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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."
TACO Bell
I probably am just thinking about this too simply, but can't something go faster than the wind if it stores some of that energy and uses it later?
If sailboats can travel faster than the wind, of course wind-powered carts can.
Something almost scientific that ends with an explosion?
They don't do it directly downwind, however -- they do it at an angle to the wind. This guy says he's doing it directly downwind.
I'm skeptical of this claim -- though I'd like to see their analysis of why they say it works.
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
You are just adding a complicated energy storage mechanism and then having the energy collection mechanism disable itself for part of the time. It would be slower.
You could get the car up to speed faster by having a sail that folded itself as soon as the amount of energy it was extracting dropped off. Maybe a triangle sail with the base of the triangle along the bed of the vehicle and the tip at the propellor axis. Then have it spring loaded in such a way that when wind was pushing into the sail it also resisted the spring that was trying to fold it up.
Need a Python, C++, Unix, Linux develop
For the more visual people: http://www.youtube.com/watch?v=k-trDF8Yldc
The main reason nobody believes these clowns, is because they're not good at explaining how it works. I don't even see an attempt at it. Until then, what am I supposed to believe? My gut instinct or my lying eyes?
Liberty.
...we obey the laws of thermodynamics.
I don't! :-)
The Tao of math: The numbers you can count are not the real numbers.
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.
The blades of the "propeller" (rotating sail) move sideways.
One that hath name thou can not otter
At first blush you would say if the lift/drag ratio of the sail/wing/apparatus is > 1 (plus a bit for drag) then a wind vehicle can go faster then the absolute flow speed.
The complication is that the range of possible angles of attack you can achieve gets dictated to you by trigonometry. Example, if you are on a beam reach (traveling 90 deg to the prevailing wind) and your speed is equal to the prevailing wind, the apparent flow is rotated 45 deg fwd of abeam. Now, a typical wing might give you an L/D of 20 at something like 10 degres AoA, so you would set your wing (sail) at 55 degrees from abeam. Your lift vector would be 55+90+atan(1/20) ~ 148 degrees from abeam, or 58 degrees off your bow.
Well, that's forward of abeam (90 degrees off the bow), so you have a component of lift pushing forward. It's then just a matter of getting the drag of your superstructure and rolling components down low enough to make that component sufficient to accelerate you just a bit, whereupon you are going faster than the wind.
For a boat, the "rolling components" are another wing in the water (the keel) which imposes more trigonometric limitations that make it tricky but not impossible to achieve this. Normally if it is possible it happens on a broad reach. With rolling vehicles it should be easier.
I don't know why people argue about this.
Equine Mammals Are Considerably Smaller
What you again fail to understand - the sail moves sideways (sure, a quite specific case of sideways - it rotates; but there is no difference from the perspective of the wind)
One that hath name thou can not otter
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.
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.
Those who do study history are doomed to stand helplessly by while everyone else repeats it.
OMG, we are going to hear all the same arguments here as have been posted elsewhere. Clearly the machine is going directly downwind faster than the wind. Look at the flags on the cart -- the are blowing first forward then backward. There is no gearing trickery. It's really happening. There are many videos, and you can build one yourself. I have not heard anyone build one and say it does not work except Mr. Platt and the inventors claim he built it wrong.
What's interesting to me is that argument that at a nonzero angle, "of course" this is possible, but not directly downwind. I note that the blades of the propeller are angled at a nonzero angle -- is this a factor? I really don't understand that part.
Currently hooked on AMP
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//
Exactly what everyone wants to see, a mathematical proof. Of course if you look at his free body diagram and his second equation. You'll see that he has his force vector Fp going the wrong way.
Fp is pointing in the correct direction, you merely misinterpreted the meaning of it.
Ft is the drag force on the underwater turbine. It is a drag which tends to slow down the vehicle, but the important point is that we are actively drawing energy from it. And yes, it is preforming exactly the same function as the wheels on a bike. We put a load on the wheels to extract energy.
Pt is the power that comes out f the turbine (or equivalently, the power we receive from putting a load on the wheels).
Pp is the power we supply to the prop. This is the same as the power we obtained from Pt, less some negligible percentage of loss.
Fp is the force CREATED by the power-driven prop.
The Fp pointing forwards is greater than the Ft pointing backwards, which indicates a net acceleration.
does not hold up to more than casual scrutiny.
It fails under "casual scrutiny" because the overall operation is extremely counter intuitive. However the math does work out once I managed to wrap my brain around the strange arrangement of forces and energy flow.
Your gut reaction is probably screaming that there MUST be a net energy loss in trying to extract energy from the turbine to drive the prop and that the prop's forward force MUST be less than the turbine's backwards drag. But you must remember that the wind is a source of energy relative to the water (or relative to the ground). That wind-water difference is an energy source, and that energy exists no matter how the vehicle might be moving. The trick is how to access that energy source while you're moving faster than the wind.
Note that force and power are not equivalent. Power is energy over time, and energy is force through distance. The turbine is moving through the water while the prop moves through the air. There is a speed difference (and an energy difference) between the water and the air. The turbine moves a large distance through the water. A large distance times a small force generates one unit of power. The prop is moving in the air, and even though the vehicle is moving faster than the wind the wind greatly DECREASES the apparent speed of the prop relative to the air. Because of the wind, the prop only moves a relatively small distance through the air. The prop generates a large force over a relatively small distance, which costs one unit of power.
Turbine extracting energy: small force * large distance = 1 energy extracted
Prop consuming energy: large force * small distance = 1 energy consumed
The equations balance. The large prop force accelerates the vehicle. The wind-water difference is the energy source. It's a very unintuitive arrangement, but it does successfully tap into the energy available in the wind-water difference, even when traveling faster than the wind. That energy source covers the inevitable inefficiencies in power transfer and it the pays the cost of accelerating the vehicle.
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Too bad I haven't got any mod points left. Yours is the best comment in this thread by far, illuminating the essential point :
harvesting energy from the velocity difference between the wind and the ground, not the velocity difference between the wind and the vehicle