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!"
the real issue is this (Score:5, Interesting)
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.
Re:People are stupid (Score:2, Interesting)
Re:People are stupid (Score:2, Interesting)
OK. I just want to explain this to you because you're ignorant and don't realize what's going on here. Note that I'm not jumping to the conclusion that you're dumb as a box of rocks... Just that's you're as ignorant as them at present. That's right. I consider myself teaching a box of rocks.
Think of a sail boat. Now put that sail on a wheeled land vehicle. Can we agree that the wind can move the vehicle without requiring the water? Yes? Good. Now, I want you to think of a wind mill. Notice how the wind causes the fan blades to circulate? Yes? Good. That force can be harnessed and even geared down to grind wheat or geared up to turn a generator dynamo and generate electricity? Yes? Ah, good, so we now agree that Wind Powered Generators aren't pointless. OK, now forget about that electricity. Think of how you can gear the fan's crank shaft such that it spins that little dynamo really fast. Now, imagine that instead of a generator, you've hooked those gears up to the wheels of your sail-car.
Do you think the tiny wheels can be geared to the wind mill such that they will spin relatively faster than the wind, or do you think that electrical current is not possible to create from the wind because it requires a cycles per second that is faster than the wind changes direction?
If you prove smarter than a box of rocks, then you'll agree that it's pretty damn simple to make a car that moves faster that the wind. We don't make wind generators by having a sail on a magnet and moving it back and forth by gusts of wind.
Re:the real issue is this (Score:4, Interesting)
It's explained pretty well in the wiki article [wikipedia.org], actually. As the cart approaches the speed of the wind, it stops using the push, or drag force, offered by the wind and starts using the lift imparted on the airfoil. You're no longer using the "push" of the wind, so it makes no sense to worry about the relative direction of the wind changing.
It's the same way a sailboat with a keel works. As you pick up speed, the lift on the keel becomes responsible for a great deal of the force felt.
Re:Conservation of Energy (Score:5, Interesting)
Re:can someone explain this (Score:5, Interesting)
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.
Re:Conservation of Energy (Score:4, Interesting)
And that's the point of the blackbird –while the whole is travelling downwind, individual parts are not. You can similarly see a boat travelling on a series of broad reaches interspaced with jibes to be part of a large system which is travelling directly downwind faster than the wind.
Re: Conservation of Energy (Score:3, Interesting)
This argument was to be expected. It always crops up in discussions about DDWFTTW, but it's just wrong. It's got nothing to do with the angle of the sail. The machine is a propeller-craft, plain and simple. It moves through the air like a plane with a powered propeller. The only difference is the source of power for the propeller, which in this case isn't an engine but the turning of the wheels. If you push the cart forward, the wheels turn and they turn the propeller. The wind pushes the cart forward, the wheels turn, the propeller turns, the cart moves relative to the surrounding air. Don't get confused by the headwind. It's easier if you think of the ground as moving and the air as still: Pull the ground backward, the wheels turn, the propeller turns and moves the cart forward through the air. It's really quite simple. No angles involved.