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Old 02-28-2012, 04:43 PM   #21 (permalink)
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Wind has always been a viable if not always reliable source of propulsion, and even a windsurfer or kiteboard can exceed windspeed if the person operating it is skilled enough.

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Old 02-28-2012, 08:58 PM   #22 (permalink)
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I thought the directly into the wind or faster than the wind downwind were interesting.

Wind-Powered Car Travels Downwind Faster Than the Wind | Autopia | Wired.com
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Old 02-29-2012, 12:54 PM   #23 (permalink)
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Quote:
Originally Posted by rmay635703 View Post
I thought the directly into the wind or faster than the wind downwind were interesting.

Wind-Powered Car Travels Downwind Faster Than the Wind | Autopia | Wired.com
The first case, sailing directly into the wind is easy to imagine, and as a result many windmill-powered boats have been built by enthusiasts. The windmill drives the wheels (or underwater propeller), which makes the craft move, which makes the apparent windspeed higher, which makes the craft move more forcefully,etc.

What makes the second case so interesting is that it seems so clearly impossible, (provided you haven't thought seriously about how it might be done). If you just turn the craft I described around and start downwind, as the craft speeds up, the apparent wind decreases, eventually to zero when the wind speed and craft speed are the same. There has to be some parasitic drag, so the craft can never get to wind speed... just as a sailboat cannot get to windspeed if sailing straight downwind, no matter how large the spinnaker.

The "trick" is, of course, that the propeller on Blackbird does not drive the wheels. The wheels drive the propeller. So the thing that makes the craft move, at first, is the total drag on the structure and propeller. The movement of the craft turns the wheels which turns the propeller in the direction opposite to that in which it would turn as a windmill. By moving, the propeller extracts energy from a larger piece of air.

Like any sailboat, the craft makes use of the difference between speed relative to land (or water) and air. Without that difference, there is no energy harvested: cut a kite's string, and it falls to ground.

Sailing directly downwind faster than the wind has been demonstrated before (by a guy that worked for NASA, in the mid 1960's if I recall) but not so convincingly. The reason for such little interest has to do with utility, I suppose. Conventional (which now means equipped with rigid wings, etc.) land yachts are much faster on every point of sail other than directly down wind, and can sail down wind (tacking) and achieve a similar or better velocity made good... and it is almost certainly more fun to sail at 60 knots on tacks either side of directly downwind than it is to sail at 30 knots straight downwind. (Sailors of conventional slow sailboats might think: "Shouldn't he be saying 'gybing' downwind? But, in fact, fast boats tack downwind -- the wind moves over the bow, not the stern.)

The Physics of sailing in a river are interesting too, and prompt otherwise bright people to come up with all sorts of wrong answers for how sailing works -- just as there were many who maintained strenuously that a boat could not sail downwind at greater than windspeed. Here is an interesting thread.

A related question is posed by Moronic Bumble in post 51 of that thread:

"A river runs straight from West to East at 10 knots. A 10 mile race is held: the boats sail downstream, from West to East. The first heat is held in the morning, when there is no wind. The second heat is held in the afternoon, when there is a 10 knot wind from the West. In which heat are the faster times recorded?"
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Old 03-01-2012, 09:11 AM   #24 (permalink)
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I would also move that this is some what a viable technology and it doesn't belong in the Unicorn Corral.
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Old 03-01-2012, 02:58 PM   #25 (permalink)
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I think if you wanted practical land transportation a better solution would be to use a windmill to pull a slight vacuum in a tube and let the vehicles enter the tube from the other end. I think Giffard had some research into that in the 1800s. Originally I think they even conceived subways to work that way.

If the vehicle fit in the tube with very close tolerances, then just 1 PSI of negative pressure would put considerable force on a 3 or 4 foot diameter capsule. 144 pounds of pressure per square foot of the surface area on the rear of the capsule. You could have many capsules in the same tube at the same time, and the air between the capsules would make them almost incapable of running into each other.

At the end of the tube, the air would be pulled into an opening at the top of the tube, so when the capsule passed the low pressure point it would just exit the tube onto a flat surface landing area.

Similar to the heating units in some buildings that maintain higher temperatures even with the doors open most of the time, but you would need to have some form of flapper valve to keep the vacuum coming in one direction. Should be totally doable, with mega high efficiency with the capsules not needing any internal power source.

You could have the capsules electrically powered to get them to the entrance point to the tube itself.

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Old 03-01-2012, 03:01 PM   #26 (permalink)
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Then if you really wanted to go a long distance at say 4000 MPH, just have the tubes collect passengers where they enter this.

Evacuated Tube Transport Technologies : et3 Network : Space Travel on Earth™

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Old 03-01-2012, 03:26 PM   #27 (permalink)
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Like those tubes the bank uses to take your checks at drive-thru terminals? It's interesting, but the infrastructure cost is something to investigate. I could see this sort of thing working in a tourist town where many people want to go to the same place.
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Old 03-01-2012, 11:03 PM   #28 (permalink)
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Ken Fry just fried my brain. I'll try to wrap my head around sailing down wind faster than the wind again later, when I'm not quite so tired.

My idea of harnessing wind is to have tall smoke stack type structures alongside the road with gaping open-mouthed elbows at the tops. The elbows would be articulated and have weather vane tails to keep the mouths into the wind. At the bottom there would be openings that point in the direction of travel. Viola, constant tailwind.
Yeah, we're off topic, but I blame Old Mech.

Okay, I'll go to bed now.
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Old 03-02-2012, 10:42 AM   #29 (permalink)
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Quote:
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Ken Fry just fried my brain. I'll try to wrap my head around sailing down wind faster than the wind again later, when I'm not quite so tired.

.
...analogy: because the triangle hypotenuse (Z) longer than either of its X- and Y-axes.
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Old 03-02-2012, 11:02 AM   #30 (permalink)
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Quote:
Like those tubes the bank uses to take your checks at drive-thru terminals?
Sounds like something out of Futurama

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