General Question

timtrueman's avatar

Can you explain to me how it is possible to go downwind faster than the wind?

Asked by timtrueman (5746points) November 11th, 2010

I saw this article and it’s just not making sense to me. How can it maintain or add to its energy once it’s going faster than the wind?

Please, no guessing. Only answer if you actually know and understand the problem.

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10 Answers

iamthemob's avatar

It’s probably storing excess energy as the propeller spins, and then using that energy to increase the speed.

timtrueman's avatar

No, there’s no energy storage—either that or you didn’t explain it well enough for me. Please read the article, particularly the treadmill example.

Joybird's avatar

I believe it’s because the wind may be one speed but the shape of the boat and things like sails compound the effects of the wind speed therefore pushing the boat along faster. Think of it like taking caffeine with excedrin…the excedrin alone gives one impact…caffeine compounds the effect.

Lightlyseared's avatar

If it were using a sail then it would probably be limited to the speed of the wind. However it is using a wind turbine. The turbine requires wind to be moving in order to work so if it were going at the same speed as the wind there would be no movement of air over the turbine and so it wouldn’t spin. So I’m gonna guess it’s to do with the turbine somehow?!?...

funkdaddy's avatar

My understanding is that the propeller is geared into the drive wheels.

So it’s using the wind to power itself, but not as a sail would. The propeller spins, the wheels move, and once you’re moving the cart’s inertia will maintain approximately the same speed. Anything the wind/propeller add to the equation would continue accelerating the cart until friction balances it out.

One of the tricky parts is probably gearing it so it can take off from a standing stop and still get the maximum speed when it’s rolling.

timtrueman's avatar

@funkdaddy OK, that totally makes sense but what about these two conditions:

1. Cross-over from less than wind speed to wind speed. Basically the propeller will be getting an effectively zero speed wind. How does it accelerate past wind speed? Does it have to have enough acceleration to zoom past the effectively dead wind?
2. While traveling at greater than wind speed is the propeller simply able to work both ways? The switched wind simply pushes it faster and faster? What’s the upper limit? Rolling resistance, weight and aerodynamic drag?

funkdaddy's avatar

@timtrueman – my thinking is sometime slower than wind speed is when the propeller turns from a sail capturing energy into a propeller driven by the wheels and accelerating the vehicle forward…

The upper limit would be whenever the force the propeller is generating gets balanced out with rolling resistance, weight, drag, friction from the gearing, etc…

CyanoticWasp's avatar

I like the answers from @Lightlyseared and @funkdaddy. I think they’ve nailed it. But I don’t see how the craft can go much more than 1x faster than the wind, at least “dead downwind”. What happens as the speed of the thing over the ground starts to match the wind speed is the sensation of being “pushed” by the wind gets less and less. As soon as you actually match the windspeed, then it feels (to the driver) as if there is zero ‘apparent’ wind; he’d feel a calm around him. And as soon as he travels faster than the wind, then he’d have an apparent wind… in his face. It would seem to him as if he is traveling upwind, straight into the wind.

As he travels faster than the wind at his back, then it feels as if he’s headed into a stronger and stronger headwind. And it’s going to ‘feel’ that way to the propeller blade, too. So my first question is: How does the blade continue to turn in the same direction as it did when the wind was from behind… and ‘felt’ that way?

I don’t think that we’re seeing enough from the video or learning enough from the text to understand the mechanism well enough as the thing matches and then exceeds the speed of the wind. I do agree that the videos seem to show a vehicle with the wind behind it (telltales streaming forward) and then later show a vehicle appearing to head into an apparent wind (telltales streaming aft), and I’m sure that the reason it can happen is because of the turbine effect, with the large propeller blade turning small wheels. I just don’t see how it can be “so much” effective downwind.

I’d have to know a lot more than I do about the thing.

Sailors know that a boat’s fastest point of sail is almost never “downwind”, but reaching across the wind, perpendicular to the direction of the wind. And it’s possible for some craft, such as the sail-powered wheeled vehicles shown, iceboats and hydrofoils to travel faster than the wind… across the wind. This is definitely a different animal.

Very interesting. Thanks for the introduction.

funkdaddy's avatar

The videos showing the small models on treadmills make me think the propeller is actually forcing the whole cart forward rather than the wind.

Like a “whirlibird” (not sure what else to call these) you spin with your hands but instead driven by the wheels.

gasman's avatar

This device was covered by an article in Wired magazine, where the theory is explained in more detail. Here are some quotes from the article (emphasis is mine):
The unusual wind-powered car hit a top speed 2.86 times faster than the wind during one recent run…

“The counterintuitive idea that you can travel downwind faster than the wind is casus belli for aerodynamic arguments from internet forums to college classrooms. The concept known as DWFTTW can cause world-renowned physicists to throw their Nobel Prizes in fits of rage.”
“People’s intuition is extremely strong on the topic,” [Rick] Cavallaro, an aerodynamicist and avid kitesurfer and paraglider, said. “There are literally thousands of pages of debate on internet forums about the topic.

“Skeptics think that the wind is turning the prop, and the car is turning the wheels, and that’s what makes the car go,” Cavallaro said. “That’s not the case. The wheels are turning the prop. What happens is the prop thrust pushes the vehicle.

“The wheels turn the prop, which turns the vehicle’s wheels, which turn the prop, which turns the vehicle’s wheels. Cavallaro knows what you’re thinking.

It sounds like a perpetual motion machine — but you’ve got the wind as an external power source”, he said.

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