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Going Faster Than the Wind In a Wind-Powered Cart
Shawnconna writes "Can a wind cart travel faster than the wind? A group of makers say, 'Yes!' Make: Online has published a story about the Blackbird wind cart that just set a record. This is a follow-up to an earlier story in which Charles Platt built a cart based on a viral video where a guy claimed he'd built a wind-powered vehicle that could travel downwind faster than the windspeed. Charles built one and said it didn't work. Heated debates broke out in forums, on BB, and elsewhere on the Net. In the ensuing time, a number of people have built carts and claimed success, most principally, Rick Cavallaro. He got funding from Google and JOBY to build and test a human-piloted cart. They claim success, with multiple sensor systems on board, impartial judges and experts in attendance."
If sailboats can travel faster than the wind, of course wind-powered carts can.
It is not the job of engineers or gods to figure out the science. That is for the scientists. Apples fell from trees long before Newton thought about it.
The scientists can be skeptical, they can demand reproducible tests, but once the tests have been done it is THEIR job to find an explanation, NOT that of the engineers.
These guys build something, they opened themselves up to a lot of tests, so either you make some real accusations and not just "idiot slashdotter doesn't understand so it must be fake" or start to work out the math or just accept that you are an idiot along with everyone else and leave this to smarter people.
But they do NOT have to explain to you how it works, they got far smarter people to convince, not some random kiddie on the net.
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The blades of the "propeller" (rotating sail) move sideways.
One that hath name thou can not otter
You should read the discover article on the thing. I was saying exactly the same as you before I did. The gist of it is this: Imagine the car going exactly at the speed of the wind. In the car there is no wind, except that the ground is moving. The ground moving turns the wheels powers the propellor, which rotates and gives the car a force forward. Since wind speed is 0, there is no resistance, no force to counter the propellors force, the car will now accelerate, i.e. start going faster than the wind. Once you accept that it will then go faster than the wind, the only question is how much faster. (The ground acting on the wheels will exert an very small force on the car, which can be made infinitesimal by reducing the friction)
Well said. I think it should be possible to gain some amount of velocity greater than wind speed on a sailboat with an added fan connected to an underwater turbine, but I'm not sure if the fractional speed gained will be useful. It would be a very cool demo to try, though, and even achieving 5% over wind speed would be very interesting.
I think the main problem with a boat is that you have a massive amount of friction with the ground (the water), while on a car the axle friction of the wheels can be made very small. On a boat, you need a large sail to extract a large amount of energy from the wind to overcome this friction and achieve a significant fraction of the speed of the wind, while a lightweight ground vehicle with a small sail can easily achieve a speed very close to that of the wind. On a boat, the friction with the water is hard to direct towards some kind of turbine and not towards the hull of the boat, while on a car you can easily use wheel motion almost exclusively to turn the fan (fully static friction between the wheel and ground), expending a relatively small amount of the energy as friction on the axle. I think this might very significantly undermine the ability to "port" this trick to a boat. I hope someone proves me wrong, though :)
My conclusion: This is a storm in a teapot. The guy duped everyone by using the wrong terminology; he's actually traveling upwind (into the wind) by everyone else's definition. This is confirmed by the direction of the streamers in the video embedded in TFA.
Wrong reasoning, wrong conclusion. The cart is indeed travelling downwind, i.e. in the same direction relative to the ground. Moreover, physics do not state that energy cannot be extracted from the wind when going faster than the wind, because you also need to think about the wind moving relative to the ground. That is the energy difference being extracted. The ultimate theoretical point where the cart cannot possibly accelerate any longer is when the wind speed relative to ground in the wake of the cart is zero. And finally, this experiment is not hard to do on your own in a small scale test.
True confidence comes not from realising you are as good as your peers, but that your peers are as bad as you are.
This is what I first thought. But you are thinking of speeds gained in cross-wind situations, where sail-powered crafts easily travel faster than the wind speed.
This is faster than wind-speed in downwind situations, "spinnaker legs", in other words. Took me a few minutes to get my head around the physics, but the concept is simple once you have the idea. The grandparent is a very good, if a little long-winded (oh groan) explanation.
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Yes, and iceboat races often have top speeds 3 or more times the wind speed. But it's not achieved when running downwind; it's when moving approximately at right angles to the wind. That's when the airfoil effect is the most effective. When you're aimed downwind, the sail is little more than a parachute, and can't move faster than the wind (though with the low friction of an iceboat's runners, you can get a ground speed pretty close to wind speed).
The summary claims a "downwind" speed faster than the wind. Is this physically possible? I'd think that you could build a perpetual-motion machine if you could do this.
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If you look at Rick Cavallero's replies to posts here, you'll see he directly answers that question, clarifying that there is a ratchet to prevent the propeller from directly turning the wheels (i.e. only the wheels can turn the propeller). This was how they proved to NALSA that they were not using stored energy from the propeller as a flywheel to accelerate the vehicle.