4 ms·
[EDIT: I started typing this up before smanek's post. It's essentially the same argument, but I intentionally left out the formulae.] Really cool. Here's my be
by JoachimSchipper 16y ago
[EDIT: I started typing this up before smanek's post. It's essentially the same argument, but I intentionally left out the formulae.]
Really cool. Here's my best explanation of how they achieve a stable speed faster than the wind; this isn't explained that well in the article. (Disclaimer: I'm not exactly a trained physicist/aerodynamics expert.)
They've built a lightweight vehicle with a huge propeller directly linked to the drive shaft ("directly linked" is important here!) With the vehicle at rest, the propeller catches wind and begins driving the shaft; the car itself also catches some wind. Obviously, the car speeds up.
Once the whole contraption is at or near wind speed, the wind obviously stops driving the propeller. It's clear that the vehicle can maintain this speed just by using itself as a sail.
Now, note that the wheels, directly linked to the propeller, are now driving the propeller. Running through stationary air (since the vehicle runs at wind speed, the air is stationary relative to the propeller), the propeller begins accelerating the vehicle!
Of course, as the whole contraption accelerates beyond wind speed, air and ground friction increases, etc. But all of these effects are gradual, and since we have already seen that it can, in fact, accelerate while traveling at wind speed, it will continue accelerating for a while beyond that, eventually reaching some maximum speed above the actual wind speed. Apparently, this is ~2.8x the wind speed for this particular vehicle, which is an impressive engineering feat.
The article says that this offers opportunities for power generation, but I'm not seeing those. Still, very cool.
- dirtyaura 16y agoThanks, this was the clearest explanation of the vehicle so far.
- illumin8 16y agoAre you sure that is correct, that the wheels are powering the propeller at some point? The article explained it completely differently. The article said that the effect is similar to when a sailboat tacks downwind sharply, they can briefly accelerate beyond wind speed. The propeller is essentially shaped as if each side is a sail continuously tacking, so the force from each of the two "sails" drives the propeller in a circular motion. I don't believe the wheels are providing any energy into the process, but I could be wrong. I think it is the shape of the propeller that causes the effect. The article also said it could most likely work on water as well. The wind energy applications of this propeller design are intriguing.
- stygianguest 16y agoYes, the wheels drive the propeller directly. Read the articles posted in the first comment. They will explain it. Very short summary: The prop creates an air cushion that acts as a virtual sail.
- deleted 16y ago[deleted]
- JoachimSchipper 16y agoThe interesting part is not that they can briefly go faster than the wind, but rather that they can keep this speed up. It's easy to briefly go faster than the wind: use some gears to connect a windmill to a heavy piece of metal rotating on the end of a long pole. Assuming a strong wind, low-friction gears, and that you've chosen the gears such that the metal rotates faster than the windmill, the machinery will soon be spinning quite rapidly. Now, just put a hapless pig in the way of the rotating piece of metal, and you suddenly have a slightly-dented pig flying at greater-than-wind speed. ("With sufficient thrust, pigs fly just fine", although this will definitely annoy the pig.) I'm sure you can think of other ways to store the wind's energy and use it for a speed boost; the interesting thing is that this vehicle can consistently outrun the wind. Also note that my explanation does not require the vehicle to be on the ground instead of in the water - think "water wheel". With respect to "tacking": you'll find that Chu-Caroll's article, which smanek linked to, provides the same explanation as I did (skip the initial part where he explains his error and start reading after "let's look at a new diagram"), and mentions the similarity to "tacking" (start at "Once it's moving, what's going on can be described in a bunch of different ways.") However, note that not even the "perfect" sail that Terence Tao assumes in the article linked above achieves more than 2x wind speed; clearly, there is something about this real-world vehicle that makes it move faster than even theoretical (fixed-)sail-powered cars. I think my comment above is a plausible explanation for how this can happen.
- deleted 16y ago[deleted]
- barrkel 16y ago
- peterwwillis 16y agoWhat I don't get is why people thought this was some heretical nonsense? It makes total sense, and is kind of obvious when you actually consider the design. A car with a big sail on it would only be powered by the wind and probably not exceed the wind speed, because nothing else is driving it. But it was clear that this thing had two power sources: the force of the wind moving it forward, and the force of the prop pushing the air backward (that may sound totally retarded, I know nothing of physics) and propelling itself forward. It's as if people just stopped thinking once they hit the conundrum part of it and purely started trying to form reasons why it could not work, and didn't really question or analyze it with the possibility of them being wrong.
- deleted 16y ago[deleted]
- klochner 16y agoWhat I think you're missing is that the wind never drives the propeller - the wheels drive the propeller. When it first starts moving, the propeller moves the opposite direction from what you would expect - so that the propeller is actually pushing back against the wind from t=0.
- JoachimSchipper 16y agoYou're right - the propeller may help early acceleration by acting as a sail, but it's never rotated by the wind. Thanks for the improvement.