Down wind faster than the wind

[sol invictus]

All we have is your assertion that it works.

Well if there was any doubt whether you were a troll, that obliterates it.

Spork and co have provided multiple videos, including some specifically addressing people's concerns, construction plans that would let you build the thing for yourself, and detailed explanations of the physics (which is basic, standard, high school level Newtonian dynamics).

The other side has provided absolutely nothing other than incoherent rants and a pig-headed refusal to understand one of the most important and fundamental (and yet incredibly simple) principles of nature - which, once understood, makes it totally obvious that this thing works just fine.

 

[tsig]

That's correct.



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.

So the wheels spin the prop that makes the cart go that spins the wheels.

 

[sol invictus]

So the wheels spin the prop that makes the cart go that spins the wheels.

Time for a "time out", tsig. You're on ignore.

 

[tsig]

Well if there was any doubt whether you were a troll, that obliterates it.

Spork and co have provided multiple videos, including some specifically addressing people's concerns, construction plans that would let you build the thing for yourself, and detailed explanations of the physics (which is basic, standard, high school level Newtonian dynamics).

The other side has provided absolutely nothing other than incoherent rants and a pig-headed refusal to understand one of the most important and fundamental (and yet incredibly simple) principles of nature - which, once understood, makes it totally obvious that this thing works just fine.

Calling me a troll is an ad hom. The thing works or it does not.

 

[tsig]

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

There is no wind on the treadmill. Your calling troll on anyone who disagrees with you is a sign of woo.

 

[dv82matt]

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.

The upwind case is the more difficult engineering challenge and would, but for the quirky nature of human intuition, be the more impressive case IMO.

Nontheless I do see your point and I suppose there would be logistical difficulties as well. You can't exactly run it on a treadmill in a still room to demonstrate it, and putting a fan at the top of the treadmill or running it outdoors are likely to generate objections that the demonstration is insufficiently controlled.

Anyway I do think the upwind case (DUWFTTW) is worth attempting if only for the sake of completeness.

 

[Michael C]

There is no wind on the treadmill. Your calling troll on anyone who disagrees with you is a sign of woo.

You didn't answer this question:

What is Wind?

 

[sol invictus]

The upwind case is the more difficult engineering challenge and would, but for the quirky nature of human intuition, be the more impressive case IMO.

Not really - under many circumstances upwind is easier. Think about a sailboat on water. It would take a very fast boat to jibe back and forth on a path that averages to directly downwind faster than the wind (if it's possible at all on water). But any sailboat can sail upwind, and can travel "directly" upwind by tacking.

See here: http://www.nextenergynews.com/news08/next-energy-news8.29.08b.html

 

[tsig]

You didn't answer this question:

What is Wind?

If you don't know what wind is then you are not qualified to enter into this conversation.

 

[H'ethetheth]

If you don't know what wind is then you are not qualified to enter into this conversation.

Think again and then answer the question, please.

 

[Michael C]

The upwind case is the more difficult engineering challenge and would, but for the quirky nature of human intuition, be the more impressive case IMO.

Yes, intuition plays funny tricks on us. A few people have said to me that they find the idea of a machine that goes upwind easier to grasp than one that goes faster than the wind downwind.

Why is this? Maybe these people are thinking of the wind like an arm that pushes against the cart. They can imagine the cart working it's way up the arm that is pushing against it, but they think it's impossible for the cart to move away from the arm that pushes. What they don't realise is that the wind is like a long arm that extends past the cart both backwards and forwards (just like my ruler ). The cart can work its way along the arm in either direction.

 

[Michael C]

If you don't know what wind is then you are not qualified to enter into this conversation.

I know the answer. I want your answer. You assert that "There is no wind on the treadmill". This is incorrect, so I'd like to see what definition of "wind" brings you to this false conclusion.

 

[tsig]

I know the answer. I want your answer. You assert that "There is no wind on the treadmill". This is incorrect, so I'd like to see what definition of "wind" brings you to this false conclusion.

where does the wind come from?

 

[H'ethetheth]

where does the wind come from?

If I say "from the motion of the treadmill", will you tell us what you think wind is?

 

[dv82matt]

Not really - under many circumstances upwind is easier. Think about a sailboat on water. It would take a very fast boat to jibe back and forth on a path that averages to directly downwind faster than the wind (if it's possible at all on water). But any sailboat can sail upwind, and can travel "directly" upwind by tacking.

I think that's due to fast sailboats being optimised to tack upwind rather than downwind. Tacking upwind is much more useful since the virtual wind (that's the wind relative to the fast moving sailboat) tends to blow from that direction.

See here: http://www.nextenergynews.com/news08/next-energy-news8.29.08b.html

That is interesting but I'll note that the winner the Stuttgart Ventomobile only managed 64% of the wind speed directly against the wind.

 

[sol invictus]

I think that's due to fast sailboats being optimised to tack upwind rather than downwind. Tacking upwind is much more useful since the virtual wind (that's the wind relative to the fast moving sailboat) tends to blow from that direction.

But that's precisely the point!

That is interesting but I'll note that the winner the Stuttgart Ventomobile only managed 64% of the wind speed directly against the wind.

I think you're confusing two things here. Going any speed in between at rest with respect to the ground/water and at rest with respect to the air is one thing. Going upwind or "upground" is another.

Going upwind at wind speed (so that the apparent wind is twice as strong) is nothing particularly special - that would be analogous to going downwind twice as fast as the wind. If you can go directly upwind at any speed at all it demonstrates the same principle that allows the cart to go downwind at a speed faster than the wind.

 

[dv82matt]

Yes, intuition plays funny tricks on us. A few people have said to me that they find the idea of a machine that goes upwind easier to grasp than one that goes faster than the wind downwind.

Why is this? Maybe these people are thinking of the wind like an arm that pushes against the cart. They can imagine the cart working it's way up the arm that is pushing against it, but they think it's impossible for the cart to move away from the arm that pushes. What they don't realise is that the wind is like a long arm that extends past the cart both backwards and forwards (just like my ruler ). The cart can work its way along the arm in either direction.

That's a good analogy. I think there's also something beguiling about the apparent simplicity of the demonstration. It's simple enough that we think we can grasp it intuitively instead of laboriously thinking about it. Unfortuanetly our intuitions seem to channel Aristotle rather than Newton or even Galileo.

 

[Christian Klippel]

For the people who think that only math can explain it, take a look here:

http://home.san.rr.com/tadhurst/DWFTTW.htm

If you want some source-code:

http://www.btinternet.com/~sail/dwfttw02.htm

Greetings,

Chris

Edit: Note that the second link is about DWFTTW in general, and more relating to boats than to the cart in question.

 

[screensnot]

That is interesting but I'll note that the winner the Stuttgart Ventomobile only managed 64% of the wind speed directly against the wind.

You say that like you think the Ventomobile is 36% short of actually traveling upwind.

If you have a 10mph wind coming directly from the north, and you build a cart that can use that wind to make your cart move north, you are going directly up wind. If you can attain 0.1mph, you are going directly upwind at 1% of windspeed. The Ventomobile can do this, but >60x faster! 60 times faster!

 

[dv82matt]

But that's precisely the point!

I don't see any point of contention then. I agree that sailboats that are optimised for tacking upwind may be better at tacking upwind than downwind. What I don't see is how it's relevant.

I think you're confusing two things here. Going any speed in between at rest with respect to the ground/water and at rest with respect to the air is one thing. Going upwind or "upground" is another.

Going upwind at wind speed (so that the apparent wind is twice as strong) is nothing particularly special - that would be analogous to going downwind twice as fast as the wind. If you can go directly upwind at any speed at all it demonstrates the same principle that allows the cart to go downwind at a speed faster than the wind.

I think you are mistaken. The upwind scenario will have greater losses due to drag, but other than that the two scenarios DDWFTTW and DUWFTTW are equivalent as far as I can tell.

But be that as it may the challenge is to go directly upwind at (at least) the same speed relative to the ground as the wind is going relative to the ground.