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.
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Well if you consider the atomic energy stored in all the air molecules being moved by the wind... imagine a few grams of air being moved 20 ft. About as much as a twinkie... it could level the entire City of New York.
I posit no theories just humor. And interest in seeing what slashdot can muster here... Remember kids, we have not figured EVERYTHING out about the universe just yet.
Re:Conservation of Energy (Score:5, Interesting)
Re:Conservation of Energy (Score:5, Informative)
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What puzzles me is that if you're going 2x or more than the tailwind, wouldn't you be feeling a headwind that effectively pushes back? If so, then how is the wind assisting you at that point instead of going against you.
I know that sailboats can sail into a headwind, but they don't sail directly into it, instead they do so at an angle that causes the headwind to still push it ultimately forward due to the keel preventing angular movement.
Perhaps this is using a similar technique, only with the tailwind. Sin
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
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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
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Wind isn't pushing the craft forward. The propeller is pushing the craft forward. The wheels are powering the propeller. Wind is reducing the airspeed with respect to ground speed, thus reducing aerodynamic drag, giving an advantage that the craft can manipulate.
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incorrect. The wind can't push the craft forward - the craft goes faster than the wind, so the net force on the craft from the wind is braking it.
It's unclear whether this is merely a language quibble, or you don't yet understand how it works.
The propeller is exerting a rearward force on the air. Newtons Laws: for every action there is an equal and opposite reaction. If a force exists from the prop towards the air, there also exists and equal and opposite force from the air towards the prop. The air *is* exerting a forwards force on the cart, even when the cart is going above windspeed. And assuming we're not getting into some linguistic quibble dist
Think in terms of frame of reference (Score:4, Informative)
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.
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No, you are traveling in the same direction as the wind, or as near as you can.
The 'trick' is that the blades on the prop are at a steep angle to the wind, and that is what provides the propelling force. Google around, you'll find an article explaining the various forces involved.
And the math does all work out, as he actually made the physical device that does travel that fast.
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The Blackbird and similar carts are notorious for starting up very slowly. The initial start is from simple wind drag, with the large non-rotating prop acting much like a simple crappy sail increasing the basic wind drag.
Once the prop starts to turn, and while below windspeed, the cart slowly gains speed from a combination of simple wind drag augmented by the wheel-prop-thrust system working at a very low efficiency. I think at low rotation speed the prop blades are like an airplane wing in a stall conditio
Re:Conservation of Energy (Score:4, Informative)
quite right, and of course sailboats going faster than the wind exist too. Momentum and energy are conserved, and the wind has plenty of both to offer.
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That's what makes this thing unique, it can go downwind faster than the wind itself is going.
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Actually you are wrong. Yes, I know someone wrong on the internet. Friendly sarcasm.
Boats talk about make good speed. This means not the speed tacking in a zig zag pattern, but the speed from point A to point B in a straight line. Plenty of high performance sailboats have a make good speed in excess of the downwind speed itself. In simple terms, if you released a helium balloon in the wind, these sailboats though traveling further would get to point B well before the balloon pushed by the wind would.
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Directly downwind at the speed of the wind has zero potential energy.
Tacking is not traveling directly downwind.
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:5, Informative)
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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The wind speed relative to the ground provides power to the ground (as in a dust storm) but not to the cart.
It's the changing angle of the prop relative to the wind that provides the power to keep going after the cart reaches wind speed.
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I'm not sure if we fundamentally disagree on the operation of the cart, or if you're quibbling over a reference-frame-dependent inherent ambiguity of "how energy gets into the cart". So lets see if you agree with this:
Rip out the dive shaft. I say we can attach an electric generator to the wheel axle. This generator provides us with a several kilowatt powerline. We then attach that to an electric motor driving the propeller and consuming the several kilowatts of power
If you agree with that, then we're merel
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This is what I immediately thought. But than I read more comments about rocks... I like that perspective more. Since this thing is traveling in a straight line.
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A sailboat can reach a point downwind faster than the wind speed by traveling at angles to the wind but that is different.The blackbird is doing something else.
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Now imagine said sailing boat on land, with wheels that have no more friction than the boat in water.
Now imagine said sailing "boat" attached to a long, beam on wheels, able to travel across that beam on a guide rail.
Now you have a complete system that is travelling down wind faster than the wind, by having an element on the top of it beating backwards and forwards on a broad reach and jibing at each end.
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The only thing that makes it not a sailing boat is that I happened to use land to make the engineering easier to describe. We can trivially change it back into a sailing boat by saying "it's a really really wide catamaran, and the mast is attached to a travelling base.
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The only thing that makes it not a sailing boat is that I happened to use land to make the engineering easier to describe. We can trivially change it back into a sailing boat by saying "it's a really really wide catamaran, and the mast is attached to a travelling base.
Even if you could build such a thing that would actually strong enough and flexible enough to deal with all the forces involved while still being able to float and steer, it would no longer be fast enough to accomplish the goal. Imagine what it would take just to have a mast mounted to a moveable base which would have to have a keel whose angle could be set independently of the direction of the boat.
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true for conventional sail but there is also the "windmill sailboat" which can pull the "faster than the wind directly downwind" trick. So nothing unique about this craft, on land and ice done decades ago, and in last decade turbines and mills on boats have done it.
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I still think the blackbird is unique because it's not using a wind turbine to drive the wheels. It's using the wheels to spin the propeller, - which spin the wheels.
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a prop is just a bunch of sails on an axis 8D
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Which spins the propeller, which spins the wheels, ...
It's turtles all the way down.
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"windmill sailboat" which can pull the "faster than the wind directly downwind" trick... in last decade turbines and mills on boats have done it.
Your wording is incorrect. Direct downwind faster than wind is impossible with a windmill(turbine). Vehicles that do it must use the propeller as a fan.
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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.
can someone explain this (Score:2)
So let me throw out the question I just posed in a simple case.
when the vehicle is going downwind faster than the wind, the apparent wind is actually in the reverse direction. to apparently you can extract power from a headwind that will push you into a head wind, and you can do this in a continuous way and maintain a constant speed.
1) how is that possible as a force argument not an energy one?
but the more important question is this:
2) if there is no wind at all, and I give the cart a shove, there is now
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.
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To look at this even more simply: Since the propeller is geared to the wheels (ground), the craft can continue picking up energy as long as there is a difference between air speed and ground speed.
Re:can someone explain this (Score:5, Informative)
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.
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You can google it of course. Basically you aren't using the proper frame of reference. One of the mind bending aspects is the prop on this craft isn't driving the wheels. The wheels are driving the prop. Very high torque from the wheels was the main problem in designing the craft. It kept breaking chains from the very high torque.
Well yes, and of course not only have I "googled it" but I also read the explanations in the two sources provided and they don't actually provide an answer that I can make out. Take the test question answer. they write the following for the power going from the land to the air
P_wheel = F * v_land
P_air = F * (v_land - v_wind)
then they derate the wheel power for a loss alpha and set these equal:
P_air = (1-alpha) *P_wheel
F * v_land = (1-alpha) * F * (v_land - v_wind)
the force F drops out
mind the missing less than symbol! (Score:2)
Arggghhh. the slashdot formatter ate my less than sigh. I meant to write "consider the case V_land LESSTHAN V_wind.
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Arggghhh. the slashdot formatter ate my less than sigh. I meant to write "consider the case V_land LESSTHAN V_wind.
Is that the one of resignation when trying to write formulae on slashdot?
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Like wise you you put in that magic moment when V_land-V_wind = 0, for another silly result.
V_land-V_wind = 0 is saying V_land=V_wind. If the velocity of the air equals the velocity of the ground, then someone standing on the ground sees zero_velocity_air. So what you're looking at is the case where there's no wind blowing. No wind means no power source, which means zero force generated. The "silly result" equation was:
F * v_land = (1-alpha) * F * (v_land - v_wind)
Filling in the zeros gives:
0* v_land = (1-alpha) * 0 * (0).
Getting a force of zero and "zero equals zero" isn't unusual when you're loo
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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.
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Exactly.
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Jibbing/tacking is not traveling 'directly downwind' by definition, which you illustrated your self
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No, it's not. Real boats have demonstrated that they can get to some downwind point (velocity made good) faster than the wind speed. Racing boats do this all the time.
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The difference between the case when you push the cart and sailing downwind is this.In the case when you push the cart, imagine yourselves as an external observer. There is no wind in front of the cart (any wind the fan feels is apparent) but behind the cart there is wind in the same direction of the cart. The cart caused the wind to move in the same direction of the cart.
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No, it would be going in the direction of its heading faster than the wind speed, but would still be going slower than the wind in the direction of the wind.
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Flux just means the net flow of something through a region of space, so it's perfectly fine to describe wind that way. There's heat flux when something changes temperature, diffusion flux when something dissolves, and both mass and volumetric flux in a fluid stream. Heck, even "the garbage flux of New York" is using the word correctly (if rather strangely).
WTF? (Score:3)
DDWFTTW? WTF? UUDDLRLRBA?
You forgot BBQ (Score:2)
All the best acronyms which are way too long have BBQ at the end.
Re:WTF? (Score:4, Informative)
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UUDDLRLRBA
Unbelievable, Unexplainable, Disastrously Doubly-Long Redundant Long Redundant Bastard Acronym
Bravo. :)
But now you have to finish up and tell us what the initials S.E.L.E.C.T. S.T.A.R.T. stand for
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DDWFTTW? WTF?
No: DDWFTTW FTW!
While you're on ebay... (Score:3)
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...please check out the jet plane on a treadmill that I'm auctioning off - free shipping if it gets airborne!
Your jet plane's engine will be pulling in air through its prop and pushing it out the back. 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 angle of attack of the wing is what generates lift. So the air moving across the wing will be forced downward, under the wing, compressing the air, and causing upward lift through simple Newtonian forces. So, yes, it can get air born. It'll hover in mid air a
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Uhh.. I think you're putting way too much into this.
The wheels can spin as fast as they want and will not create a significant amount of drag on the airplane. This means the treadmill has (virtually) no effect.
Second, the engines produce thrust regardless of whether air is going across the wings. This will generate forward movement which WILL cause air to move across the wings and generate lift. But if the air is stable, the plane will go forward, not stand still.
The whole treadmill thing is a red herrin
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If it is moving with the treadmill, its airspeed will increase faster and it will become airborne faster.
If it is moving against the treadmill, its airspeed will take longer to build up. How much longer depends on the speed of the treadmill. Granted, the thrust is dependent on the engines which are independent of the wheels. Nevertheless it will affect airspeed until its effect is overcome by the thrust.
I don't mean to nitpick. But those who
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The original myth was about a guy attempting to fly by putting a treadmill on the roof of his apartment building.
He knows the minimum takeoff speed of his plane is 100 mph, so he starts up the treadmill, and when it gets to 100 mph, he throttles up his engine, moves forward, and
falls off the roof to his death.
Why? Because the tread
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Saying that they misunderstood the myth is just pedantic, they understood it just fine, and then demonstrated that the plane CAN take off on a treadmill, and will simply traverse the same lateral distance in space to take off as it would where it on a normal runway, and the treadmill has no effect on its performance.
What they did not do was crash
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I mean.. it depends on the speed of the treadmill and the friction of the wheels and the ability of the rubber to not disintegrate.
From a physics problem perspective, which is the only situation where a question like this is appropriate, this type of thing is usually ignored, and has no effect.
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"He said significant and virtually."
I also said "relatively little".
But your point is taken. Somehow my eyes skipped over the "(virtually)" in his post.
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Also: http://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/ [xkcd.com]
Like I said, the treadmill is meaningless. It's *EXACTLY* the same (assuming frictionless wheels - but relative to the engine thrust they may as well be) as if the plane is on a regular, boring, non-moving runway.
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I'm the article submitter, and I may as well provide the correct answer to the plane-on-a-treadmill while I'm at it :)
Considering that this is all about an airplane, itâ(TM)s amazing how EVERYONE badly glosses over air effects. A moving treadmill will also generate a drag force on the air above it. The larger the treadmill surface, and the longer it moves, and the faster it moves, the more it will pull the air backwards, generating a wind. This wind can and will apply a force resisting the plane's atte
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Huh? you're right, but that's what I said, too.
The treadmill can move as fast as it wants, assuming the wheels don't fall apart.
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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.
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That's turbojets.
Yes, jets. Like I said.
Turbofans, on the other hand, also have, well, the fan part in front.
Yes, turbofans have propellers. But the engine on this hypothetical aircraft is a jet, not just a generic turbine. And even if the hypothetical engine here is a turboprop, the lift is NOT provided by the air the propeller moves, it is provided by the air moving over the wing due to the forward motion of the aircraft.
They would also disagree on your poor choice of apostrophe placement.
Oh, goody, a grammar flame. How special.
Given enough thrust, that's exactly what they do.
No, that's not what they do. Not a single high-powered aircraft does this. And the reason you have to qualify your statement wi
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Thanks for helping to correct an erroneous belief I had of Bernoulli's principle. Not that he was wrong or anything just that the way in which it was taught to me.
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Thanks for helping to correct an erroneous belief I had of Bernoulli's principle. Not that he was wrong or anything just that the way in which it was taught to me.
If you're looking at this "air compression" and the wing pushing the air down as your new understanding of Bernoulli's principle, then you're worse off than you were before.
Bernoulli's principle is that air that is moving perpendicular to a surface exerts less pressure on that surface than static air, and the faster it moves the less pressure it exerts. A wing with a positive angle of attack has air moving over the upper surface faster than the air moves over the lower surface. That means there is less ai
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Air NEVER EVER PULLS. Ok there is a microscopic gravitational effect, but basically it can only PUSH. With that in mind your are talking rubbish.
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Observation != reality. Just because you think you understand why somethings happening, doesn't mean you actually do, or that you were actually observing all the inputs, as is the case here where you utterly ignore power, weight, and efficiency and pretend one is far better than any other. Clearly you figured out something that the Wright brothers ... and well, every other aeronautical engineer for the past hundred years have utterly failed to grasp ... THAT is why you're teacher was sad for you.
You're ig
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Sailing the wind (Score:2)
"I've got half a mind to close my eyes and let this string go..."
-- Loggins & Messina, "Sailing the Wind"
People are stupid (Score:2)
Anybody that claims this violates the laws of physics is a sufferer from the Dunning-Kruger effect, i.e. they are stupid but do not realize it. This is not even high-school physics.
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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
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I like rocks, sails, and sail-cars =) Thanks! (Because I will admit this explanation worked for me in understanding this)
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Do you think the tiny wheels can be geared to the wind mill such that they will spin relatively faster than the wind,
I was good up to this point - once the wind mill is moving at the same rate as the wind the wind mill will stop spinning because the effective wind-speed will be zero. I don't see how any amount of gearing can make zero into non-zero.
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The wind is still moving and the windmill is still moving against it. Pressure still has not decreased. Apparently there is an equilibrium somewhere probably you could break it down to friction. At this point it cannot overcome resistance somewhere and stops accelerating.
*I know I said I would not posit theories, but I do not want to wait the time it takes to -AC this.
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Upwind. I have no idea how this thing works downwind =)
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I assume it's "intuitive" to you how the car gets energy to move in the case where it's standing still relative to the road, and the wind is rushing by: the car uses the force of the moving wind against its windmill structures to extract some energy, which can be used to turn the wheels and start moving forward.
The same logic actually works to explain what's happening when the car is already moving at wind speed --- standing still in the air's reference frame, with the road rushing by underneath. In this ca
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I think you forgot to mention the go faster faeries that helped push it. Their invisible so I understand how you'd miss that part.
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Shhh! Stop giving away the secrets to my free energy perpetual motion machine! I usually just tell people it's "cold fusion" to make it sound more sciency and protect my real methods.
This idea isn't even new. (Score:2)
I guess bicycles and lacrosse sticks... (Score:5, Funny)
...violate the laws of physics, too. Because that's really all that this is: a form of leverage that multiplies speed while decreasing force.
I'd buy it... and (Score:2)
Stick it in my front yard. Besides the HOA disapproved my architectural modification to put a WWII howitzer with flag pole in the front yard, so this may be approvable.
Thank you /. - this maid my day (Score:3)
I would like to thank the original poster, this is one of the best story I've read for weeks. I should have known this earlier but I didn't... Thanks again for pointing out this enlightening discovery.
First I really doubt it was real, then I wondered how it could be possible, now I understand how it works !
My short explaination: the propeller harvests the wind energy (relative to ground) that was in front of the vehicle. The more it moves forward, the more it stops different masses of air, accumulating wind's forward momentum.
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Nothing. Requires some basic physics understanding though to grasp why.
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As a former aerospace engineer I always marveled at the discussions of planes on treadmills. It's all pretty straight forward once you look at what makes a plane fly (and wheels have nothing to do with it.)
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Of course, if you are like me you begin to think about the speed rating of the tyres and bearings, but that is beyond the scope of the stupid question.
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And a normal treadmill, being only a little longer than the plane, would not be anywhere near long enough.
Try putting a Cessna on a 30 foot long treadmill, and then mount the treadmill on the back of a ship. Throttle up, the plane moves forward, and falls right in the drink. Every time.
The mythbusters totally misunderstood this.
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a normal treadmill
A normal treadmill is about three feet long. Obviously, the only aircraft that can take off in the space of three feet are helicopters and VTOLs. The point is that airspeed and groundspeed are independent of one another.
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In this case, it's wheels driving a propeller, but I think the other way could work too. See this animation for a good intuitive reason as to why it works
http://sifter.org/~simon/journal/20101107.h.html [sifter.org]
It's the movement of the wind relative to the ground that's important (to avoid the perpetual motion stuff), and the relative forces required in air vs on-ground. The animation clearly shows how a simple geared machine can move in the opposite direction to the motive force (and faster), aor move in the same d