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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."
It should be easier than in a sailboat. After all, all you need to do is find a steep enough hill.
When our name is on the back of your car, we're behind you all the way!
Not necessary to store energy to go faster than the wind.
The reason this works is that the propeller is able to "push off" against the tail wind. Think of it like sitting on a skateboard and pushing off from a moving wall behind you with your arm. The difficulty in making it work is that you need very little drag and a very efficient propeller. But the energy equations for traveling faster than the wind do balance and there is no violation of energy conservation.
When all else fails, run.
Something almost scientific that ends with an explosion?
I think they are overcoming that particular limitation with the propellor which is technically approaching the wind indirectly.
Nuclear war would really set back cable. - Ted Turner
it is possible, if what you do is to extract energy from the speed difference between the wind and the ground instead of that between the wind and the vehicle. Consider this greatly simplified concept: Build an enormous wheel, and set it up so that it has large sails around its circumference, between the thread and the shaft. Sat things up so that the sail will be closed or parallel to the wind when on top of the wheel, and perpendicular to it when on the bottom. The wind will push the sail, that will lever against the ground and cause the wheel to roll forward. Since the shaft is above the sail, it can travel faster than the wind even if the sail is slower,, and if the resistance of all the setup is small enough, you have something that travels faster than the wind, even if it's actually pushed by it
In a sailboat or iceboat, to travel faster than the wind you head about 45 degrees off of the direction from which the wind is coming (called 'reaching'). The sails then work as airfoils, creating lower pressure on the outside of the sails, which in conjunction with the keel propel you forward damn fast if you choose (iceboats sometime 4-5 times the speed of the wind). However, when you are 'running' (heading directly downwind) the sails are not working as airfoils, but function merely as a wall the wind hits that propels you forward. You don't go faster than the wind in this case. The article specifically mentions heading directly downwind.
For the more visual people: http://www.youtube.com/watch?v=k-trDF8Yldc
Here's an analysis performed by Mark Drela of MIT (http://web.mit.edu/aeroastro/people/drela.html)
http://www.boatdesign.net/forums/attachments/propulsion/28167d1231128492-ddwfttw-directly-downwind-faster-than-wind-ddw2.pdf
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.
MMO Quests are like orgasms:
You may solo them, I prefer them in a group.
Nope, you've got it backwards, the GP got it right, and this is absolutely the key to understanding how this works.
The car isn't using the propeller as a turbine as a source of energy to power the wheels. That, indeed, would be impossible, because once you reach wind speed the force exerted on the propeller is zero.
Instead, it works the other way around, as a fan to push air backwards and accelerate the car. The energy is transfered from the wheels to the fan.
Assume that, to begin with, the car is moving at wind speed. The wheels are spinning (because the car is moving) and you can use that energy (i.e. brake the car) to push the propeller. The propeller blows air backwards, which propels the car forwards. If your mechanism is efficient enough, that push more than counteracts the braking action on the wheels and the car actually accelerates forwards. As it accelerates, the efficiency drops and it eventually stabilizes at some speed, faster than the wind.
Now everyone is shouting "Perpetual motion! You're producing more energy with the fan than you're getting out of the wheels!". Nope. That's the final bit. Let's say that wind speed is 10km/h. If the car is moving at 11km/h (faster than the wind), then the motion on the wheels relative to the ground is 11km/h. However, the fan only has to push air backwards at 1km/h, as the wind is doing the rest and providing the base 10km/h of forward motion. This difference in velocity is what offsets the inevitable energy losses: the ground speed is whatever you're generating with the fan plus the velocity of the wind "for free". This "free velocity" goes down (as a fraction of total velocity) as you accelerate, until it matches the (in)efficiency of the system (energy loss), and this is the stable velocity that the car achieves, faster than the wind.
This really isn't an issue with perpetual motion. It's easy to see that you could use a stationary turbine to generate (say, electric) power from the wind, and then use that power to accelerate a car (say, powered by a laser, so it is not tethered) in a different (windless) location faster than the original wind. Output velocity can be greater than input velocity. The difficulty lies in grasping the interesting mechanics and interactions of the downwind-faster-than-the-wind car uses to achieve this within the original wind itself. It's a mechanics puzzle, not an energy conservation puzzle. Another way to look at it is that the energy lies in the difference between the velocity of the wind and the ground, and the car always has access to both of these moving entities via friction (friction with the wind, and friction of the wheels with the ground), and thus can harness that power regardless of what its own velocity is.
On a treadmill, if the vehicle is moving forward (relative to the observer, not the treadmill belt), then it is moving faster than the wind (which is moving at velocity zero relative to the observer). It is simply a change of frame of reference. If you place the observer on the treadmill's belt, then the wind is blowing forwards as fast as the outside world is moving forwards, and the vehicle is moving forwards faster than that. On the flip side, if you take the real-world DWFTTW vehicle example, and place the observer on a balloon moving with the wind, then (just as in the treadmill scenario) the wind is moving at zero velocity relative to you, the ground is moving backwards (just like a giant treadmill), and the vehicle is moving forwards faster than you (just like in the treadmill example the car moves forwards relative to an outside observer, even though the treadmill moves backwards).
To answer the GP, see my post above. Everyone (including myself at first) immediately assumes this is a turbine-powered car using a wind turbine to drive the wheels. That's backwards, it's a sailcar (simply pushed by the wind) which in addition to that uses the wheels as generators to drive a fan (not a turbine) to push air backwards and increase thrust, thus actually achieving faster than wind speed.
It's simple: the vehicle must be able to move forwards faster than the wind forever, as long as the wind keeps blowing. In other words, the energy stored in the moving parts must not decrease and eventually cause it to stop working. Or in yet other words: the system must achieve a steady state where energy is flowing in and out at a constant rate, while traveling faster than the wind.
For a race where time matters, energy input initially into the system is relevant. However, for the purposes of proving that DWFTTW is possible, it isn't. Any amount of energy added initially will by necessity be dissipated in the friction losses of the system - you can't run a car forever on a fixed amount of energy. If it can, in fact, run forever on a steady wind, then you can discount any initially applied or stored energy, and conclude that it is being powered solely by the wind. If it does that while going faster than the wind, then you can conclude that DWFTTW is possible.
You do realize that NALSA certified it, right? As in they checked all these things?
They installed a bracket on the shaft to ensure the propeller never drives the wheels, so all the momentum of the propeller is going to be able to do is allow the propeller to continue spinning. It never, ever, drives the wheels.
Anyway, it's way beyond theoretical. The current land sailing speed record is 3.15 times wind speed 126mph in a 40mph crosswind, fast!), set with a traditional land sail in a crosswind. It was set the same day NALSA certified the first DDWFTTW record.
Here's the explanation of the physics: http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind
Security is mostly a superstition... Avoiding danger is no safer in the long run than outright exposure. - Helen Keller
So here's a question for everyone: could you make it work in a boat?
Yes.
We're all born with nothing.
If you die in debt, you're ahead.
Why is it that you are comfortable with a device that travels upwind at 3 times the wind speed, but think that one that travels downwind at 3 times the wind speed is a perpetual motion machine?
Clearly, any wind powered vehicle that travels faster than the wind in ANY direction must be harvesting energy from the velocity difference between the wind and the ground, not the velocity difference between the wind and the vehicle, or it would be a perpetual motion machine.
http://xkcd.com/756//