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Newton's third law problem.

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Kleinstein:

--- Quote from: electrodacus on November 29, 2022, 07:34:24 pm ---It will be your job to prove the equation is wrong as that equation is used by almost all engineers to build things that work in real world as predicted by it.

--- End quote ---


--- Quote from: electrodacus on November 29, 2022, 03:13:20 am ---This will be that equation
P = 0.5 * air density * (area * drag coefficient) * (wind speed - vehicle speed)3

--- End quote ---
Luckyly the equation is mainly used by fools and not be engineers, because it is flawed.
The part sometimes used is the special case the vehicle speed = 0 and with a slightly different coeffcient - that it get right.  Adding the vehicle speed is a point making a flawed.

It would actually your part to prove the equation, not my part to proof it is false.

I can still give you the proof by showing how the backbird vehichle works and gets power out from the wind, when driving at the speed of the wind. So giving an example that poofs the equation wrong.

A prop running in still air needs a  given power P to provide a thrust force of F. We actually don't care very much how good the prop is, it just has to work in the ideal case with no extra friction loss.
The force from the prob can than be used to have equal force on the wheels to counter-act the movement. This way the wheels provide a power of  vehichel speed times force-. This power can than be used to drive the Fan and excess power could be used for what ever other task. That excess power would be generated from the wind, as the wind is the only power soruce to the system.
To make this there needs to be excess power available. So one need   F*V_vehicle > P_prop  or  V_vehicle > P_prop / F. So given enough speed for the vehicle = wind this condition can be met.
For only somewhat realistic prop quality it does not even need a high speed, nowhere near the speed of sound where you can expect the equations to break down.

So this gives a proof that one can get power at the speed of the wind and thus that the equation with  (wind speed - vehicle speed)3 is wrong.

PlainName:

--- Quote ---The minimum wind speed is related to wind turbine. Wind speed needs to be above a minimum threshold 3 to 4m/s in order to be enough wind power to cover the turbine friction losses.
--- End quote ---

It is not a wind turbine, it's a propeller. And the effect it more pronounced the fast the wind is because the propller thrust is proportional to the vehicle speed (not the wind speed). Thus the faster the vehicle goes over the ground, the greater the power sucked up  by the wheels and the greater the thrust.

Think about it (I know, you need someone else to do if for you, but ask them nicely) - at zero ground speed there is no propeller power regardless of how fast the wind is blowing, or how many megawatts your favoured equation says there is.

electrodacus:

--- Quote from: Kleinstein on November 29, 2022, 08:07:29 pm ---
--- Quote from: electrodacus on November 29, 2022, 07:34:24 pm ---It will be your job to prove the equation is wrong as that equation is used by almost all engineers to build things that work in real world as predicted by it.

--- End quote ---


--- Quote from: electrodacus on November 29, 2022, 03:13:20 am ---This will be that equation
P = 0.5 * air density * (area * drag coefficient) * (wind speed - vehicle speed)3

--- End quote ---
Luckyly the equation is mainly used by fools and not be engineers, because it is flawed.


--- End quote ---

That is not good enough.
If you want to say that equation is wrong you need to provide the correct one so we can compare the prediction each of them makes.
The other equation that floats around is wrong and is so easy to show it is as it will predict zero power available for a stationary vehicle also the values for available power will be way off what is measured in real world.


That calculator comes from "electromotive engineering & consulting" they use the same equation and they must have at least one engineer based on the company name so equation is used by engineers.

You can not move a vehicle with the brakes engaged so when you want to start moving you need to disengage the brakes thus you need to deal with the power applied by the headwind if you want to move in that direction.

IanB:

--- Quote from: Kleinstein on November 29, 2022, 07:01:34 pm ---The problem is that AFAIK there is no such simple, easy to accept equation for the maximum available power.  At least I don't see such an equation that I would take for granted. So it would still need a proof or at least good plausible explanation. 
Aerodynamics can be be quite tricky and I would consider getting such an euqation and proving it rather hard. So the way via the available power is a hard one.
--- End quote ---

There are some ways of making estimates, however.

It doesn't help Electrodacus that he makes assertions that are demonstrably untrue and then relies on them to derive conclusions. For example, saying that a sail is nearly 100% efficient at extracting wind energy:


--- Quote from: electrodacus on November 29, 2022, 04:53:41 pm ---You are correct that a wind turbine is no more than 40 to 45% efficient but a sail is way more efficient approaching ideal.
--- End quote ---

We covered this in another thread, and it appears that a simple square sail is not very efficient at all. A sail self-evidently has zero efficiency when stationary, and zero efficiency when moving at the same speed as the wind. It seems the maximum efficiency of a simple square sail occurs somewhere in between, and is not much more than 15% at best. See this post for an explanation:

https://www.eevblog.com/forum/chat/mess-with-your-minds-a-wind-powered-craft-going-faster-than-a-tail-wind-speed/msg3888830/#msg3888830

Yacht sails are clearly much more efficient, but that is because they use angles and aerodynamics to do a better job.

electrodacus:

--- Quote from: PlainName on November 29, 2022, 08:10:09 pm ---It is not a wind turbine, it's a propeller. And the effect it more pronounced the fast the wind is because the propller thrust is proportional to the vehicle speed (not the wind speed). Thus the faster the vehicle goes over the ground, the greater the power sucked up  by the wheels and the greater the thrust.

Think about it (I know, you need someone else to do if for you, but ask them nicely) - at zero ground speed there is no propeller power regardless of how fast the wind is blowing, or how many megawatts your favoured equation says there is.

--- End quote ---

This is getting old. How many times I need to mention that there are two very different vehicle being discussed.

Direct downwind uses a propeller for propulsion
Direct upwind uses a propeller as a generator so wind turbine.

There is power available for a vehicle at 0 speed while wind is blowing and in fact that is when the max wind power is available for a direct down wind vehicle.
By applying the brakes you transfer all this wind power directly to earth.

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