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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!"
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.
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.
Some drink at the fountain of knowledge. Others just gargle.
...violate the laws of physics, too. Because that's really all that this is: a form of leverage that multiplies speed while decreasing force.
"How to Do Nothing," kids activities, back in print!
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.
For the unenlightened: Unbelievable, Unexplainable, Disastrously Doubly-Long Redundant Long Redundant Bastard Acronym. (It's a "bastard acronym" because it's actually an initialism, and will give you extra lives if you recite it.)
Bio questions? Ask me to start a Q&A journal. Computer analogies available for most topics!
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.
Prediction for end of Universe #42: Fencepost error in Quantum_bogosort.cpp
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.
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.