Down wind faster than the wind

[Dan O.]

I thought I would never say this but humber is correct.

His description of the cart on the treadmill being in balance is absolutely correct.

What are the forces acting on the cart? Here is a simplified diagram:



The external forces acting on the cart are gravity, a normal force from the surface, belt push due to the energy the cart is extracting from the motion over the surface and thrust from the acceleration of the air by the propeller. When the system is in balance, the thrust generated by the propeller equals the push from the surface plus the residual of the gravity and normal forces.

If the cart is able to balance when on an inclined surface, there will be a net forward force with the inclination removed so the cart would accelerate. What energy source is supplying this extra force?

Velocities are all shown as differences relative to the cart

 

[H'ethetheth]

I've put a new demonstration on YouTube. Inspired in particular by Modified and Brian M, I've made a simple mechanical cart that moves under the ruler faster than the ruler:



The large wheel, turning against the direction of movement of the ruler, is very similar to the propeller of the wind-powered cart, turning against the wind.

Two things:

(1) Your avatar isn't quite clear enough; are you Koko or Terry?

(2) Have you ever thought about narrating childrens stories?

 

[H'ethetheth]

I thought I would never say this but humber is correct.

His description of the cart on the treadmill being in balance is absolutely correct.

What are the forces acting on the cart? Here is a simplified diagram:

[qimg]http://www.internationalskeptics.com/forums/imagehosting/thum_1514449426363349b8.jpg[/qimg]

The external forces acting on the cart are gravity, a normal force from the surface, belt push due to the energy the cart is extracting from the motion over the surface and thrust from the acceleration of the air by the propeller. When the system is in balance, the thrust generated by the propeller equals the push from the surface plus the residual of the gravity and normal forces.

If the cart is able to balance when on an inclined surface, there will be a net forward force with the inclination removed so the cart would accelerate. What energy source is supplying this extra force?

The treadmill. What is your problem exactly?

 

[Michael C]

Two things:

(1) Your avatar isn't quite clear enough; are you Koko or Terry?

Neither one nor the other: the avatar is a portrait of Mo, who hasn't yet had the honour of appearing on YouTube. Maybe I'll give him a starring role in the next instalment.

(2) Have you ever thought about narrating childrens stories?

Not as such, but I've already done something like it: I'm a musician (conductor + pianist), and I've had a lot of fun presenting concerts for children. I did a series where my partner was a green hippopotamus called Linda. Linda would ask innocent but tricky questions about the music we were playing and I would try to answer them.

 

[H'ethetheth]

Neither one nor the other: the avatar is a portrait of Mo, who hasn't yet had the honour of appearing on YouTube. Maybe I'll give him a starring role in the next instalment.

Not as such, but I've already done something like it: I'm a musician (conductor + pianist), and I've had a lot of fun presenting concerts for children. I did a series where my partner was a green hippopotamus called Linda. Linda would ask innocent but tricky questions about the music we were playing and I would try to answer them.

I will count that as a yes.

Good job, is what I wanted to say.

 

[sol invictus]

You'd never believe the debate we had with Wayne Whiteman of GA Tech. This guy has a PhD and is a professor of Mech Eng. Check his credentials here: http://www.me.gatech.edu/faculty/whiteman.shtml

He assures us that the treadmill bears no relation whatever to a road with a 10 mph tailwind. But it gets better. He also tells us that no test would ever satisfy him that our cart is faster than the wind. He said even if we had our cart race a neutrally buoyant balloon on a direct downwind 5 mile course, and our cart reached the finish line in half the time of the balloon, that would still prove nothing. He admitted that if an ice boat started and completed the course at the same time and place as our cart, it WOULD prove the ice boat is faster downwind than the balloon - but it would NOT prove our cart was. He assures us that 10 of his colleagues with vast experience in fluid dynamics have reviewed the problem and hold his same opinions.

Wow. What an idiot.

I don't know a lot of engineering profs. But I do know many physicists, and I really can't imagine any of them not understanding the point about the treadmill. Physicists are trained to think in more general terms, to abstract problems down to their essentials, and they're used to all kinds of truly bizarre and counterintuitive physical systems (quantum mechanics etc.) So that probably makes it easier to get past the initial non-intuitiveness of this.

Personally I was almost immediately convinced, but it probably helped that I have some experience with sailing and windsurfing. But for me the best way to make the argument is just to point out that no laws of physics are violated, since there is a nearly unlimited amount of energy available at the air/ground interface. Then it becomes purely an engineering problem, and not a very hard one.

 

[Dan O.]

The treadmill. What is your problem exactly?

I don't have a problem. Just trying to help some members figure out theirs.

Since all the velocities listed are relative to the cart, the diagram could equally represent the cart going up the side of a hill with a matching tail wind. Without the hill there would be a net force accelerating the cart forward. Now, what is the source of the energy causing the cart to go faster than the wind?

 

[H'ethetheth]

I don't have a problem. Just trying to help some members figure out theirs.

Since all the velocities listed are relative to the cart, the diagram could equally represent the cart going up the side of a hill with a matching tail wind. Without the hill there would be a net force accelerating the cart forward. Now, what is the source of the energy causing the cart to go faster than the wind?

The treadmill. On the road it's the fact that when moving at wind speed there's this continuous supply of road passing by to power the propeller.

 

[humber]

These are some truly bizarre statements. You seem to think that wind tunnels can only be used to model wind, whereas in the real world they're almost always used to model still air, which is what makes them tricky to make, not to mention huge.

How do you come to that conclusion, H'ethetheth? Cars are often simply restrained while air is blown over them to simulate road conditions.
That cannot be done without moving air, right?
My point was that "equivalency" is not a premise for their existence.
If possible, the entire tunnel could be put in a vehicle, and driven at some speed. Unless there is interaction with the external environment, nothing will change. The measurements of the interaction between the car and windtunnel air, will remain unchanged.

I have addressed F1 cars before. Sometimes, the belt is a dyno, so they can run the engine. They use ground effect, and unless the motion between the car and the ground is also modeled, the results will not be sufficiently accurate. In this article, they are studying overtaking, (and into the wind), so the motion of the car is also important.
But, there they still have wind, right? If the car remains in place, and they want to test for performance at 100kph, then the wind will still be 100kph, moving belt or not. I am not sure how they are conducting this test, perhaps the car always remains in place on the belt. To actually have the car move forward any distance would be impractical at F1 speeds, but perhaps not laterally.
Computers also model aerodynamics, and they have no 'velocities' at all. From direct experience in acoustics, I can tell you that sometimes media other than air are used, because shorter wavelengths, mean smaller models. One day, when the mathematical models are good enough, goodbye expensive windtunnels. They are all just models, and equivalence plays no part. The model is constructed to test for the desired outcome, and not because of equivalency. That is not involved at all. That is certainly BS, and an unnecessary embellishment.

In my drawings I have included some intermediate cases of belt and wind, finally reaching the limiting case of the current treadmill. If you look, all velocities are on the same line, which for the last case, results in (0,0) and not +20 (windspeed) as is claimed. There is a discontinuity. That needs to be explained. This is evidence that the treadmill is not what it claims to be! It is on the x-axis, all the time, hovering around (0,0). The explanation is clear. Increasing the friction will definitely send it back down the belt, as the graphs indicate.

One last try:
If we test a car in a wind tunnel on a treadmill, and the wind speed in the wind tunnel is equal to the speed of the belt, this is equivalent to driving in still air. If you think this is a ludicrous idea, I advise you to take it up guys like these

That is not what the belt is for, as I hope I have explained above Nothing is 'ridiculous' in that sense, H'ethetheth. Leave that to Sol_inviticus. It is common to use that ad hom, but I am immune to it.
I always imagine the person typing it. That image makes them look ridiculous, in the correct sense of the word.
The treadmill works, or it does not, that is all there is to it.

If we turn off the wind tunnel, and keep everything else running the same, what is it equivalent to?

The car will move off the belt, carried by its momentum, because the restraining force of the wind has been removed. There will be feedback here. A control loop that monitors the car's position, while constantly controlling the belt speed. Not at all like a treadmill!

If your answer is not "driving on a road with a tail wind that matches the speed of the car", you have the wrong answer. And the kind of car does not matter in this respect, so this is valid for sailboats, chickens, corn flour, aprons, squirrels, beef, and the cart.
If you don't see this, I'm pretty sure I can't help you see it.

Belts do not make wind, H'ethetheth. There is a big wind generator in use in that F1 case, but I think I understand what you mean. I have covered all possible cases in my last post, from 0 to fullspeed belt. That is not the point, surely. What have I left out? The intermediate cases are perhaps pointless, but valid.
You are arguing for the general case, whereas the treadmill is a specific case, with no wind at all, and a claimed velocity that is not apparent, nor supported. It is not correct, and that's for sure.


My turn. You are at an airport, where there are those long moving walkways. Place your luggage on the belt. If you view it from the belt, or walk along the floor instead, how does that change anything?
If you put a cart on the belt, do you think that it would suddenly shoot off at beltspeed? Do you think the propeller would even turn?

(Christian, if you are interested, please answer that too)

 

[humber]

Wow. What an idiot.

I don't know a lot of engineering profs. But I do know many physicists, and I really can't imagine any of them not understanding the point about the treadmill. Physicists are trained to think in more general terms, to abstract problems down to their essentials, and they're used to all kinds of truly bizarre and counterintuitive physical systems (quantum mechanics etc.) So that probably makes it easier to get past the initial non-intuitiveness of this.

Personally I was almost immediately convinced, but it probably helped that I have some experience with sailing and windsurfing. But for me the best way to make the argument is just to point out that no laws of physics are violated, since there is a nearly unlimited amount of energy available at the air/ground interface. Then it becomes purely an engineering problem, and not a very hard one.

I wrote to said professor. Here is his reply;

"Sorry….I bowed out of that debate quite a while back. Let me assure you that what’s been attributed to me below (Sporks's quote) is not accurate. The discussion became far too heated, emotional, and folks weren’t listening to rational explanations."

'Nuff said

 

[Modified]

I've put a new demonstration on YouTube. Inspired in particular by Modified and Brian M, I've made a simple mechanical cart that moves under the ruler faster than the ruler:



The large wheel, turning against the direction of movement of the ruler, is very similar to the propeller of the wind-powered cart, turning against the wind.

Well done. You might want to add a treadmill test to the video as well (very low speed, of course).

The "screw" version should be even more convincing, if someone can build one. The only place I can think of to get an appropriate screw would be from a post hole digger or ice auger.

 

[Marcus]

Marcus, it was not me who made the assertion you state above, but I can provide you with a link to a discussion I was involved in on the physics forum. In this thread, the Mentor/Moderator eventually closed the thread because our critics wouldn't address his questions.

http://www.physicsforums.com/showthread.php?t=272437&highlight=resolved

He then opened another "closed" thread where he only took questions via PM and answered them in that thread. That thread is here:

http://www.physicsforums.com/showthread.php?t=274416

Hope that helps.

JB

Thanks for the links.

I haven't really kept all the players straight. Is there just one group of people, including yourself, responsible for the videos we have seen on this thread, or is there more than one?

 

[ThinAirDesigns]

Thanks for the links.

I haven't really kept all the players straight. Is there just one group of people, including yourself, responsible for the videos we have seen on this thread, or is there more than one?

There are several folks that have developed DDWFTTW carts and videos have been posted. I think other than Ynot(who isn't quite sure if he has actually accomplished the feat or not), Sport and I are the only two contributing to the thread that have.

JB

 

[H'ethetheth]

How do you come to that conclusion, H'ethetheth? Cars are often simply restrained while air is blown over them to simulate road conditions.
That cannot be done without moving air, right?
My point was that "equivalency" is not a premise for their existence.
If possible, the entire tunnel could be put in a vehicle, and driven at some speed. Unless there is interaction with the external environment, nothing will change. The measurements of the interaction between the car and windtunnel air, will remain unchanged.

It is true that for most purposes the spinning of the wheels does not affect the aerodynamics enough to go through the trouble of having a treadmill in a wind tunnel, but that's completely beside the point. Putting the car on stationary ground is still a concession to practicality over accuracy.

* ETA: The specific answer to your question how I came to that conclusion is this quote:

Windtunnel wind is not 'like' the wind it IS a wind.

Windtunnel wind is sometimes not like wind, but exactly like still air. That's the point.

Computers also model aerodynamics, and they have no 'velocities' at all. From direct experience in acoustics, I can tell you that sometimes media other than air are used, because shorter wavelengths, mean smaller models. One day, when the mathematical models are good enough, goodbye expensive windtunnels. They are all just models, and equivalence plays no part. The model is constructed to test for the desired outcome, and not because of equivalency. That is not involved at all. That is certainly BS, and an unnecessary embellishment.

Equivalence plays no part? I'm not sure how you can even think that! A model is a simplified representation of a physical phenomenon. The whole point of any model is precisely that at least the aspects you want to look into are close enough to equivalent. If you concede too much to simplicity, it will be unable to reliably tell you anything about the modelled phenomenon.
A still plane in a wind tunnel is equivalent enough to a still plane in an atmosphere sized wind, which is equivalent to a plane flying through an atmoshpere.

The car will move off the belt, carried by its momentum, because the restraining force of the wind has been removed. There will be feedback here. A control loop that monitors the car's position, while constantly controlling the belt speed. Not at all like a treadmill!

First of all, its acceleration is not because of its momentum; The car is standing still relative to the wind tunnel so it has no excess momentum. It will start accelerating because the energy input into the engine is too large for the current equilibrium.

Secondly, I don't care about the transition between wind and no wind. For all I care, we could shut the whole thing down, have lunch inbetween and maybe a game of chess, or come back the next week and then start up the treadmill and the car only. I'm talking about steady states.

Thirdly, I asked what it is analogous to, not what will happen in the wind tunnel. Do you think that if a car on the road suddenly gets a huge tail wind it will not accelerate forward?

Belts do not make wind, H'ethetheth. There is a big wind generator in use in that F1 case, but I think I understand what you mean. I have covered all possible cases in my last post, from 0 to fullspeed belt. That is not the point, surely. What have I left out? The intermediate cases are perhaps pointless, but valid.
You are arguing for the general case, whereas the treadmill is a specific case, with no wind at all, and a claimed velocity that is not apparent, nor supported. It is not correct, and that's for sure.

There is a wind generator for an F1 car because these cars generally move through air that is not moving at the same speed as they themselves do! If they would, they would most certainly be tested in still air on a treadmill.

My turn. You are at an airport, where there are those long moving walkways. Place your luggage on the belt. If you view it from the belt, or walk along the floor instead, how does that change anything?

Not in terms of speeds, no. However, this has nothing to do with the equivanlence between standing on the treadmill and standing by the road.

If you put a cart on the belt, do you think that it would suddenly shoot off at beltspeed? Do you think the propeller would even turn?

You haven't read my posts very well. I've covered this already, and it's irrelevant to the steady state situation of moving at windspeed. I does not matter how this situation came to be.

Anyway, it's really very simple: If you put the cart on a treadmill and then turn on the treadmill it will move backward. It's moving backwards through the air, so the wind it experiences (from the back) will decelerate it (i.e. accelerate it forward, you see?), and at some point it will come to a standstill and move back up the treadmill, like it does on the small treadmill.

My turn again: You want to know what happens aerodynamically when a F1 ferrari gets a 150 kph tailwind while travelling at 300 kph. The test track is booked full, and all you have at your disposal are a wind tunnel and a treadmill. Moreover, your boss wants to know the effects of the spinning wheels on aerodynamics.
How would you model this situation? Specifically: how fast is the treadmill running, and how fast is the wind blowing?


...and a 300 kph tailwind?

 

[spork]

I wrote to said professor. Here is his reply;

"Sorry….I bowed out of that debate quite a while back. Let me assure you that what’s been attributed to me below (Sporks's quote) is not accurate. The discussion became far too heated, emotional, and folks weren’t listening to rational explanations."

The professor in question is lying. JB and I had him on speaker phone, and both heard him quite clearly. Ask him if he believes our cart can go downwind faster than the wind.

I just sent that professor a PM (on another forum). I reminded him that he was on a speaker phone with two witnesses. I'll be curious to hear how he defends his lie. If he has any common sense at all (which has yet to be seen), he'll admit that he's come to understand we were right all along. Still, lying about his earlier stance is as shameful as the fact that he's allowed to teach.

 

[mender]

My turn. You are at an airport, where there are those long moving walkways. Place your luggage on the belt. If you view it from the belt, or walk along the floor instead, how does that change anything?
If you put a cart on the belt, do you think that it would suddenly shoot off at beltspeed? Do you think the propeller would even turn?

(Christian, if you are interested, please answer that too)

Let's have a look at that. If you are standing on the walkway, the airport appears to be moving by. If you are standing on the floor, the walkway (and the people and luggage on it) appear to be moving by. Since you can't be in two places at once, you have to choose a frame of reference.

Which frame of reference allows you to easily read the tag on the luggage on the walkway? Right, the one that has the observer at rest with respect to the luggage. In either case, the luggage is moving with respect to the airport.

Let's reverse the situation and put the luggage on the floor, where you forgot it when you stepped onto the walkway. Someone sees what happened, grabs your luggage and runs ahead and stops, holding your luggage up so you can verify what the tag says. To stay stationary relative to your luggage and read the tag, either you have to walk in the opposite direction as the walkway is moving or the other person needs to walk in the same direction as the walkway. Both situations will satisfy the condition of having your luggage stationary relative to you.

But, you say, they are not equivalent, because the energy expended comes from different sources. That is right. But if you are saying that you can read the tag in one frame of reference and not in the other, that's wrong. You were able to achieve the same relative motion in both frames of reference between you and your baggage. Kinetic energy is not an issue either, because if either of you bump into a person that isn't walking, you still get a collision and the baggage is no longer at the same speed as you.

Also, we've been assuming that in both cases, the person doing the walking is moving at the same speed as the walkway in order to cancel out the relative movement. What happens if the walking person moves faster than the walkway?

Now let's address the small cart on the walkway. Let's use 10 mph for the walkway speed. Let's start with a person standing on the 10 mph walkway holding the cart, facing away from the direction of travel so that the still air in the airport feels like a tailwind. They set the cart down. The cart feels the air moving by at 10 mph. When the cart is released, the propeller is stationary and the air movement across the propeller and cart drags the cart along with the air. As the cart starts to move, the propeller turns against the direction of the air travel but the force required to turn the propeller forward is less than the force required to turn the wheels forward, so the cart moves forward.

This is a key point, and Michael C's ruler demonstration clearly shows that it does work.

The cart continues to be pushed by the air movement and accelerates because of the unbalanced force. Within a short time, the cart reaches 10 mph relative to the walkway and the person who released the cart, and is seen by people standing beside the walkway (in still air) to be stationary relative to them. However, the person on the walkway (who is still experiencing a 10 mph "tailwind") saw the cart move away from them after releasing, and unless their depth perception is quite acute will not be able to tell exactly when the cart reaches that 10 mph relative speed.

A bystander on the floor leans over and picks up the cart. The propeller, not having a power source anymore, slows down and stops. The bystander then places the cart back on the walkway, holds it for a few seconds to allow the propeller to get back to the speed it was at before and releases it again. The cart still has an unbalanced force on it and attempts to move forward relative to the person on the floor. They decide they want to look at it some more and poke it with a spork to hold it in place. Once they are finished, they allow the cart to continue on and the cart accelerates until the force generated by the spinning propeller is matched by the drag of the cart. That is at a finite speed, likely about 14mph relative to the walkway and 4 mph relative to the floor and still air of the airport.

When the cart gets to the end of the walkway, the cart wheels now encounter a surface that is stationary instead of moving and are forced to slow to 4 mph, the propeller transfers what little momentum it has to the cart, increasing the cart speed by a ratio equal to the difference in the momentum of the propeller and drive train and the entire mass of the cart. That causes a momentary spurt of speed, after which the cart immediately slows down and stops.

If the carbon tube was a rubber band, the spurt of speed could be experienced over a longer time period but since it is carbon fiber and very rigid, it doesn't. The momentum transfer is essentially instantaneous and the cart jerks forward a little bit, then slows to a stop.

 

[Marcus]

In fact all you have to do is check the physics blogs and forums to find that most physicists laugh at the idea that this can be done.

Thanks, this is the information I was looking for. If the idea hasn't been accepted generally by physicists, it is all the more interesting. It means there is more work to be done, like designing tests that will convince most physists.

One of these doubting physicists would ideal for answering the questions posed earlier about the treadmill tests: how to explain the performance going up an incline, tensioning a string, ect.

 

[spork]

One of these doubting physicists would ideal for answering the questions posed earlier about the treadmill tests: how to explain the performance going up an incline, tensioning a string, ect.

It's plenty easy to explain the performance of the cart climbing an incline. There is plenty of energy available (as has been show) at the ground/wind interface to cause the cart to climb an incline, accelerate beyond the wind speed, or both. We show all three in our videos.

If you hope to get any meaningful answers from the "physicists" that claim this is not possible, I wish you luck. I've yet to find one such physcist that understands the most basic principle of equivalence of inertial frames.

 

[ThinAirDesigns]

Marcus, if you want to watch the process that we've been describing, go the following link.

Here we have a professor of Physics who wrote a blog aimed at spork and I saying that DWFTTW is impossible and for very clearly stated reasons -- clear, but unbelievably flimsy.

The conversation is going on live and now.

http://blog.dotphys.net/2008/12/physics-and-directly-downwind-faster-than-the-wind-dwfttw-vehicles/

JB

PS: To learn about Rhett the professor, click on the "about" at the top of the blog.

 

[spork]

I thought I would never say this but humber is correct.

His description of the cart on the treadmill being in balance is absolutely correct.

I read VERY little of what humber posts. While I've never seen him be right on anything at all (or even close), it stands to reason that he'd stumble onto something right with the reams he posts here.

Yes, when JB and I go to great pains to get the cart to exactly balance on an inclined treadmill, it is in fact in balance on an inclined treadmill. That seems obvious enough to me.

If we simply place it on our treadmill at max speed and max incline, the cart simply climbs the incline and heads straight off the front of the treadmill.