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

the real issue is this

[goombah99]

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.