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

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."

Comments

Debate?

[Mikkeles]

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

Re:Debate?

[Rigrig]

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?

[dtmos]

Ah, no. I can do no better than to quote ThinAirDesigns:

The key thing to remember is that due to the tailwind, the wheels are traveling over the ground much further than the propeller is traveling through the air[1] -- thus using the force x distance calculations for work and power it's easy to see that when we are traveling the speed of the wind, we can gain more power from the wheels (faster moving ground) than we have to expend in the air (slower moving air).

________
[1] As he is about to mention, this is best considered at the moment when the car is moving at the speed of the wind. In this case, there is no wind over the propeller, since the car and the wind are traveling at the same rate in the same direction; however, the wheels are moving relative to the ground at the speed of the wind, and therefore turn the propeller and supply additional thrust to the car, accelerating it.

Re:Debate?

[SerpentMage]

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:Debate?

[dtmos]

The rotating sails (i.e., propeller blades) are inclined to the wind; it's how all propellers work -- when rotated, they provide the force to the car that accelerates it to speeds higher than the wind. However, the power flow is from the wheels to rotate the propeller, not the other way around.

He states quite clearly, several times, that they don't use the "sail" word, because it confuses people into thinking that the power flow is in the opposite direction. Like all propeller-driven craft, the power flow is from the craft to the propeller.

The energy comes, ultimately, from the wind. One might as well ask, "Where would the energy to move a sailboat faster than the prevailing wind come from?"

Re:Debate?

[Anonymous Coward]

The wheels power the propeller. Consider a situation without wind: You push the car. Then you'd expect that the propeller can not propel the car faster than you're pushing it. The mechanism takes energy from the wheels, opposing your pushing, and expends it turning the propeller, helping your pushing. Conservation of energy means that the force it creates with the propeller is countered by the opposite force on the wheels.

Now add tail wind. If you ignore all losses but aerodynamic drag, moving the car at a speed above ground increased exactly by the wind speed will take the same amount of power as moving it at the lower speed before. You just move the entire frame of reference with the speed of the wind and the elaborate construction of you pushing and the wheels turning the propeller is irrelevant: You put a fixed amount of power into the thing.

But we're supposed to be sailing, so no more pushing. What's the source of energy when you're sailing? The wind. What's the limit of conventional sailing? Wind speed. If you move at wind speed (in the same direction as the wind), the airspeed of the vessel is zero, so the wind can no longer exert a forward force on the vessel. But not with this vehicle: The airspeed of the vehicle is changed by the propeller. That's why the wind can keep "pushing" the vehicle even at speeds exceeding the wind speed.

Re:Debate?

[sznupi]

All the energy there comes from the wind. The wheels act as keel, providing resistance against the surface. The propeller (rotating sail, really) is what tacks to the side (by rotating...but it doesn't make any difference from "to the side" as far as wind is concerned), so the whole vehicle doesn't have to and can move "forward", with the wind.

DWFTTW--Except where the car couples to the wind!

[karlandtanya]

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:What debate ?

[Dracker]

I'm not trolling, honestly. How is this not perpetual motion?