
Wind-Powered Car Travels Downwind Faster Than The Wind - mkuhn
http://www.wired.com/autopia/2010/06/downwind-faster-than-the-wind/
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stcredzero
A good visual analogy:

[http://www.youtube.com/watch?v=E7vcQcIaWSQ&feature=relat...](http://www.youtube.com/watch?v=E7vcQcIaWSQ&feature=related)

~~~
mey
[http://www.youtube.com/watch#!v=JJVsoLcdGMY&feature=rela...](http://www.youtube.com/watch#!v=JJVsoLcdGMY&feature=related)

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cma
General explanation of how this is possible:

[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/)

~~~
Groxx
I remember that one. And this comment:

 _The author does a good job of taking a complicated subject and explaining in
such a way that it becomes extra extra complicated._ [
<http://news.ycombinator.com/item?id=1323665> ]

which still seems accurate to me.

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Groxx
I can see why it's a hard thing to come up with, but I'm puzzled as to
people's problem accepting it.

The blades create backward-thrust, pushing against the wind, accelerating the
car, creating more thrust, pushing harder against the wind, etc. It stops a
seemingly-endless increase when you hit the break-even point.

The reason it goes _faster_ is because, when it finally catches up to the
speed of the wind (which is obviously possible), the blades are _already
spinning_ , thrusting the car forward beyond just the wind speed. If they
weren't spinning at all, 1x wind speed would be the max, fairly easily
approached (though not _achieved_ ). This video is particularly telling of
this effect:
[http://www.youtube.com/watch?v=dgHBDESd38M&feature=relat...](http://www.youtube.com/watch?v=dgHBDESd38M&feature=related)

~~~
barmstrong
In this case you could surpass it momentarily but the average speed of the
vehicle would still never go faster than the wind speed, like one of the
posters mentioned above.

~~~
Groxx
Both the article and the linked video would imply the exact opposite. And I
see no such post. The propeller generates thrust, and _both_ result in faster-
than-wind movement.

It's not a momentary burst of acceleration - the propeller spins relative to
your wheels. If it's going fast enough to burst you, it will _stay_ at that
burst speed because its speed is tied to the craft's.

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mtinkerhess
It seems like they're using the propeller as a combination flywheel /
accelerator. If you're allowed to store energy as rotational momentum, why not
in a battery?

~~~
ThinAirDesigns
There is no stored energy involved in the acceleration -- it is all powered by
slowing the air down relative to the surface(as all wind powered craft do).

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te_platt
From the article: “I thought people would say, ‘That’s cool,’ but they didn’t.
They said, ‘Wow, you’re an idiot.'"

If I had a dollar for every time I heard that ...

~~~
CapitalistCartr
I have learned to quickly sort people out by telling them about whatever is
the most improbable project I'm fiddling with at the moment. The project is
unimportant; what is telling is their response. I get one of three responses,
no interest, that's cool, or you're an idiot. Quickly tells me whether they're
worth further time or not.

~~~
DrSprout
Which ones do you keep around? The yes men, the indifferent, or the naysayers?

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herdrick
No explanation within the article. That was annoying.

Any ordinary sailboat sails faster than the wind when going roughly
perpendicular to the wind direction. But this is apparently something
different.

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gvb
No, it is the same thing. What is counter-intuitive is that the body of the
car is going directly downwind. The thing is, the _sails_ of the car (the
propeller) are _not_ going directly downwind, they have an angle of attack
relative to the wind because they are angled relative to the car body. Ignore
the car body and draw a propeller blade, the actual wind, and the apparent
wind WRT the blade.

An iceboat is a better example than a sailboat. They go substantially faster
than the wind. They can do this because their sails are very efficient in
terms of being able to sail a very low angles of attack and because they have
very low friction WRT their motion.

According to this article,
[http://www.idniyra.org/articles/IceboatSailingPerformance.ht...](http://www.idniyra.org/articles/IceboatSailingPerformance.html)
the iceboat in question could sail 8 degrees off the wind and achieve a speed
7 times the wind speed.

Picture the propeller of the vehicle being a sail at a very small degree angle
of attack (e.g. 8 degrees) while it is "sailing" in circles. Rather than
pushing the vehicle body directly like with an iceboat, the energy is coupled
to the wheels to drive the body at an arbitrary (but fixed for the vehicle in
question) angle relative to the "sail" angle of attack.

~~~
electromagnetic
Could you shed light on how well this vehicle would perform 'sailing into the
wind' as it were? (Obviously with modifications to its transmission)

~~~
ThinAirDesigns
It is likely that the team from San Jose State University will use this
vehicle to answer that very question during the next academic year, but it
will involve a bit more work than just transmission changes.

While it is true that with only ratio changes the vehicle can be made to drive
directly upwind, it would not be all that efficient as the airfoils currently
on the vehicle are propeller blades and not turbine blades. A set of properly
cambered blades will need to be fabricated and installed and then the test
could be done.

One thing to remember is that when traveling upwind, the apparent wind over
the chassis will be higher for any given multiple of wind speed. Upwind at 1x
WS gives the same apparent wind as going 3x downwind.

The current 'record' for directly upwind is ~64% of wind speed.

[http://www.gizmag.com/the-first-race-for-wind-powered-
vehicl...](http://www.gizmag.com/the-first-race-for-wind-powered-
vehicles/9953/)

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tocomment
When I follow this link in my iPhone I get redirected to wireds mobile site.
Is anyone else experiencing this?

It really grinds my gears when websites do this! Grrr

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Semiapies
_" Yes, it’s possible that SJSU, Google, and Joby Energy are all being hoaxed
here. What do you think?"_

Could be, especially considering the video linked on this page to the
treadmill demonstration. I'd like to see this "blackbird" actually "spread its
wings" on more than a short sprint, after a thorough inspection. I suspect its
average speed would end up below that of the wind.

~~~
ThinAirDesigns
@Semiapies:

A: if the vehicle can accelerate under the power of the wind to several
multiples of the windspeed, what would suddenly make it slow down? Are you
suspecting stored energy is being used somehow?

B: How long of a run would convince you?

~~~
Semiapies
A) It _is_ admittedly using stored energy - the propellers are spun by the
wheels, which are spinning due to the vehicle's forward motion. I suspect that
they may have come up with a clever trick to manage a burst of speed before
slowing down back below the speed of the wind. The treadmill demo video shows
the model speeding up, then slowing down.

B) I'm not sure how long the temporary burst of speed would last on a larger
model; I'd prefer an engineer coming up with that. My WAG would have that
period be rather less than an hour.

I'm reminded of perpetual motion scams, especially by someone creating a login
to start defending them and demanding what would convince someone. Trying to
put skeptics on the defensive and trying to force them to disprove the
remarkable claim put forth is a classic shtick.

And I think the perpetual motion bit is fair - if we take the treadmill video
on its face, this is a vehicle that, in still air, _would go uphill under its
own lack of power - and even accelerate for a while_ after an initial push.
This has nothing to do with apparent wind velocity.

~~~
ThinAirDesigns
@Semiapies:

>It is admittedly using stored energy - the propellers are spun by the wheels,
which are spinning due to the vehicle's forward motion.

I'm sorry, but by that logic your car is also "admittedly using stored energy"
to accelerate (as the wheels and other parts are turning due to the vehicles
forward motion) and we know this isn't the case. Just because there _exists_
stored energy in a system, doesn't mean that this stored energy is being used
and in fact, as the vehicle accelerates, _more_ energy is stored rather than
energy being used.

In the case of this vehicle, it _can't_ be using the stored energy in the
propeller without it turning progressively slower and the videos show the
propeller simply continuing to spin faster as the vehicle accelerates.

The gearing between the prop and the wheels is fixed ... there's no way to
slow the prop down and speed the wheels up -- that would require a variable
ratio between the two.

Semiapies: <<Trying to put skeptics on the defensive and trying to force them
to disprove the remarkable claim put forth is a classic shtick.

I don't recall attempting to force you to disprove anything. I simply asked
period of time would convince you. You can call it "shtick" if you like -- I
call it attempting to learn your position.

~~~
Semiapies
_"it can't be using the stored energy in the propeller"_

Note that I didn't say it was. Sorry to get you off-script.

 _As claimed by the builders_ , the propeller is powered by the wheels.
Assuming no other stored energy source, the stored energy is the motion of the
vehicle after it's been started off by the wind. I'm not clear how they do it,
but going from the treadmill video, I suspect they manage a temporary burst of
speed followed by a slowdown.

 _"I don't recall attempting to force you to disprove anything"_

You rushed in to try to get a layman to give explicit parameters for
disproving it. That's a common tactic, usually followed by assurances that
this system can _totally_ do that, and we'll see video of it doing exactly
that Real Soon Now.

At this point, I'm waiting for the normal course of events: the other shoe
will drop months from now and hardly make a blip on the blogs and other venues
currently hyping this story.

~~~
ThinAirDesigns
Me: "it can't be using the stored energy in the propeller"

You: "Note that I didn't say it was. Sorry to get you off-script."

Wow -- you make a clear claim and then two posts later are denying you even
said it. I'll refresh your memory: This is you claiming that the vehicle uses
stored energy: "It is admittedly _using_ stored energy - the propellers are
spun by the wheels,"

You: "You rushed in to try to get a layman to give explicit parameters for
disproving it."

Nope -- I merely asked what would convince you -- big difference.

~~~
Semiapies
Two posts? You're contradicting yourself within your single post, right here.

I _never_ said the energy was stored in the propeller. You even just quoted me
saying exactly the opposite, twice!

Real, working projects don't have shills - sorry, _evangelists_ \- running
around here and other forums trying to shout down people who are dubious about
the very physical possibility of what they've made. They don't have to - they
have working machines!

~~~
ThinAirDesigns
Wow -- just wow.

Moving on.

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alex_h
A helpful way to understand sailing into the wind (for me at least) is to
think of a stationary wind turbine producing eg. 2MW of power. Now if you were
to mount that turbine on the roof of your electric car, that 2MW is more than
enough to power the car at great speed in a windward direction. This seems to
be the same concept except with a pure mechanical linkage.

~~~
eru
Not really. Here the propeller isn't generating the power, but doing the
acceleration.

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wdewind
This seems to be the same issue as horsepower: If the engine has horsepower
doesn't that mean it can only go as fast as horses do?

Of course not, that's the intuitive "apples and oranges" type mistake, the two
numbers aren't really correlated like that. It's like leaving your foot on the
gas at the same level will keep it accelerating until it hits the most it can
do with that much gas.

