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diy solar

Faster than the wind going downwind?

Real or Fake?

  • Real

    Votes: 10 76.9%
  • Fake

    Votes: 2 15.4%
  • On the fence

    Votes: 1 7.7%

  • Total voters
    13
The comments on that video are all over the place, but most think not possible... and sideways motion is irrelevant...
Hand waving...
 
That's how sailboats do go faster than the wind and upwind and looks good to me, but it's not for how the land vehicle goes faster than the wind downwind.

I would have left a Bernoulli comment on it, but about a million people beat me to it! (It's also why I'd love to see a retractable Magnus effect keel (lower overall forward resistance without increasing leeway (drifting downwind), Not that I'd want one, not until batteries are cheap anyway)).

I like how they said the Veritasium video "sort of waved his hands over the technical bits" ... that's how I felt about it.

Hopefully, someone more knowledgeable will do a video on it.
I think what's in #20 is correct, but I have a failing in that I always think I'm right. Actual mileage varies and is usually lower than I like. ;)
 
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I would have left a Bernoulli comment on it, but about a million people beat me to it!
I think caution needs to be applied when invoking Bernoulli as an explanation of lift, primarily because this is not a closed system (in the way that fluid flow in pipe is more of a closed system).
 
I rejected their cute illustration of two sailboats on a cylindrical world as a propellor because sailboats on a broad reach still have an apparent windspeed.

The propeller blades of this cart also have apparent wind, even when the cart doesn't. The propeller blades are not moving straight downwind, but across the wind, like the tacking sail.

The other factor is that their propellor isn't affected by the force of the wind as a sail is. The propellor is turned by the wheels and pushes back against the wind. So, that whole analogy made no sense to me.

The wheels & transmission are forcing the blades to move across the wind, just like the keel of the sailboat forces the sail to move across the wind.

Here the vectors for the boat:

downwind_vectors_en_3.png

And here for the propeller blade:

downwind_propeller_vectors.jpg
 
The propeller blades of this cart also have apparent wind
See post #20 for my mental evolution regarding the cart mechanics.

...I rejected their cute illustration of two sailboats on a cylindrical world as a propellor because sailboats on a broad reach still have an apparent windspeed....
Nice set of Newtonian-style vector drawings!
But, I still reject the illustration they used for the reason mentioned, if there's no wind on a boat sail, there can be no force. When the apparent wind is zero, the force is zero. For example, a free-spinning wind turbine mounted on a truck will not spin when the truck speed is moving at the same speed as the wind.
 
When the apparent wind is zero, the force is zero.
Yes, but the apparent wind is never zero at the propeller blades, just like it is never zero at the sail going diagonally to the wind.

You have to differentiate the apparent wind at the vehicle chassis (which is zero at wind-speed) from the apparent wind at the spinning propeller blades (which can never be zero, if there is true wind). Below is the vector diagram showing the vectors for the propeller blade, when the vehicle reaches windspeed:

downwind_propeller_vectors_ws.png

For example, a free-spinning wind turbine mounted on a truck will not spin when the truck speed is moving at the same speed as the wind.

But this is not a free-spinning wind turbine. It is propeller that is coupled to the wheels.
 
But this is not a free-spinning wind turbine. It is propeller that is coupled to the wheels.
Yup, came to peace with it in #20 after a whole lot of headbanging. The free-spinning analogy refers to the cylindrical world metaphor and why it's misleading/incorrect.
 
The free-spinning analogy refers to the cylindrical world metaphor and why it's misleading/incorrect.

The analogy they show is correct, but unfortunately they don't mention the role of the boat's keel which is key here. The keel drives the boat across the wind, just like the wheel drive the propeller blades across the wind.
 
the role of the boat's keel which is key here. The keel drives the boat across the wind, just like the wheel drive the propeller blades across the wind.
When headed directly downwind (which is the only time you can get a zero apparent wind) the only function of the keel is drag as there are no lateral forces. In fact, sailors typically pull the daggerboard up when "running" (a downwind point of sail) to eliminate it.
 
When headed directly downwind (which is the only time you can get a zero apparent wind) the only function of the keel is drag as there are no lateral forces.
Correct, but in their analogy the boat is not moving directly downwind, but diagonally downwind. This is equivalent to the motion of the propeller blade, which also has a velocity component perpendicular to the wind.
 
... the boat is not moving directly downwind, but diagonally downwind...
Yup, one more reason to discount it since the apparent windspeed > 0. As you can see from #20, I'm not arguing the land vehicle can't go faster than the wind downwind, or how propellors work... just that metaphor breaks down when apparent windspeed = 0. At least I don't see how it supports their case for DDWFTTW.
 
Yup, one more reason to discount it since the apparent windspeed > 0.
The same is true for the propeller blades of the DDWFTTW cart, so I don't see the reason to discount it. Again, the analogy is not between the boat and the cart. It is between the sail and the propeller blade.

At least I don't see how it supports their case for DDWFTTW.
It demonstrates that DDWFTTW can be achieved if you allow for moving parts, For example like this:


The propeller is also a moving part that allows it in a more continuous and compact manner.
 
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...It demonstrates that DDWFTTW can be achieved...
I don't really question DDWFTTW since the explanation in post #20.
Perhaps we're not talking apples to apples, what do you think the boat on the cylindrical world is demonstrating in regards to DDWFTTW?
 
A high level conceptional overview. We tend to focus of the velocity of the wind. But there is a force applied to the sail/prop/vehicle. Assume any given windspeed, and a sail of a given size. The sail is sized such that a vehicle of a certain mass will travel at the speed of the wind. If we increase the size of the sail, the force is increased, but the velocity is not. It is easy to understand that we can move a heavier vehicle with a larger sail. But there is nothing preventing us from using a larger sail, and converting the extra force that is applied into velocity. Doing so does not break any laws of energy. It is just a question of designing a machine that can convert that extra force into velocity. The propeller adds size to the "sail" allowing the force to push against greater resistance. The gearing from the wheels to the propeller turn that extra force into a greater velocity.
 
Look the vector diagrams in post #25. It's the same aerodynamics.
We're still not talking the same thing, that's okay. I believe there are no aerodynamics when the apparent wind speed is 0 which I think you agree with.

A high level conceptional overview.
The bit with the size of the sail could be confusing as once you go faster than wind-speed, in parallel to the wind, bigger would be more drag. Also, no sail on the vehicle which might also confuse some. But it sounds like you have it, if not 100% sure see post #20.
 
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I believe there are no aerodynamics when the apparent wind speed is 0 which I think you agree with.
Yes, but it's irrelevant. The apparent wind at the propeller blades can never be zero with this setup
Also, no sail on the vehicle which might also confuse some.
The propeller blades are the analogue to the sail.
 
Best quote, "In my defense, I didn't present the information good enough in the first video". Makes me feel a whole lot better that I wasn't the only one that had a hard time believing it. That video bothered me for a week.

When I first saw the video I was saying the same things as the professor and found the same faults with the treadmill model. The stab at a force analysis in #20 doesn't quite match up with 12:14 (no infinite force problem), but I'd say the video has a better rendition.

Loved the Fox model, ~15:00 she's handling it on the treadmill that blows the arguments in the OP about the model out of the water.
 
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