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Down wind faster than the wind

Okay, I have now come up with a model of a device similar to the OP that is simple enough to analyze and looks like it should work. It doesn't use a propeller, it uses a blower.

You need two same-sized wheels, wheel 1 and wheel 2. Wheel 2 is geared to 1 so that it turns in the opposite direction and half the angular speed of 1. (Yes, half, not twice.)

Wheel 1 will be rolling on the ground, wheel 2 will be fitted with blades and its upper portion will be exposed to the wind (think water-mill-wheel). The rest will be made as aerodynamic as possible.

The wheels are so geared that the blades are always moving, with respect to the ground, at half the speed of the whole device. Subsequently, when the device is moving at the speed of the wind, the blades are still moving slower than the wind, and the drag of the air will push them forward, accelerating the device.

The ideal device would move at twice the air speed; practically, its speed will be lower because of the drag of the aerodynamic part, ground friction, etc. etc. - but if these are lowered enough, it should still move faster than the wind.

So what's the tradeoff here? The tradeoff is that it will be twice as hard for the wind to accelerate the device (or for you, if you push the exposed blades forward by hand). It's really like putting a bike into high gear.

And no, it's not over-unity and energy does not come from nowhere.
 
Thabiguy said:
Okay, I have now come up with a model of a device similar to the OP that is simple enough to analyze and looks like it should work. It doesn't use a propeller, it uses a blower.

You need two same-sized wheels, wheel 1 and wheel 2. Wheel 2 is geared to 1 so that it turns in the opposite direction and half the angular speed of 1. (Yes, half, not twice.)

Wheel 1 will be rolling on the ground, wheel 2 will be fitted with blades and its upper portion will be exposed to the wind (think water-mill-wheel). The rest will be made as aerodynamic as possible.

The wheels are so geared that the blades are always moving, with respect to the ground, at half the speed of the whole device. Subsequently, when the device is moving at the speed of the wind, the blades are still moving slower than the wind, and the drag of the air will push them forward, accelerating the device.

The ideal device would move at twice the air speed; practically, its speed will be lower because of the drag of the aerodynamic part, ground friction, etc. etc. - but if these are lowered enough, it should still move faster than the wind.

So what's the tradeoff here? The tradeoff is that it will be twice as hard for the wind to accelerate the device (or for you, if you push the exposed blades forward by hand). It's really like putting a bike into high gear.

And no, it's not over-unity and energy does not come from nowhere.
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But is is paradoxical, I think
If the propeller's speed is proportional to the 'wind', to increase that speed, the angular momentum of the 'propeller' must be increased as the square of that speed.
 
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humber said:
But is is paradoxical, I think
If the propeller's speed is proportional to the 'wind', to increase that speed, the angular momentum of the 'propeller' must be increased as the square of that speed.
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Nope, there's no paradox. It's important to realize that the device will not accelerate indefinitely: there is an equilibrium and the system will tend towards it.

Ideally, the equilibrium would be (for this particular gear factor) at twice the wind speed. Practically, the equilibrium will be somewhere below that - at the point when the forward force of the wind pushing against the exposed blades matches the resistance of friction, unwanted drag etc.

When the equilibrium speed is reached, the device will no longer accelerate, and in fact will slow down from speeds higher than that.
 
Thabiguy said:
So what's the tradeoff here? The tradeoff is that it will be twice as hard for the wind to accelerate the device (or for you, if you push the exposed blades forward by hand). It's really like putting a bike into high gear.
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Your design won't work because of the same reason why the propellor won't work. Drag.
 
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Drag is what makes the device work.

When the wind is blowing and the device is at rest, all of the drag of the wind on the device pushes the device forward.

When the device is at wind speed, the drag of the wind on the device is zero, except for the exposed blades, where the drag results in forward force. So the device will still accelerate.

In other words: drag of the air on the exposed blades is a forward force dropping towards zero (at twice the air speed), while drag of the air on the rest of the device is a forward force dropping towards zero at the air speed and then a rising backwards force. The equilibrium (ignoring other kinds of friction) is where the forward and backward forces become equal - somewhere between the air speed and twice the air speed. By making the exposed blades more "draggy" and the rest of the device more aerodynamic, you push this equilibrium closer towards twice the air speed.
 
Thabiguy said:
Drag between what and what?

(P.S.: I wish you stopped editing your post so much. :) )
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The propellor/blower/whatever and the air.
When the wind is blowing and the device is at rest, all of the drag of the wind on the device pushes the device forward.
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No. If that wheel starts turning then you have the drag from the wheel pushing to stop it from rotating.
 
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Thabiguy said:
I think it could work - going downwind faster than the wind is directly equivalent to sailing against the wind (air -> water, ground -> air), which we know can be done. Whether this particular device can do it, I can't tell, but I don't think it matters in principle - even if this device couldn't do it, you could definitely build another one which could.
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Just connect two sailboats, iceboats, or land yachts with a long sliding bar with a seat in the middle, and let them tack back and forth in opposite directions. The seat and the center of mass of the system could move straight downwind faster than the wind, or move straight upwind.
 
Modified said:
Just connect two sailboats, iceboats, or land yachts with a long sliding bar with a seat in the middle, and let them tack back and forth in opposite directions. The seat and the center of mass of the system could move straight downwind faster than the wind, or move straight upwind.
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Good idea! Hadn't thought of that.

technoextreme said:
Right but the air is also pushing it backwards.:)
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Are you talking about the internal drag and friction of the device? Sure, that is why the equilibrium will not be at twice the air speed, but lower. It won't, however, prevent the device from working. - You may ask yourself, what is the equilibrium speed when the wind is blowing on the device.
 
A vehicle has a 1:1 wheel/fan ratio ,and travels approaching windspeed x. You say that if you gear it down by two, the speed may approach 2x. You are ignoring the fact that only the same amount work is available in each case. The fan must obey the principle that pressure is inversely proportional to velocity.
This is another way of saying what has already been said by others. One gain cancels out another, etc.

The mass of the fan is not important, but to illustrate that you need more energy than is available, even on an incremental basis.
The filmed model probably exploits the flywheel effect. The builder remarks that the wind is variable. A gust will accelerate the vehicle, and store energy in the mass of the fan. There's a lot of energy in that fan. If the wind falls again, the vehicle is powered by the wind and the stored energy. In this way, the velocity is averaged, perhaps close to the speed of the gusts, which are higher than the measured or perceived average. The sock is useless indicator in this regard.
 
my_wan said:
For power you need a difference somewhere to take advantage of. This craft takes advantage of the difference between road speed and wind speed.
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The wind power doesn't come from the difference between the wind speed and road speed, but from the wind speed relative to the device.

If the device is traveling faster than the wind, then it's effectively travelling upwind.

It's possible, in theory, to create a wind-propeller powered vehicle that travels upwind, but because of friction, it must be geared down to work. If you have a 5 km/h headwind, then the device must travel at less than 5 km/h in the opposite direction. If this wind speed is caused by the motion of the vehicle, it will slow down to a stop.

(If it slows down, there is less headwind, which makes it slow down more, which means less headwind... and so on.)
 
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Hey folks,

JB and I are the ones that posted the recent DWFTTW vehicle videos on YouTube. We've described and debated the theory on several forums. For about three years people on various forums called us charlatans and fools for thinking such a thing would ever be possible. Finally, JB insisted we build the darn thing and put the questions to rest. You can find the videos we've posted on youtube at "spork33".

There's no particular rhyme or reason to the vids. Some are just tests and demos. Others were made to answer specific questions people have posted on other forums (such as "can it self start with a tailwind?"). If you have a specific test you'd like to see just let us know. If you think we're faking it somehow, tell us how we're faking it. We'll do whatever we can to eliminate any possibility of fakery.

The thing is real, it's simple, it goes downwind faster than the wind, and it has no real purpose. And NO - it's not perpetual motion. If the wind stops, it stops.
 
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Okay, here's my final analysis of the device in the OP.

The key characteristics of the propelling assembly (be it a propeller or a blower) is the leverage factor f = vprop / vdev : if you forcefully pushed the device forward at speed vdev (with respect to the ground), the propelling assembly would attempt to move the air forward at speed vprop (with respect to the ground).

For the blower, f depends on the gear transmission; for the propeller, it depends also on the angle of the blades. f ranges from minus infinity to plus infinity and is independent on the speed of the device or the wind.

There are three main forces acting on the device:

1. The effective propelling force of the wind pushing the device forward, which equals Fwind * f, where Fwind is the forward force exerted by the wind on the blades. This force acts to make the device move at forward speed vwind / f (with respect to the ground).

2. Ground friction, internal friction and internal drag - this force acts to make the device move at forward speed 0 (with respect to the ground). The designer of the device will want to minimize it.

3. Force exerted by the wind on the non-propelling structure of the device - this force acts to make the device move at forward speed vwind (with respect to the ground). The designer of the device will want to minimize it.


Depending on the leverage factor, the following scenarios are possible (we will assume that the wheels of the device always have contact with the ground):

A. f = +infinity: The device will not move; its propelling assembly acts as a brake on the wheels.

B. f > 1: The device will slowly move forward at low gear; the equilibrium speed will be below the wind speed (minus losses), but the force driving it towards that speed will be greater than the force of the wind. (The device will more easily overcome obstacles, for example.)

C. f = 1: The propelling assembly acts as a simple sail. The equilibrium speed will be the wind speed minus losses.

D. 0 < f < 1: The device will move forward at high gear; the equilibrium speed will be vwind / f minus losses, making it possible for the device to move at higher than wind speed (with less "oomph", though). However, as f approaches zero (the gear is set too high), the force that drives the device towards vwind / f will get progressively lower, to the point that it will begin to lose to the friction and drag forces - and this will lower the equilibrium speed. So there is a sweet-spot f in this range that maximizes the equilibrium speed for a given wind speed. This depends on the specific construction of the device.

E. f = 0: "Infinite gear"; the effective force of the propelling assembly is zero and does not affect the device. The only forces acting on the device are friction, drag and air pushing against the non-propelling elements, placing the equilibrium speed somewhere between 0 and vwind.

F. -1 < f < 0: The device will move backwards, at high gear. Because of the friction and drag forces, it will only actually move backwards when the gear is not too high (i.e. when f is sufficiently below zero). The equilibrium speed will be vwind / f, minus losses. Note also that the losses will be greater than in the forward movement scenarios, because the force exerted by the wind on the non-propelling structure of the device now works against us even more. As in scenario D, there will be a sweet-spot f in this range, maximizing the backward equilibrium speed for a given wind speed.

G. f < -1: The device will move backwards, at low gear. Again, backward equilibrium speed will be less than the forward speed of the wind (minus losses), but the force driving the device towards it will be higher than the force of the wind. The losses will be greater than in scenario B.

H. f = -infinity: The device will not move; its propelling assembly acts as a brake on the wheels.

And that's it.
 
Cousteau's "Alcyone" could sail directly upwind. Which I know is different from moving faster than the wind, but it would imply that it could do so at right angles to it, which I do not know to be the case.
 
spork said:
I refuse to believe that's your FINAL analysis. :D
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Why?

ETA: Unless I made a grave error somewhere, it does indeed conclude my analysis of the device. It confirms to my satisfaction that your device should theoretically work as advertised, and that there is no reason to assume that your videos are fake. If I may, I congratulate you on such a brilliant idea (I wouldn't have thought of that) and thank you for this very inspiring exercise in theoretical mechanics.
 
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