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Own the Controversy! Blackbird DDWFTTW Up For Auction!
Alsee writes "Center of flaming controversy across the Internet and here on Slashdot for claiming to travel 'Directly Downwind Faster Than The Wind, Powered Only By The Wind, Steady State' (DDWFTTW), the Blackbird is now up for auction on Ebay. It has been certified by the North American Land Sailing Association and Guinness World Records to have reached 2.8 times wind speed directly downwind and was subsequently modded to also achieve more than double windspeed directly upwind. It has been the subject of an MIT physics paper and was included as a model problem in the International Physics Olympiad, yet many still argue it would violate the laws of physics. Let the bidding (and debate) commence!"
I love the DDWFTTW controversy because I initially was convinced it had to be rubbish then revised my opinion as I convinced myself it was possible. As you note from an energy conservation argument it isn't that bothersome. TO see this just imagine the following. Stop the vehicle. Let it suck up some energy. Then let it power itself downwind. It's easily possible that the net downwind speed averaged over the stops could be faster than the wind. Now you just have to extend that to the infinitesimal limit. Thus energetically no problem.
The problem is that it's mind bending to figure out the forces involved. How can wind push anything faster than the wind? Even if you rationalize that with the angle of attack on the proellor or something, you then have to ask, well then doesn't the apparent wind (the wind as seen by the moving cart) lead to a positive feedback loop (faster than the wind --> more power to go faster than the wind --> increased speed faster then wind --> .... ). Like wise how come a cart that is not moving at all, could not be pushed to create some apparent wind, then propel itself using that? Clearly, the gain on that feedback loop has not only to be less than unity, but it has to have a very special curve that leads to net integral such that a cart that is shoved on a windless day cannot go faster (on average) than the shove would provide. Otherwise I think you have a paradox.
It's this latter subtely that I can't connect all thr way through all the complicated force arguments.
Now when the wind is blowing, we know the force arguments have to be valid for a very simple reason. We already know that sailboats not heading directly downwind can go faster than the wind in the net downwind direction. They do this by jibing (i.e tacking down wind) in a zig zag path. If you were to drop a large black box over such a sailboat then you would not be able to see the actual motion of the boat, but you would see a black box going directly downwind faster than the wind. thus we know this happens empirically. It's not some werid stored energy issue. the forces directly allow this. but it's hard to figure. Even the apparent wind effect of increasing the effective windspeed on a sailboat is real.
So it's only truly mindbending at the second order level of how somehow the force argument still has to conserve energy.
Some drink at the fountain of knowledge. Others just gargle.
...violate the laws of physics, too. Because that's really all that this is: a form of leverage that multiplies speed while decreasing force.
"How to Do Nothing," kids activities, back in print!
Take a can of your gasoline. Say this can of gasoline is the sun. Now, you spread a thin line of it to a ball, representing the earth. Now, the gasoline represents the sunlight, the sun particles. Here we saturate the ball with the gasoline, the sunlight. Then we put a flame to the ball. The flame will speedily travel around the earth, back along the line of gasoline to the can, or the sun itself. It will explode this source and spread to every place that gasoline, our sunlight, touches. Explode the sunlight here, gentlemen, you explode the universe. Explode the sunlight here and a chain reaction will occur direct to the sun itself and to all the planets that sunlight touches, to every planet in the universe.
Bio questions? Ask me to start a Q&A journal. Computer analogies available for most topics!
The story is quite simple. The propeller pushes against the air, its positive effect is affected by the difference in speed between the craft and the air. The propeller is driven by the wheels, so its negative effect on the craft is due the the difference in speed between the craft and the ground.
If you have a wind, the craft-to-ground speed is different from the craft-to-air speed. The vehicle can extract energy from this difference - like any sailboat, really - and pull ahead of the wind.
1. Force equations? The force backwards on the wheels is proportional to the groundspeed, the force forwards on the propeller is proportional to the airspeed. If groundspeed exceeds airspeed, as it does travelling downwind, there is an unbalanced force. If losses could be eliminated, the craft could travel at infinite speed (until relativism takes effect!)
2. If you give it a shove, without wind, airspeed == groundspeed, so there is no unbalanced force. Losses are all there is, so it slows down.
Prediction for end of Universe #42: Fencepost error in Quantum_bogosort.cpp
Thanks. :) That was from the "climax" of Plan 9 from Outer Space. It's worth seeing precisely once. More than once and you come away feeling less whole than you started out.
Bio questions? Ask me to start a Q&A journal. Computer analogies available for most topics!
The shortest answer, the key insight is that the wheels drive the propeller.
Therefore, it isn't the wind speed relative to the vehicle that matters. It's the wind speed relative to the ground. Energy is extracted from the vehicles ground speed, producing a backward force X. That energy is transferred to the surrounding air producing a forward force Y. Since the surrounding air is moving more slowly than the ground relative to the vehicle, so long as your propeller is sufficiently efficient, Y will be larger than X.
To answer your second question, in that situation the air and the ground will be moving at the same speed. No matter how efficient your system is, there's no speed differential to extract energy from.
It's a thing that has to do with where the wind interacts to provide power to this particular vehicle. Although the wind is hitting the vehicle flat on (dead downwind), it's not doing that relative to the actual blades of the wind turbine powering the vehicle. When you look at where the wind is hitting the blade surface, it's still deflecting off at an angle. And considering that the blades are rotating, the effect is the same as having a regular sail which is moving at some angle to the wind. (It's all relative to how you look at it. And most people think the principles of relativity is just a nuclear physics kind of thing.)
Also the power here comes from the airspeed difference relative to the ground, and the same concept also works on water. (Because like the ground, the water isn't going to be moving at the same speed as air.) Interestingly enough, if you can exploit the airspeed difference over a wind-shear boundary the same concept may even work for aircraft. Something that may work is using tractor kite or turbine on a tether at a different altitude where winds are much higher, or perhaps flying at the very boundary of a strong wind shear and using the mechanics of that to gain momentum.
Directly downwind at the speed of the wind has zero potential energy.
Article submitter speaking. Your statement is the common objection, however it requires a very crucial correction: Directly downwind at the speed of the wind has zero potential energy between the cart and the air.
However energy does exist between the wind and the ground. That energy does not cease to exist simply because the cart is moving. How to tap into that energy source while the cart is at windspeed is a tricky engineering problem, not a laws-of-physics problem and not a perpetual-motion-machine problem.
The key to the engineering solution is that the cart is connected to the air via the prop and connected to the ground via the wheels. The energy of wind-moving-relative-to-ground still exists, and by having the wheels and prop driving in the opposite of the way you intuitively assume, this energy becomes visible/accessible in the force and motion of the ground pushing backwards against the wheels. The cart is *in* the wind, driven forwards. From the point of view of the cart, the wheels are extracting power from the ground. At windspeed the prop feels like it's in still air. Propellers are extremely efficient at generating a forwards thrust when driven in still air. The prop can generate enough thrust to overcome the drag of the wheels on the ground because the wind-over-ground is a true and existing source of energy. That energy is contributing to and powering the system.
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