A California team recently tested a wind-powered car that can actually outrun the wind, adding more fuel to a lingering physics debate.
In a test two weeks ago, the car hit a top speed 2.86 times faster than the wind, according to its creators. Some physicists say this should be impossible, but car-builder Rick Cavallaro says that's exactly what happened on May 16. What gives?
Cavallaro had funding from Google and Joby Energy to build the car, basically an aerodynamic foam chassis attached to a wind turbine, with the San Jose State University aerodynamics department.
Wired's Autopia blog features the car and Cavallaro's quest to prove it is possible to travel directly downwind faster than the wind.
The wheels turn the car's propeller, and the prop thrust pushes the car, which turns the wheels, Cavallaro says. It's not a perpetual motion machine, because the prop is also using wind energy as an external power source.
He says the toughest part was designing a transmission system to transfer power from the wheels to the propeller. Design and construction took almost a year, and Cavallaro has detailed photos and schematics on his blog.
His team made eight test runs May 16 at New Jerusalem Airport in Tracy, Calif. The next step is a land-sailing record certified by the North American Land Sailing Association, Cavallaro says.
Not too hard to figure out. You use the wind speed to propel the vehicle, but if this is designed properly you only need to use a fraction of the energy from the wind to propel it to the winds speed. With the extra energy you store it and then use that energy accelerate the vehicle beyond the winds current speed.
My "explanation" of how it works in the article is a bit unfortunate. That was intended to be my explanation as to why so many people think it should be impossible.
@Setarip: We don't make any use of stored energy. This vehicle can go more than 2.5X windspeed, directly downwind, steady-state.
Ah ok, Sounds pretty interesting. What was the actual windspeed during this?
Yes KH, that is a very good point (about sailing not used car salesmen), but that isn't always true. Wait... never mind, that is always true, you can't go faster than the wind compared to the direction of the wind.
The thing that makes me uneasy is that the car goes downwind, and that, as KH said, would make "You lose the power of the wind when you reach the windspeed..." My only explanation for getting any faster would be from the inertia of the propeller, which could go faster than the wind, because it still has energy from the wind when the wind (relative to the speed of the car) was faster. This wouldn't be anything special, heck, I could do that in my back yard. I'd just say it is a clever energy capturing storing device, with a big fly wheel.
i'm sure it works. wind varies in angle. it kinda snakes around. plus, it blows in bursts. i'd guess that during the trip downwind that the propellor blades would have moments where it slowed down and sped back up. good job.
Thank you Setarip for the logical explanation - there's the energy that the wind transfers into spin on the huge propeller blades, and there's the energy required to move the prop assembly (the car) forward in a straight line. There's no reason that the two should have to cancel each other out at the speed of the wind that powers the blades - the wind itself is not linearly pushing this car.
>> you can't go faster than the wind compared to the direction of the wind.
Indeed you can. Ellison's speed in the recent Americas Cup on a direct downwind projection was nearly 3X the windspeed.
>>My only explanation for getting any faster would be from the inertia of the propeller
I assure you the inertia of the propeller does not come into play. This cart can go DIRECTLY downwind at better than 2.5X windspeed continuously.
The wind blows against the back of the car and propeller. The wheels turn, which is geared to turn the propeller for forward thrust, against the direction of the wind. ie. backwards. Once the speed of the wind is reached, the backward thrust pushes the car beyond that speed. I'm just guessing. It's the only way that I can think that it would work.
You know, I didn't believe it at first. But it makes an odd kind of sense if you think about the work done on the air being at a different speed than the work done by the air.
Found some videos on youtube.
The answer is simple, the faster you go against the wind, more power will come from the wind.
Ex: If the wind is at 50km/h and you go downwind at 50km/h this mean you have the power of a wind at 100km/h.
(sorry, english isn't my native language
Skeptics think that the wind is turning the prop, and the car is turning the wheels, and that’s what makes the car go,” Cavallaro said. “That’s not the case. The wheels are turning the prop. What happens is the prop thrust pushes the vehicle.
This is a quote from the Autotopia article, and it doesn't make sense to me. If the prop is directly connected to the axles, and if the car starts moving with no outside intervention than the statement is incorrect per se. If you push it to start it then the next question would concern thrust.
As far as I know air can only flow one way at a time across an airfoil, so if the wind comes from the rear of the vehicle than wouldn't the thrust vector also be downwind, the opposite to the movement. By this reasoning you wouldn't get any thrust until you exceeded the wind speed.
It may be that they have found a new principle at work and simply haven't recognized it yet. Of course they said bumblebees couldn't fly, but it would be nice to see some numbers.
I didn't mean to say that the car can't go faster than the wind. I intended to point out that the same angular forces which cause the boat to go at a higher speed apply to the car as well... it just isn't as seemingly logical due to the change of where the angular forces are being applied.
You're quite right.
>> Why doesn't someone put a model of this car into a wind tunnel?
Wind tunnels are for vehicles that go into the wind. We put this on a treadmill - so it will advance slowly (even on an inclined treadmill) when going just above wind speed.
Initially the vehicle is pushed by the wind as a bluff-body. As the cart begins to move, the prop begins to spin. As it approaches wind speed, more and more of the prop becomes unstalled (starting at the tip). At some point well below wind speed the prop is creating significant positive thrust. This continues to well above wind speed (2.86X is our best run so far - several runs in the 2.75X range).
>>It may be that they have found a new principle at work and simply haven't recognized it yet.
Nope. We use all the old tried and true principles - and actually understand them quite well.
I'm skeptical and suspect the vehicle is just storing and releasing energy to briefly exceed the wind speed. The data presented on their website shows no steady state. It shows the vehicle accelerating, but the vehicle brakes _immediately_ after obtaining its maximum speed. The videos corroborate the posted data. One can see it braking as soon as it hits its maximum speed.
Do you really have any steady state runs with data?
>>I'm skeptical and suspect the vehicle is just storing and releasing energy to briefly exceed the wind speed.
In point of fact we don't even have a way to use stored energy to accelerate. To do that we'd have to slow the prop down as we increase the speed of the wheels. But our gearing is fixed.
>> The data presented on their website shows no steady state. It shows the vehicle accelerating, but the vehicle brakes _immediately_ after obtaining its maximum speed.
I don't recall exactly what data we posted on the blog, but I can tell you the only good data runs we have to date were done on the runway at New Jerusalem. I had to brake before running off the runway for some of the runs (I was still accelerating), and I topped out briefly (at around 40 mph if I recall correctly) for another couple - before having to brake to avoid running off the relatively short runway).
>>Do you really have any steady state runs with data?
As I mentioned, we have a couple of runs where I topped out briefly before having to brake. We also have small scale models that run indefinitely on the treadmill - even climbing an incline. But more importantly, the analysis shows that it can clearly exceed the wind speed by a large margin - steady state.
In effect you have created a greater drag than a propeller's area would have. The propeller is pushing against the wind. The two forces are creating this action. It would be easy to suggest that a plane can travel faster than wind since it has a propeller and motor. In this approach the wind is both the sides powering the action.
>>In point of fact we don't even have a way to use stored energy to accelerate. To do that we'd have to slow the prop down as we increase the speed of the wheels. But our gearing is fixed.
You can still store energy and accelerate beyond the wind speed with fixed gearing. This is because you are using props with a variable pitch. This effectively does change the gearing (as far as thrust is concerned). You can start with a pitch that is favorable to getting up to speed. Then once you get up to the wind speed, the drive extracts this rotational energy by changing the pitch of the props to produce rearward trust. Your website says the pitch is adjustable from the cockpit.
More importantly, your claim of steady steady has not be demonstrated. A steady state period that last longer than the entire earlier phase of the experiment is needed to prove the vehicle isn't just storing and releasing energy. Carl Sagan said, "extraordinary claims require extraordinary evidence."
When this car goes downwind, wind does not rotate propeller blades. Blades are rotated by the wheels through transmission. The propeller creates a thrust against the direction of the wind (i.e. backwards). This is how it goes faster than the wind.
OK....my 2 cents
The article was interesting and even a bit entertaining.
But after reading 23 comments debating the physics involved I now have a splitting headache.
Off to the medicine chest for me.
I believe it is possible after reviewing the physics of sailing and watching this video www.youtube.com/watch?v=aJpdWHFqHm0
The pitch of the props are equivalent to a sailboat sailing at close reach which allows it to go faster than the wind.
It would still be cool to see the full scale model go for a long time.
Sailboats leverage the wind against the water to travel faster than the wind, but at an angle. Wheeled vehicles with sails do the same thing.
Going directly downwind, the propeller would slow as it reached wind speed, because there is no relative wind. Even with gearing or pitch control, no wind is no wind. This is crap.
It also cannot propel itself directly into the wind, although that would seem more plausable, and would have a better chance of duping people, which I'm sure is what they are trying to do. Soon you will see a call for investors. Maybe they should buy Moller International's 'investor' list.
So the wing starts in stall. If the vehicle acts as a bluff body then you have a mechanism to prevent counter rotation. Is that the ratchet mechanism I saw referred to on your site? Okay so far, so early on it's a kite. Now as the speed increases the prop unstalls gradually until it provides thrust. To increase beyond wind speed the thrust must add to the force of the wind. So now it's like swimming with the tide. To go any further I have to assume that the total energy output is equal to the force of the wind on your vehicle, minus losses else wise we are in perpetual motion territory. This implies to me that your are using the prop to recover the lost energy due to friction of one type or another. Effectively you are using your prop to increase your efficiency. Do you have any numbers on that.
@poindextrose: "You can still store energy and accelerate beyond the wind speed with fixed gearing. This is because you are using props with a variable pitch."
Nope -- the only way to use the stored energy from the drivetrain (prop, etc) to accelerate is to slow down portions of the drivetrain (say the prop) and use that energy to speed up the wheels. With fixed gearing that is impossible.
Change the pitch of the prop all you wish, but you'll never be able to use any of that stored energy with the fixed gearing.
Nice MAKE though.
>>You can still store energy and accelerate beyond the wind speed with fixed gearing.
Actually, you can't without dragging the wheels along for the ride. To USE the energy stored in the spinning prop you have to slow it down. This will slow the wheels down. Unless you skid the wheels, the cart will slow down.
>> More importantly, your claim of steady steady has not be demonstrated....
There's no period of time that conclusively demonstrates steady-state. But we've done it ridiculously long with our model on a treadmill. And again, the analysis shows it can do so.
The prop is driven by the wheels. When the body of the vehicle is at windspeed the prop is climbing through the air faster than that.
Look at it this way. If you're standing along the track with a windspeed of 10 (pick your preferred units) this cart drives past you with at 20. Its prop is climbing through the air at 25. The air along the track just before the cart is moving at 10. Just after the cart passes it's moving in the same direction at 5.
They're slowing the wind down just like any other windmill by extracting energy from it.
@PhillnYork The prop doesn't slow down because it is directly attaching to the wheels via gearing. It's not a one way thing; if the wheels turn, the prop turns, if the prop turns, the wheels turn. What you are saying is that it's one way reaction that the prop turns the wheels but the wheels don't turn the prop..which isn't this case.
btw thanks ziploc for responding to all the comments with your information! This has the most genuinely conversational comments that aren't involved in a flame war that I've seen on here.
It's not a one way thing; if the wheels turn, the prop turns, if the prop turns, the wheels turn. What you are saying is that it's one way reaction that the prop turns the wheels but the wheels don't turn the prop..which isn't this case.
Actually, for practical reasons we do have a custom machined ratchet in the middle of the prop-shaft that prevents the prop from driving the wheels in the forward direction. This was done in part so we wouldn't have to rapidly brake the prop when braking the cart. This isn't really much of an issue as it happens. And now that we have a variable pitch prop we do just the opposite - we use the prop to brake the wheels at the end of the run (before applying the disk brakes).
The other reason for that particular ratchet is related to a transmission approach we considered (spool and take-up reel) but didn't need to implement in the end.
Downwind faster than true wind speed is easy, just like with an actual sailing vessel. In light to moderate breeze, the vessel's downwind velocity causes the wind velocity, relative to the vessel, to increase and the airfoils' angle of attack to decrease; sailors would say boatspeed has caused the apparent wind to shift forward.
On the wind-car, the apparent wind to the prop blades is quite a bit higher than the true wind and the angle of attack is similarly decreased, as prop & vehicle speed increases. Obviously, once the vehicle speed equals true windspeed, the apparent wind to the prop will still be significantly higher than true windspeed, allowing the vehicle to further accelerate.
What I'd really like to know is what happens when the wind car heads up, i.e. turns into the wind. Assuming the car doesn't capsize, reaching across the wind should be easy & it should also be able to sail at slightly less than a 90 degree angle to the true wind.
Thin Air guys, any interest in getting some hiking straps and big, empty parking lot so you can build us some polars? ;)
@ziploc Oh, so..it's really the propulsion of the propeller that's pushing the car? The only function of gearing the two together was for braking? (I'm just vocalizing my thought process) Or, wait, the wheels turn the prop which generates thrust. I think it's that. I'm sure it's said somewhere in the article/comments, I just needed to wrap my head around it.
I need to pay attention in my classes more..