Heya DL, do you have any more information on this device?

wonder if it's also going down hill?

My initial thoughts on the device would be it has to be going down a hill at anytime that's it's going faster than the wind, or, fluctuations in wind speed. IE a gust gets it up to a speed and then the wind drops, hence for a period it is traveling faster than said wind. At the end of the day energy in = energy out (of said system) it's as simple as that. There is no way it can travel faster than the wind on a dead flat surface in a 100% constant wind.

The treadmill scenario is a no brainer. It's a treadmill on a slope, the cart is going to want to roll down the slope, however if you managed the force of the cart wanting to roll off the treadmill with the force created by the treadmill turning the wheels, turning the prop etc then of course it will 'sit there' or 'climb up'.

However to start that experiment they HAVE to hold the cart still with the treadmill going at the outset. Otherwise the cart will simply just roll off. The energy required to overcome the initial startup friction etc is to great.

So at this stage i'm highly skeptical :D

You people wth brains...tsk....

Thanks for the nuclear bomb, machine guns, mustard gas and automobiles, guys....where would we be without you?

hi there,

I am pretty sure Make Magazine made one of these and found it didn't work, i will have a search through my collection today.

If you think a cammed race sail is technical to set up then check this out:





... after a bit of a search here is some discussion and stuff from the Make article:

http://forums.makezine.com/comments.php?DiscussionID=5123

cheers cw
cheers cw

while it is possible that there is a remote control unit to steer the thing (black box in the middle) the concept is bull. directly off the wind...impossible, that is almost to perpetual motion stuff... 120 degrees to the wind, possible

That's a good one DL. The portion of the wheel where it touches the ruler is moving at ruler speed. If you replaced the ruler with wind, the top of the wheel would be able to move a little slower than the wind, extracting the energy for the device from the wind. All OK energy wise. The sails or spinnakers mounted to the wheel would have to fold and unfold as they rotated with the wheel otherwise the drag of the lower bit going faster than the wind would stop it. But I suppose it only has to go a ****teenth faster than the wind to prove the point and in a very slick system it might do that without furling and unfurling. The wind profile would help.

So I'll change my mind, it's theoretically possible to go downwind faster than the wind, but I can't see that the one in the other video is using any clever gearing up techniques.

Still think it's a gust and lull trick in the first video. The energy in the wind is proportional to the cube of windspeed, that's why they measure wind very carefully before installing a wind farm, if they're out by 10% in mean wind estimation the farm's output is down by ~30%.

It's going to be particularly gusty in a suburban street.

you guys need to get out sailing more!!!!

Thanks DL.. that got us thinking for the afternoon yesterday!

Check out the youtube videos on : http://peswiki.com/index.php/Directory:Downwind_Faster_than_the_Wind_(DWFTTW)

Spork runs his one on a treadmill, simply because it is easier to use a treadmill than it is to find a smooth straight piece of road with a constant tailwind (and it all works out the same anyway once you account for frames of reference). Whats even more impressive is that he has set the treadmill up so that the device not only stays ahead of the treadmill, but actually climbs UP a hill at the same time.

It certainly seems possible to me - it even makes sense, but it is definitely NOT intuitive!

Spork is so sure that it works that he's thrown out an open bet - at 10:1 odds. If you say it's Bvllshirt, put up $10k. If he's proven wrong, he's pays you $100k.

oldie said...


So the wind pushes against the breeze from the prop, making the cart faster than the wind speed.

That makes perfect sense to me now.

So the prop is actually spinning in the opposite direction to what I expected it to. This explains why you can see the prop change directions in this video (0:10):


1) Wind blows the whole cart downwind
2) wheels turn against the road
3) wheels are geared to the prop
4) prop produces forwards thrust by blowing air backwards against the wind
5) cart speeds up.

Yeh it was easy until I saw the spot on that other treadmill. So out in the open the propellor throws air backwards slowing down the wind, extracting energy. The tips of the blades are on a broad reach, but the whole device goes directly downwind.

With a cyclic pitch control out of a helicopter it might go well in other directions as well. Drag might kill it on water but blow carts and ice yachts might give it a go.

Good one DL.

wouterd said...

ice-sailors can go downwind faster than the wind.

Have you a URL on this? I would be curious to read & understand this.

pierrec45 said...

wouterd said...

ice-sailors can go downwind faster than the wind.

Have you a URL on this? I would be curious to read & understand this.

The critical phrase in the above line which you didn't quote is "VMG-wise". I don't think he/she is saying that an ice sailor can travel faster than the wind downwind with the apparent wind at 180deg to the direction of travel of they wouldn't have added that.

I think the whole treadmill 'proof' is totally irrelevant to the situation because that is purely a function of balancing friction/potential energy(if treadmill is tilted) against prop force, something which is very easy to do. As far as I see it anyway.

The whole prop is creating a kind of rear 'boundary layer' which the wind is effectively 'pushing' on is an interesting theory. Need to think that one through some more :)

CJW said...

pierrec45 said...

wouterd said...

ice-sailors can go downwind faster than the wind.

Have you a URL on this? I would be curious to read & understand this.

I think the whole treadmill 'proof' is totally irrelevant to the situation because that is purely a function of balancing friction/potential energy(if treadmill is tilted) against prop force, something which is very easy to do. As far as I see it anyway.

At one stage of the thought process I'd thought the finger was the element that made the treadmill irrelevant - because if the force of the finger opposed the direction of treadmill travel the blocking finger would force the wheels to turn faster speed up the fan and add energy to the system. But the treadmill is going away from the finger so the finger slows down the angular speed of the wheels (and fan) on the device - the finger adds drag and hence absorbs energy.

I've concluded that the treadmill proof is valid.

From our frame of reference the treadmill is providing the power - which ends up stirring up the still air in the room. From the outdoorsie frame of reference the wind provides the power. Doesn't matter which frame of reference you choose, the physics will give the same answer.

Or from a bipartisan's perspective you could say the device is extracting energy from the velocity discontinuity between the two mediums. If you left the device on the treadmill long enough the air in the room would tend to circulate such that the relative windspeed on the treadmill would decrease - ie the energy in the discontinuity would be absorbed and transferred to the device.

The old grey matter has had a bit of a workout today... Oh I did also go down to the lake on a promising BOM obs but the wind faded.

CJW said...
I think the whole treadmill 'proof' is totally irrelevant to the situation because that is purely a function of balancing friction/potential energy(if treadmill is tilted) against prop force, something which is very easy to do. As far as I see it anyway.

I can't see that it's irrelevant.. the only reason they've tilted it is to demonstrate that it is actually doing 'work' and has the potential to exceed the speed of the surrounding airmass (and also I guess to stop it accelerating clean off the end of the treadmill).

If you take that tilt/load away, and unbalance the friction/potential energy what happens? Will that disprove the proof?

All that would prove is that the device can exceed the speed of the surrounding air, which is the whole point after all!