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!"
Conservation of Energy (Score:3, Insightful)
Seems like a simple conservation of energy problem to me. Why compare wind speed to a vessel's speed? Wind would be better measured in terms of flux. If it can impart enough energy on the vessel, of course the vessel could go faster than the wind.
Re:While you're on ebay... (Score:4, Insightful)
Your jet plane's engine will be pulling in air through its prop and pushing it out the back.
Jet planes don't have props. They have compressors.
This means the relative air movement through the engine and across the wings must exist in order for it to begin to roll forward on your treadmill.
The wings have nothing to do with the forward motion, only the air being pulled into the engine and thrust out the back does. In fact, before the aircraft starts the takeoff roll, there can be zero wind over the wings. But it isn't until there is a wind across the wings that the aircraft can actually fly. That "wind" comes from the forward motion of the aircraft created by the thrust.
It'll hover in mid air above the treadmill if it allowed to get up to speed and enough wind is moved across the wing,
If there is no static wind then there will be no wind over the wings of an aircraft that is not moving wrt the earth. It doesn't matter how much air the jet itself moves, if the aircraft is in some way prevented from moving forward to create an apparent wind, it will not fly. That includes having a surface below the aircraft providing sufficient friction through the wheels to balance the thrust from the engine.
If you do that experiment you will notice that as the helicopter's propeller spins up
The difference is that a helicopter's "propeller" provides lift, where a jet engine provides thrust. Lift is up. Thrust is whatever direction you point it. Yes, a Harrier can "fly" with zero airspeed because the jet engine thrust balances the weight. A helicopter can fly with zero airspeed because the blades provide sufficient lift to balance weight. But, a jet aircraft with a normal engine pointed the normal way will not fly just because the jet engine is running, it requires the lift generated by air moving across the wings. If the aircraft is not moving wrt the air, there is no lift.
As the helicopter hovers just above the scale's surface, but not touching it, examine the numbers on the scale -- They're the same as before the helicopter became airborn.
That's not true. Prior to starting the engine, the entire weight of the helicopter will be supported by the scale. After takeoff, and equivalent mass of air will be accelerated downward, but it will not be focussed on the scale, it will act on a larger area. Since the same mass occurs over a larger area, the "weight" will be less on the smaller area. And, of course, once the helicopter moves out of ground effect, the weight on the scale will be zero.
Additionally, the shape of a plane's wing does not cause much of the lift. It's the angle of attack.
NACA, and it's successor, NASA, would disagree. This is why there have been many studies on the most effective shapes for wings. True, at some angle of attack, any wing will have zero lift, but at any given angle of attack some shapes will have more lift than others. That's why they don't just use a flat slab of aluminum for a wing.
Re: Conservation of Energy (Score:2, Insightful)
If you were stuck with a fixed sail, yeah, you have to go at an angle to the wind. But why would you limit yourself to a fixed sail, when you can have a fan with the axis parallel to the direction of travel, such that each blade is always moving at an angle to the wind (combination of (wind velocity - vehicle velocity), which is parallel to vehicle motion, and the tangential motion of the rotating fan, which is perpendicular to it. The resulting helix is always at an angle (given constant vehicle and wind velocities, a constant angle) to the wind, and as a bonus there's no time wasted tacking.