Down wind faster than the wind

[Smackety]

I think the disagreement is about what can be considered a treadmill. Clearly when you are running on a treadmill there is no wind, right? (you MUST run the speed of the treadmill or you fall off) Explain how a wind powered device can continue to generate power in the absence of wind?
A gigantic, never ending treadmill/people mover/escalator would let you simulate any wind speed you wanted as long as you didn't move. When you reached the speed of the treadmill there would be zero air movement, and again, the question of how to generate wind power in still, unmoving air becomes relevant.

 

[spork]

I haven't followed this thread well enough to even ask a question, but let me ask a few.

Fair enough. Serious questions are worth answering even if they are repeats.

1.What is the general consensus? It seems like it is that a device that could travel downwind faster than the wind is possible. Is that the consensus with perhaps some doubters?

I don't feel qualified to speak about the consensus. I know there are plenty of people that believe and understand. I know that many of them started as non-believers. I don't think ANY have started as believers and gone the other way. Quite apart from the question of consensus, I can tell you it's true, and that you can easily test it for yourself from a parts list and build notes I've posted in several places.

2. Assuming that a iceboat on a reach can have a downwind component velocity component faster than the wind can the plausibility of this device be established by just imagining a single vehicle that consists of two iceboats reaching and tacking downwind tied together?

That is a perfect and accurate analogy. That's all that should be required. This should be the shortest thread there ever was. But it hasn't worked out that way. I've put forward that analogy, and thought I had made it pretty clear - but maybe it only seemed clear to me.

3. Is the consensus that the treadmill is a valid way to test the device? Has any attempt been made to do so?

My answer to this is identical to #1. The proof that it is valid is simply that it is one of the most basic principles of physics that goes back more than 3 centuries. It's known as Galilean Relativity. It's says that all inertial framewords are identical and indistinguishable.

 

[Michael C]

No the treadmill has no wind. All the power comes from the treadmill.

3. There is no wind on a treadmill, sorry. At least, not for very long.

Question of the day

What is Wind?

Please give a simple answer, in one sentence.

Hint: if you answer "Wind is moving air", you're missing something.

 

[spork]

Explain how a wind powered device can continue to generate power in the absence of wind?
A gigantic, never ending treadmill/people mover/escalator would let you simulate any wind speed you wanted as long as you didn't move. When you reached the speed of the treadmill there would be zero air movement, and again, the question of how to generate wind power in still, unmoving air becomes relevant.

If you want to understand we're happy to explain. If you want to explain that we're wrong - have at it.

 

[Smackety]

Okay. On a 10 mph treadmill, with the device moving at 10 mph, and thus encountering zero wind, how much energy is being input to the device, and by what means?

 

[spork]

Okay. On a 10 mph treadmill, with the device moving at 10 mph, and thus encountering zero wind, how much energy is being input to the device, and by what means?

The amount of energy being "put into" the device is equal to the tangential force of the belt against the wheels times 10 mph. This of course is "power" or rate of energy. This also happens to be the same force with which the prop pushes on its shaft.

By what means that energy is "put into" the system depends entirely on the frame of reference we choose for our analysis. If we choose the wind as our reference frame, all energy is provided by the interaction between the wheels and treadmill tread.

If we choose the treadmill tread as our inertial reference frame in which we perform our calculations, then the energy is provided by the action of the thrust forcing the cart forward via the prop (in other words the wind provides the energy in this case).

This is the nature of energy. Unlike mass, it's not an intrinsic property. It's more like a method of bookkeeping to be used when performing such analyses. It's also not like force. The forces will be the same no matter the frame in which their measured.

Force and momentum tend to be far more intuitive for most people than energy. If you consider the kinetic energy of a bullet, it's given by:
1/2 x M x V^2.

If you're standing in the road and the bullet is coming at you at 1000 feet/sec, you'll experience that energy in an awful way. If on the other hand, you're moving down the road at 999 ft/sec when the bullet approaches, it's relative velocity will only be 1 ft/sec. Consequently it will have very little kinetic energy in this frame. It would be as if someone carefully handed the bullet to you. Given the choice, I'd recommend option 2.

 

[Smackety]

And if I was moving at 1000 feet/sec the bullet would not hit me at all, there would be no kinetic energy.
If I understand you correctly, once the cart reaches 10 mph, the same at the treadmill, energy input drops to zero? How does it continue to accelerate at this point?

 

[dv82matt]

It seems to me that the question of whether DDWFTTW is possible is resolved, the cart works by sacrificing torque for velocity. When the cart is motionless on the ground with a 10 mph tailwind the wind acts on the cart in two ways. First it attempts to push the cart forward, second it attempts to spin the propeller.
Since the cart is set up so that these two forces oppose each other the stronger force prevails but the torque is reduced. If one were to lift the wheels from the ground they would start spining backwards driven by the propeller so we can see that when the cart is in forward motion (but at less than wind speed) the linkage to the wheels is forcing the propeller to turn in the opposite direction that it would if it were spinning freely (at above wind speed it is forcing it to spin faster than it would otherwise).
The sacrificed torque is thus used to increase velocity resulting in a flatter acceleration curve and a higher top speed but lower torque.

But I'm curious as to whether the cart could (with some minor tweaking such as adjusting gear ratios and/or optimizing propeller/turbine efficiency) go directly upwind faster than the wind.

In the downwind case the prop acts like a propeller and is powered by the wheels. In the upwind case it's the reverse as the prop would act as a wind turbine delivering power to the wheels.

 

[Smackety]

It seems to me that the question of whether DDWFTTW is possible is resolved, the cart works by sacrificing torque for velocity. When the cart is motionless on the ground with a 10 mph tailwind the wind acts on the cart in two ways. First it attempts to push the cart forward, second it attempts to spin the propeller.
Since the cart is set up so that these two forces oppose each other the stronger force prevails but the torque is reduced. If one were to lift the wheels from the ground they would start spining backwards driven by the propeller so we can see that when the cart is in forward motion (but at less than wind speed) the linkage to the wheels is forcing the propeller to turn in the opposite direction that it would if it were spinning freely (at above wind speed it is forcing it to spin faster than it would otherwise).
The sacrificed torque is thus used to increase velocity resulting in a flatter acceleration curve and a higher top speed but lower torque.

But I'm curious as to whether the cart could (with some minor tweaking such as adjusting gear ratios and/or optimizing propeller/turbine efficiency) go directly upwind faster than the wind.

In the downwind case the prop acts like a propeller and is powered by the wheels. In the upwind case it's the reverse as the prop would act as a wind turbine delivering power to the wheels.

Are you saying that the propeller reverses direction when the cart accelerates past wind-speed?

 

[dv82matt]

Are you saying that the propeller reverses direction when the cart accelerates past wind-speed?

No. The propeller is linked to the wheels and so cannot reverse direction unless the cart reverses direction.
However, if the propeller were spinning freely (which it is not) then it would change direction depending on whether it were moving at above or below wind speed.

 

[Smackety]

No. The propeller is linked to the wheels and so cannot reverse direction unless the cart reverses direction.
However, if the propeller were spinning freely (which it is not) then it would change direction depending on whether it were moving at above or below wind speed.

Allright, I understand now that the propeller is linked to the wheels.
When you say that "the two forces oppose each other" do you mean that the propeller spins the wrong way, i.e. attempts to slow the cart down?

 

[spork]

And if I was moving at 1000 feet/sec the bullet would not hit me at all, there would be no kinetic energy.

That's correct.

If I understand you correctly, once the cart reaches 10 mph, the same at the treadmill, energy input drops to zero?

Nope. It seems like it should. That's what makes this such a good brainteaser. But there are several ways to think about it. When the cart is moving along at 10 mph in a 10 mph tailwind, the cart feels no wind (this is true). The prop blades do still feel wind, because they aren't simply moving downwind with the cart, their also spinning around (i.e. moving crosswind). But let's come back to that.

For now we're going downwind at wind speed - thus as a passenger we feel no wind. However, the road is still passing beneath our wheels at 10 mph (think treadmill). This is a perfectly good energy source. And if we use this energy source, we can spin the prop to provide the necessary resistance in the tailwind to keep us still while the treadmill belt continues to spin our wheels at 10 mph. If there were no velocity differential between the wind and the treadmill belt, we would never be able to extract enough energy from the road (i.e. treadmill belt) to pull this trick off. But that very differential between road and wind puts our prop in a medium that's easier to push against. So the energy available at the wheels is greater than the energy needed at the prop.

Even so, we have to remember that whether the energy comes from the road or the wind is really only a matter of how we do our analysis. This doesn't really make any difference though, because no matter what frame we do our analysis in the experimental results are the same (i.e. speed of the cart, forces on the wheels, prop, prop-shaft, transmission, etc.).

Energy is just a very different animal. In some ways it's like velocity. When the vehicle is going downwind at 10 mph in a 10 mph tailwind, how fast is it going relative to the road? 10 mph. But how fast is it going relative to the air? 0 mph. How can it be both? Velocity only has meaning relative to the reference frame in which it's measured. Kinetic energy is the same in this respect.

 

[spork]

When you say that "the two forces oppose each other" do you mean that the propeller spins the wrong way, i.e. attempts to slow the cart down?

If the cart were sitting still in a tailwind, and you could put the transmission in neutral, the wind would indeed spin the prop in the opposite direction to which it actually does spin in operation. I don't think we can say the prop acts to slow the car down, because it is this interaction of the prop, transmission, and wheels that enable the cart to operate.

It's also this business of opposing torques (i.e. what the shaft makes the prop do vs. what the wind would make the prop do) that defines our prop as a prop (and not a turbine or windmill). In other words, the prop is creating thrust - rather than being turned by the wind.

 

[spork]

However, if the propeller were spinning freely (which it is not) then it would change direction depending on whether it were moving at above or below wind speed.

This is true, but could be slightly misleading. At all speeds both above and below wind speed (in normal operation) the prop acts as a prop, and is turned by the shaft (rather than turning the shaft).

I'm guessing you know that, so I'm just trying to add clarification for those that might miss the point.

But I'm curious as to whether the cart could (with some minor tweaking such as adjusting gear ratios and/or optimizing propeller/turbine efficiency) go directly upwind faster than the wind

By simply reducing the size of the wheels the cart becomes an upwind vehicle. With the right gear ratio and low enough losses the cart could go upwind faster than the wind. The parameter that governs the upwind/downwind operation is the advance ratio of the overall cart. This is the distance the prop would theoretically advance in still air in a single rotation divided by the distance the wheels would move in that same single rotation of the prop. If that number is smaller than 1.0 we have a downwind cart. If it's greater than 1.0 it's an upwind cart. The closer to 1.0 (from either side) the faster (but weaker) is the cart.

 

[H'ethetheth]

Cart moves prop, prop moves cart..

Let me try to help.

What the wind does is push the cart to a speed where the wheels spin the propeller. When this happens the propeller pushes the cart to a higher speed. This is true for all speeds up to wind speed. Above wind speed the relative wind reverses direction and will cancel the propeller's thrust at some point. How's that?

 

[spork]

Above wind speed the relative wind reverses direction and will cancel the propeller's thrust at some point. How's that?

It's true that the free-stream reverses its direction relative to the prop disk above the wind speed. But it's important to understand that the relative wind hitting the prop blades *always* hit the back of the blades. There is no magical switching point when we reach wind speed. In fact the flow through the prop becomes front-to-back well before the cart reaches wind speed.

The advance ratio of the prop relative to wheels defines the theoretical maximum (kinematic) velocity the cart can achieve. This is the speed at which the prop would advance through the air in a no-slip condition. Beyond that speed (which is above wind speed) the prop would produce negative thrust. Of course with real-world losses, we never actually achieve that theoretical max speed.

 

[dv82matt]

Allright, I understand now that the propeller is linked to the wheels.
When you say that "the two forces oppose each other" do you mean that the propeller spins the wrong way, i.e. attempts to slow the cart down?

No, well not exactly, the propeller does in a way "attempt to slow the cart down" by removing forward momentum via the wheels but the converse is also true in that the propeller "attempts to speed the cart up" in the same manner that an airplane prop does. Other than losses from friction and drag the total energy in the system is not affected one way or the other, but torque is sacrificed and velocity is gained. The effect is similar to changing to a higher gear in a car or bicycle.

By simply reducing the size of the wheels the cart becomes an upwind vehicle. With the right gear ratio and low enough losses the cart could go upwind faster than the wind. The parameter that governs the upwind/downwind operation is the advance ratio of the overall cart. This is the distance the prop would theoretically advance in still air in a single rotation divided by the distance the wheels would move in that same single rotation of the prop. If that number is smaller than 1.0 we have a downwind cart. If it's greater than 1.0 it's an upwind cart. The closer to 1.0 (from either side) the faster (but weaker) is the cart.

Thanks that's interesting and about what I would expect. Maybe that would be an interesting experiment to try out next.

 

[spork]

Maybe that would be an interesting experiment to try out next.

The upwind case seems somewhat boring to me. But it should be easy enough to try. I just have to see if I have any wheels that are small enough. Maybe a couple of the tail-wheels.

 

[tsig]

Compelling as this "proof by absolute (and repeated) assertion" is - I'm going to have to say TROLL.

All we have is your assertion that it works.

 

[John Freestone]

I think the disagreement is about what can be considered a treadmill. Clearly when you are running on a treadmill there is no wind, right? (you MUST run the speed of the treadmill or you fall off) Explain how a wind powered device can continue to generate power in the absence of wind?
A gigantic, never ending treadmill/people mover/escalator would let you simulate any wind speed you wanted as long as you didn't move. When you reached the speed of the treadmill there would be zero air movement, and again, the question of how to generate wind power in still, unmoving air becomes relevant.

I think you have the second point wrong, which may be why you ask the first part. If you "didn't move" on the escalator (if you mean relative to the ground), you still have no wind, and "when you reached the speed of the treadmill" (if you mean moving along with the tread surface) then you don't have "zero air movement" relative to yourself, but a wind of whatever speed the treadmill is going at (if there's no wind relative to the ground).

I think that one of the simple confusions people have (maybe not you) comes from the fact that spork or JB or whoever it is testing a cart on a treadmill has to hold the cart still until its wheels and prop are spinning before letting go. This adds to the impression that there's "no wind" in the experiment. There isn't any then, because the cart is now "moving at windspeed". It represents exactly the situation in a road test where the machine has run along and accelerated to windspeed. It's just that to show that on a treadmill (to put the cart on at tread-speed and let it accelerate to windspeed) would need a very long treadmill, steering on the cart, etc., and isn't really important. The important thing is that when they do this and let go, it doesn't then do what most objects would do, start being pushed backwards, but progresses forwards. It does this because, although there is no windspeed for the cart, there is for the prop, being driven at an angle through the air.