Down wind faster than the wind

my_wan

Graduate Poster
my_wan
Joined
Feb 24, 2007
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This is a wind powered craft that purports to travel directly down wind faster than the wind powering it.

It has also been posted on mythbusters.
http://forum.mythbustersfanclub.com/index.php/topic,12948.0.html

In essence the prop and wheels are directly connected through a drive belt. I defended the idea on physicsforums but unfortunately spork (the OP poster) wouldn't refrain from rants about being attacked and the thread got locked twice. This guy spork made his own version.
http://www.youtube.com/watch?v=1BRvYZd81AQ&fmt=18

Why I think it works:
It wasn't presented as perpetual motion and I agree that it's not. The source of energy is well define by the difference between wind speed and ground speed. This is regardless of the motion of the craft due to that difference being maintained by a direct belt connection between the prop and the wheels. With a constant power source at a given wind speed the velocity is limited only by the friction/wind resistance on the craft itself via Newton's first law.

My debate with spork concerned how the power was distributed through craft and the effects that had on efficiency. I could well have been wrong on some points of the wind powered version. However, the specifics in that regard are likely determined by prop to wheel ratio and prop efficiency. There is a difference in the treadmill case and wind powered case irrespective of the validity of the Galilean relativity argument.

What do you think? Will it work and why?
 
The experiment doesn't prove anything because it an oddball variation of the airplane on a treadmill myth.
his is regardless of the motion of the craft due to that difference being maintained by a direct belt connection between the prop and the wheels.
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Problem. Someone actually did the experiment correctly. You need to swap the direction of rotation which isn't done in this movie.
 
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The vehicle is effectively tacking the wind. The propeller's blade is presented at an angle to the prevailing wind.
 
technoextreme said:
The experiment doesn't prove anything because it an oddball variation of the airplane on a treadmill myth.
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True, the videos proves nothing. However, you seem to be implying that an airplane on a treadmill will not take off. Is that your claim here? If so that alone is false. Furthermore, it is not related to an airplane on a treadmill at all. An airplane has no drive connection between the prop and the wheels as in this case. An airplane is also powered by an engine. This craft is powered solely by the difference between the ground air speed and the ground, which doesn't change when the craft moves because the wheel are directly tied to the prop. Neither does this craft ever leave the ground. It can't or it loses power.

The craft being spoke of here is not designed to operate on a treadmill. The treadmill was only used by that guy as a proof of concept. The first video show what the craft is actually supposed to do.

technoextreme said:
Problem. Someone actually did the experiment correctly. You need to swap the direction of rotation which isn't done in this movie.
Click to expand...

Who did what experiment? In this craft the wheels and the propeller are directly connected for a reason. Swap the direction of rotation of what? The connection between the wheels and the prop? Since I don't even know what you claimed and/or objected to here I'm at a loss to answer you. Presumably I need to answer it sense you called it a problem so explain.

Points of fact:
This craft has no motor and is essentially a fancy wind sail.
This craft has a direct drive connection between the prop and wheels, the wheel and prop cannot turn independently of each other (planes do not).
This craft uses force feedback to continue drawing power from the wind road differential even after the craft itself exceeds the speed of the wind.

I don't really care for the treadmill video sense it only purports to be a proof of concept rather than the actual operation. This relates to a debate I had and spoke of in the OP. The first video shows how the actual craft is supposed to perform. As a point of physical fact a windmill is perfectly capable of producing enough power to propel a vehicle faster than the wind driving the windmill. This design simply incorporates the windmill on the craft itself.

Please answer the above questions so I can even have a clue what you claimed.
 
humber said:
The vehicle is effectively tacking the wind. The propeller's blade is presented at an angle to the prevailing wind.
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Yes there is truth to that statement. I still haven't figured out how to define the craft analytically. The force vectors get quiet complex and take on a different character at above and below wind speed. When you consider force connection between the wheel rpm and prop rpm it get even more complex. The direction of torch can go either way depending on the ratio of the connection and relative efficiencies. It appears to me in the first video that the propeller and wheel RPM is locked at a one to one ratio, or nearly so.

I can identify the source of power but until I can get a handle on the direction of the torch vectors on all the craft components I don't know how to go about maximizing efficiency.
 
my_wan said:
Yes there is truth to that statement. I still haven't figured out how to define the craft analytically. The force vectors get quiet complex and take on a different character at above and below wind speed. When you consider force connection between the wheel rpm and prop rpm it get even more complex. The direction of torch can go either way depending on the ratio of the connection and relative efficiencies. It appears to me in the first video that the propeller and wheel RPM is locked at a one to one ratio, or nearly so.

I can identify the source of power but until I can get a handle on the direction of the torch vectors on all the craft components I don't know how to go about maximizing efficiency.
Click to expand...

Stored momentum.
 
my_wan said:
However, you seem to be implying that an airplane on a treadmill will not take off.
Click to expand...

Depending on which version of this problem you mean it may very well be true. It's certainly possible to keep an aircraft stationary on a conveyor if you purposely set out to do that and as long as you've got an idealized airplane that will not break and an idealized conveyor with unbounded traction and unbounded power output.

The wheels of the plane have some non-zero moment of inertia, which means that even if they spin completely freely and are in no way tied to the means by which the plane generates thrust the conveyor must still act on them with some non-zero force in order for them to start spinning. If the wheels are frictionless there exists some constant acceleration(not speed; acceleration) of the conveyor belt(and corresponding angular acceleration of the wheels) such that the traction force acting on the wheels is equal and opposite to the thrust of the airplane.

If there is friction in the wheels the speed of the conveyor increases until the friction torque in the wheels eventually rise up to cancel out the torque imposed on the wheel by the conveyor belt. At that point the wheel will be spinning with some constant angular velocity.

In the real world many other outcomes can be imagined(e.g. the conveyor might reach super sonic speeds and generate wind strong enough to give the air plane lift, the wheels might not have enough traction and the airplane will burn rubber and start lurching forwards, the wheels might spin so fast they fly apart etc.).
 
@my wan
Having though it over, I would expect that efficiency alone would prevent the vehicle from reaching, let alone exceeding, the wind.
It may not be a hoax, but the measurements are so crude. Windspeed is determined by voice-over and a sock. There may well be wind bursts that exceed the average velocity of the vehicle. Is the road level? There will be some input from the wind bearing directly on the chassis, etc.
 
Hi

If you aren't going directly downwind, you can develop what landyachters call a, "virtual wind," which can get you going significantly faster than the wind.

I wouldn't be much surprised if the same virtual wind would work on a propeller as well as a sail.
 
@my wan
If you look at the video of the smaller belt test, you will see that the propeller is allowed to come to speed before being released. You would expect that it would then travel along a stationary belt even if the propeller were to be a simple flywheel. It is likely that the propeller stores excess energy from wind bursts as it progresses.
Ack. Mhaze
 
Just thinking of the first demo. When the vehicle's velocity reaches wind speed, it is essentially motionless with respect to the air and there should be no force on the propeller. With no force there is no acceleration. Can't see how it exceeds the windspeed.
 
True, the videos proves nothing. However, you seem to be implying that an airplane on a treadmill will not take off. Is that your claim here? If so that alone is false. Furthermore, it is not related to an airplane on a treadmill at all. An airplane has no drive connection between the prop and the wheels as in this case.
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No. It isn't. It's exactly airplane on a treadmill and it's exactly what I would expect. The simplified free body diagram would prove that it is airplane on a treadmill.
This craft has a direct drive connection between the prop and wheels, the wheel and prop cannot turn independently of each other (planes do not).
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And there's the problem. In order for the propeller to act like a windmill and viceversa you have to change the direction of spin of the wheels which isn't done in the video.
 
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The concept of the propeller vanes 'tacking' is interesting.

But don't think of it as an airplane prop, think of it as a helicopter rotor. Hmmm, needs a gross amount of cyclical pitch like a helicopter rotor to get the 'tack effect' on both apparent upwind and downwind strokes of the vanes, (top and bottom of rotor). You'll want the vanes at top and at bottom in parallel planes, and in horizontal planes when horizontal. Plus the ability to twist the mast to adjust the prop for best apparent wind, and also collective pitch to get maximum power to the wheels. You'll want big blades because the wind is slower than rotors usually spin- it would look like a windmill (DUH!), or, to keep it nautical looking, a Dutch windmill with sailcloth vanes.

Generally, it would only be practical on water. The rotor would be too large for use on highways. And the cost/benefit of the mechanisms vs sails makes it impractical on the water. Marconi rigged boats go everywhere now. And unless the target is directly down wind, they tack without all the mechanism.
 
It’s got no significant energy storage so power in must equal power out. Power in is a function of wind speed + craft speed and propeller efficiency. The wheel and air resistance will determine how fast the craft will go on that much power. So, basically, wherever these two curves intersect is how fast the craft will go.

The key seems to be an efficient propeller. It needs to convert a good portion of the wind resistance it creates back into power for forward propulsion. All the wind would need to do is overcome the losses in the system. Since most of the wind resistance will come from the propeller the fact that it mostly cancels should allow it to move with relatively little added power.

Optimizing the propeller is going to be a tough problem to handle analytically, but trail and error on a computer model should work just fine and it shouldn’t stray that far from what we already know from wind turbines.

Anyway, despite being an interesting little device, I’m not sure what practical use it would have. I can’t see how it could move with the wind, and it doesn’t really do anything you can’t already do with a sail driven craft, which can already exceed wind speed when they are traveling perpendicular to the wind.
 
casebro said:
Generally, it would only be practical on water. The rotor would be too large for use on highways. And the cost/benefit of the mechanisms vs sails makes it impractical on the water. Marconi rigged boats go everywhere now. And unless the target is directly down wind, they tack without all the mechanism.
Click to expand...

The advantage of a sail is that it’s most effective going perpendicular to the wind, which allows you to tack upwind or downwind with relative ease. This drive system would do little going with the wind, and you’d probably be better off locking it in place and treating it as a crude sail.
 
my_wan said:
What do you think? Will it work and why?
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I think it could work - going downwind faster than the wind is directly equivalent to sailing against the wind (air -> water, ground -> air), which we know can be done. Whether this particular device can do it, I can't tell, but I don't think it matters in principle - even if this device couldn't do it, you could definitely build another one which could.

It isn't going to revolutionize transportation, but if someone can turn this mechanism into a successful toy, there is some potential money to be made here.

What I don't understand is why this thread is tagged "over unity".
 
The is technically off topic but I am responding due to the claim (I think) that it is related.

soylent said:
Depending on which version of this problem you mean it may very well be true. It's certainly possible to keep an aircraft stationary on a conveyor if you purposely set out to do that and as long as you've got an idealized airplane that will not break and an idealized conveyor with unbounded traction and unbounded power output.
Click to expand...

No it's not possible under any motion of the conveyor to prevent the airplane from taking off.

soylent said:
<snip>If the wheels are frictionless there exists some constant acceleration(not speed; acceleration) of the conveyor belt(and corresponding angular acceleration of the wheels) such that the traction force acting on the wheels is equal and opposite to the thrust of the airplane.
Click to expand...

There may be a slight moment of inertia while accelerating the wheel but it translates into angular momentum of the wheel and places a nearly unmeasurable drag on the forward motion of the planes itself.

A plane accelerates against the air and the air alone, the same way it does after taking off. A plane does not accelerate against the wheel in any way. The small friction from the wheel is no different when the wheel is spinning or even accelerating because it just represents a tiny more acceleration. It would be like claiming that if you lowered the wheel in flight and spun it fast enough the plane would fall out of the sky.

The motor powered flight of the airplane in no way relates to the wind powered craft described in the OP. I don't even like the conveyor version of the OP craft in the second video because it doesn't even represent the actual claimed abilities. The first video does and that is the OP subject.
 
Did anyone notice that the propeller rotates the way a plane propeller rotates, and not like a generator? I mean, when the wind blows on it, the prop should go clockwise, not the counterclockwise being observed.

I also noticed after the first push when the car slowed to a stop, it started creaping forward again, but only when the wind died down. Could there be a battery pack on this thing, and the wind was too strong pushing the prop in the beginning for the motor to operate?
 
humber said:
@my wan
Having though it over, I would expect that efficiency alone would prevent the vehicle from reaching, let alone exceeding, the wind.
Click to expand...

Prior to reaching wind speed the wind increases efficiency greatly. A stationary windmill can produce many times the power needed to drive the family car many times faster than the wind creating that power. The scale might be more than what's feasible on this craft design but the fundamentals are there for it to work.

humber said:
It may not be a hoax, but the measurements are so crude. Windspeed is determined by voice-over and a sock. There may well be wind bursts that exceed the average velocity of the vehicle. Is the road level? There will be some input from the wind bearing directly on the chassis, etc.
Click to expand...

Agreed. Wind variability, momentum, etc., is a major issue with reading too much into that video. However, the fundamentals of having a well defined power source to actually accomplish what is claimed is there.

Consider Newtons first law of motion. This law means that without friction you can accelerate a refrigerator for instance to near light speed with the constant power of a watch battery given enough time. The only limit to the speed of the craft then is determined by when friction and wind resistance matching the power. Up until you reach wind speed the wind resistance part actually works in your favor. Bearing and road friction can be designed extremely efficiently.
 
humber said:
@my wan
If you look at the video of the smaller belt test, you will see that the propeller is allowed to come to speed before being released. You would expect that it would then travel along a stationary belt even if the propeller were to be a simple flywheel. It is likely that the propeller stores excess energy from wind bursts as it progresses.
Ack. Mhaze
Click to expand...

Yes, good point. Just one of the many reasons I don't care for the conveyor belt version. I guess I shouldn't have posted it but I felt that due to my argument with the guy that made it he deserved the recognition.
 
Very fishy video. The video isn't clear enough, nor is there a close up of the vehicle, to determine that there's no motor on it. But it's probably being towed, seeing as how it keeps steering straight.
 
Acleron said:
Just thinking of the first demo. When the vehicle's velocity reaches wind speed, it is essentially motionless with respect to the air and there should be no force on the propeller. With no force there is no acceleration. Can't see how it exceeds the windspeed.
Click to expand...

Not quiet. Once the vehicle reaches ground air speed the prop tied to the wheels then creates thrust that effectively increases the air speed relative to the craft. Of course this places more drag on the wheels but because ground speed and actual air speed still don't match the power is there to pay for this drag. The available power is the difference between air speed and ground speed regardless of the motion of the craft or extra air speed created by the prop.
 
Don't ya see, it's not the wheels on the road acting like a keel, to cause lateral resistance, thereby allowing tacking at greater than wind speed. It's the drive train resistance to the prop shaft's turning that gives the resistance that allows the vanes to tack. Thereby allowing the rotational vector to be greater than wind speed. Then it's a matter of rolling resistance/hull speed as to whether distance made good is farther than the wind speed.

I think.
 
technoextreme said:
No. It isn't. It's exactly airplane on a treadmill and it's exactly what I would expect. The simplified free body diagram would prove that it is airplane on a treadmill.
Click to expand...

Still don't have a clue what you claimed here, much less got an answer for what you claimed previously. Is a "simplified free body diagram" like those lines they draw around murder victims on TV? You did make it clear this time that you think this is the "airplane on a treadmill" but I responded to that last time on assumption and you simply repeat yourself.

How is this the same as the "airplane on a treadmill" when the propeller of an airplane is not tied to the wheels in any way? You can spin the wheels on an airplane without turning the prop. On this craft if you turn the wheel without letting the propeller turn with it you'll break it. The wheels are tied directly to the prop with a drive shave or belt.

technoextreme said:
And there's the problem. In order for the propeller to act like a windmill and viceversa you have to change the direction of spin of the wheels which isn't done in the video.
Click to expand...

I asked what problem you were referring to so you replaced:
Problem.
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With:
And there's the problem.
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And replaced:
You need to swap the direction of rotation which isn't done in this movie.
Click to expand...
With:
In order for the propeller to act like a windmill and viceversa you have to change the direction of spin of the wheels which isn't done in the video.
Click to expand...
And you never did answer what experiment you said was already done. Even more confusing is you responded this way as if responding to a quote of me saying the wheels and prop are tied together.

If the "spin of the wheels" is reversed then it reverses the spin of the propeller and make the propeller blow air into the direction the craft is supposed to go. It would be like shifting your car into "R" for race. I'm not even sure if you are claiming the spin of the wheels should be reversed by design or something you do when the craft reaches wind speed or even what you think this reversal is supposed to accomplish. The wheels turn one direction at all speeds: forward. The prop turns one direction at all speeds: the direction that drives air toward the back of the craft. Whether or not the air is already moving in that direction or not makes no difference. It cannot be any other way.
 
casebro said:
Generally, it would only be practical on water.
Click to expand...

It would even work properly on water unless tie the prop rotation to the different rate water was flowing under the boat. That differential is the power source. The drag and slippage of doing this in water would make it so inefficient as to be totally useless. Standard sails are extremely efficient and that's what boats need to stick with.

The advantage this has on land is that the tie between the prop and wheels allows this craft to continue taking full advantage of the wind speed relative to the road at all speeds for the power source. Sail boat power is are limited by wind speed even if tacking can get you some speed above wind speed.
 
humber said:
The vehicle is effectively tacking the wind. The propeller's blade is presented at an angle to the prevailing wind.
Click to expand...

Not really, remember that the faster it goes the lower the effective wind speed will be, until as it ggoes faster thant he wind it will be blowing against it.

If you can go faster than the wind in its own dirrection why do you need a wind?
 
Thabiguy said:
I think it could work - going downwind faster than the wind is directly equivalent to sailing against the wind (air -> water, ground -> air), which we know can be done. Whether this particular device can do it, I can't tell, but I don't think it matters in principle - even if this device couldn't do it, you could definitely build another one which could.
Click to expand...

It is also like using wind power on a still day, as once you start moving you will be going against a wind, and you can use that for your power right?


The problem is that the faster you go in the dirrection of the wind the slower the effective wind speed is for you, once you go faster than the wind it is just drag.
 
so a couple of questions. First (thanks JWideman) how is this thing steering? Second, why does this thing slow down and stop so drastically in the end?

I think this is a fake. I think there is either a battery driven motor and steering servo on the car, or the car is being towed behind something.

If you bring up the fact that it takes a couple of tries, so that proves it is real, then you really haven't been around long. That is an old trick. Oh, and Grandma couldn't be in on the prank, right?


BTW, this operation has nothing to do with tacking the wind. The prop is not being blown by the wind, it is being driven by the wheels. As cool as the plane on a treadmill is, and what we can learn from it, this video here is nothing like it, and doesn't make sense.
 
ponderingturtle said:
If you can go faster than the wind in its own dirrection why do you need a wind?
Click to expand...

You need the wind, because the wind means that the ground is moving with respect to the air. When you're moving at the same speed as the air, you can use the moving ground to propel yourself against the air - even in the opposite direction.

If there was no wind, then if you were at rest with respect to the air, you would also be at rest with respect to the ground and you could use nothing to move.

ponderingturtle said:
It is also like using wind power on a still day, as once you start moving you will be going against a wind, and you can use that for your power right?
Click to expand...

No, it is not like that at all. Do you know how a sail can sail against the wind? Well, in this case it's switched: instead of moving air to push you, you would have to use the moving ground to push you, and instead of water to push against, you would have to use the air to push against.

It's perfectly possible to construct a vehicle that will move faster than the wind - in a zig-zag pattern, but still in the direction of the wind. Identically to a sail moving against the water in the opposite direction of the wind, the vehicle will move against the air in the opposite direction of the ground movement - i.e. faster than the wind (only by a small amount though).

As an exercise, look up the sailing technique and figure out how the land vehicle would have to be constructed to utilize an analogous technique.

So the question is not whether it's possible, the question is whether this device could do it. And I have to admit, this is difficult for me to determine. I have considered several simple models, but no conclusion yet.
 
lomiller said:
It’s got no significant energy storage so power in must equal power out. Power in is a function of wind speed + craft speed and propeller efficiency. The wheel and air resistance will determine how fast the craft will go on that much power. So, basically, wherever these two curves intersect is how fast the craft will go.

The key seems to be an efficient propeller. It needs to convert a good portion of the wind resistance it creates back into power for forward propulsion. All the wind would need to do is overcome the losses in the system. Since most of the wind resistance will come from the propeller the fact that it mostly cancels should allow it to move with relatively little added power.

Optimizing the propeller is going to be a tough problem to handle analytically, but trail and error on a computer model should work just fine and it shouldn’t stray that far from what we already know from wind turbines.

Anyway, despite being an interesting little device, I’m not sure what practical use it would have. I can’t see how it could move with the wind, and it doesn’t really do anything you can’t already do with a sail driven craft, which can already exceed wind speed when they are traveling perpendicular to the wind.
Click to expand...

Agreed that the particular design these guys used is essentially just a curiosity. However, the principles of operation that allow using the road to wind differential as a constant power source at any craft speed opens the door for much more practical designs. Designs that may change the propeller type altogether along with some other characteristics. The fastest bicycle speed is about 127 mph. That would probably be about matched by a 20 mph wind with 75 square feet effective area at 30% efficiency. I think alternative prop designs could pay off big and trade a little efficiency for torque.

Yes the analysis for optimization is a bigger headache than what I first assumed. I'm working on it but haven't gotten very far yet. I think I've figured out some ways that might simplify the basics. Your thinking there is very close to mine when I started thinking about it. I'm considered dragging out the giant fan and building a homemade wind tunnel.
 
Thabiguy said:
What I don't understand is why this thread is tagged "over unity".
Click to expand...

I debated that.. but couldn't think of any other tags. Many have claimed that this craft would require perpetual motion but that simply isn't so.
 
my_wan said:
If the "spin of the wheels" is reversed then it reverses the spin of the propeller and make the propeller blow air into the direction the craft is supposed to go. It would be like shifting your car into "R" for race. I'm not even sure if you are claiming the spin of the wheels should be reversed by design or something you do when the craft reaches wind speed or even what you think this reversal is supposed to accomplish. The wheels turn one direction at all speeds: forward. The prop turns one direction at all speeds: the direction that drives air toward the back of the craft. Whether or not the air is already moving in that direction or not makes no difference. It cannot be any other way.
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Congratulations. You just posted the reason why it doesn't work. *Soft Claps*
 
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ponderingturtle said:
If you can go faster than the wind in its own dirrection why do you need a wind?
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For power you need a difference somewhere to take advantage of. This craft takes advantage of the difference between road speed and wind speed. Speed is relative and Galilean relativity is sufficient in this case. The difference in road to wind speed stays constant no matter what speed the observer is going, even when going faster than the wind itself. It can't be done without that difference and no wind equals no difference.
 
my_wan said:
For power you need a difference somewhere to take advantage of. This craft takes advantage of the difference between road speed and wind speed. Speed is relative and Galilean relativity is sufficient in this case. The difference in road to wind speed stays constant no matter what speed the observer is going, even when going faster than the wind itself. It can't be done without that difference and no wind equals no difference.
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Ahhh finally found the explanation as to why it doesn't work that you can comprehend. What happens when you are going faster than the wind? You have a headwind. What does the headwind do? It pushes the propellor. In what direction does the headwind push the propellor? The wrong direction. What will happen? The cart will stop and start going the other direction.
The prop is not being blown by the wind, it is being driven by the wheels. As cool as the plane on a treadmill is, and what we can learn from it, this video here is nothing like it, and doesn't make sense.
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Not the cart video. The cart on the treadmill. The cart that the author says proves that he device works.
 
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Perhaps the videos are red herrings, but this is the part that I don't understand.

my_wan said:
Not quiet. Once the vehicle reaches ground air speed the prop tied to the wheels then creates thrust that effectively increases the air speed relative to the craft. Of course this places more drag on the wheels but because ground speed and actual air speed still don't match the power is there to pay for this drag. The available power is the difference between air speed and ground speed regardless of the motion of the craft or extra air speed created by the prop.
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I see it this way.
The prevailing wind exchanges momentum with the propeller, driving the vehicle forward. Even if the vehicle reaches wind speed, it can go no further, because at least at that point, it must loose momentum in order to accelerate the air impinging upon the propeller's leading face.
I think the terminal velocity will be determined at the point where the load line of the propellor and resistive forces intersect. I see the gears as a transformer, that will allow for a higher terminal velocity, but one that remains below windspeed.
 
my_wan said:
Yes the analysis for optimization is a bigger headache than what I first assumed. I'm working on it but haven't gotten very far yet. I think I've figured out some ways that might simplify the basics. Your thinking there is very close to mine when I started thinking about it. I'm considered dragging out the giant fan and building a homemade wind tunnel.
Click to expand...

I’d start by locking the prop and plotting the wind resistance at different wind speeds and then the prop efficiency at different wind speeds.

Provided the prop efficiency is high enough and the wind is strong enough there should be some craft speed at which (wind speed + craft speed) * resistance matches the power output of the prop at that windspeed + craft speed. (Unless I’ve screwed up my units that is…)

BTW I’m not saying it’s not a fake, but I think it’s conceivable it could work if the prop were efficient enough. The devil is, as they say, in the details.
 
Thabiguy said:
You need the wind, because the wind means that the ground is moving with respect to the air. When you're moving at the same speed as the air, you can use the moving ground to propel yourself against the air - even in the opposite direction.
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Ok so you are moving with the wind, how do you use your speed over the ground to increase your speed over the ground? You are getting energy from nothing here, because you are using your kinetic energy to increase your kinetic energy.

The only way you could possibly to it is have some method of storing energy when the vehical is stationary, to go faster than the wind for a while.
If there was no wind, then if you were at rest with respect to the air, you would also be at rest with respect to the ground and you could use nothing to move.
Click to expand...

You can not use your speed with respect to the ground to accelerate.


Here are some free body diagrams

Slower than wind

<-------
Wind
______
|Car |
-------
------->
Friction

You can see here that the force of the wind could exceed friction and make it go faster,

Here is the free body diagram of the car when it is moving with the wind

______
|Car |
-------
------->
Friction

Here is the diagram when it is moving faster than the wind

------->
Wind
______
|Car |
-------
------->
Friction

There is no source of energy for the car to accelerate once it is moving at the speed of the wind.
No, it is not like that at all. Do you know how a sail can sail against the wind? Well, in this case it's switched: instead of moving air to push you, you would have to use the moving ground to push you, and instead of water to push against, you would have to use the air to push against.
Click to expand...

You can't sail dirrectly against the wind, you sail at an angle to it and tack back and forth. This permits you do use aerodynamic effects.

You can sail much faster than the wind, if you sail at a right angle to it, see iceboats.
 
my_wan said:
I debated that.. but couldn't think of any other tags. Many have claimed that this craft would require perpetual motion but that simply isn't so.
Click to expand...

It is so, you are getting more momentum out in a way that you can not put it in, and also increasing the energy as a result.

You are accelerating faster when the only force on you is friction.
 
technoextreme said:
Not the cart video. The cart on the treadmill. The cart that the author says proves that he device works.
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There is nothing wrong with useing a treadmill, other than it can permit people to confuse themselves.

If this device worked, droping in onto a treadmill in a room with still air would mean it would accelerate against the treadmill.
 
ponderingturtle said:
Ok so you are moving with the wind, how do you use your speed over the ground to increase your speed over the ground?
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In the same way that the sail, moving with the water, can use its speed against the air to increase its speed against the air. Please try to think about it.

ponderingturtle said:
You are getting energy from nothing here, because you are using your kinetic energy to increase your kinetic energy.
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No, that is just wrong. The energy does not come from nothing - it comes from the kinetic energy of the wind. When two objects are moving with respect to each other (air vs. ground), then braking them against each other releases energy, which can be utilized - in this case, to propel you forward.

You want a simple example how that works? Imagine yourself in a car on a highway. You are moving at the same speed as the cars in your direction. You take a ball and throw it at a car going in the opposite direction. It bounces from it, then it bounces from a car behind you in your direction, and then you recatch it. Guess what? The ball, when you recatch it, will be travelling forward faster than when you threw it, giving you net increase in momentum and kinetic energy (do the calculation if you don't believe me). The momentum and energy did not come from nowhere - it came from the two cars that the ball hit, slowing them with respect to each other. Your speed is now higher than the speed of other cars in your direction - but you needed the cars going in the opposite direction for this to work.

ponderingturtle said:
The only way you could possibly to it is have some method of storing energy when the vehical is stationary, to go faster than the wind for a while.
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ponderingturtle said:
You can not use your speed with respect to the ground to accelerate.
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ponderingturtle said:
There is no source of energy for the car to accelerate once it is moving at the speed of the wind.
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These statements are not true. Sorry.

ponderingturtle said:
You can't sail dirrectly against the wind, you sail at an angle to it and tack back and forth. This permits you do use aerodynamic effects.
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So what? The point is that your net movement is in the direction opposite to the wind. In the same way, you can move faster than the air - at any given moment, you'll be approaching your target faster than a piece of paper freely blown by the wind.
 
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