Merged DDWFTTW in a “Real” Wind

[I Ratant]

Christ, you're stubborn. There are a range of people from barely above ignorant (me) to highly knowledgeable folks including the designer and creator of the cart (Spork) who have tried to explain this to you. Why won't you listen?

Request @ Spork: Please correct this one time without acrimony.

.

some years back I saw this one... looked like it worked to me...

 

[spork]

On the cart..
I've found that a propellor (10x6 APC electric prop) loose on a shaft, free to rotate in a wind, goes faster rpm wise with the wind coming from the rear of the prop. Maybe 15% or so.
Turning the cart around might need some alteration to the prop pitch, to get the same grunt forward, unless it's merely the inefficency of the prop in the downwind situation.

I Ratant - you don't get it, you don't want to get it, and you've already told us it's a pointless discussion. Thanks for stopping by. Bye now.

I am not interested in your "opinion". Produce a text reference for the claim of tacking props.
It is highly unlikely that such behavior would go unrecorded

I love the fact that humber trys to peddle all manner of complete horse-*****, but won't accept that objects fall in the presence of gravity without a peer reviewed article to back it up.

 

[D'rok]

.

some years back I saw this one... looked like it worked to me...

It does work. But unless I am completely misunderstanding your explanations, you have the mechanism wrong.

From Goodman himself wrt that cart:

The key to understanding DDWFTTW, is that the wheels are turning the propeller and that the propeller need only produce enough lift in still air to overcome the forces required to turn it.

http://www.boatdesign.net/forums/at...wind-ddwfttw_from_catalyst_n23_jan_2006-1.pdf

 

[spork]

It does work. But unless I am completely misunderstanding your explanations, you have the mechanism wrong.

Of course he's getting it wrong. We're talking about a guy that's amazed to find when using a prop as a turbine, that it helps to have the camber on the correct side.

 

[Llyricist]

Okay time to re post this.

@ I Ratant, do you disagree with the motions shone? Assuming the cart is accelerated to 10 MPH wrt the ground before being dropped on the walkway, will it keep going at that speed?
\


ETA: One step at a time

 

[I Ratant]

It does work. But unless I am completely misunderstanding your explanations, you have the mechanism wrong.
]Quote]:
The key to understanding DDWFTTW, is that the wheels are turning the propeller and that the propeller need only produce enough lift in still air to overcome the forces required to turn it.
From Goodman himself wrt that cart:

.
That's what I said.
The wind on the prop starts the process.
If there were no connection between the prop and the wheels, the prop would achieve wind speed immediately, and drag the cart down wind at less than the wind speed, due to the various frictions and losses in the prop/airstream.
The connection between the prop and the wheels slows this acceleration to where the prop must generate lift (as if it were a sail), and pull the cart, while rotating.
(There's an animation showing this.)
As the cart moves downwind, the prop continues to move the cart, thru the gearing to the wheels, which turn and reduce the load on the prop, so the machine can accelerate.
Should the machine reach a speed where the headwind drives the prop, it will not go faster than that. The zero slip speed for the prop.

 

[I Ratant]

Okay time to re post this.

[qimg]http://www.internationalskeptics.com/forums/attachment.php?attachmentid=21575&stc=1&d=1301194790[/qimg]

@ I Ratant, do you disagree with the motions shone? Assuming the cart is accelerated to 10 MPH wrt the ground before being dropped on the walkway, will it keep going at that speed?
\


ETA: One step at a time

.
I don't care about this.
One insoluble problem at a time, until the weather improves and I can go outside and play with my toys.
I have not yet seen a sensible description, without acrimony and injured egos, listing the process from the start. What the prop does, what the wheels do, what are the limits... that would cause me to doubt my version.

 

[Llyricist]

.
I don't care about this.
One insoluble problem at a time, until the weather improves and I can go outside and play with my toys.
I have not yet seen a sensible description, without acrimony and injured egos, listing the process from the start. What the prop does, what the wheels do, what are the limits... that would cause me to doubt my version.

So you are as immune to learning as humber.... that's a shame.

 

[D'rok]

.
That's what I said.
The wind on the prop starts the process.
If there were no connection between the prop and the wheels, the prop would achieve wind speed immediately, and drag the cart down wind at less than the wind speed, due to the various frictions and losses in the prop/airstream.
The connection between the prop and the wheels slows this acceleration to where the prop must generate lift (as if it were a sail), and pull the cart, while rotating.
(There's an animation showing this.)
As the cart moves downwind, the prop continues to move the cart, thru the gearing to the wheels, which turn and reduce the load on the prop, so the machine can accelerate.
Should the machine reach a speed where the headwind drives the prop, it will not go faster than that. The zero slip speed for the prop.

It sounds to me like you are hung up on one error - namely that you think the prop drives the wheels. It doesn't. The wheels drive the prop.

 

[spork]

.
I don't care about this...

Yes, we know you don't care about any of this. It's all a waste of time, which you frown upon, even though it was OUR time.

And yet, you keep coming back to waste more time.

One insoluble problem at a time, until the weather improves and I can go outside and play with my toys.

Nonsense. Why aren't you on the uphill skiing forum until the whether improves (at which time you can go play with your blue collar toys and stop bothering us)?

I have not yet seen a sensible description...

That has to do with your definition of "sensible". No, I'm sure you haven't seen a description that matches your sensibilities. That's because we understand how it works.

...without acrimony and injured egos

Good point. Why all this acrimony when people that don't, can't, and/or won't understand it come along simply to insult us for even having the conversation - and then decide to stay and insert themselves into the conversation as well. If only you had the gene to recognize irony.

listing the process from the start. What the prop does, what the wheels do, what are the limits... that would cause me to doubt my version.

Oooh - can I go ahead and write out that detailed description just for you? Will you tell me I've got it all wrong and then throw in a couple of insults for good measure? I'm just dying to get in on this deal.

 

[Michael C]

.
I'm missing diddly.
Starting with square one... which way does the prop rotate?
Finger out that.
Then square two.
Which way must the wheels turn for the direction of motion?
Any gearing between the two can't oppose this, or it can't move at all!

Look at the "Under the ruler" video I posted. You're making the same mistake as Coco, who thinks that the cart won't move at all because the motion of the blue wheel opposes the motion of the other wheels. And yet the cart does move, faster than the ruler, precisely because of this opposition.

The prop cart works in a similar way: right from the start the prop and the wheels are in opposition. That holds both for Goodman's cart and for spork's. Here's what Goodman himself has to say about it:

"The key point is that the propeller is a propeller, not a wind vane, and when the cart is rolling, the wheels are powering the propeller, not the other way around. With the right gearing, the propeller will always push backwards against the air, whether or not the air is moving forwards or backwards relative to the cart. The tailwind and the propeller action combine to make the wheels spin fast enough to keep the whole system rolling faster than the wind". Definitely counter-intuitive."​

(From http://www.flixxy.com/sailing-yacht-research.htm)

The forwards thrust from the propeller and the backwards force from the wheels continue to oppose each other while the vehicle accelerates through speeds below wind speed, to wind speed and above. The vehicle reaches a steady state speed when these two forces are equal.

 

[RossFW]

I thought up waffle-bottomed skies.. the waffling permits traction when going uphill, and lies flat on the bottom for the downhill. A ratcheting effect, as it were.

You mean like the cross-country skis that have been in use for almost 100 years?


You should patent that.....

 

[lionking]

I've been following this thread with fascination, but not a lot of understanding. Are there any other tests planned which may put humber's criticisms to bed?

I guess probably not.

 

[zooterkin]

It sounds to me like you are hung up on one error - namely that you think the prop drives the wheels. It doesn't. The wheels drive the prop.

Ok, so what is making the wheels go round?

 

[spork]

I've been following this thread with fascination, but not a lot of understanding. Are there any other tests planned which may put humber's criticisms to bed?

I guess probably not.

That's a complicated question. But the most important parts are...
- We've LONG since given up the idea of trying to convince humber
- There is no such thing as a test that would convince humber. I'm being quite literal here.

We will be doing a different set of tests this summer. We'll make the cart go directly upwind faster than the wind. This is not for humber's benefit of course. It's just the next step.

Ok, so what is making the wheels go round?

The thrust of the prop is greater than the retarding force on the wheels. Initially, the wind hitting the cart from behind causes it to move forward. This causes the prop to turn. As the cart moves faster, the prop turns faster. This produces thrust. Ultimately, that thrust causes the cart to exceed the wind speed - directly downwind.

We realize this sounds like perpetual motion - but it's not. It's a wind powered cart. If the wind stops - the cart stops.

The reason the prop can produce more thrust than the drag induced on the wheels has to do with the gearing and the fact that the prop is operating on the air while the wheels are operating on the ground. Because of the tailwind, the cart moves through the air more slowly than it moves over the ground. This allows it to do what any lever does - trade a small force over a large distance (the wheels over the ground) for a larger force over a smaller distance (prop through the air).

 

[zooterkin]

The thrust of the prop is greater than the retarding force on the wheels. Initially, the wind hitting the cart from behind causes it to move forward. This causes the prop to turn. As the cart moves faster, the prop turns faster. This produces thrust. Ultimately, that thrust causes the cart to exceed the wind speed - directly downwind.

We realize this sounds like perpetual motion - but it's not. It's a wind powered cart. If the wind stops - the cart stops.

The reason the prop can produce more thrust than the drag induced on the wheels has to do with the gearing and the fact that the prop is operating on the air while the wheels are operating on the ground. Because of the tailwind, the cart moves through the air more slowly than it moves over the ground. This allows it to do what any lever does - trade a small force over a large distance (the wheels over the ground) for a larger force over a smaller distance (prop through the air).

Thanks. I'm sure these questions have been asked before, but I couldn't see the answers. So, is this a stable situation or will the cart speed up and slow down? What would happen in a constant wind? What's the upper limit on the speed? If the cart is being pushed by the wind, and it's going faster than the wind, what is then pushing it?

We will be doing a different set of tests this summer. We'll make the cart go directly upwind faster than the wind. This is not for humber's benefit of course. It's just the next step.

I'm not sure I should be asking this yet, my head might explode. But, anyway; is this going upwind from a standing start? What is making the wheels go round in this case?

 

[spork]

I'm sure these questions have been asked before, but I couldn't see the answers.

No worries. Always happy to answer questions. Not so excited about insults.

So, is this a stable situation or will the cart speed up and slow down?

In a steady wind, the cart would end up at a steady speed well above the wind speed. In about 12 mph wind, we can maintain somewhere in the neighborhood of 35 mph.

What's the upper limit on the speed?

This cart was designed to go 36 mph in an 18 mph wind. I think the fastest we've gone is 54 mph. I guess I would be comfortable in it up to maybe 70 mph on a really smooth surface(?)

Ultimately, you end up with the prop tips achieving a significant percentage of the speed of sound (greater than 30% - 40%), and that causes lots of drag due to the compressibility of air at those speeds.

In theory the cart can do any multiple of wind speed - if it were efficient enough. But that would only work in lower and lower winds, as you still have the compressibility problem at relatively high speeds.

If the cart is being pushed by the wind, and it's going faster than the wind, what is then pushing it?

It's not really being "pushed" by the wind any more than an airplane flying downwind is being pushed by the wind. It's pulling itself through the air using a propeller - same as the airplane. It just works out that we can get more thrust than we have drag because it's moving more slowly through the air than it is over the ground (due to the tailwind). But it's a mistake to think of it as being pushed by the wind once it's above wind speed.

I can show you with vectors how the air/wind interacts with the prop blades to enable them to do this if you like.

is this going upwind from a standing start?

Yes.

What is making the wheels go round in this case?

In the upwind configuration, the wind turns the turbine (which looks pretty much like our current prop). And the turbine turns the wheels. Pretty much the opposite of the downwind case.

 

[zooterkin]

No worries. Always happy to answer questions. Not so excited about insults.

It's ok, I'm not planning any of those. Thanks for the answers.

It's not really being "pushed" by the wind any more than an airplane flying downwind is being pushed by the wind. It's pulling itself through the air using a propeller - same as the airplane. It just works out that we can get more thrust than we have drag because it's moving more slowly through the air than it is over the ground (due to the tailwind). But it's a mistake to think of it as being pushed by the wind once it's above wind speed.

Ok, but in the case of an airplane, it's clear what is making the propeller go round. When the cart is going faster than the wind, what is keeping it going? Where is it getting energy from?

I can show you with vectors how the air/wind interacts with the prop blades to enable them to do this if you like.

It would be interesting to see, yes please, though they might be wasted on me.

In the upwind configuration, the wind turns the turbine (which looks pretty much like our current prop). And the turbine turns the wheels. Pretty much the opposite of the downwind case.

Ok, I think I follow that; what speed can be reached compared to the wind speed?

 

[spork]

Ok, but in the case of an airplane, it's clear what is making the propeller go round. When the cart is going faster than the wind, what is keeping it going?

How the prop interacts with the air to create thrust is a separate question from where the power comes from to turn the prop. As far as the aerodynamics goes, the prop on our cart is no different than the prop on the airplane. Also, it's important to realize that, like an airplane, the prop is being turned by its shaft - not by the wind.

Where is it getting energy from?

The overall machine is exploiting the velocity differential between the air and ground (i.e. the wind). It does this by accelerating the air aft - in the cart's frame. But in the ground's frame, it's actually slowing the wind down. That's where the energy comes from.

It would be interesting to see, yes please, though they might be wasted on me.

Alrighty - I uploaded a vector diagram and explanation here:

http://www.yousendit.com/download/eUREa3NlK3g3bUEwTVE9PQ

This explains how an ice-boat can achieve and maintain a downwind velocity component faster than the wind while on a broad reach. That's exactly what the blades of our cart are doing. Have a look at it and let me know if you have any questions. Then we can talk about how that analysis relates directly to our prop.

what speed can be reached compared to the wind speed?

Similar story to the downwind case. In theory we can achieve any multiple of wind speed - as long as we avoid the compressibility effects. In the real world we'll be very happy if we're efficient enough to achieve 2X wind speed directly upwind.

 

[humber]

I love the fact that humber trys to peddle all manner of complete horse-*****, but won't accept that objects fall in the presence of gravity without a peer reviewed article to back it up.

Oh, gravity is ubiquitous. What is missing, is your peer-reviewed paper.
Don't you have the rocks?