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A wind-powered vehicle that can travel twice as fast as the wind itself

businessinsider.com

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Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#41

Post by Retric »

jrs235 wrote: post #37
sopooneo wrote: post #16

Earlier quoted context omitted.

You're right that it falls far short, but it corrects one of the fundamental misunderstandings many people have about the vehicle: the wind does not spin the propeller, the wheels do.

So, did/does the vehicle in the desert start with a push (from a force other than the wind) or did it really get started moving by the wind pushing it? Edit: FYI in the video he does sort of cover his claim even if it got started by a non wind force by stating the vehicle can maintain a speed faster than the wind pushing it. But I'm still curious, was a non wind force needed in the desert to get it started?

No, it’s light enough for the wind to start pushing it.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#43

Post by foerbert »

Valgrim wrote: post #28
sopooneo wrote: post #16

Earlier quoted context omitted.

You're right that it falls far short, but it corrects one of the fundamental misunderstandings many people have about the vehicle: the wind does not spin the propeller, the wheels do.

Then what is spinning the wheels?

The wind, I think. My current impression is that when it is starting, the whole thing is basically just acting as a big sail. You could lock the propeller and prevent it from spinning and it would still move forward.

After that is where I start to get more fuzzy on the matter. On the one hand it makes sense that by using some of the speed from the wind to do other things, you can extract more total energy from the wind. And by doing propulsion as that "other thing" it makes sense you get up to a higher speed.

But then it seems to me like there would be a problem once you go faster than the wind, because it's no longer pushing you. I'm not sure if some other effect takes over, or if I'm thinking about the wind in an incorrect way, or if my whole line of thought about this is wrong.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#44

Post by mumblemumble »

jrs235 wrote: post #37
sopooneo wrote: post #16

Earlier quoted context omitted.

You're right that it falls far short, but it corrects one of the fundamental misunderstandings many people have about the vehicle: the wind does not spin the propeller, the wheels do.

So, did/does the vehicle in the desert start with a push (from a force other than the wind) or did it really get started moving by the wind pushing it? Edit: FYI in the video he does sort of cover his claim even if it got started by a non wind force by stating the vehicle can maintain a speed faster than the wind pushing it. But I'm still curious, was a non wind force needed in the desert to get it started?

Any wind that's strong enough to overcome static friction would do the trick, and I can't see why 10mph wouldn't be enough.

The small models that you can build at home (sorry, can't find the plans anymore; this was 10 years ago now) do not require a push to get started.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#45

Post by bxbb »

DebtDeflation wrote: post #17
michaelt wrote: post #13

> The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades — they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward. That's a terrible explanation - no wonder people…

Yeah, my first inclination is that since the wind is turning the propeller and the propeller is turning the wheels via a gear system, the speed of the vehicle will be determined by the propeller speed, the gear ratio, and the tire diameter (ignoring friction and air resistance for a moment). The gear ratio is the critical component. It's not like the propeller is a simple sail. I could be completely missing something…

The prop is spun by the wheel, not the wind. The tailwind provide push.

Think of the wheels as a power bank: It convert and temporarily store linear force to rotational force. As long as it accumulated enough power to counter the rolling resistance, headwind drag, gravity, gearing loss, etc; the excess rotational force can be used to spin the prop. Which in turn generate additional push to be converted and stored.

Within this "loop", the vehicle got two linear input, tailwind and propeller. This will allow continuous acceleration once the vehicle goes faster than tailwind up to the point where the additional force from the prop is completely negated.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#46

Post by jrs235 »

Retric wrote: post #41
jrs235 wrote: post #37

Earlier quoted context omitted.

So, did/does the vehicle in the desert start with a push (from a force other than the wind) or did it really get started moving by the wind pushing it? Edit: FYI in the video he does sort of cover his claim even if it got started by a non wind force by stating the vehicle can maintain a speed faster than the wind pushing it. But I'm still curious, was a non wind force needed in the desert to get it started?

No, it’s light enough for the wind to start pushing it.

Even with 100+ pounds (50+ kg)?

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#47

Post by jrs235 »

mumblemumble wrote: post #44
jrs235 wrote: post #37

Earlier quoted context omitted.

So, did/does the vehicle in the desert start with a push (from a force other than the wind) or did it really get started moving by the wind pushing it? Edit: FYI in the video he does sort of cover his claim even if it got started by a non wind force by stating the vehicle can maintain a speed faster than the wind pushing it. But I'm still curious, was a non wind force needed in the desert to get it started?

Any wind that's strong enough to overcome static friction would do the trick, and I can't see why 10mph wouldn't be enough. The small models that you can build at home (sorry, can't find the plans anymore; this was 10 years ago now) do not require a push to get started.

Understandable that it doesn't take much wind for a vehicle well under a pound. But what about one that weighs well over 100 pounds?

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#49

Post by marcan_42 »

michaelt wrote: post #13

> The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades — they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward. That's a terrible explanation - no wonder people…

Here's another take:

Step 1: a propeller is like a wall or (straight on, no wing stuff) sail moving forwards forever. When the wind pushes on a propeller it's like it's pushing against a sail or wall, but instead of moving forward it turns in place. Virtually, it's still like a wall moving forward. So it converts linear push to a rotational motion.

Step 2: Wheels are the opposite for solid surfaces: they couple rotational motion to linear motion on the ground.

Step 3: the propeller on the vehicle is connected to the wheels. The gearing makes the "virtual wall" of the propeller move backwards as the vehicle moves forwards. This cancels out the forward motion of the vehicle from the point of view of the air.

So from the point of view of the wind, the propeller is like a wall or sail that isn't moving with the vehicle. It's stuck stationary on the ground, or moving slower.

The reason why the wind can't push a vehicle faster than the wind is that once you get to wind speed, there is no more speed difference to impart a force on the vehicle. Here, even when the vehicle is at wind speed, there is still a difference between the (slower) virtual speed of the propeller and the wind. So it still gets pushed, and can further accelerate.

TL;DR the vehicle uses the wheels and propeller to make itself appear to be running slower relative to the wind, and so it can continue to accelerate when its real speed has reached the wind speed.

Another way to visualize it is: instead of a traditional propeller, imagine it like a water wheel, or an impeller, with vanes the air hits. As the vehicle moves forward, the vanes move backwards. With the vehicle at wind speed, the vanes moving backwards cancel out some of its speed, so that there is still a relative difference between the wind speed and the vane speed, so there is still a force exerted.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#50

Post by mumblemumble »

Valgrim wrote: post #28
sopooneo wrote: post #16

Earlier quoted context omitted.

You're right that it falls far short, but it corrects one of the fundamental misunderstandings many people have about the vehicle: the wind does not spin the propeller, the wheels do.

Then what is spinning the wheels?

Asking what is spinning what is maybe framing it in a way that impedes understanding.

A quick summary of the basic forces involved are: The wind pushes the propeller, the propeller pushes the wheels, the wheels push the ground, the ground pushes the wheels, the wheels push the propeller, and the propeller pushes the wind. Or perhaps it's better to say that everything is dynamically interacting with everything else.

I think that what this all adds up to is that, by introducing this mechanical linkage, the cart is extracting energy from the difference between the wind speed and the ground speed. This runs counter to our intuition, which assumes that it should be getting its energy from the difference between the wind's speed and its own speed.

That's what allows it to go faster than the wind. If it were based off the difference between the wind speed and the cart speed, then the forward force would go to zero as the cart's speed approaches the wind speed. But the difference between the wind speed and the ground speed is not related to how fast the cart itself is moving. So the forward force on it doesn't disappear as it approaches the speed of the wind, and instead it will continue to accelerate until the forward force balances with drag and rolling friction.

A kite might be a good starting intuition pump here? Kites generally don't do much of anything useful unless they're mechanically tethered to the ground.

From there move on to sailboats. The reason why this cart can go downwind faster than the wind isn't all that far off from the reason a sailboat can sail into the wind (albeit not directly into the wind), but, critically, only if it has a keel or centerboard.

(Edit: Another detail to point out is that the linkage causes the propeller to turn in the opposite direction it would if it were spinning freely. And remember that Newton's third law works in both directions at the same time.)

(Edit again -- Another observation that might help change one's intuition is that, while the cart as a whole may be moving relative to the ground, the part of the cart that's currently touching the ground at any given moment is more-or-less stationary with respect to the ground. And is also mechanically linked to the propeller.)

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