A wind-powered car has been clocked in the US traveling down wind faster than the wind. In a recent run at New Jerusalem in Tracy, California, the car reached a top speed of more than 2.85 times faster than the wind blowing at the time (13.5 mph) powered by the wind itself.
This is similar to a ship concept I had involving a Voith-Schneider propeller and a Darius wind turbine, but I convinced myself it couldn't work. Oh well!
Boy what a lousey graph. They give the relative wind speed (absolute value of velocity) which makes understanding it a bit on the tortured side. Other than that it makes sense.
Sail-craft going 2-3 times as fast as the wind, propelled by a side wind, and not unknown. The wind turbine drive strikes me as an impractical gimmick. Also, I see no indication of wind direction or a direct reading of ground relative wind speed.
hanelyp wrote:Sail-craft going 2-3 times as fast as the wind, propelled by a side wind, and not unknown. The wind turbine drive strikes me as an impractical gimmick. Also, I see no indication of wind direction or a direct reading of ground relative wind speed.
Look at the vane on the nose of the vehicle in the series of pictures. When the vehicle is parked at the end of the runway, the vane points directly behind the vehicle. The wind is from directly a-stern.
Also, you can get a sense of the basic wind speed by looking when the vehicle is at zero velocity and by subtracting the ground velocity when they unit is being towed "up" the runway.
The confusing part is that the relative wind velocity is not shown (speed doesn't go negative) during the run.
Last edited by KitemanSA on Thu Jun 10, 2010 1:30 am, edited 1 time in total.
The relative wind speed continues the stay zero even though the car is accelerating, which is odd. You would think it would measure something non-zero. When the car is stationary or towed the wind fluctuates by 5units (mph i assume), but does not in the acceleration phase.
kcdodd wrote:The relative wind speed continues the stay zero even though the car is accelerating, which is odd. You would think it would measure something non-zero. When the car is stationary or towed the wind fluctuates by 5units (mph i assume), but does not in the acceleration phase.
I think that is a combination of the variability of the wind and the interaction of the anamometer and the propeller.
In the "push-by-hand" acceleration phase, the wind speed drops as the vehicle ground speed increases. Then there is that period where they are engaging the wheel-propeller linkage and the vehicle is undergoing conversion from being a simple sail vehicle to being a propeller driven vehicle, and that is where the weirdness lies. I don't think their measurement system is up to that variability.
Last edited by KitemanSA on Thu Jun 10, 2010 1:31 am, edited 1 time in total.
From the comments it was clarified that it always runs with the wind (downwind), and that the wind pushes the vehicle, which turns the wheels, which turns the prop, which pushes wind back, which pushes the vehicle ...
His comments indicate that when the wind stops so does the vehicle, but when the wind is blowing the vehicle is able to move up to 2 or 3 times the actual windspeed. Wheel-to-propeller gearing and propeller area/propeller air flow are apparently the key aspects of this.
Thanks for your comments. To me a lot of it seems reasonable. There remains a niggling feeling of some perpetual motion rule being broken, but I can't put my finger on it - perhaps I'm just being over-cautious.
hanelyp wrote:a direct reading of ground relative wind speed.
I imagine that would be section "3: waiting through low wind cycle"
kcdodd wrote:The relative wind speed continues the stay zero even though the car is accelerating, which is odd.
The only place I see zero wind speed is in section "5: vehicle at wind speed".
The vehicle speed and wind speed seem to invert between section 4 & 5, which makes sense.
btw, I just noticed you can click the graph to open it on its own and zoom in, if anyone wanted a closer look.
In theory there is no difference between theory and practice, but in practice there is.
BenTC wrote:There remains a niggling feeling of some perpetual motion rule being broken, but I can't put my finger on it - perhaps I'm just being over-cautious.
I had the same feeling, but after mulling it over I decided it is not totally founded. Clearly, in either rest frame of the wind, or the road, there is energy to be had. Either look at it as getting energy from the kinetic energy of the wind, or kinetic energy of the road (earth), because in either frame the other thing is moving. Now, why would this potential energy (so to speak) really be frame dependant? You could build a power station in one frame and transfer the energy to the other frame. It seems all they have done is found a way to extract the energy directly from a frame where neither wind nor road are stationary. I would even bet money that it is possible to make the vehicle go upwind for that matter.
I once saw a video of a dead fish swimming upstream in a water tunnel. I believe it had to do with how the flexibility of the fish's body interacted with the von Kármán effect...
Making a wind-turbine go upwind is pretty much a no-brainer once you know how sailboats tack. Sailboats go more or less upwind all the time. Ice boats can hit remarkable speeds doing so, and they do it in a tack, not running downwind. I think the official record stands at 84 mph. Unofficially they can break 100 mph. A similar craft operated on land definitely has, hitting 126.2 mph.
A wind turbine in this configuration would essentially be like a sail on a tacking boat, the sail running across the wind, but the geared drive allowing the vehicle to drive into the wind.
The downwind approach sounds fishy. There is energy to be had, but I can't buy the description. The turbine itself must provide the power
kcdodd wrote: I would even bet money that it is possible to make the vehicle go upwind for that matter.
I don't think it could go upwind. The car already needs to be moving downwind before the propeller has any effect. The wind pushes the body of the car downwind, which turns the wheels, which turns the propeller, which then provides additional push to the car. (Although in the experiment the push start the car. I wonder if it could overcome starting friction by itself.)
In theory there is no difference between theory and practice, but in practice there is.