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// Vehicle has otherwise no moving parts/engine. // |
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... and no pretentions to efficiency, either. |
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Yes, it will work. Just about. Sort of. You're going to need a big, very lightweight air tank (probably titanium). |
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It will be incredibly noisy. |
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Thrust and therefore speed will be difficult to control without a complex valve and some electronics as the feed pressure falls continuously. |
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My guess is that it should be as efficient as airboat. |
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Noisy ? yes. But that may be a solvable problem. simply covering the nozzle with sound absorbing material may be a good start. |
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About the tank size, : I was thinking same tank size as any other air powered car tank size. May be too optimistic. |
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An air motor isn't terribly efficient, and it's still far more effecient than an air jet. |
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// Vehicle has otherwise no moving parts // |
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Not even the vehicle itself. |
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The title could use the word "jet", perhaps. |
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// My guess is that it should be as efficient as airboat. // |
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An airboat has a prop diameter of around six feet. The diameter of an air jet is very much a factor in speed and efficiency--whatever nozzle you rig is going to be much smaller than the air jet created by a prop. |
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Put it this way: Take your compressed air and run it into an airboat engine, and make it an airmotor prop boat, and you will go a lot further (and if you feed the compressed air in, then add gasoline, you'll do even better). |
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Oh, hell. Go down to the toy stores, especially the science-toy stores. You'll find some air-jet balloon vehicles, but they are damned small and silly and run on strings. You'll also find some AirHog airmotor vehicles, with pistons and all, which are bigger and go further.. |
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This is one of those things that are fairly easy to think of, but a bit hard to explain why they don't work. I'm just going to say that if it did work, it would already be done. |
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It could be done, but there's no reason to. Well, giggles and fart noises, maybe. |
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//brakes// ... perhaps not the most necessary accessory imaginable ... |
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A variable geometry nozzle would help quite a bit. |
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It's been done better using the compressed air to drive a (rather impressively powerful) motor. Or looking at it another way, hovercraft, areoplanes, sailing boats or any other vehicle driven by wind is in effect, a compressed-air vehicle, it's just the minor details of collection and storage that are different. |
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[zen_tom] Just for the sake of being über picky, compression of air (in the sense of a change in volume due to a change in pressure) is not required to explain how those vehicles work, and is often omitted from first-order calculations of lift and so forth; obviously it occurs, but it's relatively unimportant, at least at moderately low speeds. Near and above the speed of sound, on the other hand, compression is very important. |
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[spider] But compression can be omitted from the motive calculations on how a compressed air jet works, as well. You need it to determine air-speed out the nozzle, but the actual movement is a simple momentum calculation. Replace air speed out of the nozzle with air speed from the fan, and it's the same calculation that a fan driven vehicle uses. |
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Indeed; but compression is vital to the working of a compressed air jet (obviously), whereas it barely enters into the performance of those other vehicles at all. |
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To put it another way, it's possible to predict the aerodynamics of subsonic bodies with great accuracy by testing a model in water rather than air, scaling the velocity appropriately; the vast difference in modulus is not a great source of error, because compression is almost entirely unimportant. The same is (obviously) not the case with a compressed water vs a compressed air jet. |
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To put it another another way, the simple form of Bernoulli's law, which assumes an incompressible fluid, is routinely used to describe subsonic lifting bodies, much as Newton's laws are used for speeds well below light speed - it is an approximation, but a very accurate one. |
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// When you just use a free jet like this most of the gas is expanding after its left the jet, typically in all directions// |
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I think jet has spread of approx 30 degrees. Otherwise sprays would never work. |
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Efficiency of this was a just a wild guess, although I did not add adjective "wild" then. |
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I think this scheme might work on Go-karts or mopeds but not on heavier vehicles. |
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I've made little steam-rocket boats on this principle. Nothing more than a used CO2 cannister, part filled with water, suspended over a candle, all on a floating base. Steam shoots out the back, boat goes forwards. It works, but, yes, it's slow and inefficient. |
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My dad at one time added a 3-foot propellor to a 1.5 hp Briggs and Straton engine, which he set on the back of one of his Quadracycles (4-wheel recumbent bike) for the fun of it. It did about 40 MPH. |
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For slower vehicles than jets, for air-propelling it's simply much more efficient to move a lot of air more slowly than a small amount of air very quickly, hence the AirHog's (and my father's) implementation of a piston-powered propellor. |
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The Mythbusters built a boat powered by two compressed
air cylinders. The two giant cylinders propelled the boat
forward about 20 feet at a whopping 5kts. |
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Using a large industrial CO2 bottle (70# or so), this scheme
would move a go-cart (with passenger) about ten feet.
Adapting it for use on a moped would be a very interesting
way to hurt yourself. |
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Compressed gases alone do not make for effective (forget
efficient) rocket
propulsion in atmosphere. If they did, we'd send the Space
Shuttle into
orbit with air instead of A-P. |
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Yeah, compressed air is one of the less-efficient means of storing power, even disregarding heat losses during compression. And tankage is a bugger. |
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// Noisy ? yes. But that may be a solvable problem. simply covering the nozzle with sound absorbing material may be a good start. // |
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Um, no. It's the air swirling in air that makes the noise. |
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I think he meant covering the nozzle so that the air doesn't swirl out? |
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If you closed off the nozzle completely, that would
solve the noise problem without greatly reducing
the efficiency. |
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An ideal nozzle design producing a laminar stream
would both minimize noise and maximize efficiency.
It's still going to be far less efficient than the
simplest air motor driving the axle. |
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I once rigged an air motor as a pre-rotator on a gyroplane. It could spin the heavy rotor up to skull-cracking speeds. When we wanted to empty the air tank, we'd just yank the hose and let the air jet out--nothing ever went anywhere. |
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I've used many an air hose and nozzle as a duster, and never had any troubles with reaction. And that was about as nice a jet nozzle as is easily available. If you want to build augmenters and silencers, you might as well switch over to pistons. |
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I think you could improve the efficiency without adding moving part is you used a venturi. This converts the small volume flow of high pressure air to a much larger volume, lower pressure stream. Does anyone know if this could approach the efficiency of an air motor attached to a fan? |
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See link. At the end of the "air mover" .pdf file, there is a warning: "Secure the air blower in place prior to turning on the air supply or it will tend to move from its intended position". I was hoping for a warning that was a little more dire. Personally I'd prefer a vehicle that did more than "tend to move". |
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It's more accurate to say that a venturi device converts high velocity, low flow rate into low velocity, high flow rate. Pressure potential has already been converted into kinetic energy by the primary nozzle. |
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A group who made pulse jet engines and powered go-carts with them found that a venturi cowl increased the performance, especially at low speeds, but they emphasised that the overall efficiency was still lower than that of a continuous jet engine, let alone a propellor. So I think the answer to your question is no. |
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You need more reaction mass. Something heavy to lob backwards... |
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A prop works great as it grabs loads of air and pushes it backwards. Also, it's not limited to the mass it carries. Whereas the air cylinder idea is. |
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Compressed air can be used to push other fluids out of a nozel and hence provide more reaction mass. Water is great as it's much heavier than air. By extension mercury is much better still. |
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Powered wheels are even better, as the reaction mass is effectively infinite. |
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Limited to the mass of the earth, surely? |
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[pocmloc] No, actually. Imagine driving on a tiny asteroid. The asteroid rotates a bit, but that simply pulls the destination (elsewhere on the asteroid) towards you, so you don't get there any later. Mutatis mutandis, ceteris paribus, increasing the mass of the Earth does not increase your acceleration relative to the surface of the earth. Therefore, the earth's mass is _effectively_ infinite in this context, Q.E.D. |
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//By extension mercury is much better still.// |
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[spidermother], thanks for that description. I will remember it next time I am poling my canoe up a river. |
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