# Downwind faster than the wind

Discussion in 'General Science & Technology' started by billvon, Jun 4, 2013.

1. ### eramSciengineerValued Senior Member

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Yeah, I only watched the earlier part.

I'm fine with a DDWFTTW vehicle, though I still think that Cavallaro's explanation is "troll-science-ish".
The correct explanation is probably more complex.

3. ### A.T.Registered Member

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His explanation is correct. But there are always different ways to explain something.

Look at the small models. Not very complex really.

Last edited: Jun 8, 2013

5. ### AlseeRegistered Member

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First, to be sure this important point is nailed down, this is NOT a windmill-powered-car.
There is no windmill. Never. The prop does not drive the wheels. Never. The wheels do not drive it forwards. Never.
This is more like a "wheel-powered-airplane".
The prop is a fan driving it forwards. The wheels drive the prop. The wheels are trying to drag it slower.

Let's confirm that this does indeed extract power from the wind.
The prop is a fan, and it blows backwards at the wind. If you picture it from the ground, you can see that this causes the wind to slow down. If the wind is slowing down then it is losing kinetic energy. Conservation of energy says that this energy *must* go somewhere. The only place for it to go is into the cart. I'll do the math below, but the short answer is that this energy is going to appear as extra thrust at the prop. The wheels are dragging backwards, the prop is thrusting forwards, and the energy being drained from the wind shows up as excess thrust at the prop. The prop thrust will be greater than the wheel drag. The energy being drained from the wind becomes a net forwards force. The wind is pushing forwards on the cart even when the cart is going faster than the wind.

How can the wind be pushing forwards on the cart, when the wind is slower than the cart? The prop is pushing backwards on the wind. Newtons laws, for every action there is an equal and opposite reaction, for every force there is and equal and opposite force. **If the prop applies a backwards force on the wind, then and equal and opposite forwards force must exist from the wind onto the prop.** We now have a forwards force from the wind onto the cart, and we can apply this force against the ground to spin the wheels, which spins the prop, which establishes the the link between the prop and the wind. Yes, this is a circular loop, which seems like a perceptual motion machine, but the wind is losing energy in the process. The wind's energy is feeding into the loop, the energy drained from the wind is paying all the costs.

Consider it at below windspeed. A forwards force from the wind exists on the prop. We have taken that forwards force and bounced it off the ground, reflecting it back so it hits the prop sideways (rotational). The hard-gear link between the wheels and the prop means it's impossible for the wind to move the prop forwards without also causing the prop to rotate. This happens below windspeed, and it continues to happen at above windspeed.

And for those who like calculations: The cart a going 30 m/s to the left. The wind is 20 m/s to the left. The cart is going downwind, faster than the wind, at double windspeed. The cart feels an apparent 10 m/s headwind. Now let's establish a 1 Newton drag at the wheels. How much power does this remove from the wheels? Power = force * distance. The force is 1 N, and the wheels are dragging over the ground at 30 m/s. This gives us power = 1 N * 30 m/s. The power removed from the wheels is 30 N-m/s.

Now lets send that power to drive the prop. How much thrust do we get? Again, power = force * distance. Flipping that to isolate force gives to force = power / distance. Note that the prop is applying this force against the air! The velocity between the prop and the air is the apparent 10 m/s headwind. And the power was 30 N-m/s. So we get force = (30 N-m/s) / (10 m/s). The prop thrust is 3 Newtons.

The existence of the wind causes the ground velocity under the cart to be unequal to the air velocity over the cart. We used this velocity difference to leverage ourselves into unequal forces at the prop and wheels.

We have a 1 Newton drag at the wheels and a 3 Newton thrust at the prop. A net thrust of 2 Newtons. Even if our cart has a net efficiency of merely 34% we still obtain a positive net thrust. The cart is going downwind, faster than the wind, and accelerating.

It's a brain-bender design, but the math confirms it. Which reminds me of the joke:
Yeah yeah, they built it and it works in practice... but can it work in theory??

7. ### KitemanSARegistered Senior Member

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I think I would call it more of a wheel powered aircart. Like an airboat, but a cart!

8. ### PeteIt's not rocket surgeryRegistered Senior Member

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Welcome to Sciforums, Alsee.
After an opening post like that, I really hope you stay around.

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Ibid

10. ### A.T.Registered Member

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I guess you meant:

power = force * velocity

because that is what you actually correctly use later.

11. ### AlseeRegistered Member

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Yep, that was careless typing. It definitely should be force*distancevelocity. I would edit it to fix it, but there's no edit button. Searching the forum I see a post indicating new users don't get an edit button right away. If some moderator wants to fix it that would be cool. I'm not a fan of leaving botched calculations lingering around

Last edited: Jun 20, 2013
12. ### A.T.Registered Member

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I guess you meant: force * velocity

13. ### Motor DaddyValued Senior Member

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I guess you meant power=(force*distance)/time or power=work/time. Is that what you meant?

14. ### AlseeRegistered Member

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Doh, doh, doh, double doh^2!

In my original post I meant to say "force*velocity".

In my correction post I meant to say "it definitely should be force*velocity".

Gah, I'm an :m:idiot:m:. lol.

Anywho, I was thinking it would be cool to find a physics simulator software that can support the cart. There's tons of free packages out there, but most of them don't seem to handle propellers. The only one I found (so far) explicitly mentioning propeller support was Physics Abstraction Layer (sciforums won't let me post links yet, but it's at adrianboeing dot com/pal/index.html). Unfortunately it's the site only has it as a source code download, no executable packages. Does anyone know of a suitable free simulator that can handle the cart?

15. ### Motor DaddyValued Senior Member

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Wiki says:

A joule is a unit of measure of WORK, and a second is a unit of measure of TIME. POWER=WORK/TIME. How do you get velocity from that?

16. ### Aqueous Idflat Earth skepticValued Senior Member

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Joules are the units of energy. Work is a form of energy but not all energy is work.

Power = energy / time.
Energy = force * distance
Power = (force * distance) / time
Power = force * (distance / time)
Power = force * velocity.

(All of this assumes force is constant and uniform, otherwise the equivalent calculus applies.)

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18. ### Motor DaddyValued Senior Member

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So let's apply your logic to a simple example:

You lift a 100 lb rock 10 feet away from the center of the earth. How much energy is that, 1,000 joules?

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21. ### Motor DaddyValued Senior Member

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Can you be more specific, and explain using a simple example?

Why not just link to this so as to give me an even greater understanding?

22. ### A.T.Registered Member

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Can you be more specific, and explain what is still unclear?

23. ### Motor DaddyValued Senior Member

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A joule is a unit of measure of WORK, and a second is a unit of measure of TIME. POWER=WORK/TIME. How do you get velocity from that?