Can a Wind-Powered Vehicle Travel Downwind Faster Than the Wind?

[atyy]

That's correct. If our treadmill were long enough we could start the vehicle moving backward at the speed of the belt (equivalent to setting it down on a road with a tailwind). Eventually the relative wind would get the vehicle up to speed and it would finally outpace the wind and advance on the treadmill

It' not obvious to me this is like the wind. A vehicle in the wind doesn't need wheels, and the wind will just blow it along. The less friction between the ground and the vehicle, the easier it is for the wind to get the vehicle moving.

This seems more devious (I mean that as a compliment!) to me. Here friction between the vehicle and the treadmill is essential. With wheels there is the ability to make friction useful. The treadmill exerts a torque on the wheels and this is converted into the propellor movement which pushes the vehicle up. The question is how do you get friction to point in the right direction? Is holding the vehicle in place at the start necessary to do this? Does the propeller sort of "replace the hand" once it has sufficient speed?

 

[rcgldr]

Either the prop is powering the wheel or the wheel is powering the prop.

The wheels are turning the prop. The prop is generating a small amount of thrust, enough to allow the cart to move slightly faster than the wind.

 

[spork]

It' not obvious to me this is like the wind.

But it is. Imagine you were on your sailboat in the middle of the ocean. You wake up, go up on deck, and feel a 10 knot breeze on your face. How do you know if that's "real" wind? You don't. Maybe there is no current and the wind is blowing 10 knots, or maybe there is actually no wind, and there's a 10 knot current. To the boat (and to your face) it's all the same thing. And that's all we're doing here, we move the road and let the air sit still. This makes it much easier to prove the concept indoors with very controlled conditions.

A vehicle in the wind doesn't need wheels, and the wind will just blow it along. The less friction between the ground and the vehicle, the easier it is for the wind to get the vehicle moving.

Yes, but that vehicle can never go directly downwind faster than the wind. Because this vehicle extracts the needed energy from the air/ground interface, it must have a way of working against the ground. You could use skate blades, wheels, or other things.

The treadmill exerts a torque on the wheels and this is converted into the propellor movement which pushes the vehicle up.

You could look at it that way, but it's probably better to see it as the treadmill (or ground) holding the bottom of the wheel still so the cart can produce a torque on the wheels by pushing them forward at the hub.

Is holding the vehicle in place at the start necessary to do this?

No, but our treadmill would probably have to be over 100' long if we just set it on the belt. It would be going backward until it reached wind speed. It then starts advancing on the treadmill when it exceeds wind speed.

The wheels are turning the prop. The prop is generating a small amount of thrust, enough to allow the cart to move slightly faster than the wind.

To say the wheels are turning the prop oversimplifies it slightly (in my opinion). Not only does the prop have to create the thrust to overcome rolling friction and other transmission losses - it also has to create enough additional thrust to cause the wheels to turn.

The whole thing is simply caught between the ground and the wind. The prop pushes the wheels forward so they have to turn, and the wheels are geared to the prop so it has to turn when the cart moves forward. I know, it sounds like perpetual motion, but it's extracting that energy from the ground/air interface. Wouldn't do a thing without it.

 

[rcgldr]

The wheels are turning the prop. The prop is generating a small amount of thrust, enough to allow the cart to move slightly faster than the wind.

To say the wheels are turning the prop oversimplifies it slightly (in my opinion). Not only does the prop have to create the thrust to overcome rolling friction and other transmission losses - it also has to create enough additional thrust to cause the wheels to turn.

The wheels interact with the ground and turn the prop. The prop interacts with the air to generate thrust that affects the entire cart. The drive train torque from the wheels is opposed by an equal an opposite drag torque from the prop. This should answer my_wan's question.

simple explanation of the DDWFTTW cart

Ignore the land sail comparason for these carts, here's a much simpler explantion. Power input = equals the force at the driving wheels times the speed of the cart relative to the ground, because the wheels interact with the ground. Power output equals the thrust at the prop times the speed of the cart relative to the air, because the prop interacts with the air. The ratio of (power output) / (power input) = (thrust times air speed) / (force times ground speed) and with a ratio well less than unity, the cart can still go DDWFTTW.

To compensate for rolling resistance and other drag factors, the thrust from the prop is greater than the force at the driving wheels, but as long as the speed between prop and air is relatively smaller than the speed between wheels and ground, the power output is less than the power input, and the cart works.

Given this perspective, a rotating prop isn't required. Any thrust producing device would work as long as it was similarly efficient to a prop. Regarding the prop, a range of pitches will probably work, changing the pitch changes both the thrust and drag torque from the prop, and these are offsetting, depending on the lift to drag ratio of the prop.

 

[spork]

The wheels interact with the ground and turn the prop. The prop interacts with the air to generate thrust that affects the entire cart. The drive train torque from the wheels is opposed by an equal an opposite drag torque from the prop.

That's a perfectly fair characterization.

This should answer my_wan's question.

I will be surprised and impressed if it does.

Ignore the land sail comparason for these carts, here's a much simpler explantion.

Everyone's got a "simpler" explanation, because everyone had a different point where it clicked for them. For me the land sail explanation is the most accurate and intuitive given. If another explanation works better for you - that's great - as long as it's accurate.

Power input = equals the force at the driving wheels times the speed of the cart relative to the ground, because the wheels interact with the ground. Power output equals the thrust at the prop times the speed of the cart relative to the air...

This suggests there is no power output when it's going downwind at exactly wind speed. Depending on how you define your system this can be true. Is this how you're looking at it? I would be more inclined to say the power output is equal to the work the prop actually does on the air (which will be greater than thrust * speed of cart relative to air).

...but as long as the speed between prop and air is relatively smaller than the speed between wheels and ground, the power output is less than the power input, and the cart works.

True - as long as that difference is enough to allow for the inefficiencies of the prop, rolling resistance, and drivetrain losses.

Given this perspective, a rotating prop isn't required. Any thrust producing device would work as long as it was similarly efficient to a prop...

...and indexed to the ground in a way similar to the prop. This is required to extract the energy from the ground/air interface so that we can go faster than the wind.

Regarding the prop, a range of pitches will probably work, changing the pitch changes both the thrust and drag torque from the prop, and these are offsetting, depending on the lift to drag ratio of the prop.

Agreed - as long as the efficiency is still sufficient and the advance ratio of the prop vs. wheels is less than 1.

 

[ThinAirDesign]

Ignore the land sail comparason for these carts, here's a much simpler explantion.

Ok, let's hear your simpler explanation:

Power input = equals the force at the driving wheels times the speed of the cart relative to the ground, because the wheels interact with the ground. Power output equals the thrust at the prop times the speed of the cart relative to the air, because the prop interacts with the air. The ratio of (power output) / (power input) = (thrust times air speed) / (force times ground speed) and with a ratio well less than unity, the cart can still go DDWFTTW.

You didn't pass the "simpler" test ... my girlfriend says the above is all gibberish. :-)

JB

PS Meant in good fun Jeff.

 

[schroder]

It has taken me a while, but for what it’s worth, I finally understand the principle of this thing. It is not a pmm at all. I won’t apologize for being dense, but if I directly insulted anyone I sincerely apologize for that. As I understand it now, the cart is employing positive feedback, not unlike a turbo charger, from output device (propeller) to input device (wheels). In that sense, the cart is being driven by both the wheels and the prop at the same time. My mental block concerned advancing against the treadmill, while drawing all its power from the treadmill. If the device were using two wheels to extract power from the tread and an additional set of wheels working against the tread, clearly that would not work. What finally convinced me was to imagine a threaded rod run parallel to the treadmill, and instead of a propeller a worm gear was used to drive the cart along the threaded rod. That would certainly work, and the cart would accelerate until it either burned itself up from friction and heat, or it reached some terminal velocity dictated by the available energy, mass and friction forces. I then extrapolated that idea to the propeller drive and found it is not much different. Of course, saying all this after seeing the very convincing video does not establish me as a good intuitionalist but at least my brand of formalism does allow some flexibility. But all of these concessions are to the machine on the treadmill. As a formalist, I still have reservations about the ability to outrun the wind, going directly downwind. As Atyy pointed out, any contact with the ground must introduce friction forces which would not be present with the wind alone, but those same friction forces can be used to advantage as well. In that case, from both an intuitionalist and a formalist point of view, it does seem rather questionable as to which of the forces would be greater. However, having been proven wrong about the treadmill I concede that I may well be wrong about the wind as well.

 

[spork]

I won’t apologize for being dense

An apology is never required or expected for simply not quickly understanding unintuitive things.

..but if I directly insulted anyone I sincerely apologize for that.

Thank you. I accept.

If the device were using two wheels to extract power from the tread and an additional set of wheels working against the tread, clearly that would not work.

Agreed. The cart extracts its energy from the road/air interface. As such it has to have something working against the air.

What finally convinced me was to imagine a threaded rod run parallel to the treadmill, and instead of a propeller a worm gear was used to drive the cart along the threaded rod. That would certainly work, and the cart would accelerate until it either burned itself up from friction and heat, or it reached some terminal velocity dictated by the available energy, mass and friction forces.

Perfectly accurate.

I then extrapolated that idea to the propeller drive and found it is not much different.

Also true.

Of course, saying all this after seeing the very convincing video does not establish me as a good intuitionalist but at least my brand of formalism does allow some flexibility. But all of these concessions are to the machine on the treadmill. As a formalist, I still have reservations about the ability to outrun the wind, going directly downwind.

But the two cases are identical. The same experiment is being performed in two different inertial frames. As such the results must be identical.

However, having been proven wrong about the treadmill I concede that I may well be wrong about the wind as well.

It *has* to work in the wind - because wind across a stationary road is identical to a road moving beneath a stationary mass of air.

 

[ThinAirDesign]

Schroder, I commend not only your ability to learn and in return model and describe your observations, but even more so your ability to stand up and apologize -- that demonstrates a quality that really matters.

Kudos.

JB

 

[ThinAirDesign]

my_wan, I respect knowledge and you clearly hold more physics knowledge than I. I must say however that if you are correct, the Physics world will be rocked by your discovery and you will likely get a Nobel.

You are holding that there is now a test which can be performed which will yield varying results based on differing 'amounts' of purely linear motion.

I'm quite surprised frankly that there are not more folks on a physics forum piping up and taking a position on a debate so fundamental to one of the most basic and long-standing physics principles.

JB

 

[spork]

I'm quite surprised frankly that there are not more folks on a physics forum piping up and taking a position on a debate so fundamental to one of the most basic and long-standing physics principles.

No need frankly. They can wait for you and I to go and provide physical proof once again by taking the cart out on the road in a smooth tailwind, to show it behaves the same way.

 

[OmCheeto]

In response to... "How can a vehicle move faster than the wind that is powering it?"

OmCheeto wrote:
"It cannot go faster than the wind when going directly downwind."

Then I built a cart that does exactly that, made a video, and posted it.

Yes you did, but as I pointed out, the parameters of the experiment changed.
For you next experiment, I think you should remove the RC steering control from the first vehicle, add it to your new mini-me "faster than wind" vehicle, and put it back on the treadmill. I bet it would go even faster.

Now OmCheeto incredulously writes:

hmmm... I thought I'd deleted my statement that the reason they called you charlatans was because you are charlatans.

For the record, you used the term before I did.

and my_wan tells me I haven't been insulted, I may not have the stomach for this, and this is "how science works".

There's a kind of twilight-zone sort of entertainment to this I suppose.

I agree. I keep unsubscribing to this thread, yet somehow I keep getting sucked back into this vortex.

I guess it's like forcing ones self to read some poorly written mystery novel, because no matter how bad it gets, you just have to know how it ends.

 

[spork]

...but as I pointed out, the parameters of the experiment changed.

And as I pointed out - you're wrong. I defined the parameters of the experiment. I concieved of the solution. JB and I built the cart, and we demonstrated it doing EXACTLY what we claimed.

For the record, you used the term before I did.

In point of fact I NEVER used to word "charlatan" except to complain about people calling me that.

I agree. I keep unsubscribing to this thread, yet somehow I keep getting sucked back into this vortex.

Next time you feel you're about to get sucked back in you really should call a friend and have them talk you down.

I guess it's like forcing ones self to read some poorly written mystery novel, because no matter how bad it gets, you just have to know how it ends.

I'll save you the trouble - I'm right. Sorry for the spoiler, but now you have no need to get "sucked back in" just to hurl insults at the ones who HAVE proved their point.

 

[cristo]

Some members have a very bizarre definition of an "insult." Let me remind you that comments against someone's ideas, or someone's experiments are completely different to comments which are meant to solely put a member down, or insult them. The latter set are not permitted, as per the global guidelines.

The OP has given out way more than he has received, and in fact threw insults in his very first post here at PF. Thus, any remarks along the line of "self defence" are not going to wash.

Since there is clearly no hope of a civil discussion of this topic, this thread is done.