Can a Cart Move Faster Than the Wind?

[russ_watters]

Previous threads on the subject have gone badly and/or have moved quickly and gone round in circles without really getting anywhere. The question is whether a cart with a propeller powered by the wheels (or vice versa) can move downwind faster than the wind. It has (supposedly) been demonstrated on the ground and on a treadmill. The debate has taken two directions, first whether the two scenarios are identical and second, whether they are really showing what they appear to show (whether they are hoaxed).

The scenario is actually relatively simple, but it is counterintuitive and easy to misunderstand (I did when I first thought about it). The only way to get it straight is to draw a diagram and make sure it is clear to everyone what is happening. And the only way to ensure that people keep focused is for me to control the thread. So that's what's going to happen. If you want to post, PM me and I'll post applicable parts here. No direct posting will be allowed.

Now I will be away for a few days around Thanksgiving and may not have computer access and I don't have access to PF at work. So we'll just have to see how it goes.

So here goes...

I've started with a diagram, similar to one I posted in the last thread. I have gotten a few responses that tell me that I need to back up a step, so this one is not exactly the same as the one I posted before. Describing it in words:

The cart is sitting on a surface and we are working from the frame of reference of the cart (as if you were sitting in it). What the surface is is irrelevant at this point. The surface is moving from the right to the left, past the cart. The cart's wheels and propeller are spinning because they are geared together. The cart feels no wind from any external source. An important things to note, as it has been misunderstood previously:

*The cart does not, in either scenario, start off stationary wrt the surface. When the scenarios start, the wheels are spinning and the cart and surface moving wrt each other because in either case (whether on the ground or on the treadmill) a person pushes it up to speed.*

Does everyone understand and accept this as a starting point for the problem?

[edit] Heh - made a mistake in the diagram. Fixed at 5:55 pm EST.

 

[russ_watters]

Let me re-emphasize: this thread will proceed as directed by me. I will not post responses that fail to address the scenario I have posted. Accept it and go with it, because this thread will go no other way. That said, schroder, only two things in everything you sent to me are relevant here:

In the case of the cart on the treadmill, we have been shown it advancing against the tread in one frame, but have never been shown it advancing downwind faster than the wind. [emphasis added]

That's a critical point - and quite wrong - but you haven't examined my diagram yet, so you don't see why. However, you are saying here that you believe that what the cart on a treadmill video shows is really happening. Ie, there is nothing at all hoaxed in that video. Is my interpretation of your position correct?

Based on this actual observation [the cart on the treadmill], the claim is that when placed into a 10m/s wind, it should do 19 m/s because of equivalence [actual numbers unimportant] [emphasis added]

[separate part, discussing cart on road] Prop is driven by wind...

Wrong! This is the critical flaw in your understanding that I highlighted in my first post above. The cart is not "placed" on the road, it is pushed to get it up to speed. If (using my numbers), you push a cart at 5mph in the same direction as a 5mph wind, you get the scenario in my diagram above. It is also not oriented "into" the wind, it is pointed downwind. Whether the cart can be made to drive itself upwind with no change in orientation of the prop (not sure, but don't feel like spending the mental energy on it right now) is not relevant. No, schroder, the propeller is not driven by the wind in either scenario: If the cart is rolling at 5mph in the same direction as a 5mph wind, a wind anemometer mounted on the cart (away from the propeller) "feels" and measures no wind. But at the same time, the propeller is being driven by the wheels.

Please acknowledge that you understand this point and understand the diagram. It is critical and the discussion can go nowhere until you do.

 

[russ_watters]

This post is jumping ahead slightly, but partially covers the same point:

For DDWFTTW prop driven carts, the prop needs to be treated as a prop, not as a wind turbine. The prop uses power from the wheels to generate thrust. The thust is used to drive the entire cart forward, and is opposed by the force from the driven wheels, and the energy losses in the system.

Yes! It is critical to understand, in each scenario, that the propeller is generating thrust, it is not being spun like a turbine by the relative wind over the cart.

There was more to the post, dealing more with efficiencies, but that's not relevant yet. We need to establish the theoretical possibility first before worrying about if the propeller's thrust can overcome the friction in the drive system. We'll get there (hopefully).

 

[russ_watters]

Ehh, I've gleaned a little more that I think is relevant:

Prop is providing propulsion in the same direction as tread is going.

No! That's the opposite of what we want the cart to do, so why would the cart be set up that way? As you seemed to acknowledge, we want the cart to advance against the treadmill, so the propeller must provide thrust to propell the cart against the direction the tread is going. This objection of yours baffles me - is it because you think the cart needs to go in the same direction as the treadmill in order to equate the two scenarios?

scrhoder, your misunderstanding is actually pretty similar to the one I had when I first looked at the Youtube videos. Drawing the diagram - and making sure it matched what I was seeing in the video - helped. Please try to approach this by analyzing my diagram.

 

[russ_watters]

By request, here are links to the videos:

Cart on a treadmill:
Cart on the ground: http://www.youtube.com/watch?v=aJpdWHFqHm0&feature=related

Note with the cart on the ground, there is a wind sock hanging behind the propeller and you can also see the pitch of the propeller. It is clear that the propeller is spinning in a direction to produce thrust, not in a direction to be a turbine.

 

[russ_watters]

Someone had an issue with my wording here:

...we want the cart to advance against the treadmill, so the propeller must provide thrust to propell the cart against the direction the tread is going.

The propulsion of the cart is against the direction of the tread. The mass of air being pushed by the propeller moves in the same direction as the moving tread.

I think my wording was ok.

 

[russ_watters]

I would like to suggest a few guidelines.

When the car is at speed (i.e. at the same speed as the wind):

-There is no relative wind, so there is no air drag on what would be called the 'body' of the cart.
-The only tendency to 'slow' the cart (neglecting effects of the prop in air) would be frictional forces in the drivetrain, and the wheel to ground friction.
-The drivetrain consists of all of the bearings in the wheels, and the gearing/bearing to drive the prop.
-We should neglect all friction from these sources in the first analysis.

Agreed.

The 'friction' that cannot be neglected is that of the prop in the air. I would suggest that we examine the prop as a perfect prop, one that doesn't stir the air but only ends up contributing to the forward motion at first.

In this setup we would have to analyze the torque on the propeller that transfers angular momentum (of the prop) to linear momentum of the air particles. This change in linear momentum per unit time could be summed to give the forward force on the cart - at the propellor mount. We would then have to find the effect that the torque induced on the propellor has on the drivetrain, namely, how it all works out in the gearbox to change the rotation speed of the wheels.

I think that might get a little complex for a first pass, but I think you do have the right idea.

 

[russ_watters]

No Russ. Look at the video carefully..very carefully if you want to catch the hoax and not be fooled! As the video starts, we see the cart getting pushed forward and the propeller turns CCW. That is not surprising because the piych of the prop will be turned CCW when getting pushed forward. At 0:15 seconds the cart stops, it is clearly not going anywhere in that configuration. At 0:21 seconds it is pushed again (just a side not here; how mant times are you allowed to push a wind powered vehicle) now you need to pay close attention to the video between 0:26 and 0:27 seconds because the picture is getting “jumpy” and it really is difficult to tell what the propeller is doing. It almost appears as if the propeller is changing direction, but it could also be a bad splice/edit job. At 0:28 seconds the propeller is clearly spinning CW.

Two pretty obvious problems here, schroder:

1. When something is spinning on video, the framerate of the video affects the look of the spinning object. It's an elementary optical illusion and you see it everywhere. See also: strobe light. http://www.wisegeek.com/why-do-spinning-rims-appear-to-be-rotating-backward.htm
2. You didn't exactly acknowledge the point, but what you are suggesting is that they changed the setup midway through to make the cart not work! You still have not specifically acknowledged the critical point here: when the cart is first pushed, the thrust is pushing the car in the direction it is moving. Ie, The propeller is acting as a propeller, not a turbine.

Sorry, guys, I have to go catch a train. I may be able to get back on tonight or tomorrow, but I'm not sure. I didn't get all the way through the last batch of PMs. In particular, schroder, you think I took your quotes out of context. I'm not so sure, but I'll go back and take a look. I also realize I didn't specifically address your diagrams. I will - however, the direction that the discussion has gone should make it clear where the errors in your diagrams is: you have the propeller acting as a turbine and it isn't.

Please consider: If the device were set up they way you are claiming it is set up, it would not work. You are correct about that. Now, please consider if the device would work if it was set up the way the builders are claiming it is set up. You are arguing against something that isn't being claimed. It's a strawman.