Can a wind powered vehicle travel down wind, faster than the wind?

[Belgian_Draft 0]

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I understand that is what they are claiming, but you cannot use the wheels to power the prop to push the vehicle to spin the wheels to power the prop to push the vehicle to spin the wheels.....

It's not a cycle. the wind pushes on the vehicle(and blades) which rolls the wheels that spin the prop. The prop bites into the incoming wind that would provide drag on the prop and decreases any drag that would otherwise halt acceleration beyond wind speed and throw it back towards the tail-wind(the only real source of power). the tail-wind plus the positive pressure at the back of the blades and vacuum in front of the blades allows the chasis to speed up past the speed of the tail-wind. The wind is pushing on the cushion of air as a result of the spinning of the blades now. Not the blades themselves. I believe that if there wasn't any loss of energy from the wheels turning the prop, the vehicle could theoretically accelerate to infinity instead of meeting some speed equilibrium.

If that were the case the vehicle should be able to propel itself across a level surface in windless conditions, something we all know is impossible.

HAMMER:
Originally employed as a weapon of war, the hammer nowadays is used as a
kind of divining rod to locate the most expensive parts adjacent the
object we are trying to hit.

[billvon 2,393]

>Yes, but if and only if neither of the belts causes an opposing force
>when the vehicle trys to exceed that belt's speed.

?? Two frictionless belts will impart no force to any device between them, so that condition makes no sense. Friction is _required_ for any such vehicle to move at all; indeed, that's how belts (conveyor, drive etc) work.

> But that can never happen unless energy is brought into the
> system from outside.

You seem to be reaching two different conclusions for two similar cases here.

If a device that uses two (high friction) moving belts can exceed the speed of both belts, then a device that uses two (high friction) moving mediums can exceed the speed of both mediums. It doesn't really matter if the medium is a rubber belt, a dry lakebed or the air, provided there is sufficient friction to get good coupling.

As an additional thought experiment, ask yourself if a device can go faster than those belts if they are both moving at the same speed.

[billvon 2,393]

>If that were the case the vehicle should be able to propel itself across a
>level surface in windless conditions, something we all know is impossible.

That is quite possible - as long as the two surfaces (wind and air) are moving relative to each other. The absolute speed doesn't matter; it is the _relative_ speed that matters.

[Belgian_Draft 0]

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>Yes, but if and only if neither of the belts causes an opposing force
>when the vehicle trys to exceed that belt's speed.

?? Two frictionless belts will impart no force to any device between them, so that condition makes no sense. Friction is _required_ for any such vehicle to move at all; indeed, that's how belts (conveyor, drive etc) work.

??Who said anything about frictionless??

> But that can never happen unless energy is brought into the
> system from outside.

You seem to be reaching two different conclusions for two similar cases here.

If a device that uses two (high friction) moving belts can exceed the speed of both belts, then a device that uses two (high friction) moving mediums can exceed the speed of both mediums. It doesn't really matter if the medium is a rubber belt, a dry lakebed or the air, provided there is sufficient friction to get good coupling.

Difference being that between the belts there is nothing to prevent the object from exceeding the speed of both belts. A vehicle operating in the air cannot move faster than the column of air that is pushing it in the same exact direction. If something forces it to then it meets air resistance that opposes that increased speed.

As an additional thought experiment, ask yourself if a device can go faster than those belts if they are both moving at the same speed.

Nope.

HAMMER:
Originally employed as a weapon of war, the hammer nowadays is used as a
kind of divining rod to locate the most expensive parts adjacent the
object we are trying to hit.

[Belgian_Draft 0]

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>If that were the case the vehicle should be able to propel itself across a
>level surface in windless conditions, something we all know is impossible.

That is quite possible - as long as the two surfaces (wind and air) are moving relative to each other. The absolute speed doesn't matter; it is the _relative_ speed that matters.

In windless conditions the air and ground are not moving relative to each other.
A wind powered vehicle moving at the speed of the wind in the same direction has no air moving across it's prop unless the wheels drive it to turn. If they do then the energy returned to the vehicle by the prop cannot exceed that imparted to the prop by the wheels. Since the energy available to the wheels is restricted to the amount given to the vehicle by the prop it is a losing situation.

HAMMER:
Originally employed as a weapon of war, the hammer nowadays is used as a
kind of divining rod to locate the most expensive parts adjacent the
object we are trying to hit.

[billvon 2,393]

>Difference being that between the belts there is nothing to prevent the
>object from exceeding the speed of both belts. A vehicle operating in the
>air cannot move faster than the column of air that is pushing it in the
>same exact direction.

And I argue that there is no fundamental difference between the two cases. If a device can move faster than the two belts with both belts applying a "stopping" force that tends to keep it at the same speed as the belt (i.e. friction) then a device can move with a belt on one side and a device for 'coupling' to the wind on the other (i.e. a prop.)

>>As an additional thought experiment, ask yourself if a device can go
>>faster than those belts if they are both moving at the same speed.

>Nope.

Agreed! It is the _difference_ in the two belt speeds that make it possible to extract energy from them - even though, from the point of view of the device, both belts are only 'slowing it down' (i.e. moving more slowly than the device.)

[bfilarsky 0]

And Belgian Draft, brush up on Frames of reference. I know it hasn't been talked about in a couple pages, but the video I posted of the cart on the treadmill IS proof that this works - the physics are exactly the same with moving ground, still air, as they are with still ground, moving air.

Just like the physics of a wind tunnel are the same as the physics of flight.

[Calvin19 0]

This is a frame of reference problem, obviously a little too logistically complex for some.

-SPACE-

[Belgian_Draft 0]

And thus is why there are so many threads on internet forums debating the issue. If it were a simple problem there would be no debate, only agreement.
Though I respect your opinion I will cling to my own until there is undeniable proof otherwise.

HAMMER:
Originally employed as a weapon of war, the hammer nowadays is used as a
kind of divining rod to locate the most expensive parts adjacent the
object we are trying to hit.

[jakee 1,254]

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And thus is why there are so many threads on internet forums debating the issue. If it were a simple problem there would be no debate, only agreement.

9/11

Do you want to have an ideagasm?

[Belgian_Draft 0]

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And thus is why there are so many threads on internet forums debating the issue. If it were a simple problem there would be no debate, only agreement.

9/11

Ok, point taken.

HAMMER:
Originally employed as a weapon of war, the hammer nowadays is used as a
kind of divining rod to locate the most expensive parts adjacent the
object we are trying to hit.

[ryoder 1,381]

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And thus is why there are so many threads on internet forums debating the issue. If it were a simple problem there would be no debate, only agreement.

9/11

Rudy?
Rudy Giuliani?
Is that you?

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[ryoder 1,381]

Here is another interesting counter-intuitive device:

http://en.wikipedia.org/wiki/Brennan_torpedo

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[champu 1]

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And thus is why there are so many threads on internet forums debating the issue. If it were a simple problem there would be no debate, only agreement.
Though I respect your opinion I will cling to my own until there is undeniable proof otherwise.

Sticking with the "relative wind" approach that you prefer...

Having a wheel-driven propeller and having it be the dominant mechanical coupling path from the wind to the vehicle allows the vehicle to travel downwind at a speed greater than the wind while maintaining an apparent relative wind at the coupling interface.

The steady-state top speed of the vehicle will always be a function of the wind speed passing through the origin. The two ways to increase this top speed are to increase the dominance of the propeller (e.g. by streamlining the rest of the vehicle) or increase the apparent relative wind at the coupling interface at the steady-state top speed (e.g. by adjusting the gearing between the propeller and wheels.)

[kallend 1,623]

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And thus is why there are so many threads on internet forums debating the issue. If it were a simple problem there would be no debate, only agreement.
Though I respect your opinion I will cling to my own until there is undeniable proof otherwise.

As any racing yachtsman knows, the DOWNWIND COMPONENT of velocity of a sailboat on a reach can exceed the windspeed. Consequently even if the race includes a leg that is directly downwind, they will usually sail some 30 degrees off the direct downwind direction because the net result is a faster leg.

A propeller is just like a rotating sail.

You don't even have to analyze a propeller in detail (which is very difficult). Imagine you have an ''actuator disk" that can change the momentum of the air, linked to a reversible motor/generator wired to another motor/generator attached to the wheels. Can you arrange things so that the change in momentum of the air results in extracting enough energy from the wind to provide the force necessary to keep the entire thing going when going downwind faster than the wind?

The answer is: yes , you can.

...

The only sure way to survive a canopy collision is not to have one.

[kallend 1,623]

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Even when travelling at 10 times the wind speed, wind powered vehicles such as wind surfers, ice boats etc all travel at less than the wind speed in the downwind direction.

That is not what my colleague and Olympic triallist in Soling class tells me.

...

The only sure way to survive a canopy collision is not to have one.

[Belgian_Draft 0]

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And thus is why there are so many threads on internet forums debating the issue. If it were a simple problem there would be no debate, only agreement.
Though I respect your opinion I will cling to my own until there is undeniable proof otherwise.

As any racing yachtsman knows, the DOWNWIND COMPONENT of velocity of a sailboat on a reach can exceed the windspeed. Consequently even if the race includes a leg that is directly downwind, they will usually sail some 30 degrees off the direct downwind direction because the net result is a faster leg.

A propeller is just like a rotating sail.

You don't even have to analyze a propeller in detail (which is very difficult). Imagine you have an ''actuator disk" that can change the momentum of the air, linked to a reversible motor/generator wired to another motor/generator attached to the wheels. Can you arrange things so that the change in momentum of the air results in extracting enough energy from the wind to provide the force necessary to keep the entire thing going when going downwind faster than the wind?

The answer is: yes , you can.

Now THERE is an explanation I can understand!

HAMMER:
Originally employed as a weapon of war, the hammer nowadays is used as a
kind of divining rod to locate the most expensive parts adjacent the
object we are trying to hit.

[nigel99 143]

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And thus is why there are so many threads on internet forums debating the issue. If it were a simple problem there would be no debate, only agreement.
Though I respect your opinion I will cling to my own until there is undeniable proof otherwise.

As any racing yachtsman knows, the DOWNWIND COMPONENT of velocity of a sailboat on a reach can exceed the windspeed. Consequently even if the race includes a leg that is directly downwind, they will usually sail some 30 degrees off the direct downwind direction because the net result is a faster leg.

A propeller is just like a rotating sail.

You don't even have to analyze a propeller in detail (which is very difficult). Imagine you have an ''actuator disk" that can change the momentum of the air, linked to a reversible motor/generator wired to another motor/generator attached to the wheels. Can you arrange things so that the change in momentum of the air results in extracting enough energy from the wind to provide the force necessary to keep the entire thing going when going downwind faster than the wind?

The answer is: yes , you can.

thanks excellent answer - along with Bill's explanation earlier in the thread.

Experienced jumper - someone who has made mistakes more often than I have and lived.

[strop45 0]

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Can you arrange things so that the change in momentum of the air results in extracting enough energy from the wind to provide the force necessary to keep the entire thing going when going downwind faster than the wind?

The answer is: yes , you can.

Understood, but where does the energy required to change the momentum of the air come from? It comes from the air driving the vehicle downwind. If the propeller is adding 10 units of energy in the direction opposite the wind, it must be removing more than 10 units in the other direction, hence the overall result is a loss of energy driving forward?


Imagine a person on a trolley being powered by someone throwing them coconuts which stop dead and fall to the ground. If they catch them and throw them back they go faster. If its a machine rather than a person, the best that can happen is that 100% of the energy is captured by the vehicle. Trying to do anything else, e.g. pushing it backwards requires you to use energy (and therefore waste some) resulting in less energy transfer to the vehicle?

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein

[billvon 2,393]

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Understood, but where does the energy required to change the momentum of the air come from?

From the power available at the wheels. Since the wheels are going "artificially fast" there is more power available than if the vehicle were standing still with respect to the air. (Power is speed times torque, so you can transfer torque from a higher speed system to a lower speed system without violating conservation of energy.)

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Imagine a person on a trolley being powered by someone throwing them coconuts which stop dead and fall to the ground. If they catch them and throw them back they go faster. If its a machine rather than a person, the best that can happen is that 100% of the energy is captured by the vehicle. Trying to do anything else, e.g. pushing it backwards requires you to use energy (and therefore waste some) resulting in less energy transfer to the vehicle?

This is a somewhat different issue, but there is nonetheless a way to improve things here as well without adding additional energy. If a person catches them and they fall dead to the floor of the trolley, the trolley gets an impulse of X. If instead they are fired at a trampoline mounted sideways on the trolley, so that they bounce back instead of falling straight down onto the trolley, the trolley gets a larger impulse.

[strop45 0]

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Understood, but where does the energy required to change the momentum of the air come from?

From the power available at the wheels.

So where is the energy to turn the wheels coming from? This must be the true definition of a circular argument?

I understand what you are saying, I just don't agree.

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein

[strop45 0]

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Even when travelling at 10 times the wind speed, wind powered vehicles such as wind surfers, ice boats etc all travel at less than the wind speed in the downwind direction.

That is not what my colleague and Olympic triallist in Soling class tells me.

Perhaps you get your colleague to explain why most boats sailing downwind will use spinnakers (if permitted under their rules).

Answer: because they are very large sails and give best speed.

Problem: they require the relative wind to be behind them to work and stay inflated, i.e. the boat's downwind velocity is always less then the wind speed.

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein

[billvon 2,393]

>So where is the energy to turn the wheels coming from?

From the motion of the vehicle.

>This must be the true definition of a circular argument?

Because it involves a wheel? Perhaps!

[billvon 2,393]

>Perhaps you get your colleague to explain why most boats sailing
>downwind will use spinnakers

Because boat sails can't move relative to the boat.

[Belgian_Draft 0]

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Understood, but where does the energy required to change the momentum of the air come from?

From the power available at the wheels.

So where is the energy to turn the wheels coming from? This must be the true definition of a circular argument?

I understand what you are saying, I just don't agree.

I think you are having the same issue understanding it that I did until i put Bill & Kallend's explanations together.
Maybe this will help...
The wheels drive the prop. The prop spins and part of its energy is lost to aero drag, but enough is converted to thrust to propel the cart ahead. Now come the catch. Due to the incoming air having additional energy to impart to the prop the prop has more than enough energy to do it's job. The enrgy comes from the air moving past the prop at one speed and the ground moving under the cart at another. I know it don't sound right, but if Bill and Kallend say it does then that's good enough fer me.

HAMMER:
Originally employed as a weapon of war, the hammer nowadays is used as a
kind of divining rod to locate the most expensive parts adjacent the
object we are trying to hit.