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Gravity Plane

It is quite likely that it will float quite far in the other direction, before it can glide forwards.:D
 
Hunter estimates a gravityplane that can carry the same payload as a Boeing 747 would be roughly 50% larger than the current 747.
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(my italics)

I would have expected a plane designer to be calculating his design parameters, rather than estimating them.
Particularly if he wants the likes of me to get on board it.
 
The tanks required to displace enough air to cause the thing to float are enormous. Assuming that the thing weighs about 1/2 a 747 (200 tons empty) and that perfect vacuum is achieved then you'd need around 80 000 cubic metres of storage.

Or put another way two sausages 10m in diameter and 1 kilometer long would be needed at sea level, even more at altitude. I think the gentleman's having a laugh. It also looks like he hasn't bothered to do spme basic buoyancy calculations. Aero engineer my big fat ass.

This design is complete garbage, and it doesn't even consider the enormous amounts of energy required to generate the vacuum
 
Looks like an unfortunate mating attempt between two zepplins and a stealth fighter.

He doesn't explain properly how he'd contain a vacuum of that 'volume' (or should that be displacement?). You'd need some pretty hefty infrastructure to get anything that low pressure without it collapsing.

That isn't addressed at all in the discussion about the 'challenge' of making the pontoons.
 
To help test and refine his designs, Hunter plans on building a scaled-down, three-man submarine version of his gravityplane over the next five months.
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As anyone will tell you, building a submarine is an important part of testing any new aircraft design.
 
I, for one, have no desire to ride in a plane that was made to be a submarine, nor a submarine that was supposed to be a plane.
 
OK, that article is old hat. Now for dismantling :D:

1) As already mentioned the buoyancy sections must be dimensioned MUCH larger than depicted. You get about 1.3 kilograms of lift per cubic meter helium at sea level, but og course you also need to lift the tanks.

2) Making vacuum tanks with positive lift is impossible with known technology.

3) Making wind generators with 4 times better efficiency than present designs is probably impossible with current technology (otherwise why does he not sell the idea to the wind generator industry).

4) Even if all snags could be ironed out, the craft needs a power source. Making vacuum and compressing helium requires exactly the same amount of energy as can be gained from the changes in lift obtained, plus losses. This is because buoyancy is really about pressure. A body that is lighter than its surrounding medium has lift because the pressure difference between its lower and upper survafaces exceeds its weight. To alter that by changing the pressure inside (part of) it, you have to excert a corresponding pressure on the medum in it, and presto, you loose what you gain.

Hans
 
He'd have gotten away with it too...........if it wasn't for those pesky laws of physics
 
MRC_Hans said:
3) Making wind generators with 4 times better efficiency than present designs is probably impossible with current technology (otherwise why does he not sell the idea to the wind generator industry).
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Totally OT, but I saw a fascinating attempt in Peter's port harbour on Guernsey a few weeks ago ... a full size catamaran with the mast removed and replaced with a danish style wind turbine. Presumably it was driven by some sort of electric motors, unless the whole get up was mechanical.

I was trying to work out whether the power generation would be sufficient to get any sort of speed, but it gave me headache so I went for more beer instead. I suppose a boat like that would need about 150 hp to move at a leisurely 12-15 knots, that would be about 11kW. I don't know how much those turbines generate ... 3 blades, about 15 m diameter.

I would have taken a photo, but I think everyone would accuse me of having photoshopped it together. I'd imagine it would be one hoopy sight coming towards you in the channel.

Edited to add: Ah well here it is
 
To move a sailboat at moderate speed does not require 150hp. I used to have a 26ft yacht weighing in at two tons. It could run 6 knots with a 6hp outboard.

I don't think a wind turbine to propeller arrangement (whether electric or direct drive) is nearly as efficient as a sail, but it should be able to move a boat. An advantage would be the ability to sail directly into the wind.

Hans
 
MRC_Hans said:
To move a sailboat at moderate speed does not require 150hp. I used to have a 26ft yacht weighing in at two tons. It could run 6 knots with a 6hp outboard.

I don't think a wind turbine to propeller arrangement (whether electric or direct drive) is nearly as efficient as a sail, but it should be able to move a boat. An advantage would be the ability to sail directly into the wind.

Hans
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I'm a bit ropey at that, I'm used to steel and wooden things without sails, not grp hulls. This looked more like 40ft twin hulled, but I suppose it would still lightly skip through the water like a yacht.

Last boat I piloted was more like 18 tons, 63 ft with about 40hp that struggled to get it to 5 knots. All the hydrodynamics of a rusty bathtub.

I suppose with buffering batteries they could get smoother progress, with none of that irritating listing you yachties like so much.
 
MRC_Hans said:
To move a sailboat at moderate speed does not require 150hp. I used to have a 26ft yacht weighing in at two tons. It could run 6 knots with a 6hp outboard.

I don't think a wind turbine to propeller arrangement (whether electric or direct drive) is nearly as efficient as a sail, but it should be able to move a boat. An advantage would be the ability to sail directly into the wind.

Hans
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Seems like there'd be conservation of momentum issues. Suppose you're sailing into the wind. You want to increase the energy of the boat, so you have to decrease the energy of the wind. The wind is blowing towards you, so you need to impart a forward momentum on the wind. To do this, you must impart a backwards momentum on the boat. I think you'd have to have the boat go at a fraction of the wind speed to counter this.
 
MRC_Hans said:
2) Making vacuum tanks with positive lift is impossible with known technology.
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So I see I was right about this craft violating entropy.

#2: Couldn't a balloon be made that has two layers; one inside the other and tied together at many points with kevlar cords. The inner sphere is airless and the space between the two cloth spheres is pressurized to two or three bar?
 
SkepticJ said:
#2: Couldn't a balloon be made that has two layers; one inside the other and tied together at many points with kevlar cords. The inner sphere is airless and the space between the two cloth spheres is pressurized to two or three bar?
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I'm not sure what you mean here. If it's a balloon, the atmospheric pressure would collapse it.
 
Art Vandelay said:
Seems like there'd be conservation of momentum issues. Suppose you're sailing into the wind. You want to increase the energy of the boat, so you have to decrease the energy of the wind. The wind is blowing towards you, so you need to impart a forward momentum on the wind. To do this, you must impart a backwards momentum on the boat. I think you'd have to have the boat go at a fraction of the wind speed to counter this.
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Mmm, I'm not sure what you mean by this. The winf blows against the turbine, providing energy (at the cost of locally slowing down the wind, there will be a zone of relative lee behind the turbine). This energy you use for driving a propeller, which makes the boat move forwards. If you steer directly into the wind, you will, of course, experience maximum resistance from the head-wind, slowing down the boat, but at he same time, moving against the wind, part of this (but NEVER all) is compensated by your relative wind strenght becoming higher (wind speed + boat speed). I don't see where conservation of momentum comes into it.

It is really not different from a conventiona sailboat tacking into the wind, except that a conventional sail loses efficiency when going too close to the wind, whereas the turbine does not.

Hans
 
SkepticJ said:
So I see I was right about this craft violating entropy.

#2: Couldn't a balloon be made that has two layers; one inside the other and tied together at many points with kevlar cords. The inner sphere is airless and the space between the two cloth spheres is pressurized to two or three bar?
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Sounds like a nice idea, but the forces won't balance unless the inner balloon is a lot smaller than the outer sphere, because the cords will pull inwards on the outer shell along with the outside atmosphere. If the inner balloon has a surface area A, and the outer ballon has a surface area B, and the pressure in the intermediate region is P, then the net outward pressure from the intermediate gas is going to be P*(B-A)/B, not P. So if you pressurize it with 2 atmospheres, you need the inside area to be less than half the outside area to be stable. If it's half the area, it's sqrt(2)/2 times the radius, for a sphere. And that means the volume of the inside sphere is less than 1/2 the volume of the outside sphere, which means that the air you need to keep the shell stable is actually going to weigh MORE than the air it displaces. So even with a weightless membrane for your balloons, this idea won't end up working. It's a neat problem to figure out why it doesn't work, though :)
 
SkepticJ said:
So I see I was right about this craft violating entropy.

#2: Couldn't a balloon be made that has two layers; one inside the other and tied together at many points with kevlar cords. The inner sphere is airless and the space between the two cloth spheres is pressurized to two or three bar?
Click to expand...
Yeah, back on topic, heheh. Yes, that might be one way to create a light but strong container, but the atmosheric pressure is 1kg per square centimeter, so even on a normal CRT, the pressure is several tons. The pressure a large container (and we are talking thousands of cubic meters) would have to withstand would be astronomic.

So, in your double-layer structure, pressure perpendicular to the surface would be translated to pressure parallel to the surface, and countered by the pressure between the two layers. But while the perpendicular pressure from the surrounding air adds up with increasing area, your parallel pressure does not (because the cross section of the space between the layers is constant), so you would have to increase the pressure between the layers as the container got bigger. Even with a very moderate container, this pressure would soon become impossibly high. I even suspect the weight of the thus compressed air might outweigh the buoyance of the vacuum.

Hans
 
MRC_Hans said:
I even suspect the weight of the thus compressed air might outweigh the buoyance of the vacuum.
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It does indeed. I gave a specific example above (2 atmospheres in the intermediate region of a spherical balloon), but it's actually true for any shape and pressure arrangement.
 
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