Down wind faster than the wind

[humber]

I agree that I have not demonstrated that such a vehicle can accelerate to the wind speed; only beyond it (and then only to a point dictated by the efficiency). That is not a particular problem for me since we already know other ways to get to wind speed; an ordinary sail is one of them.

Won't the problem of efficiency that limits above windspeed velocity, also limit the opportunity to get there?

The force is only constant for a given velocity, but yes, this produces an acceleration when at wind speed. As the vehicle velocity increases, the propeller force decreases until it balances with the force from the roadway, at which point the vehicle is at steady-state.

But if it already has that force or velocity available why wouldn't already be at that velocity? That is, the motive force would already have the inherent capacity to apply itself at that velocity.

 

[Dr. Trintignant]

True, to get a force with no work, the air would have to be solid. As we generate thrust by accelerating air, the work the propeller does is transferred into kinetic energy in the air + losses.

My initial plausibility argument did assume a "perfect" propeller, but the later part takes efficiency into account (in a simple fashion). However, it remains true that we can create an arbitrarily "forceful" propeller at zero air speed simply by making it bigger. As I mentioned, moving twice the air at half the velocity produces the same force for half the work.

- Dr. Trintignant

 

[Dr. Trintignant]

Won't the problem of efficiency that limits above windspeed velocity, also limit the opportunity to get there?

Not necessarily, but I would need to think about it more. All that needs to be proven is that the net force is always positive, up to the efficiency limit. I strongly suspect this is the case, but haven't run the math yet.

But if it already has that force or velocity available why wouldn't already be at that velocity? That is, the motive force would already have the inherent capacity to apply itself at that velocity.

Because it's a dynamic system, but it's hard to analyze that way, so we pick several "interesting" points and analyze those. I picked two: when the vehicle is at wind speed, and when the forces balance (i.e., zero acceleration). At wind speed, I showed that we should expect a force imbalance, and therefore acceleration. At zero acceleration, I showed that the velocity should be higher than the wind speed. Taking these two together is enough to demonstrate that the vehicle is reasonable.

- Dr. Trintignant

 

[John Freestone]

Think again about what you just said.

There's no difference between a stationary plane in still air, and a plane moving forward at the same speed as the wind. For the propeller, the situation is absolutely identical. In fact, if you were sitting on the wing and could only observe the air, there would be no way to tell which situation it is.

Think about it.

Yes, sorry. Talking garbage.

 

[Dan O.]

In the same simple world, I can accelerate an oil tanker as I please.
Two airstreams, 10kmh, relative to each other. Reverse their velocities. Now the relative velocities have increased. What has happened?

humber, If you can't do simple addition (which is all this problem required) and instead continue your attempt to derail every argument by posting irrelevant blather I'll be forced to ask the moderators to assist by removing you from this thread. Other posters in this thread do want to learn how the cart works and I am trying to give them the resources they need to find that understanding.

 

[ynot]

Haven’t been keeping up with this thread lately so hope these suggestions haven’t already been offered . . .

Creat a large flat disc and place the cart toward the outer edge and tether it to the centre as shown. Rotate the disc as shown as see if the propeller ever developes enough thrust to overcome the rolling resistance and travel in the opposite direction as the spinning disc. Essentially a treadmill without ends.

To keep the cart inline on the tread stretch a taught nylon line (shown in red) down the tread and anchored off either end. Fix figers at either end of the cart that extend down so the line runs between them (shown as red dots). Fix stops (shown as blue) toward either end off the line so the cart can’t travel off the ends of the treadmill.

Images are not drawn to any particular scale


ETA mods - how come I can only see the images when I'm logged on?

 

[spork]

humber, If you can't do simple addition (which is all this problem required) and instead continue your attempt to derail every argument by posting irrelevant blather I'll be forced to ask the moderators to assist by removing you from this thread.

BRILLIANT!!!! I wish I'd thought of that about 37 pages ago.

 

[Dan O.]

To make the cart function properly on the disk you need to modify it slightly. The drive wheels should be inline with the axis of the disk and the inside wheel needs to be smaller to account for the shorter track it will be following.

These changes could be continued by transforming the treadmill disk into a cylinder with the cart and propeller spinning around the same axis...




This transformed cart will still demonstrate traveling faster than the wind either by rotating the base in still air or fixing the base and rotating the air around it.

 

[ThinAirDesigns]

Creat a large flat disc and place the cart toward the outer edge and tether it to the centre as shown. Rotate the disc as shown as see if the propeller ever developes enough thrust to overcome the rolling resistance and travel in the opposite direction as the spinning disc. Essentially a treadmill without ends.

Ynot, that's a really great idea. Hadn't thought of it. Still not sure I'll build it, but don't interpret that as a rejection of the concept -- I think it's clever and very cool. NICE!

To keep the cart inline on the tread stretch a taught nylon line (shown in red) down the tread and anchored off either end. Fix figers at either end of the cart that extend down so the line runs between them (shown as red dots). Fix stops (shown as blue) toward either end off the line so the cart can’t travel off the ends of the treadmill.

Don't confuse our 'no strings attached' position with 'we don't know how to attach strings". We know the results of any such tests already and you are one of only about two people who have asked for string tests -- I hope you enjoy them when you get to them yourself.

JB

 

[spork]

Haven’t been keeping up with this thread lately so hope these suggestions haven’t already been offered . . .

Creat a large flat disc and place the cart toward the outer edge and tether it to the centre as shown....

Shoot! I do like that idea. I'm not sure I'll do it, but I want to think about it a bit. And yes, it would very definitely work.

 

[ynot]

Shoot! I do like that idea. I'm not sure I'll do it, but I want to think about it a bit. And yes, it would very definitely work.

Get Mythbusters to build a 20 - 30 foot diameter spinning circular track. They seem to like to spend heaps on over engineering things. They would want to blow it all up at the end though.

Instead of the tether string the cart could run loosly on a guide track that could also steer it.

 

[Dan O.]

The guide string down the center of the treadmill is a valid idea. At most, it would represent a small additional wind drag on the cart frame so should not be objectionable. A similar guide wire is used for the junior solar racer competition: <http://www.nesea.org/education/jss/rules.html>.

I would even suggest that builders expect to use the guide wire and follow the dimension guidelines for the solar racers. We may want to hold downwind competitions some day and the rules would insure that all the carts are in the same class.

 

[mender]

You are seeing the test as a failure because you presume the outcome.

When a test excludes a possible outcome whether I presume it or not, the test is invalid. You however reject the treadmill test because it demonstrates a different outcome than you presume. That's called denial.

Ynot's proposal however is quite good. One thing to add would be a method of reducing scrub, possibly by having a spring loaded steering mechanism or adjusting the fixed wheel to run in an arc that is slightly bigger than the disc. The tension on the guide wire or track would be minimized. Also, if the drive wheels are connected solidly as they are now, the wheels will either have to be sized differently or a differential used (R/C car) to again reduce scrub.

I see this as a very good way of validating the cart's performance. It will allow a speed measurement above windspeed and will duplicate the outdoor test under controlled conditions. Excellent suggestion!

 

[ynot]

More on the rotating disc - The cart could have only two (or maybe even one) wheel and be rigidly fixed to a centre pole with bearings. Haven’t got time to fully explain as I’m moving faster than the time I haven’t got. Hope the picture helps to explain what I mean.

 

[spork]

I see this as a very good way of validating the cart's performance. it will allow a speed measurement above windspeed...

I agree completely.

... and will duplicate the outdoor test under controlled conditions.

I agree completely, but I guarantee the skeptics would not. Even so, the idea is stuck in my head. Must think of a really big existing example... some kind of merry-go-round or park swirly thing...?

The cart could have only two (or maybe even one) wheel and be rigidly fixed to a centre pole with bearings.

This approach has a number of advantages, and satisfies me completely. But the critics would say "what is that thing, and what's it supposed to prove?"

 

[mender]

And would say that the shaft is powered, making the cart move in any way that the tester felt like showing. Unfortunate as a demonstration but the best way I've seen to thoroughly investigate this.

I wonder what Charles Platt would say?

 

[ThinAirDesigns]

Spork:

But the critics would say "what is that thing, and what's it supposed to prove?"

-
Mender:

I wonder what Charles Platt would say?

-
"what is that thing, and what's it supposed to prove?"

JB

 

[spork]

I wonder what Charles Platt would say?

What do you mean!? He'd just build a really lousy version that didn't work and then call JB and I fakes and scammers.

 

[ynot]

I agree completely.



I agree completely, but I guarantee the skeptics would not. Even so, the idea is stuck in my head. Must think of a really big existing example... some kind of merry-go-round or park swirly thing...?



This approach has a number of advantages, and satisfies me completely. But the critics would say "what is that thing, and what's it supposed to prove?"

I’m a sceptic and I suggested it. Shame that this has become a “them and us’ debate rather than a persuit of the truth.

 

[spork]

I’m a sceptic and I suggested it. Shame that this has become a “them and us’ debate rather than a persuit of the truth.

I think you're misunderstanding me. I know you're a sceptic, and I know you suggested it, and I think it's a great idea.

I even know you sometimes seem to think that just maybe this thing is possible (in between insulting us for being dumb enough to think it's real ourselves).

But here's the real deal. EVERY sceptic has a specific test they say would satisfy them - have you seen the ridiculous notion Platt has for a test? The problem is that the test in question would always be more frought with problems for the other sceptics than the simple videos we've attempted.

The difference with this one - at least for me, is that I really like it. If I could figure out an easy enough way to do it, I'd do it in a heartbeat, video tape it, post it on youtube, and then take all the credit.

Incidentally, this is definitely not a "pursuit of truth" thing for me. I understand quite well how this thing works, and I've done my best to describe it in as many ways as I can to anyone willing to listen (which is a surprisingly small number of people).