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

[bfilarsky 0]

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You are still trying to create energy.

How so? Power in>Power out. I explained it farther up.

[Belgian_Draft 0]

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You are still trying to create energy.

How so? Power in>Power out. I explained it farther up.

Say the wheels can develop 4 hp. Transmit that 4 hp to the prop and just for sake of argument we'll neglect losses in the transmission. I'll even give you a very efficient prop and let 90% of that 4 hp be converted to thrust. That's only 3.6 hp being used to drive the vehicle. Where do you get the other .4 hp? It can't come from the wind since the vehicle is moving at the same speed and in the same direction. The prop cannot develop thrust and transmit power back to the wheels at the same time since it needs a relative wind to do that and there is none.
So...where does the power come from to replace that lost through drag and friction?

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 1,590]

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The prop cannot develop thrust and transmit power back to the wheels at the same time since it needs a relative wind to do that and there is none.

One more time: THE WHEELS POWER THE PROP, AND NOT VICE VERSA.
The force generated by the spinning prop is applied to the vehicle as thrust against the prop's mounting pylon.
That thrust moves the vehicle.

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

[Belgian_Draft 0]

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The prop cannot develop thrust and transmit power back to the wheels at the same time since it needs a relative wind to do that and there is none.

One more time: THE WHEELS POWER THE PROP, AND NOT VICE VERSA.
The force generated by the spinning prop is applied to the vehicle as thrust against the prop's mounting pylon.
That thrust moves the vehicle.

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.....
There has to be energy coming into the system from somewhere. As long as the vehicle feels an apparent wind against the prop it can work...to an extent. But as soon as the vehicle and the wind are at the same speed in the same direction, there IS no apparent wind...from ANY direction... and all source of energy is lost. Any stored in the system as momentum is quickly lost through friction and drag.

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.

[nanook 1]

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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.

_____________________________

"The trouble with quotes on the internet is that you can never know if they are genuine" - Abraham Lincoln

[ryoder 1,590]

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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.

Once the vehicle exceeds wind speed, it also starts picking up aerodynamic drag which will geometrically increase with increasing speed.

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

[kallend 2,129]

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The prop cannot develop thrust and transmit power back to the wheels at the same time since it needs a relative wind to do that and there is none.

One more time: THE WHEELS POWER THE PROP, AND NOT VICE VERSA.
The force generated by the spinning prop is applied to the vehicle as thrust against the prop's mounting pylon.
That thrust moves the vehicle.

Why wouldn't that work in the absence of any wind? Just give it a push and off it goes.

...

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

[kallend 2,129]

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You are still trying to create energy.

How so? Power in>Power out. I explained it farther up.

I think you are neglecting Newton's 3rd Law. What of the energy imparted to the airstream by the prop?

...

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

[billvon 3,092]

>That's only 3.6 hp being used to drive the vehicle.

Yes. But 3.6hp at 20mph (assuming some kind of normal transmission) results in a completely different torque than 3.6hp at 40mph.

Here's another thought experiment. Say you have two gears spinning near each other at different speeds. Can you design a device that fits between them that spins faster than either one? Sure; that's a common transmission design. How can the device go faster than either gear? What provides that "power?"

[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.

Once wind speed is matched there is no longer a tailwind. Without energy being added somewhere to the system it will not go any faster.

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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>That's only 3.6 hp being used to drive the vehicle.

Yes. But 3.6hp at 20mph (assuming some kind of normal transmission) results in a completely different torque than 3.6hp at 40mph.

Here's another thought experiment. Say you have two gears spinning near each other at different speeds. Can you design a device that fits between them that spins faster than either one? Sure; that's a common transmission design. How can the device go faster than either gear? What provides that "power?"

Sure, it's called a planetary gearset.
There is no extra power involved in spinning the gears any faster. You gain speed but reduce torque while maintaining the equation horsepower=(torque*rpm)/5250. No extra power is needed nor added to achieve the goal (neglecting additional friction losses due to higher bearing speeds).
But that situation is not similar at all to a wind powered vehicle.

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.

[nanook 1]

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Once wind speed is matched there is no longer a tailwind. Without energy being added somewhere to the system it will not go any faster.

that is in relation to which direction the wind source is blowing; at the tail. Relatively there's no tail-wind, but to another observer the wind is still blowing. The column of air is still moving. Overall, the wind provides all; the wheels and props are like some sort of lever.

_____________________________

"The trouble with quotes on the internet is that you can never know if they are genuine" - Abraham Lincoln

[billvon 3,092]

>neglecting additional friction losses due to higher bearing speeds

You can't neglect friction; it occurs in every transmission. Yet real world transmissions can run at faster than their input speeds.

>You gain speed but reduce torque while maintaining the equation
>horsepower=(torque*rpm)/5250

Exactly! Another way to express that is horsepower=(force*distance/time.) If you reduce the speed of one "medium" (gear/belt/wind) then the distance/time (which is the same as speed) changes to balance the equation. This is how transmissions work; it's also how you can extract power from two mediums operating at different speeds.

>But that situation is not similar at all to a wind powered vehicle.

OK, we've agreed that a transmission can extract power from two dissimilar-speed gears, and can spin its output faster than either one. Now we get to an example that's closer to the vehicle:

Instead of two gears you have two belts running at different speeds. One is below, one is above. Can you design a vehicle that will run between the belts but go faster than either one?

[ryoder 1,590]

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Once wind speed is matched there is no longer a tailwind. Without energy being added somewhere to the system it will not go any faster.

You are still missing the point that the WIND DOES NOT SPIN THE PROP.
The prop is spinning AGAINST the direction the wind would spin it.
Propulsion if provided by the force of the prop against the vehicle.
Movement of the vehicle turns the wheels.
Some power is drawn from the wheels to turn the prop.
Since the prop is turning in an air mass that is moving forward, the thrust force against the prop's mounting pylon is greater than the force that would exist if the air was stationary wrt the ground.

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

[bfilarsky 0]

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You are still trying to create energy.

How so? Power in>Power out. I explained it farther up.

I think you are neglecting Newton's 3rd Law. What of the energy imparted to the airstream by the prop?

I already addressed that on the last page:

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First off, the power output from the propeller must be less than the power input to the wheels. I think everyone can agree on that. You have losses from friction, drag, and the energy the propeller imparts into the air in the form of kinetic energy.

... Being generous to the naysayers, we'll assume a 50% efficiency - we'll say that only 50% of the power input to the wheels is actually put back into the machine via the propeller. If this is the case, we have 2250 Watts of Power delivered to the vehicle.

That's why they're using a huge prop: large prop = High Mass Flow Rate. High Mass Flow rate means putting less kinetic energy into the air to get the same momentum.

[bfilarsky 0]

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Why wouldn't that work in the absence of any wind? Just give it a push and off it goes.

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This explains why the machine is NOT perpetual motion. It does need that wind to make it move. If the wind speed is 0, then the speed in reference to the propeller will be equal to the speed in reference to the ground, meaning the force at the propeller will simply be the force input to the wheels minus losses, giving a net loss in energy and a deceleration.

[bfilarsky 0]

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>That's only 3.6 hp being used to drive the vehicle.

Yes. But 3.6hp at 20mph (assuming some kind of normal transmission) results in a completely different torque than 3.6hp at 40mph.

Someone gets it!

[bfilarsky 0]

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The prop cannot develop thrust and transmit power back to the wheels at the same time since it needs a relative wind to do that and there is none.

One more time: THE WHEELS POWER THE PROP, AND NOT VICE VERSA.
The force generated by the spinning prop is applied to the vehicle as thrust against the prop's mounting pylon.
That thrust moves the vehicle.

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.....
There has to be energy coming into the system from somewhere. As long as the vehicle feels an apparent wind against the prop it can work...to an extent. But as soon as the vehicle and the wind are at the same speed in the same direction, there IS no apparent wind...from ANY direction... and all source of energy is lost. Any stored in the system as momentum is quickly lost through friction and drag.

Yes, that's the point - there is no apparent wind from any direction, yet they have ground speed. This delta is where the energy comes from.

[kallend 2,129]

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You are still trying to create energy.

How so? Power in>Power out. I explained it farther up.

I think you are neglecting Newton's 3rd Law. What of the energy imparted to the airstream by the prop?

I already addressed that on the last page:

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First off, the power output from the propeller must be less than the power input to the wheels. I think everyone can agree on that. You have losses from friction, drag, and the energy the propeller imparts into the air in the form of kinetic energy.

... Being generous to the naysayers, we'll assume a 50% efficiency - we'll say that only 50% of the power input to the wheels is actually put back into the machine via the propeller. If this is the case, we have 2250 Watts of Power delivered to the vehicle.

That's why they're using a huge prop: large prop = High Mass Flow Rate. High Mass Flow rate means putting less kinetic energy into the air to get the same momentum.

Surely you are extracting kinetic energy from the air, not adding it.

...

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

[bfilarsky 0]

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Surely you are extracting kinetic energy from the air, not adding it.

Absolutely - the net energy exchange has to extract energy from the air, as that is where the energy for the vehicle comes from.

I was referring to propulsive efficiency, where higher mass flow rates give better efficiency.

[strop45 0]

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Why wouldn't that work in the absence of any wind? Just give it a push and off it goes.

This is a key point in understanding why a wind powered machine can't sustain travel downwind at speeds greater than the wind.

Remembering that wind is simply the difference in the speed of the air mass relative to the ground, then if a wind powered machine can sustain travel faster than the wind, then by definition it can move in zero wind, i.e. with no energy source.

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.

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

[billvon 3,092]

>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.

?? Did you mean to say that? Wind powered vehicles _can_ travel at 10 times windspeed when moving close to downwind. Fixed-wing sails can't get beyond windspeed when running _directly_ downwind.

[ryoder 1,590]

Anther article with a different video:

http://sailmagazine.com/racing/running_faster_than_the_wind/

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

[Belgian_Draft 0]

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Once wind speed is matched there is no longer a tailwind. Without energy being added somewhere to the system it will not go any faster.

that is in relation to which direction the wind source is blowing; at the tail. Relatively there's no tail-wind, but to another observer the wind is still blowing. The column of air is still moving. Overall, the wind provides all; the wheels and props are like some sort of lever.

Exactly! The column of air is moving, the vehicle is moving with it...but it cannot move faster than it if it is going in exactly the same direction, only if it is at an angle so as to take advantage of a crosswind component...something non-existent while moving in the same direction.

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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>neglecting additional friction losses due to higher bearing speeds

You can't neglect friction; it occurs in every transmission. Yet real world transmissions can run at faster than their input speeds. I didn't say there was no additional friction, I was only saying we could neglect it for the purposes of discussion since I assumed you knew higher bearing speeds mean a greater loss due to friction.

>You gain speed but reduce torque while maintaining the equation
>horsepower=(torque*rpm)/5250

Exactly! Another way to express that is horsepower=(force*distance/time.) If you reduce the speed of one "medium" (gear/belt/wind) then the distance/time (which is the same as speed) changes to balance the equation. This is how transmissions work; it's also how you can extract power from two mediums operating at different speeds.

Agreed. But it doesn't mean you can create energy, something that the faster than wind vehicle would have to do.

>But that situation is not similar at all to a wind powered vehicle.

OK, we've agreed that a transmission can extract power from two dissimilar-speed gears, and can spin its output faster than either one. Now we get to an example that's closer to the vehicle:

Instead of two gears you have two belts running at different speeds. One is below, one is above. Can you design a vehicle that will run between the belts but go faster than either one?

Yes, but if and only if neither of the belts causes an opposing force when the vehicle trys to exceed that belt's speed. That is what happens when trying to directly downwind faster than the wind...the vehicle starts to push against the wind instead of being pushed by it. But that can never happen unless energy is brought into the system from outside. The only way to do that is through the wheels...but they only have a finite amount of energy stored in them and in the mass of the vehicle.

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.