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

[Belgian_Draft 0]

Let me make this as simple as possible.
On any self propelled vehicle there are two forces in oppossition to each other, thrust and drag. In a normal automobile thrust is provided by an engine via the driveline and tires, any tailwind component, and gravity if traveling downhill. Drag is provided by rolling resistance, gravity if traveling uphill, and aero drag including headwind component. We can leave out gravity for the comparison and just discuss a level roadway.
In a normal car thrust is generated via an energy source that is withing the car...it is internal and not dependent upon speed. The driver uses the throttle to balance thrust against drag. As long as there is more power available than is lost though drag the vehicle can accelerate. If both are equal then the car cruises along at a steady speed. If, for some reason, drag suddenly increases then the car slows down.
A wind powered vehicle is different in that it's power is speed dependent. It is dependent on the speed of the relative wind. Once it is at the speed of the wind (assuming it is traveling in the same direction) it no longer has a wind to produce additional thrust. No additional thrust = no acceleration = no speed beyond wind speed. The wheels, having relied upon the popelorr to move the vehicle forward, no longer have that energy input and therefor cannot provide any thrust beyond the energy stored in their rotating mass. Even if we assume no frictional losses and 100% efficiency in the prop (which, btw, is also impossible) the best the vehicle can do is hold it's speed.
No relative wind means no energy input means no acceleration. The ground moving past the car is not the same as in the treadmill because the treadmill provided the thrust, here the vehicle provides the thrust if and only if there is a relative wind.

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]

Don't dig that hole too deep.

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

[jakee 1,583]

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A wind powered vehicle is different in that it's power is speed dependent. It is dependent on the speed of the relative wind. Once it is at the speed of the wind (assuming it is traveling in the same direction) it no longer has a wind to produce additional thrust.

But the prop, which is always moving crosswind does still feel a relative wind.

Do you want to have an ideagasm?

[Belgian_Draft 0]

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A wind powered vehicle is different in that it's power is speed dependent. It is dependent on the speed of the relative wind. Once it is at the speed of the wind (assuming it is traveling in the same direction) it no longer has a wind to produce additional thrust.

But the prop, which is always moving crosswind does still feel a relative wind.

Once the vehicle...and the prop...reach wind speed there is no air moving across the prop. None. Zero. Nada. Nil. No air moving across the prop means no energy conversion.
The prop can only move crosswind if the entire vehicle moves crosswind. That would negate any claims of DWFTTW.

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.

[shropshire 0]

Oh I'm glad I started this thread ... I have nowt to add ... because it seems counter intuitive to me but I really don't know either way ....

WE NEED MYTH BUSTERS (or at the least the really cute lady with the red hair)

(.)Y(.)
Chivalry is not dead; it only sleeps for want of work to do. - Jerome K Jerome

[nigel99 604]

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A wind powered vehicle is different in that it's power is speed dependent. It is dependent on the speed of the relative wind. Once it is at the speed of the wind (assuming it is traveling in the same direction) it no longer has a wind to produce additional thrust.

But the prop, which is always moving crosswind does still feel a relative wind.

Once the vehicle...and the prop...reach wind speed there is no air moving across the prop. None. Zero. Nada. Nil. No air moving across the prop means no energy conversion.
The prop can only move crosswind if the entire vehicle moves crosswind. That would negate any claims of DWFTTW.

Someone call Kallend and ask him to get his arse out of speakers corner for 5 mintes

I have no problem that relative winds can be used to increase speed - but to me the vector sum of +1 and -1 = 0 which is roughly what they are contradicting.

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

[Belgian_Draft 0]

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Someone call Kallend and ask him to get his arse out of speakers corner for 5 mintes

Professor! Your input is needed!

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 1,583]

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The prop can only move crosswind if the entire vehicle moves crosswind.

The surface of each blade of the prop is moving. It is not stationary.

Do you want to have an ideagasm?

[Belgian_Draft 0]

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The prop can only move crosswind if the entire vehicle moves crosswind.

The surface of each blade of the prop is moving. It is not stationary.

Yes. But what drives the prop?

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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Once the vehicle...and the prop...reach wind speed there is no air moving across the prop. None. Zero. Nada. Nil. No air moving across the prop means no energy conversion.

Wrong.
The prop is powered by the wheels, and is turning in the opposite direction it would be turning if it was freewheeling and being spun by the wind.
When the vehicle is moving at wind speed, the prop is pulling and applying a force to it's pylon.

You need to let go if this fixation on wind speed.
Power is extracted from the wind and applied as force to the vehicle.
There is no law that says force must be directly proportional to the speed of the power source, else levers and transmissions could not exist.

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

[jakee 1,583]

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Once the vehicle...and the prop...reach wind speed there is no air moving across the prop. None. Zero. Nada. Nil. No air moving across the prop means no energy conversion.

Let me put it this way.

What happens when a spinning propellor with an appropriate pitch angle sits in a still room? It does work on the air around it. It is not a stationary sail, it is moving.

In the same way, as long as it's moving the propellor on this machine, even when there is no relative wind, will do work on the air. Or from another frame of reference, the air will do work on it.

(I don't know if I'm right about this, I just don't feel that you're thinking about it properly)

Do you want to have an ideagasm?

[bluetwo 0]

I totally would've said no, the the original question, but wow... there's some crazy stuff in this thread!!

_______________________________________

[Belgian_Draft 0]

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Once the vehicle...and the prop...reach wind speed there is no air moving across the prop. None. Zero. Nada. Nil. No air moving across the prop means no energy conversion.

Wrong.
The prop is powered by the wheels, and is turning in the opposite direction it would be turning if it was freewheeling and being spun by the wind.
When the vehicle is moving at wind speed, the prop is pulling and applying a force to it's pylon.

You need to let go if this fixation on wind speed.
Power is extracted from the wind and applied as force to the vehicle.
There is no law that says force must be directly proportional to the speed of the power source, else levers and transmissions could not exist.

The way it is supposedly working is that the wheels turn the prop that creates thrust that pushes the vehicle which causes the wheels to turn....
problem is, when the vehicle is at wind speed there is no energy available to replace that which is lost through drag.
Levers and gears (which are just rotary levers) can increase force at the expense of distance, or increase distance at the expense of force, but they can never increase both.

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]

I figured it was: the wind pushes at the back of the prop thus pushing the whole vehicle forward thus rolling the wheels that are mechanically linked to the prop that starts to spin. the spinning prop's condition has a positive pitch that produce a cushion of positive-pressure air to add to the tail-wind thus adding total push on the back of the blades. I figure it's a perfect balance of prop (RPM?) condition and friction caused by the linkage system, rolling wheels, and resistance caused by the bite of the blade slicing through the air.

Someone tell me why I may be wrong.

_____________________________

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

[Belgian_Draft 0]

Lacking any other source of propulsion the vehicle must rely on one of three things; the wheels to drive it forward, the prop to create thrust, or the wind to push it.
For the wheels to drive they need power from the prop. That is no problem as long as there is air moving past the prop similar to a wind tower. That moving air diminishes as the vehicle approaches wind speed until it reaches zero. At that point there is no air movement to turn the prop. If the vehicle, by jibing, can exceed wind speed and the vehicle is then turned to go directly downwind, they now have a realtive headwind that works against them no matter what they do. It can be used to turn the prop, but drag losses would quickly surpass any energy harnessed. The prop can create thrust if driven by the wheels. Problem is, the wheels have to be driven by something other than the prop (it's that darn energy conservation thing again). That is where the wind pushing it (like a sail) comes in. But once again, this diminishes as wind speed is approached until there is zero relative wind to give the "push".
I have no doubt their vehicle can reach the speeds they claim, but only if it is jibed like a sailboat. It doesn't take much, just a few degrees off wind direction will allow speeds faster than the wind. But it still is not directly downwind.

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.

[bfilarsky 0]

OK, I thought about this quite a bit today, and finally got the physics a bit more narrowed down in my head.

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.

Power is the rate at which work is done. So more work must go into the wheels per time than comes out of the propeller per time.

Work is Force X Distance. Second equation on this page, if you're in doubt: http://en.wikipedia.org/wiki/Work_%28physics%29

Therefore, Power is Force X Speed. Second equation on this page if you're still in doubt: http://en.wikipedia.org/wiki/Power_(physics)#Mechanical_power

And THAT, is exactly why this machine works. Take a snapshot of the vehicle at the point where it is traveling directly downwind, with a 10 m/s wind speed and a 15 m/s vehicle speed, giving it a headwind of 5 m/s.

Since the wheels are turning, and being turned by the machine as it travels, they are receiving power at a rate of vehicle ground speed * force to turn wheels. Let say this force is 300 Newtons. That means the power input into the wheels is 300N * 15 m/s = 4500 N*M/Sec (Watts)

Now because of the tailwind, the velocity at the propeller is only 5 m/s. 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.

Using our previous formula of speed * force = power, we can also say that force = power/speed. Hence, 2250/5 = 450 N. So, we have 450 Newtons of thrust being exerted by the propeller onto the vehicle.

Combing those two forces, we get a net 150 N forward thrust on the vehicle (implying acceleration), despite going faster than the wind around it, and therefore actually have a headwind component.



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.

Hope that clears things up finally?

[Belgian_Draft 0]

First off I am not an aero engineer and will leave those calculations to someone who is.
Second, you cannot create energy. What I mean is, you can't generate 300 units of energy from the wheels and transform it into 450 units at the prop. That violates the laws of physics which is why this machine cannot operate the way they claim.

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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Lacking any other source of propulsion the vehicle must rely on one of three things; the wheels to drive it forward, the prop to create thrust, or the wind to push it

And this is the thing that gets me: It appears that the only source of propulsion is the wind. the wheels do nothing but provide the "rollers" for the chassis to roll on and it spins the propellers. The blades will always provide an area to get pushed by the wind, regardless of it's spinning.

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That is no problem as long as there is air moving past the prop similar to a wind tower.

Issue two: What pitch setting is the blades? If the leading edges are toward the rear of the vehicle, I can see that the rotation of the blades turn the wheels and speed is dependent on the wind's push and rotation of the wheels. All this time I assumed that the pitch was towards the front, and that the wind resistance of the bulk of the vehicle prevented the inital backwards windmilling as it would make the wheels move in reverse towards the wind(if the gearbox is designed in reverse of the rear-pointing pitch). A flat pitch would mean that it can only travel the speed of the wind and the rotation caused by the wheels have no effect(except slowing the chasis itself due to friction). rotation or not, the blades are still in the way of the wind.

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At that point there is no air movement to turn the prop

Here it seems that the wind is pushing the chasis and the blades and the wheels are the ones rotating the props. If there is a negative pitch, I can see your argument. I doubt that the wheels have any input to the power of propulsion. It's just there to rotate the prop.

A positive pitch would cause the props to thrust against the tailwind. That is, the wind is now pushig against the positive pressure of thrust caused by the blade rather than the actual blade itself. There would be a problem of initial windmilling in the wrong direction from the stop, but helping it would be the fact that windmilling will cause the wheels to turn backward and the vehicle will push against the wind.

I'm not an engineer though, and I am probably arguing against a system that I cannot see or comprehend. it just makes sense in my non-engineer world.

_____________________________

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

[bfilarsky 0]

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First off I am not an aero engineer and will leave those calculations to someone who is.
Second, you cannot create energy. What I mean is, you can't generate 300 units of energy from the wheels and transform it into 450 units at the prop. That violates the laws of physics which is why this machine cannot operate the way they claim.

You are absolutely correct. You cannot generate 300 units of energy at the wheels, and transform it into 450 units of energy at the prop.

You are capturing 4500 units of energy per second at the wheels, and putting 2250 units of energy per second back into the system through the propeller.

The numbers 300 and 450 are units of FORCE, not energy. This is very important, and the two can not be confused.

Think about a lever (we'll assume its perfect just for simplicity). If you have a 5 to 1 mechanical advantage lever, then the force exerted by the lever will be 5 times the input force. However, the distance that force travels will be 5 times less than the input force. Since Work = Force X distance, The input work equals the output work. Of course in the real world, the output work will be slightly less than the input due to losses.

So - we know that an input force can be multiplied, and the output force higher than the input force. However, the output work can not be higher than the input. With these things in mind, read through my last post again, and note that I only used half the work input into the system, yet was still able to increase force. If this machine were perpetual motion, this would hold true all the time. It does not - only when the relative wind is moving slower than the ground (ie there is wind, which provides an energy source).

[kallend 2,131]

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Someone call Kallend and ask him to get his arse out of speakers corner for 5 mintes

Professor! Your input is needed!

Lots of people practicing physics without a license in this thread.

Simplest analysis: is the machine doing work on the wind, or is the wind doing work on the machine?

...

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

[billvon 3,095]

>No relative wind means no energy input means no acceleration.

Sailboats can get very close to the wind, and can accelerate far beyond wind speed. Since they have a fixed sail, they can't maintain this while going directly downwind. The vector sum in the forward direction goes to zero.

However, if they had a moving sail, one that was able to generate a slight sidewards motion, they could head directly downwind. The motion of the sail would cause the wind vector as seen by the sail to move, and you could sail straight downwind at speeds far greater than the windspeed, since you could generate two large non-aligned force vectors, and the small angular difference would result in a forward force.

A sailboat cannot, however, move its sail very far before it "runs out of room." A propeller _can_ keep moving its "sail" (airfoil) for as long as you like.

[Belgian_Draft 0]

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>No relative wind means no energy input means no acceleration.

Sailboats can get very close to the wind, and can accelerate far beyond wind speed. Since they have a fixed sail, they can't maintain this while going directly downwind. The vector sum in the forward direction goes to zero.

However, if they had a moving sail, one that was able to generate a slight sidewards motion, they could head directly downwind. The motion of the sail would cause the wind vector as seen by the sail to move, and you could sail straight downwind at speeds far greater than the windspeed, since you could generate two large non-aligned force vectors, and the small angular difference would result in a forward force.

A sailboat cannot, however, move its sail very far before it "runs out of room." A propeller _can_ keep moving its "sail" (airfoil) for as long as you like.

But what happens when that sail is going the same exact direction and same exact speed as the wind? Observation is that there is no relative wind available to do anything...the sail and the air around it are moving together and a person on that boat would feel no wind (but this obviously can't happen because of the drag of the water).
Am I missing something?

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.

[Remster 30]

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Am I missing something?

Yes. Since the start... The propeller blades movement is not directly in line with the wind.

Remster

[Belgian_Draft 0]

You are still trying to create energy.

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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Am I missing something?

Yes. Since the start... The propeller blades movement is not directly in line with the wind.

No, I get that part. But once the prop is moving in the same direction at the same speed it doesn't matter what direction the blades face or their angle of attack....there is no air moving across them to generate power unless the tires turn the prop, and that robs more power than you get back.

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.