Own the Controversy! Blackbird DDWFTTW Up For Auction! 266
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!"
Re:Conservation of Energy (Score:4, Informative)
quite right, and of course sailboats going faster than the wind exist too. Momentum and energy are conserved, and the wind has plenty of both to offer.
Re:Conservation of Energy (Score:2, Informative)
Actually you are wrong. Yes, I know someone wrong on the internet. Friendly sarcasm.
Boats talk about make good speed. This means not the speed tacking in a zig zag pattern, but the speed from point A to point B in a straight line. Plenty of high performance sailboats have a make good speed in excess of the downwind speed itself. In simple terms, if you released a helium balloon in the wind, these sailboats though traveling further would get to point B well before the balloon pushed by the wind would. Effectively little different than the Blackbird managed just using a different mechanism to accomplish.
Re:WTF? (Score:4, Informative)
Re:Conservation of Energy (Score:5, Informative)
Re:can someone explain this (Score:5, Informative)
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.
Think in terms of frame of reference (Score:4, Informative)
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.
Re:Conservation of Energy (Score:5, Informative)
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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