Wind-Powered Car Travels Downwind Faster Than The Wind
wired.com
wired.com
http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wi...
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.
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...
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.
EDIT: Also that video is hilarious.
If I had a dollar for every time I heard that ...
In a way, we are in such a system. This is what makes startups so dandy!
Any ordinary sailboat sails faster than the wind when going roughly perpendicular to the wind direction. But this is apparently something different.
Contrary to popular believe, the spinning rotor is not a turbine (as you describe), but rather a garden variety prop which is driven by the wheels.
But any type of "sailing" that isn't directly downwind depends on the speed difference between the air and the surface (land or water). If you have this type of speed differential your vehicle speed is limited only by friction and sail size.
What they've done is applied this principle to direct-downwind sailing. While you can't get any energy from apparent wind in this situation, you can use the speed of the wind (and the vehicle) relative to the ground to extract some energy, which can then be used to increase the speed of the vehicle.
From an energy perspective, it's not unlike a having lever fixed to the ground at one end, with a sail in the middle, and the vehicle at the other end. In this situation the vehicle can clearly travel faster than the sail without assuming any kind of perpetual motion.
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... 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.
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...
It really grinds my gears when websites do this! Grrr
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.
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?
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.
>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.
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.
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.
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!
Moving on.
No, it wouldn't. The vehicle extracts energy from the velocity difference between the air and the ground, which it converts into forward motion. This is essentially the same mechanism used by sailboats tacking against the wind, and despite being counterintuitive at first is well-established. The speed is limited only by gearing and mechanical efficiency--an idealized device could accelerate arbitrarily.
While the Wired link doesn't explain how the device works, the same idea has made the rounds on the internet many times before; links to detailed descriptions were already posted in the comments here, which I assume you didn't bother to read before implying that this is a scam?
"Trying to put skeptics on the defensive"
Sticking with a knee-jerk reaction against something while ignoring further information that supports it gives "skeptics" a bad name.
Tacking is different from this. This is using ground-speed to generate thrust, as even the shills acknowledge.
"The speed is limited only by gearing and mechanical efficiency--an idealized device could accelerate arbitrarily."
Arbitrary magic acceleration inside an atmosphere should be your first clue that something's extremely wrong here.
And yes, I looked at the various forum links that turned up, which made very clear the very typical full-court astroturf press going on (like the argument above with someone who registered only to argue with unbelievers in this thread). You might take a look.
"Sticking with a knee-jerk reaction against something while ignoring further information that supports it gives "skeptics" a bad name."
Skeptics get a bad name because they don't join in the squee.
All I ask is that you look at the further information. Check back on this story in months and years and see whether it pans out.
...in other words, either you didn't actually read the explanations or you didn't understand them. The energy is being extracted from a velocity difference between two media. And no, tacking works on exactly the same principles in order to sail upwind. Some manner of mechanical linkage decelerates one or both media with respect to the other, extracting usable energy. In the case of a boat it's the combination of the keel and sail; for this vehicle, it's clever fixed gearing between the fan and wheels. In either case, the velocity of the vehicle relative to either medium is nearly irrelevant.
Check back on this story in months and years and see whether it pans out.
The idea behind this vehicle has been floating around the internet for at least three years, by the way.
If you're really so confident that you're correct, I'm sure one of the "shills" would be happy to make a wager with you, mediated by a neutral third party. What kind of odds would you accept for a wager that the vehicle doesn't work?
"If you're really so confident that you're correct, I'm sure one of the "shills" would be happy to make a wager with you"
Funny; demands to make a bet seem pretty common around this whole matter. No, I'm not going to waste my time dickering over the terms of a bet with shills or true believers. I'm going to wait for this farce to fall apart.
Given a steady source of energy, you can accelerate as much as you like, up to the limit of losses due to friction. What do you think is going to prevent that? More energy, more acceleration.
And again, remember that the ability to extract energy from the velocity difference is independent of the vehicle's speed. The acceleration isn't infinite of course, because the energy has to come from somewhere--but so long as the velocity difference between the media persists, energy can be extracted. In this case, that means "as long as the wind is blowing", since the energy to move a lightweight vehicle around isn't going to make much of a difference compared to the total energy in the atmosphere.
Funny; demands to make a bet seem pretty common around this whole matter.
Demands? More like an offer of free money, wouldn't you say? I mean, you'd just have to prove that their clearly impossible machine indeed doesn't work, right?
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.