I was trying to explain that you had left something out of your description - the ground moving the wheels. That's the "engine" that drives the propeller.
-- Roger
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Merged DDWFTTW in a “Real” Wind
- Thread starter ynot
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I was trying to explain that you had left something out of your description - the ground moving the wheels. That's the "engine" that drives the propeller.
-- Roger
The "wind" and the "air" are two different things. Wind is air moving relative to the ground.
When the cart is moving at "wind speed," the air around it isn't moving. As you said, to the propeller on the cart, it's the same as if the cart were in still air.
To the cart, there is no wind, but there is something making its wheels spin which gives it energy to turn the propeller and speed up. That's the picture that finally made everything clear to me about how the cart works.
-- Roger
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Ynot's questions
What part of the system are you having trouble understanding, Ynot?
Maybe we can help. Maybe not!
Regards,
WindGrins
What part of the system are you having trouble understanding, Ynot?
Maybe we can help. Maybe not!
Regards,
WindGrins
The debate isn’t about whether propellers work or not. It’s about how the cart continuously receives energy from the wind when the cart is moving at or faster than the wind. I think we all understand the mechanics of how the propeller is spun.
ThinAirDesigns
Graduate Poster
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Then you don't believe in reality -- namely the thing that happens day in and day out all over the world. Sailboats leave regular lie to your "belief".
Some folks never learn.
JB
Hey ynot, I thought you'd been around a long time, and had a more complete understanding of this thing?
Try inverting the problem, how is it that a windmill moving at the speed of the ground (by way of being bolted to it) can harness energy from the motion of the air? Then, if you could bolt something to the air why wouldn't you be able to harness energy from the motion of the ground? What is the difference here? Ok, it is hard to bolt something to the airmass, but not actually that hard to get the same effect, it will always slip given it is a fluid, but not necessarily too much. Right?
BTW, sorry to here about the losses in NZ.
There are discussions of "headwinds" and the "propeller tacking" and all that which, I think, cause a lot of confusion.
Just keep reminding yourself that the propeller is getting energy from the turning of the wheels. Don't get hung up on the speed of the wind - it's not important. As long as the wind is blowing, the cart will move; the cart's wheels will turn; and the propeller will spin.
The wheels are the "engine" that turns the propeller.
The "engine" is running all the time when the cart is moving slower, the same, or faster than the wind's speed. As long as the wind is blowing - no matter at what speed - the engine is driving the propeller.
-- Roger
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The exact point at which the cart receives energy continuously from the wind when the cart moving at or faster than the wind.
It has been claimed that this point is where the propeller blades hit molecules of apparently still air. As the blade and the air molecules are essentially moving at the same speed before they hit each other the only energy that can be transferred to the cart is half the energy of the impact. But no energy from the wind.
I shouldn‘t have to keep adding “directly downwind” when that‘s what the whole debate is about.
"The wind" is air moving relative to the ground. Therefore, in order to understand how the cart receives energy from the wind, you can't only look at the interaction between the cart and the air. You also have to look at the interaction between the cart and the ground. You're basically saying, even though the ground is moving relative to the cart when the cart is at windspeed, that movement of the ground past the cart couldn't possibly help the cart speed up. Why couldn't it?
I don't understand what you mean here.
I think I can help but let me know if I do!
My claim would be that if you look at a single blade rotating through the air at any point in time (let's say the cart is faster than the wind), then you would find that the blade is operating like a sail being blown by the apparent wind formed by the true wind and the "wind of rotation".
The reason this is more profound than it looks at first blush is that if you removed the true wind part of the combined wind vector, the cart would not experience the "push" from the total wind "true + rotational". The blade would not be harvesting any energy and would slow down due to drag.
This is basically the picture of the wind vectors in word form.
If you compared a picture of the blade with the true wind and one without, you would see that the one with the true wind shifts the vector just enough that the vector is not directly on the leading edge of the blade but behind it (or below it depending upon how you define this)-like an airplane wing with an angle of attack--thus generating lift.
Is this helpful so far?
WindGrins
My claim would be that if you look at a single blade rotating through the air at any point in time (let's say the cart is faster than the wind), then you would find that the blade is operating like a sail being blown by the apparent wind formed by the true wind and the "wind of rotation".
The reason this is more profound than it looks at first blush is that if you removed the true wind part of the combined wind vector, the cart would not experience the "push" from the total wind "true + rotational". The blade would not be harvesting any energy and would slow down due to drag.
This is basically the picture of the wind vectors in word form.
If you compared a picture of the blade with the true wind and one without, you would see that the one with the true wind shifts the vector just enough that the vector is not directly on the leading edge of the blade but behind it (or below it depending upon how you define this)-like an airplane wing with an angle of attack--thus generating lift.
Is this helpful so far?
WindGrins
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When above WS, the cart sees a headwind. The cart travels faster than the wind ahead, and will lose energy to that wind as it accelerates it.
The proposal is the the cart can "harvest" the energy of its own motion to
go faster. All the stories are to cover up for that fact, which they know is a show-stopper.
There are only two choices for the cart. Accelerate the air forwards, which produces a reaction that opposes the cart, so that's no good.
Or to do the opposite, and slow it. How is that done, when the cart is traveling faster than that air? Pushes back on it? A childish notion to fill the void.
Also, don't get hung up about how a propeller works.
The details of how a propeller works, also, are not important to understanding how the cart works.
You already said that you understand that a propeller can accelerate a vehicle whether the air is still or moving. That's what the propeller is doing for the cart.
Think of the cart having an engine onboard to power the propeller. You can see how the propeller could make the cart move. As long as the wind isn't so strong that it knocks the cart over or blows it backwards or whatever, the propeller will be able to drive the cart forward.
For the DDWFTTW cart, replace the engine with a connection from the wheels to the propeller so that the turning of the wheels drives the propeller. The wheels cause the propeller to spin. The spinning propeller moves the cart forward.
-- Roger
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All true Jadebox
Jadebox. This is all true but I don't think it lends any insight into where the energy to do all that comes into the system. From Ynot's question, he seems to want to know the entry point into the overall system of the wind energy I think. It's a completely reasonable question because its not so obvious until one looks at the wind on the prop with and without an actual wind in the picture.
Regards,
Windgrins
Also, don't get hung up about how a propeller works.
The details of how a propeller works, also, are not important to understanding how the cart works.
You already said that you understand that a propeller can accelerate a vehicle whether the air is still or moving. That's what the propeller is doing for the cart.
Think of the cart having an engine onboard to power the propeller. You can see how the propeller could make the cart move. As long as the wind isn't so strong that it knocks the cart over or blows it backwards or whatever, the propeller will be able to drive the cart forward.
For the DDWFTTW cart, replace the engine with a connection from the wheels to the propeller so that the turning of the wheels drives the propeller. The wheels cause the propeller to spin. The spinning propeller moves the cart forward.
-- Roger
Jadebox. This is all true but I don't think it lends any insight into where the energy to do all that comes into the system. From Ynot's question, he seems to want to know the entry point into the overall system of the wind energy I think. It's a completely reasonable question because its not so obvious until one looks at the wind on the prop with and without an actual wind in the picture.
Regards,
Windgrins
ThinAirDesigns
Graduate Poster
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Yep, and as was pointed out to you the last time we went through this (only a few pages ago), place a telescoping bar between two sailboats on mirror tacks and sit in the middle -- now you are going DDW with no part of the device going slower than the wind.
As I (and others) said -- sailboats leave lie to your belief.
JB
Talking about wind on the propeller confuses the issue. Ynot already understands that a propeller works whether the air is still or not.
It's better, I think, to say that the wind pushes the cart just as it would push a balloon or a sail boat - the propeller doesn't matter.
When the cart reaches "wind speed," the air around the cart isn't moving from the vantage point of the cart. But, the wind still pushes the cart so that the cart is moving at wind speed.
From the vantage point of the cart, the air is still, but the ground is moving backwards and turning the wheels. That's where the energy to spin the propeller and accelerate the cart comes from.
I thnk it's actually easier to understand when you use the treadmill analogy. Switch your point of view so that the air is still and the treadmill moves under the cart. The treadmill turns the wheels that power the propeller.
-- Roger
A problem with this explanation
Once you get past the point of no wind=WS, the cart itself feels a headwind so this clearly is not the wind that allows the cart to accelerate up to 2.8X windspeed if you think about it. In the description I gave, it is always true below WS, at WS, and above WS. Eventually one will reach a practical limit in that there is not enough "angle of attack" of the blades to overcome the losses.
Regards,
WindGrins
Once you get past the point of no wind=WS, the cart itself feels a headwind so this clearly is not the wind that allows the cart to accelerate up to 2.8X windspeed if you think about it. In the description I gave, it is always true below WS, at WS, and above WS. Eventually one will reach a practical limit in that there is not enough "angle of attack" of the blades to overcome the losses.
Regards,
WindGrins
recursive prophet
Graduate Poster
That’s just the point I was making. So explain the physics of why such angles slow the cart down. Mender gave a good reply, but I still fail to see how a 90 degree wind angle would reduce the prop’s thrust to 0. I get the idea I think-goes out at same angle it comes in-but a gif showing the air foils at that 90 degrees would be helpful. Or not.
It goes in the opposite direction into the wind hitting it. But what about the apparent headwind? What the headwind sees is that the prop is moving CCW like a turbine, right? Yet this somehow has no effect if my understanding is correct.
Yes, this concept of how the prop is somehow bolted to the wind is confusing, to say the least. 
I still haven’t gotten any feedback on why I Ratant’s cart went slower than the wind. Here’s a link again to the video: http://www.youtube.com/watch?v=zqW6BFCypoA I assume the answer will be he didn’t have it geared properly? And what about the foam popcorn as a way to measure wind speed??
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