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## Google-Backed Wind-Powered Car Goes Faster Than the Wind393

sterlingda writes "A wind-powered car has been clocked in the US traveling downwind 2.85 times faster than the 13.5 mph wind. The definitive research by Rick Cavallaro of FasterThanTheWind.org is being funded by Google and Joby Energy. The run should now settle the DWFTTW (downwind faster than the wind) debate that has been raging for some time on the Internet about whether or not such a feat was possible."
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## Google-Backed Wind-Powered Car Goes Faster Than the Wind

• #### Debate? (Score:5, Interesting)

on Sunday June 06, 2010 @05:45AM (#32474252)

Sailing vessels can go faster than the wind, why shouldn't a car be able to?

• #### Re:Debate? (Score:5, Insightful)

by Anonymous Coward on Sunday June 06, 2010 @06:09AM (#32474338)

Sailing vessels only go faster than the wind when they travel with the wind coming from the side. No matter how fast the vessel goes, the wind keeps blowing from the side and delivering energy to the vehicle. When you try to go faster than the wind in the direction of the wind, the relative motion to the air goes down to zero and then you start going against a head wind. Obviously the wind can not be the propelling force beyond the point where you go as fast as the wind in the conventional sailing sense, because at that speed there is no wind (motion is relative). The described device uses the sailing force to accelerate and then produces its own faster wind, so to speak, by driving a propeller via a transmission from the wheels.

• #### Re:Debate? (Score:4, Insightful)

on Sunday June 06, 2010 @06:24AM (#32474400)
And the propellor is a "sail" whose surfaces are not perpendicular to the wind.
• #### Re: (Score:2)

From what I understand, this car was going against the wind
• #### Re: (Score:2)

You understand 180 degrees wrongly.

• #### Re: (Score:2, Insightful)

by Anonymous Coward

Sailing vessels can go faster than the wind ACROSS the wind. They use a keel to do this. But the keel does nothing (except drag) when they travel directly downwind. They can't go faster than the wind in the direction of the wind. If you think it through what these guys are doing is pretty clever, making the car travel faster than the wind providing it's power in the direction the wind is blowing is cool.

• #### Re: (Score:2)

They do not go faster downwind.

• #### Re: (Score:2)

But now I wonder if wind powered boats can use the same trick to go downwind faster - instead of wheels you'd use propellors or paddles in the water.
• #### Re: (Score:2, Interesting)

But sailing vessels can't go faster than the wind directly downwind, this car does.

FTFA:

the propeller is not turned by the wind. The wind pushes the vehicle forward, and once moving the wheels turn the propeller. The propeller spins in the opposite direction to that expected, pushing the wind backwards, which in turn pushes the car forwards, turning the wheels, and thus turning the propeller faster still.

• #### Re:Debate? (Score:5, Interesting)

<ChristianHGross&yahoo,ca> on Sunday June 06, 2010 @06:38AM (#32474450)

Look at the experiment I would say sure its possible because there is no resistance. As they say they can't do this on a gym floor, but a tread mill.

As a mechanical engineer who studied dynamics I would say the reason is because the resistance that is normally hit due to acceleration or keeping the thing moving is not present. Thus this thing could accelerate faster than the wind.

What I would find interesting is what are they exploiting in specific? As they say, is there some neato resistance, aero-dynamic trick that nobody has yet thought of?

It reminds me of the ram jet that below a certain speed is useless. YET at higher speeds it becomes more effective than a regular jet.

• #### Re: (Score:2)

Sure they can [wikipedia.org]. The key is to couple the wind power to an mechanism providing thrust, like the wheels in this scenario, or a propeller for a boat.

• #### Re: (Score:2)

The wheels don't provide thrust in this example. The wheels turn the propellor, not the other way around.

• #### DWFTTW--Except where the car couples to the wind! (Score:4, Interesting)

on Sunday June 06, 2010 @09:37AM (#32475240)

Read carefully the excerpt in the parent's post.
This is a demonstration of some basic physics and geometry, but it is not "DWFTTW" at the point where the car actually couples to the wind.

A science project where the participants and the public learn some interesting physics and engineering principles--or are entertained by watching--this is a very good thing. It gets the public (if you can call /. the public) talking about science.
We need more demonstrations like this--no, what we really need is another Sputnik!

Be careful.
People have been known to use the counterintuitive nature of the physical world to argue they have discovered a new way to get rich quick--and you can get in on it if you want! We like to think were too hip for perpetual motion, but a lot of folks will still hand over real green (dollars) for bogus green (environmental scams). Don't you care about the environment?

So, what is the "magic" here, and what's the physics?

The fundamental error in the statement "DWFTTW" is the fallacy of dual definitions.
This is kind of cheating--a really good science demonstrator doesn't actually lie to you; they just show you something that exposes your misconceptions. Either way, the point is to get you to say "I see it, but it's impossible!". Then you are more ready to learn some science. (or maybe to invest in a free-energy scam).
DWFTTW is simply the koan. It actually means nothing--just gets us ready to study and learn something new.

When the experimenters say "faster than the wind", they are referring to motion of the bulk (center of mass) of the car.
BUT--the wind couples to a very specific portion of the car, which has a completely different (and somewhat more complex) velocity than the center of mass of the car.
The propeller--or more specifically, the surface of the propeller that pushes against the wind.
And the part of the car that connects to the wind NOT traveling "DWFTTW".

Read the article and look at the pictures--this is why they took such care to "streamline" the car. The rest of the car (except the propeller) is built so that it presents the very minimum cross-section (drag coefficient), and is effectively transparent to the wind. So, it is the part of the propeller that pushes against the wind that matters when we try to analyze the downwind motion.

So--what is the the portion of the propeller's motion that is "downwind"?
You could say "parallel to the direction of the wind" if you like, but for this case, "downwind" works fine.

A little math (just two equations, I promise--and only to describe the geometry!):
The propeller surface has a pitch angle, theta, from zero (parallel to the plane in which the propeller rotates) to 90 degrees (parallel to the propeller shaft), and it spins at some angular velocity w (omega).
At any instant, the linear velocity, v, of a point a distance r from the shaft of the propeller is simply v=Rw
And the perpendicular (downwind) component is just v(p)=v*sin(theta).

By controlling the diameter of the propeller, the pitch angle, and the rotational speed, the experimenters cause the relevant part of the car--that is, that portion of the car that connects to the wind!--to travel downwind much slower than the wind.

But, I hear you say "We keep talking about "slower" than the wind, and cars move fast.".
This seems strange because we started with the reference frame of the road, and we compare the velocity of the car and the air. The comparative term "Faster" describes the downwind velocity of the car, which, for consistency, we continue to reference.

In Newtonian physics, there are no preferred reference frames. This is true in other cases as well, but they are not significant at the speeds this car is traveling. This means we are permitted to say "the car is traveling slower than the wind" or "the wind is traveling faster than the car" and they mean EXACTLY the same thing.

Recall again that the pertinent part of the car is that part

• #### Re: (Score:2)

they can't go downwind faster than the wind.

• #### Re: (Score:2)

Not downwind, they can't. RTFA.
• #### Re: (Score:2)

sailing vessels cannot go faster than the wind when sailing directly downwind.

• #### Re: (Score:2)

Sailing vessels can go faster than the wind, why shouldn't a car be able to?

Sail boats can sail faster than the wind... while sailing into the wind, it's their fastest tack. Let's see a car do that.

• #### Very old news. (Score:2, Informative)

It shouldn't really be a debate -- sailors have done this for decades. Essentially you turn your vessel/vehicle at an angle to the wind such that you utilize both the positive pressure from the wind and the negative pressure created by the curved sails which create an air foil. Positive pressure pushes you forward while negative pressure pulls you forward == faster than the wind. The same effect is at play with the "propeller" on the car. It's also the same principle that keeps planes in the air -- high

• #### Re: (Score:2)

Could you explain it a bit more? When you start going faster than the wind, don't you lose the "positive pressure" of the wind, since it is now against you? What am I missing?

• #### Re: (Score:2)

apparent wind angles and speed.

The faster you go the direction of wind, and it's speed changes. What is hard for most people to under stand there are really no lines on a boat that are solid. even the lines that hold the mast up can be tweaked. while any given boat has a top hull speed, you can design boats with hull speed numbers many times that of the wind. The fastest sailboat to date was clocked doing 60mph, (55knots) in a 30mph(25 knot) breeze. That's doing twice wind speed.

Sailing is areodynamics

• #### Re: (Score:2, Insightful)

So all this time, Wile E Coyote could have stood to the side, blown just as hard on his sail and gotten away from his bombs that the Road Runner turned on him?

Some genius!

• #### Re:Very old news. (Score:5, Informative)

on Sunday June 06, 2010 @06:33AM (#32474432) Homepage

You are correct, but only for boats sailing across the wind or to windward. Modern yachts cannot sail faster than the wind *downwind*. Indeed, downwind is their slowest point of sailing, which is why many yachts tack downwind rather than sail dead downwind. Sailing boats cannot do what is claimed here. What is claimed here is substantially cool.

• #### Re: (Score:2)

It is cool, but there's wiggle-room in the debate :-)

The device extracts energy from the wind, and using only that, the vehicle *as a whole* progresses downwind faster than the windspeed.

The cunning part is the prop; because it's rotating, the blades themselves aren't moving directly downwind (well, considered instantaneously, they're moving across it), and that's the "trick" of it. It's a very clever idea indeed.

• #### Re:Very old news. (Score:5, Informative)

on Sunday June 06, 2010 @07:55AM (#32474706) Journal

For sailors: By using a propeller rather than a sail, the "sail" this boat is using is simulating a continuous optimal downwind tack (the propeller blades are at a tack angle to the wind)

For cyclists: The wind is being turned into rotational force like the cranks on a bicycle. Since they now have rotational force, they can use gearing to take maximum advantage of that force.

Does that make it clearer?

• #### Re: (Score:2)

That would make sense- if they hadn't gone out of their way to state that the propeller is driven by the wheels and spinning against the wind.

• #### Re: (Score:2)

Except that is not what the DDWFTTW people are claiming. They claim that the propeller is propelling the vehicle not acting as a turbine that sends energy to the wheels.

If you think about it, at the point when the vehicle catches up to the speed of the wind the propeller can't act as a turbine as relative air speed is zero.

(Note, I'm not sure I believe their claims, but I'm pretty sure that is what they are claiming.)

• #### Re: (Score:2)

Sorry to self reply, but they also claim it can self start so to get it started the propeller can't be getting energy from the wheels either (as they are stopped). It is all very strange.

• #### Re: (Score:2)

Sorry to self reply, but they also claim it can self start so to get it started the propeller can't be getting energy from the wheels either (as they are stopped). It is all very strange.

Below wind speed, use the prop to run the wheels. Over wind speed, use the wheels to run the prop.

• #### Re: (Score:2)

No this is not old news. I thought it was this trick as well. Its not. It the direct downwind problem, not side to wind problem.

• #### Re: (Score:2)

Sailors can't go faster than the wind directly down-wind. From TFA which you didn't read:

Cavallaro explained the car is able to move faster than the wind because the propeller is not turned by the wind. The wind pushes the vehicle forward, and once moving the wheels turn the propeller. The propeller spins in the opposite direction to that expected, pushing the wind backwards, which in turn pushes the car forwards, turning the wheels, and thus turning the propeller faster still.

This isn't at all like sailing boats, or aerofoil-based boats.

• #### A million monkeys at a million keyboards... (Score:5, Informative)

on Sunday June 06, 2010 @06:31AM (#32474428)

"The run should now settle the DWFTTW (downwind faster than the wind) debate that has been raging for some time on the Internet about whether or not such a feat was possible."

You're new to the internet, aren't you, son? No amount of reality can end an internet debate.

• #### Gearbox analogy (Score:2)

From my understanding of the wheel-powered propeller system, this works basically like a gearbox that converts a lower RPM to higher. There is nothing unphysical about converting a lower velocity to a higher one this way.

Of course, in the gearbox analogy, the torque is lower in proportion, meaning less acceleration. Also, since the vehicle is now moving relatively against the wind, it needs power even to maintain that velocity.

• #### Another way to look at this. (Score:5, Insightful)

on Sunday June 06, 2010 @06:45AM (#32474472)

Firstly, ignore that it's moving.
You have 0m/s ground, and a 10m/s wind.

You put up a wind turbine - it can extract power from this 10m/s difference.

The funky part of this idea is that this still works when you're moving faster than 10m/s.

For the moment - imagine that the turbine is a pure 'airscrew'.

It describes a helix in space - like the DNA molecule.
For every meter the air moves "forward" relative to it, it turns 1m clockwise.
Considering the air as completely rigid for the moment, the airscrew goes forward in a rigid helix, unchanged by load.

So - 10m/s wind - airscrew turns at 10m/s. Simple.
You can extract - say - 100N * 10m/s = 1kW of power.

Funky part coming up.

Now. You're moving at 20m/s. Twice as fast as the wind.
Of course this will slow you down - you can't use this to make power!

Well - not quite.

If you are moving at 20m/s in the direction of the wind - for a total speed with regards to the wind of
30m/s then the blades need to be spinning at 30m/s in order to keep up.

But, you can use gearing from the wheels so that the 'base' speed of this spin is 20m/s.

That is - when you push the car along on a windless day - the airscrew creates no drag - because it is spun at exactly the right speed by gearing from the wheels. It has effectively - by rotating at the right speed - cancelled out the movement of the car.

This cancellation then allows you to ignore the speed of the car, and instead work off the speed difference between the wind and ground!

In reality - it's very far from an airscrew, and turbines have a lot of drag. It's the same basic concept though.

Another beautiful and 'obvious' when you think of it bit of physics.

• #### Not impressed (Score:5, Funny)

on Sunday June 06, 2010 @07:35AM (#32474636) Journal
Let me know when you have a solar powered car traveling faster than light.
• #### The Oracle Trimaran..250% faster than the wind... (Score:4, Informative)

on Sunday June 06, 2010 @07:35AM (#32474638)

The Oracle trimaran [bmworacleracing.com] that recently won the America's Cup had no problem exceeding wind speed due to aerodynamics, and the insanely cool carbon fiber wing that added to sail volume and power, and allowed them to use a fixed-shape sail - a huge advantage. They had no problem sailing between 16 and 24 knots upwind in 5 to 10 knots of wind—that’s 2.5 times wind speed.

They went even quicker periodically, and had a five knot downwind advantage. The first race report [boats.com] shows that the Oracle trimaran was able to almost constantly fly both outer and center hulls (amazing on a boat this big.,.I sail Hobies [hobiecat.com] and this shit is HARD) and execute some slick pre-race maneuvers (which is how you really win sailing races).

So yes, sailboats have been exceeding wind speed for a while, but not by 250%..until now. When a car does that, I'll be impressed.

• #### Re: (Score:3, Informative)

Sailing boats don't go faster than the wind when moving directly downwind. They tack at an angle which allows them to go faster than the wind speed in the downwind direction while never actually travelling directly downwind.

• #### Apparent Wind (Score:2)

Consider the apparent wind, not the actual wind.

When a sailboat or iceboat is sailing across the wind (beam reach), the apparent wind (the velocity of wind relative to the sail) diminishes more slowly than the velocity of the vessel increases. This is why it is possible for a vessel on a reach to exceed the surface-relative wind velocity.

When a sailboat, iceboat, or hoax travels directly downwind, the apparent wind is equal to the velocity of the wind minus the velocity of the vessel. The force imparted on

• #### Re: (Score:2)

In short, this did not happen. The vehicle did not reach steady state direct downwind travel above the ground-relative velocity of the wind.

You are incorrect. It did just this.

• #### Re: (Score:2)

...Which is exactly why it is so damn bothersome and confusing that this appears to have happened.

• #### Re: (Score:2)

It works because you are using a fraction of the applied power to create a counter force to your source of power.

Remember your old toy, the yoyo? Put it on the floor with the string extended to it's entirety, now pull the string until it's in your hand.

How far did your hand travel? how far did the yoyo travel?

• #### Re: (Score:2)

So what you're saying is, if you change the design substantially and it wouldn't work, this is proof of the original design not working?

Just because you can't understand how this actually works and don't understand the aerodynamics doesn't mean that it's impossible. This situation is not analogous to either of your examples, you really don't even understand how this car works.

It's a wind powered car. The wind has a lot of energy, it has been harnessed to go faster than it's own speed long ago by tacking sai

• #### Explanation (Score:2)

I think I can give a pretty good explanation how this works.

Let's start at the point where the vehicle is going at exact the same speed as the wind. The propeller is seeing exactly zero wind speed. Now look to the wheels and their link to the ground. Lets put a 100 pound load on the wheels, a force acting to slow the vehicle down. This supplies us power to drive the propeller. So now we have a 100 pound rotary force to spin the propeller. The propeller has a lift-to-drag-ratio greater than 1. 100 pounds of

• #### Re: (Score:2)

I think I explained part of that badly.

We'll assume vehicle speed is twice the wind speed. We'll imagine a 1 second period where the wind moves forwards 100 feet and the vehicle moves forwards 200 feet. Work is a force through a distance. The wheels are seeing 100 pounds of force through 200 feet of ground travel. The propeller is experiencing 100 pounds of force, but the propeller only moves 100 feet relative to the air. The wheels are doing twice as much work (and producing double the power) than the prop

• #### Conservation (Score:2)

lets look at a specific state.

If the wind is directly behind the vehicle and the vehicle is travelling at exactly the speed of the wind the wind speed relative to the vehicle would be 0.
Since it takes power from the forward momentum of the vehicle to turn the propeller that would cause the vehicle to slow down.
The propeller would push against the air with a most the same force as supplied by the wheels causing the energy to be put back into the momentum of the vehicle at most bringing the speed back up to t

• #### the propeller is going backwards? (Score:2)

If the cart is going directly downwind faster than the wind, then the apparent wind (the velocity of the air relative to the cart) is backwards. So, as the cart accelerates from slower than the wind to faster than the wind, why doesn't the propeller change direction? Shouldn't it be going "backwards"?

In any case, if this does turn out to not be a hoax, I think that the inventor in the video should thank his lucky stars that the wind happened to be blowing in exactly the same direction as the street in fr

• #### It's Easy (Score:2)

Sail boats have been going much faster than the wind for quite a few years. Measurements have been made and established. This should be quite a proof of concept as it is surely easier to do with wheels on land than with hydrofoils on water.

• #### Ice boats have beein doing this for a long time (Score:2)

Using a sail (airfoil/wing) to go faster than the wind is nothing new. All you need is low drag and a quartering wind (~4 or 8 o'clock). Dead aft does _not_ work. Iceboats reach ~4x windspeed, I believe some catamarins and windsurfers can exceed windspeed even in water.

It is all about apparent wind: as you start up on a tack known to sailors as a broad reach, and you accelerate, the apparent wind shift foreward and you trim your sails to go onto a beam reach. That accelerates further and you trim your

• #### Here's another one (Score:2)

There are many physical principles that seem to run counter to intuition.
This is why we called freshman physics lectures "Magic Shows". Here's one you can try at home:

Put a string on a spool and pull the string.
The spool will come toward you faster than you are pulling the string; it will even roll up the string as it moves forward!

Lay the spool on its side so that it can roll along a table, and wrap one end of the string around the shaft of the spool.
The string comes off of the spool at the bottom.
Make su

"There are things that are so serious that you can only joke about them" - Heisenberg

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