those heights that would slowly fill it.  A ship's freeboard keeps water out because the waterline is below its height.  If there's even a little water above it, it'll eventually fill up and sink without a bilge pump

7 minutes ago, Hyett6420 said:

Youve missed the point about the hull strength being sufficient to reflect that pressure. Same as any boat

A ships hull only needs to withstand pressure that is (on average) the displacement divided by the total surface area.  By my back-of-the-napkin calculations, a Nimitz-class aircraft carrier (134x1000 feet at the waterline displacing 100,000 tons) only has to withstand an external hull pressure of about 6-7 psi, about half that of vacuum pressure.  Interestingly, a Los Angeles-class submarine only has a hull pressure of about 3 psi when barely submerged, although that obviously starts going way up with depth...

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On 9/25/2018 at 6:17 AM, Yetti said:

So other than the guy with the lawn chair and a bunch of balloons.  Are there any experimental 1 or two person airships?   Thinking several weather balloons inside a form with a rotax snowmobile engine. 

You mean, like this:

https://www.google.ca/amp/s/globalnews.ca/news/4088865/calgary-balloon-man-new-mexico-stunt/amp/

 

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Airplanes are like ships of the sky, but better. Airplanes are faster than ships. Airplanes are more maneuverable than ships. And unlike our sea-borne brethren, we haven't left one up there yet . . . . .

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Makes heating a bag of hydrogen sound like a perfect air flotation device...

Doing a WnB calculation would determine the actual density of the collective machine...

The flight engineer would calculate the density of the air at the desired flight altitude.... and release stored ballast water to effect the desired altitude... essentially adjusting the ship’s average density...

Ever read about the graf zeppelins? 

Or see the video?

The burning blimp shows water ballast pouring out of it everywhere... as it crumbles...

Sure the hull can be turned upside down an be open at the bottom... the hydrogen will stay in place like a hot air balloon...

Keeping the H2 in place in an open hull  through all the bumps would be a challenge...

 

Go science...

 

Andrew,

a bag of fresh water will float in the ocean...

A bag of saltwater will sink in a lake

The salt mostly alters the density without changing the volume very much. The salt when dissolved fills the interstitial spaces...between molecules...

the fun part is knowing air is a density column, always changing from most dense at the bottom to least dense at the top... the altitude of the top changes in giant waves...

best regards,

-a-

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11 hours ago, jaylw314 said:

I suppooooooose....  If it was 500 miles tall, it would take a long time for air to flow in, but there is still a little gas at those heights that would slowly fill it.  A ship's freeboard keeps water out because the waterline is below its height.  If there's even a little water above it, it'll eventually fill up and sink without a bilge pump

A ships hull only needs to withstand pressure that is (on average) the displacement divided by the total surface area.  By my back-of-the-napkin calculations, a Nimitz-class aircraft carrier (134x1000 feet at the waterline displacing 100,000 tons) only has to withstand an external hull pressure of about 6-7 psi, about half that of vacuum pressure.  Interestingly, a Los Angeles-class submarine only has a hull pressure of about 3 psi when barely submerged, although that obviously starts going way up with depth...

We should post this thread to Peter Garrison of Flying magazine. He would have fun with it. 

I agree with freeboard bit a bit like salt spray filling a dinghy.  With regards to PSI on a vacuum..... the psi of air on the surface is 45 if i recall correctly, but at the top of the atmosphere surely it would not be the same, exactly like the psi on the submarine at surface versus 6 fathoms down.  So the hull would not need to withstand  that much pressure on the surface of the atmosphere because the pressure exerted on the hull would be so much less. I do agree whoever that the hull would need to eother be very tall or very broad.  

 

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6 hours ago, Hyett6420 said:

We should post this thread to Peter Garrison of Flying magazine. He would have fun with it. 

I agree with freeboard bit a bit like salt spray filling a dinghy.  With regards to PSI on a vacuum..... the psi of air on the surface is 45 if i recall correctly, but at the top of the atmosphere surely it would not be the same, exactly like the psi on the submarine at surface versus 6 fathoms down.  So the hull would not need to withstand  that much pressure on the surface of the atmosphere because the pressure exerted on the hull would be so much less. I do agree whoever that the hull would need to eother be very tall or very broad.  

Air pressure is 14.7 psi on the surface. Since you're used to QNH, isn't it 29.92" mercury = 1014 millibars?

Ships float on top of the water, because the volume of the ship underwater displaces water equal to the weight of the whole ship. Can a ship be built that will similarly float on top of earth's atmosphere? Maybe, but it sure won't be an airplane. In that sense, airplanes are more like submarines, since we never (indeed cannot!) operate at the top of the atmosphere.

But I sure do enjoy operating my atmospheric submersible, even if it is always submersed . . . .  :)

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When there are drastic density differences of the medium, things will float at the interface...

Hence the boat floating on the water...

When the medium changes density continuously, like a collumn of compressible air does... the floating ship... will find the altitude that has a density that matches the average density of the ship...

Pretty much how a balloon rises to a certain altitude and drifts around until it’s density changes. Helium balloons leak and  descend. Hot air balloons run out of heat, cool off, they descend as they cool...  not running out of heat is important to maintain control...

 

Getting a ship to float on top of a sea of air. That is an incredibly light weighted ship...

That mythical surface may be called the Karman line... an altitude of 100Km.

Some yahoo students are looking to explore reaching that altitude... with a rocket...

https://castlepointrocketry.space/

 

Senior project at Stevens Inst. of Tech.

They are looking for your support if interested...

Best regards,

-a-

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Fun Fact: Did you guys know, there are more airplanes in the ocean than ships in the sky.

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