Down wind faster than the wind

[JWideman]

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.

 

[my_wan]

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.

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.

 

[casebro]

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.

 

[my_wan]

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.

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.

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.

I asked what problem you were referring to so you replaced:

Problem.

With:

And there's the problem.

And replaced:

You need to swap the direction of rotation which isn't done in this movie.

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.

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.

 

[my_wan]

Generally, it would only be practical on water.

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.

 

[ponderingturtle]

The vehicle is effectively tacking the wind. The propeller's blade is presented at an angle to the prevailing wind.

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?

 

[ponderingturtle]

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 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.

 

[Careyp74]

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.

 

[Thabiguy]

If you can go faster than the wind in its own dirrection why do you need a wind?

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.

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?

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.

 

[my_wan]

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.

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.

 

[my_wan]

What I don't understand is why this thread is tagged "over unity".

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.

 

[technoextreme]

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.

Congratulations. You just posted the reason why it doesn't work. *Soft Claps*

 

[my_wan]

If you can go faster than the wind in its own dirrection why do you need a wind?

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.

 

[technoextreme]

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.

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.

Not the cart video. The cart on the treadmill. The cart that the author says proves that he device works.

 

[humber]

Perhaps the videos are red herrings, but this is the part that I don't understand.

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.

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.

 

[lomiller]

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.

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.

 

[ponderingturtle]

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.

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.

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.

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.

 

[ponderingturtle]

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.

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.

 

[ponderingturtle]

Not the cart video. The cart on the treadmill. The cart that the author says proves that he device works.

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.

 

[Thabiguy]

Ok so you are moving with the wind, how do you use your speed over the ground to increase your speed over the ground?

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.

You are getting energy from nothing here, because you are using your kinetic energy to increase your kinetic energy.

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.

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.

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

There is no source of energy for the car to accelerate once it is moving at the speed of the wind.

These statements are not true. Sorry.

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.

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.