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PostPosted: Tue Feb 12, 2008 8:16 am    Post subject: Downwind faster than the wind? Reply with quote #1   
Is it possible to build a vehicle that's powered only by the wind, and goes DIRECTLY downwind, FASTER than the wind, steady state?
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PostPosted: Tue Feb 12, 2008 8:20 am    Post subject: Reply with quote #2   
Nope
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PostPosted: Tue Feb 12, 2008 8:29 am    Post subject: Reply with quote #3   
I know you can sail faster than the wind speed, but not directly downwind.
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PostPosted: Tue Feb 12, 2008 8:30 am    Post subject: Reply with quote #4   
YES. Very hard to believe, but YES.
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PostPosted: Tue Feb 12, 2008 8:39 am    Post subject: Reply with quote #5   
Where does the extra energy come from?
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PostPosted: Tue Feb 12, 2008 8:59 am    Post subject: Reply with quote #6   
xerxes wrote:
Where does the extra energy come from?


Who said there's "extra energy"? Mr. Green

boarini2003 wrote:
I know you can sail faster than the wind speed, but not directly downwind.


But can you make a downwind tack in which your downwind component is faster than the wind? If so, that would be a good start.
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PostPosted: Tue Feb 12, 2008 9:13 am    Post subject: Reply with quote #7   
I would say no. Here's what I got from my 10 minutes of research:

A fast boat can sail faster than the wind, but only by using a combination of atmospheric wind and apparent wind. In short, a portion of the speed is generated by the sail shape creating 'lift'. This only works when you sail at an angle to the wind. If you start sailing directly downwind, even in a frictionless and dragless boat that harnesses the wind in a perfectly efficient manner, you will eventually match the wind speed and then there will be no air movement over the sail to create lift. By sailing downwind at an angle, you maintain some relative motion to the airstream and generate an extra 'push'.

from http://www.seed.slb.com/qa2/FAQView.cfm?ID=1098

"Typically the fastest speeds are reached with a wind coming at 130º to 145º from your course. This is the true wind, but the apparent wind is coming from 45-50 degrees most of the time. This is why you rarely see a mutlihull with the sails open and also never with a spinnaker but only a gennaker as they are always sailing relatively close to the apparent wind. For the same reason, they always need to use a highly efficient dagger board or foils to resist the leeway force generated by the sails. This is of course only true for the top racing multihulls. A cruising multihull still uses a spinnaker as it never goes faster than the wind and still receives wind from the back when sailing down-wind."

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PostPosted: Tue Feb 12, 2008 9:19 am    Post subject: Reply with quote #8   
When I am on downwind in my hang glider I'm going faster than the wind. Of course gravity is adding the extra energy. So yes, you can do this. It just requires the use of altitude and gravity.
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PostPosted: Tue Feb 12, 2008 9:22 am    Post subject: Reply with quote #9   
I certainly agree that a simple sailboat can never sail directly downwind, faster than the wind, steady state. But the question is whether any vehicle can be built that can.

You mention that the fastest tack is a downwind tack, but not directly downwind. What if we consider an ice boat - how fast could it go on one of these downwind tacks?
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PostPosted: Tue Feb 12, 2008 9:23 am    Post subject: Reply with quote #10   
Think ice boats. Cats on iceblades. Definitely go faster than the wind but not straight down wind.

Why not? Think of the sail as a wing, as you head down wind the angle of attack goes to 0 and you slow down and stop and start going into push mode to get your speed.

Now think about the sail. Its a lift generating airfoil right? The problem is, its an airfoil that stops generating lift when you approach downwind. So change the type of airfoil you are using Wink Hint hint...

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PostPosted: Tue Feb 12, 2008 9:24 am    Post subject: Reply with quote #11   
Rick M wrote:
When I am on downwind in my hang glider I'm going faster than the wind. Of course gravity is adding the extra energy. So yes, you can do this. It just requires the use of altitude and gravity.


Sorry Rick, you're not going "directly" downwind. You're losing altitude. Or if you're flying in lift there's a vertical wind component you're not matching.
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PostPosted: Tue Feb 12, 2008 9:27 am    Post subject: Reply with quote #12   
sg wrote:
The problem is, its an airfoil that stops generating lift when you approach downwind. So change the type of airfoil you are using Wink Hint hint...


Perhaps I'll make it harder for you sg, and require that it be done with typical sails moving in a typical manner. Mr. Green
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PostPosted: Tue Feb 12, 2008 9:37 am    Post subject: Reply with quote #13   
sg wrote:

Now think about the sail. Its a lift generating airfoil right? The problem is, its an airfoil that stops generating lift when you approach downwind. So change the type of airfoil you are using Wink Hint hint...


popcorn

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PostPosted: Tue Feb 12, 2008 9:58 am    Post subject: Reply with quote #14   
Rick M wrote:
When I am on downwind in my hang glider I'm going faster than the wind. Of course gravity is adding the extra energy. So yes, you can do this. It just requires the use of altitude and gravity.


Of course that brings up another interesting HG question:

Are we gravity powered, or wind powered?

I think I'll start a thread on that one later.

JB
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PostPosted: Tue Feb 12, 2008 10:00 am    Post subject: Reply with quote #15   
Check this out. I believe it is impossible and that this is a hoax but believe what ever you want....

Link
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PostPosted: Tue Feb 12, 2008 10:00 am    Post subject: Reply with quote #16   
Not sure if this is what you're thinking about, and you've got to be totally nuts to try it but:

Dynamic Soaring

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PostPosted: Tue Feb 12, 2008 10:03 am    Post subject: Reply with quote #17   
if you can go dwfttw with no other energy you just invented perpetual motion
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PostPosted: Tue Feb 12, 2008 10:06 am    Post subject: Reply with quote #18   
JDyer wrote:
Check this out. I believe it is impossible and that this is a hoax but believe what ever you want....


D'OH!!! You jumped the gun. That's not a hoax. That is one of several methods that can be done (the most practical one in fact). There are several more approaches as well.

That this is real can easily be shown mathematically - in case you don't have a DWFTTW land vehicle and a flat road handy.

Quote:
Not sure if this is what you're thinking about, and you've got to be totally nuts to try it but:


Hmmm.... Dynamic soarding requires a wind gradient. Good catch. I suspect you can beat the average wind vector directly downwind this way. You wouldn't be going directly downwind, but tacking in a sense. But with the right amount of trickery, that can probably be taken care of.

The original question intends a perfectly steady wind (no gusts or gradients).
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PostPosted: Tue Feb 12, 2008 10:09 am    Post subject: Reply with quote #19   
JDyer wrote:
if you can go dwfttw with no other energy you just invented perpetual motion


I wish it were so. Then I'd be filthy rich. It can in fact be done, and doesn't require perpetual motion. The secret is that we're extracting the real energy that arises from the difference in speed between the wind and water (or wind and ground, or wind and wind). That's why we need a "special" vehicle. A typical sailing vessel primarily extracts the energy arising out of the difference in speed between the wind and the vehicle (this is only strictly true when running directly downwind).
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PostPosted: Tue Feb 12, 2008 10:15 am    Post subject: Reply with quote #20   
So, yes, you could do it, but not sustainably.

E.g. you could store extra energy until you hit wind-speed, and then use it to accelerate past wind speed for a period of time. You could store this energy via fly-wheel (e.g. spinning propeller), or just momentum of going back/forth downwind. On the cross-wind sections you're storing energy (building up cross-wind momentum) and then releasing it as you turn back downwind. Mean speed downwind (of downwind and crosswind legs) would never be greater than that of the wind.


Last edited by gerg on Tue Feb 12, 2008 10:18 am; edited 1 time in total
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