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Mess with your minds: A wind powered craft going faster than a tail wind speed.
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Brumby:

--- Quote from: electrodacus on December 23, 2021, 07:06:37 am ---I will make an attempt to explain pressure differential.
Think about a stationary fan (stationary as in not moving but rotation blades) there will be a lower than ambient pressure on one side and higher than ambient pressure on the the side as shown in that diagram on wikipedia that I posted here quite a few times.
Now start to move this fan but at the same time the higher the fan moves the higher the rotational speed of the blades. While pressure differential will drop it will not be sudden as it is in part compensated by increased blade rotational speed.
--- End quote ---
This is pretty straightforward.  You didn't need to explain this as we are all well aware of this.


--- Quote ---Blackbird propeller swept area is a massive 20m^2 and all the energy Blackbird needs to get to that record 28mph speed is just around 6Wh easily stored with not much pressure on such a massive swept area.

--- End quote ---
Are you saying that energy is stored across the entire swept area of the propeller?
Kleinstein:

--- Quote from: electrodacus on December 23, 2021, 07:06:37 am ---
If any of you has good electrical knowledge the analogy I like to make is with an air inductor storing energy in the magnetic field it creates.
Many that do not understand fully an inductor may not consider inductor an energy storage device same way it will not think propeller can store energy in the pressure differential.

--- End quote ---
The analogy to the inductor is good and it includes 2 not so convenient effects:
1) With a normal conductor the energy storrage is for limited time only. If you short out the inductor, the current will go down with time - usually one the oder of less than a second. A similar thing happens with the pressure near the fan: one the fan is no longer creating a new pressure difference, the pressure will drop, even if the ernergy is not used to accelerate the vehicle. So in about a second or less the erngy in the air is gone, if the is no container around it. By coincidence the time scales for the usual inductors and the blackbird vehicle are comparable.

2) It is very difficult to use the erngy storred in the inductor to pump more current through the inductor. You only get the energy out from the inductor by lowering the current. Similar with the pressure behind the fan. The pressure is linked to the speed of the fan. So when making the fan spin faster, it would also cause an increase in the pressure difference. So with the fan propelling the vehicle, you can not speed up the fan from the pressure difference.
In the vehicle on the treadmil the fan speed is directly linked to the wheels.  So without some magic control for the fan pitch you can only use the energy from the pressure when slowing down.
gnuarm:

--- Quote from: Brumby on December 23, 2021, 04:21:59 am ---
--- Quote from: gnuarm on December 23, 2021, 04:01:22 am ---Yes, don't assume zero losses.  Assume some losses from the air resistance and rolling resistance.  It doesn't need to be complex.  Just consider that you can't move through the air with zero loss. 

--- End quote ---

I suggest it is better to not complicate the key discussion with real world minutiae.  It is far more practical to work out the basic principles using ideal elements.  That is, to work out if the desired result is possible. 
Once that is established, then add in the real world losses to see what sort of experimental result you can expect.

If it can't be found in the ideal situation, then there's no point in even considering losses.

--- End quote ---

Sounds great, but if "ideal" conditions prevent the analysis as was done here, then there's no way to draw any conclusions.  Did you see the part where under no losses there's no power flow and so no analysis?  That's not a key discussion.

It's all moot.  The thing was built, tested under many conditions and it works.  Anyone who wishes to say it doesn't work is a denialist without reason.
gnuarm:

--- Quote from: IanB on December 23, 2021, 04:42:22 am ---Actually, the analysis does not change if you introduce friction losses. All that happens is that any losses due to friction are overcome by the power of the motors driving the belts. You just have to assume that the motors are powerful enough to achieve the stated belt speeds, and that the wheels of the cart do not slip on the belts.

--- End quote ---

I believe you said you could not analyze the power flow because it was non-existent in the ideal case.  So I'm saying don't assume a perfectly ideal case and you can analyze the power flow. 
gnuarm:

--- Quote from: electrodacus on December 23, 2021, 07:06:37 am ---
--- Quote from: Brumby on December 23, 2021, 02:05:18 am ---
--- Quote from: electrodacus on December 22, 2021, 06:14:14 pm ---Not only the formula is correct it also shows that without energy storage no direct down wind vehicle can exceed wind speed.
--- End quote ---

Since this "energy storage" mechanism is so critical to your theory, I would ask that we take a moment to examine it.

You seem keen to find ways to help us understand, so I would invite you to put together a diagram (or series of diagrams) showing how this energy is captured, stored and released.  Include any necessary description and formulae that would be required for it to stand up to examination.

--- End quote ---

I will make an attempt to explain pressure differential.
Think about a stationary fan (stationary as in not moving but rotation blades) there will be a lower than ambient pressure on one side and higher than ambient pressure on the the side as shown in that diagram on wikipedia that I posted here quite a few times.
Now start to move this fan but at the same time the higher the fan moves the higher the rotational speed of the blades. While pressure differential will drop it will not be sudden as it is in part compensated by increased blade rotational speed.
Blackbird propeller swept area is a massive 20m^2 and all the energy Blackbird needs to get to that record 28mph speed is just around 6Wh easily stored with not much pressure on such a massive swept area.
If any of you has good electrical knowledge the analogy I like to make is with an air inductor storing energy in the magnetic field it creates.
Many that do not understand fully an inductor may not consider inductor an energy storage device same way it will not think propeller can store energy in the pressure differential.
All this is possible because air is a compressible fluid and it is much heavier than you actually imagine.
This is also the reason I insist that none of the wheels only vehicles can represent direct down wind faster than wind and all of them represent direct upwind faster than wind as there pressure differential energy storage is not used.

I saw some comments about my example with vehicle drag.
That is exactly what it seems meaning there is no wind at all but since vehicle travel trough air at 120km/h the power that vehicle will need to overcome drag is the one I calculated using the same formula.
If vehicle was to drive at just 20km/h but with a headwind of 100km/h then it will need the same amount of power to maintain that speed.
I was making a point of the fact that the equation I insist on is one of the most important equations in anything that has to deal with air from wind turbines to any type of wind powered vehicles and to any aerodynamic drag calculation.
And yes using power to define drag is absolutely the right type of unit. When you design a vehicle you need to know the drag power as that is the most significant part affecting vehicle consumption.
I see people comparing Tesla model 3 a very small super aerodynamic vehicle with a large SUV in therms of consumption at highway driving speeds and they think that the SUV was just badly designed with a very inefficient drive-train where the reality is that the larger air drag is by far the largest factor in an EV consumption.   

--- End quote ---

That was a massive fail.  He started to explain the pressure differential and how it relates to the downwind faster than wind case, then shifted gears and started talking about other stuff.  I guess that says it all. 
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