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!"

Comments

Re:Conservation of Energy

[Alsee]

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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