Hyett6420

Can aeroplanes be like ships?

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This is a serious question for all you academics out there...

A ship is made of steel (normally) and this is heavier than water and if you take a steel plate and put it in the water it will sink.  However ships float by displacing the water around them  and created an air "pocket" to achieve buoyancy.  Niw take the same scenario but buld a "plane boat".  Ie is it possible within the laws of physics to build a boat that displaces the air and creates a "space pocket", (space being lighter than air) to achieve sufficient buoyancy to float on the surface of the atmosphere?  Granted it might have to be somewhat large?

dont get confused with helium balloons because these are ineffect the same as putting a globule of fresh water inside a "balloon" and releasing it inside the sea (or is it se water inside fresh, i cant recall), ie one is less dense than the other but still the same substance, ie water

Everything in my brain says yes in theory this should work, but i would be intrigued by kther views from our academics amongst us.

Andrew

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the air in the envelope of a hot air balloon is still air, but because of heat it becomes so less dense that it floats upon the 

cool more dense air that surrounds it.

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The Helium balloon, the hot air balloon, and the "space pocket" are all the same thing.  It's just a matter of what is in it.  Buoyancy is when the item weighs less than the material it displaces it will float.  I.e. when the weight of helium + the weight of the actual ballon = less than the weight of the air it displaces then it floats.  Your analogy of fresh and salt water works the same way.  1 liter or salt water weighs more than 1 liter of fresh water so placing a balloon of fresh water in salt water will float.  Remember density = mass/volume.  Mass is the measure of the amount of matter (suff) that something is made of.  Mass is the same at sea level and at the top of Mt. Everest.  However weight is a function of mass and gravity.  While mass may be the same at sea level and at the top of Mt. Everest, the weight will be measurably different.  Though they are not exactly the same you can relate mass to weight so to keep it simple for this discussion I called it weight. So the simple answer to your question is yes and they call them blimps or derrigibles or zeppelins or airships.  But to truely answer your question, yes,  if you could build a structure with a vacuum(space) in it where the structure sufficent to hold a vacuum of that size didn't weight more than the air it displaces it would float.

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A couple of people already answered the physical aspects of the question so it saves me a bunch of thumb typing; however, there is actually some logic as to why the referenced class of “lighter than air” vehicles are called Air Ships. 

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Moreover, in order to have a vacuum bag (the buoyancy generator, or "space pocket") the bag itself would have to be rigid, in order to withstand atmospheric pressure.  That adds weight, quite a bit of it. Gas bags need not be terribly rigid since they are already at atmospheric pressure, and they can be built light.

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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. 

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Floatation is all matter of traveling medium density. To float the vehicle overall density needs to be lighter than the medium in which it travels. This why cars do not sink onto the road or boats into the water.

José

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1 hour ago, 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. 

Something that light, you probably don't need anything as big as a snowmobile engine.  You could probably get away with a weedeater engine.  There are some really powerful RC 2 stroke engines out there too now.  Flew with a guy who built an extra 300 powered by a twin cylinder 110ccc engine that swung a 25" prop if I remember right. 

That guy was a lot of fun becuase he didn't like the fixed spark advance on the engine so we modified it and built his own spark advance mechanism for the off the shelf engine he bought.  Then he decided his minivan burned too much gas to haul the plane so he bought a smart car and built a trailer for the plane.   All of this an the guy was in his mid 70s.   Really neat guy to hang out with. 

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Careful now. Density has nothing to do with a car not sinking into a road. Fluid Mechanics and Mechanics of Materials do not overlap in this (or other) related  areas

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I suppose that if you could build an envelope that could withstand a full vacuum, have a larger displacement than it's weight and be ultra light it would float in the air.  However, I do not think we have the materials for this yet.

Imagine a soda can in a tub of water and the can is filled with water.  The can will sit at the bottom of the tub.  Start removing the water form the can, i.e a vacuum with respect to the density of water, and the can will begin to float.  However, if you take the now empty and floating can deep enough the pressure of the water will crush it like a can.

I have seen substances on some type of Discovery channel that were so light and airy that they seemed to float.  However, I can't recall what it was.

Now I propose (unscientifically of course) that we are like ships or at least like wake boards or water skis.  Take the case as we accelerate our beloved air ships to 70IAS+ the relative density of the air becomes greater around the plane therefore allowing our planes to float up from the ground and do what we call flying.  Just a different way of looking at how we fly.

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7 hours ago, Hyett6420 said:

This is a serious question for all you academics out there...

A ship is made of steel (normally) and this is heavier than water and if you take a steel plate and put it in the water it will sink.  However ships float by displacing the water around them  and created an air "pocket" to achieve buoyancy.  Niw take the same scenario but buld a "plane boat".  Ie is it possible within the laws of physics to build a boat that displaces the air and creates a "space pocket", (space being lighter than air) to achieve sufficient buoyancy to float on the surface of the atmosphere?  Granted it might have to be somewhat large?

dont get confused with helium balloons because these are ineffect the same as putting a globule of fresh water inside a "balloon" and releasing it inside the sea (or is it se water inside fresh, i cant recall), ie one is less dense than the other but still the same substance, ie water

Everything in my brain says yes in theory this should work, but i would be intrigued by kther views from our academics amongst us.

Andrew

A ship puts - using your word - a "globule" of air into the denser space normally occupied by water.  The idea is to make the entire unit - displaced water plus weight of the device that displaces the water - that's the ship hull, are on average lighter than the water displaced.

So when a balloon puts a globule of lighter than air stuff - helium or hot air - in the middle of where normally thick air goes...

An alternative would be even a globule of vacuum if you could make a pressure hull light enough that it is not so heavy that despite displacing the air it on average is still heavier than the air it displaced.

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You were preceded by a couple of hundred years:

image.thumb.png.aff532522167686b1118bf7cc8052667.png

I am not sure if this is a picture of the one I read about, but it is similar enough.  The guy's idea was to use evacuated copper balloons.  He even calculated the necessary diameter of the balloon and the maximum thickness of the copper to keep the total weight within bounds.  Needless to say, the thin copper would NOT have held and would be crushed as they were evacuated, but air pressure was little understood in those days...

Edited by Ah-1 Cobra Pilot
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3 hours ago, 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. 

Solar-Ship-YouTube-0814a.JPG

whitedwarf-pedal-blimp.jpg

rousson2.jpg

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9 hours ago, Hyett6420 said:

This is a serious question for all you academics out there...

A ship is made of steel (normally) and this is heavier than water and if you take a steel plate and put it in the water it will sink.  However ships float by displacing the water around them  and created an air "pocket" to achieve buoyancy.  Niw take the same scenario but buld a "plane boat".  Ie is it possible within the laws of physics to build a boat that displaces the air and creates a "space pocket", (space being lighter than air) to achieve sufficient buoyancy to float on the surface of the atmosphere?  Granted it might have to be somewhat large?

dont get confused with helium balloons because these are ineffect the same as putting a globule of fresh water inside a "balloon" and releasing it inside the sea (or is it se water inside fresh, i cant recall), ie one is less dense than the other but still the same substance, ie water

Everything in my brain says yes in theory this should work, but i would be intrigued by kther views from our academics amongst us.

Andrew

Even if a material was available that was strong enough to contain a vacuum while being light enough to displace a sufficient weight of air to float, it would only float at one elevation.

 

PP thoughts only, not a mathematician, physicist, mechanic, metallurgist, professor, etc. :)

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9 hours ago, Hyett6420 said:

dont get confused with helium balloons

A helium balloon is much more representative of what a boat does than the vacuum filled vessel you are describing.  Inside the steel boat is air, with a lower density than the water the boat displaces.  Inside the balloon is helium, with a lower density than the air the balloon displaces.

For a fun experiment on the force constantly exerted by the air around us, try this.  Put a small amount of water in an aluminum can and some more water in a pot on the stove.  Heat the aluminum can until the water boils, displacing the air in the can with water vapor.  Then, using some tongs, flip the can upside down into the water nearby.  The water vapor in the can will condense into liquid water, leaving behind a vacuum.  At the same time, the atmospheric pressure exerted on the can will crush it instantly and spectacularly.

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20 minutes ago, N6018Q said:

Even if a material was available that was strong enough to contain a vacuum while being light enough to displace a sufficient weight of air to float, it would only float at one elevation.

Neutral buoyancy is true for any object floating in air or under water. Air ships (not balloons) are typically not lighter than air when airbourne. They are typically configured with ballast (modern = water) and remain heavier than air (the more wind or unsettled air will require them to fly heavier than vice versa) . Vectored thrust is utilized for take-off. They fly with a angle of attack to create aerodynamic lift which is balanced with aero-surfaces (including trim), weight management and thrust vectoring to maintain desired altitude; more modern ones utilize better aero shapes than football shaped blimps (that still produce lift) everyone is familiar with. More than anyone cared to know about but here's the thought stream. 

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10 hours ago, Hyett6420 said:

A ship is made of steel (normally) and this is heavier than water and if you take a steel plate and put it in the water it will sink.  However ships float by displacing the water around them  and created an air "pocket" to achieve buoyancy.  Niw take the same scenario but buld a "plane boat".  Ie is it possible within the laws of physics to build a boat that displaces the air and creates a "space pocket", (space being lighter than air) to achieve sufficient buoyancy to float on the surface of the atmosphere?  Granted it might have to be somewhat large?

dont get confused with helium balloons because these are ineffect the same as putting a globule of fresh water inside a "balloon" and releasing it inside the sea (or is it se water inside fresh, i cant recall), ie one is less dense than the other but still the same substance, ie water

As others have mentioned, a true "vacuum" space large enough to produce buoyancy needs to be strong enough to withstand 15 psi of external pressure.  While we have many things that can withstand high internal pressure, external pressure is not as well tolerated--external pressure tends to focus on any areas of a pressure vessel that deviate from a perfect spherical shape and deform the area further, which further weakens it, leading to catastrophic failure.  Internal pressure, on the other hand, tends to "smooth" out deviations and tends to strengthen it.  That's why a railroad tank car is rated for a max internal pressure of 100 psi, but you can collapse it with less than negative 15 psi (see the "Mystbusters" for the video).

That's really the only purpose of putting a light gas in balloon--it allows you to put pressure in a container to offset atmospheric pressure without adding the same weight as air.

To put it in perspective, here's the largest vacuum chamber ever built, and it sure looks like there's a lot of concrete and steel involved, so I doubt you could get it to float!

There's a Wikipedia page on vacuum airships that includes some of the (hypothetical) calculations

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56 minutes ago, N6018Q said:

Even if a material was available that was strong enough to contain a vacuum while being light enough to displace a sufficient weight of air to float, it would only float at one elevation.

 

PP thoughts only, not a mathematician, physicist, mechanic, metallurgist, professor, etc. :)

There are many ways of controlling the buoyancy of an air ship. Blimps use ballonets to control buoyancy. The same thing could be used in a vacuum envelope except you would beed a pump to empty them instead of filling them.

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5 minutes ago, N201MKTurbo said:

There are many ways of controlling the buoyancy of an air ship. Blimps use ballonets to control buoyancy. The same thing could be used in a vacuum envelope except you would beed a pump to empty them instead of filling them.

I'd also point out that the buoyancy of a vacuum is not particularly better than the buoyancy of helium.  Buoyancy is the weight of air displaced minus the weight of the displacing substance, so if you take 16 grams of air (about one mole), the equivalent volume of helium would be 4 g, so the buoyancy would be 12 g.  The buoyancy of a pure vacuum would be 16 g, or about a 25% difference.  It's even less if you use hydrogen, which would have a buoyancy of 14 g, so only a 12% difference. 

The difference is certainly not enough to make up for the weight of any pressure vessel, when hydrogen or helium only requires a lightweight bag

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1 hour ago, Freemasm said:

Neutral buoyancy is true for any object floating in air or under water. Air ships (not balloons) are typically not lighter than air when airbourne. They are typically configured with ballast (modern = water) and remain heavier than air (the more wind or unsettled air will require them to fly heavier than vice versa) . Vectored thrust is utilized for take-off. They fly with a angle of attack to create aerodynamic lift which is balanced with aero-surfaces (including trim), weight management and thrust vectoring to maintain desired altitude; more modern ones utilize better aero shapes than football shaped blimps (that still produce lift) everyone is familiar with. More than anyone cared to know about but here's the thought stream. 

So in effect a submarine in the air.  

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8 minutes ago, jaylw314 said:

I'd also point out that the buoyancy of a vacuum is not particularly better than the buoyancy of helium.  Buoyancy is the weight of air displaced minus the weight of the displacing substance, so if you take 16 grams of air (about one mole), the equivalent volume of helium would be 4 g, so the buoyancy would be 12 g.  The buoyancy of a pure vacuum would be 16 g, or about a 25% difference.  It's even less if you use hydrogen, which would have a buoyancy of 14 g, so only a 12% difference. 

The difference is certainly not enough to make up for the weight of any pressure vessel, when hydrogen or helium only requires a lightweight bag

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?

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8 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?

Yeah mate what ever you said.:huh:   I think you need to go fly your Mooney you have too much time on your hands.  :D:D

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