
Wind powered vehicle nearly 3 times faster than downwind - ck2
http://blog.makezine.com/archive/2010/11/downwind_faster_than_the_wind_black.html
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antipaganda
Beautiful. Absolutely beautiful. Ten minutes of feeling like an idiot for not
getting it, then a moment of glory when it finally became clear.

For those who still feel like idiots, here's the post on Autopia that made me
a believer:

"Let me get this straight. It is common knowledge that a sailboat can tack at
an angle to the wind sufficiently fast to reach a directly downwind point
faster than the wind itself. Yes? And yet people actually thought that merely
doing this in a straight line would be “impossible”?

Assuming the former is true and commonly accepted, all you have to do is
imagine an arbitrarily small (narrow) “zig zag” tacking pattern — which is
roughly the equivalent of your “cylinder earth” thought experiment. If the
zig-zag is small enough to be contained with in the width of your “boat”,
that’s all you need, in principle.

The rest is just engineering (and using a propeller is much better than a zig-
zagging mechanical device of some kind). I had never heard of this
controversy, and halfway through the article I’m thinking “this is just
obvious.”"

<http://www.wired.com/autopia/2010/08/ddwfttw/2/>

Tacking within the width of the boat is the key, right? Now, think about
changing the angle of the sail not by swinging it horizontally back and forth,
but by TURNING THE SAIL UPSIDE DOWN. In other words, constantly spin the sail
at the correct rate, and it's the same as tacking back and forth; and we
already know that you can beat the wind that way.

~~~
cma
[http://terrytao.wordpress.com/2009/03/23/sailing-into-the-
wi...](http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wind-or-
faster-than-the-wind/)

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Cushman
Here is a fun and straightforward demo of the principle behind this:

<http://www.youtube.com/watch?v=k-trDF8Yldc>

~~~
3pt14159
Good find. I was scratching my head for a while over this.

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ryanwaggoner
This is an interesting wikipedia article on the subject:

<http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind>

In particular, iceboats can sail at up to 5x the speed of the wind, according
to this: <http://iceboat.org/elements/faq/apparentwind.jpg>

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coffeenut
Hmm, this might be a good way to look at it (or it could be wildly
inaccurate!).

1) Assume for the sake of argument that your friction through the air is the
same as your friction of sliding on the ground (this isn't very accurate, and
is variable, but shouldn't invalidate the basic thought experiment).

2) instead of the two mediums being air and land, consider them to be two
parallel conveyer belts, going in opposite directions (i.e. moving relative to
each other).

3) Your vehicle is straddling the two conveyer belts, a set of wheels on each
belt.

4) Couple the set of wheels on one belt with the set of wheels on the other
belt, along with a transmission to engage/disengage the wheels from each
other.

5) Lock the wheels on one belt. This is effectively "going the same speed as
the wind".

6) The wheels on the other belt will be spinning furiously. Engage the gears
appropriately so that you make headway on the belt that you're relatively
stationary on. You're now going 'faster than the wind'!

Ok, it was easier to understand in my head :)

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ck2
Does 50mph in 20mph wind!

Video in action: <http://www.youtube.com/watch?v=5CcgmpBGSCI>

(looks like it's right out some post-apocalyptic anime ie. Ergo Proxy)

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RiderOfGiraffes
Related, a similar vehicle travelling directly _into_ the wind:

<http://news.ycombinator.com/item?id=1874846>

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coffeenut
The key point that's missing in this headline is that it attains that speed
'directly downwind'. Going crosswind is easy to attain speeds like this. The
confounding 'magic' is that it can do this directly downwind.

As a mind experiment to explain why this seems impossible, think about what
happens when the vehicle gets up to exactly the speed of the wind. At that
point, there's no net wind pressure on the vehicle (if you were on the
vehicle, there would be no apparent wind on your back or on your front), yet
the vehicle still continues to accelerate!

~~~
est
> The key point that's missing in this headline is that it attains that speed
> 'directly downwind'... As a mind experiment to explain why this seems
> impossible,

After reading TFA and watching this video[1], I think you are wrong.

[1]: <http://www.youtube.com/watch?v=xHsXcHoJu-A>

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yason
You could probably build such a propeller to a boat as well, having the wind
drive the underwater propulsion (only with worse efficiency than with rolling
wheels, though).

You could build a similar device that travels upwind as well, albeit slower,
right? The energy extracted from headwind by the propeller needs to be greater
than the energy needed to keep the vehicle stationary against the headwind.

Using a Darrieus or Savonius turbine on the vehicle or boat would allow for
more flexible headings without having to point the propeller to headwind
direction.

~~~
electromagnetic
The turbines when pointed at the wind are actually not facing the wind.
They're at an angle to the wind, which means you're making power from the
turbine. As long as the power made exceeds the wind resistance and rolling
friction of the vehicle.

Also you might be interest in this:
<http://www.treehugger.com/files/2007/02/windmill_sailbo.php>

I was actually looking for a horizontal turbine boat I saw, but this came up
before. However, the question with boats is whether it can beat the current as
well.

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sokoloff
Is it just me, or are other people also annoyed by the incorrect usage of "2.8
times faster than the wind"?

The actual speed record is 27.7 mph in a wind of 10 mph. That's 1.77 times
faster than the wind, not 2.77 times faster. (It's impressive and cool as
hell; I'm not quibbling with that...)

~~~
pavlov
Almost everyone says "two times faster" when they mean "twice as fast". This
is clearly wrong, but how can we ever hope to fix this? A billion-dollar
public awareness campaign? Re-education camps? Please let me know, I can
support the cause by wearing your T-shirt.

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shellerik
So the wind pushes the cart forward, which turns the wheels, which spin the
propeller, which pushes the cart forward even more - a feedback loop.
Eventually various frictions and inefficiencies equal the input energy from
the tailwind and you get a stable speed.

To go faster than the tailwind you need a gearing ratio that spins the
propeller quite a bit faster than the wheels are turning - leverage.

~~~
extension
_Yes_ , that is the intuitive way to think about it -- connect the fan to the
wheels and gear it to blow air backwards faster than the wheels are moving
forwards.

You are harnessing the speed difference between the ground and the air and
gearing it up to move yourself faster than the difference.

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dennisgorelik
I couldn't find meaningful explanation of how they do it. The only
explanations I could find sound like "Flux capacitor" explanations from "Back
to the Future" comedy.

My intermediate conclusion is that it's some sort of hoax.

~~~
eru
I did some research earlier.
<http://duckduckgo.com/?q=downwind+faster+than+wind> is a good start. It's not
a hoax.

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chrisbennet
When faced with actual evidence/proof, some people will still not accept it.
If the naysayers would spend $20 like the school kids did and make their own
model they could prove/disprove it themselves.

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kiba
This sound like an experiment that mythbuster could replicate.

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maeon3
The wheels are powering the fan blowing rearward I assume at a 1 to 1 ratio
into the wind. The cart requires an initial "push" from the human. The wind
pushes both the fan blades and the cart. So the net push from the wind > net
drag with the wheels powering the fan. The faster the cart goes, the faster
the blades spin, so the cart is being perpetually pushed by the wind on the
spinning blades no matter how fast it goes. The spinning blades appear to be
stationary from the perspective of the wind.

The blades spinning tax the speed of the vehicle, but the payback is that the
spinning blades are stationary relative to the ground.

This cart would not move if the ground was moving at the same speed and
direction as the cart. First law of thermodynamics has not been violated, the
cart is simply using the energy from the wind on a virtual stationary object
to power itself.

