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

--- Quote from: IanB on December 15, 2021, 11:12:59 pm ---
--- Quote from: thm_w on December 15, 2021, 10:29:33 pm ---Not according to this: https://www.omnicalculator.com/sports/cycling-wattage
Its ~10W at 1km/h vs ~300W at 36km/h

--- End quote ---

That's a nice calculator. A screen grab is attached below.

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Just so we are clear.  This calculator is fairly accurate.

     Even into a head-wind of 24mph, which does happen on some windy cycling days, in the lowest gear set, I am placing practically 0 power/torque (maybe a few pounds of effort) into the pedals to achieve 0mph holding my position, let alone how easy it is to pedal 1mph in this gear with that headwind.  However, on a windless day, doing 24mph in the high gear still takes a continuous output of substantial continuous torque in the highest gear.  If I was in highest gear, at 0mph, with a 24mph headwind, I would most likely be pushed backwards unless I stand still with all my weight on 1 pedal.  (This is the first clue that something interesting is at play here, otherwise walking into a headwind of 24mph should take close to 300 watts of power (bikes supposed to be more efficient than walking), and looking at the North American's general fitness, many of us should be passing out from exhaustion after ~5 minutes of walking outdoors on such days.)

     If I finish my other work by Sunday, it will be time to tackle 'electrodacus' on the wind powered craft.
Kleinstein:

--- Quote from: electrodacus on December 17, 2021, 03:46:43 am ---
--- Quote from: bdunham7 on December 17, 2021, 03:20:22 am ---
That isn't actually all that crazy, but in order for that to be a workable theory you would have to demonstrate that there is a reasonable place to store the energy and that, in fact, the Blackbird was using that energy during its above-windspeed run.  The only place where any energy can be stored is in the propeller.  Energy storage  and on-demand recovery in uncontained air is something you're going to have to affirmatively demonstrate experimentally before anyone believes that--and I don't think you'll succeed.  Anyhow, as for the propeller, it does indeed store some energy but now you have to calculate how much, and more importantly, show that the Blackbird is actually using that energy by slowing the propeller down.  You can easily put an upper bound on the energy in the propeller by estimating its mass and the tangential velocity of the tips, then positing that all of its mass is at the tips.  So throw some numbers at that and see what you get.  Does the video have a high enough framerate to determine the rotational speed of the propeller?

Even if all of that worked out and you can demonstrate that the Blackbird stores some amount of energy in the propeller, that doesn't mean that the stated principle (stated by the rest of us) isn't valid or that the Blackbird will slow down as this stored energy is exhausted.

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I will see things the other way around. I demonstrated that using correct equations an ideal 100% efficient wind powered vehicle can not exceed wind speed without energy storage.

But of course I did calculated how much energy needed to be stored for blackbird to get to 28mph with 10mph average wind speed (real wind speed average was larger but I was generous).

Blackbird is around 300kg including the driver the wheels are bicycle wheels so super low rolling resistance.
Kinetic energy of the vehicle at 28mph (12.5m/s) will be 0.5 * 300kg * 12.5^2 = 23437Ws = 6.5Wh
To get there even with no wind to start with is not that much of a problem even  3 small 3000F 2.7V super capacitors can store that about 3Wh per capacitor.
Even a cell phone battery can deliver this energy twice it just can not deliver that fast but supercapacitors can charge and discharge in seconds.
Now the propeller is just massive at 5.3m diameter 20m^2 swept area is larger than the floor area in my livingroom and not much pressure differential is needed to store this sort of energy needed to accelerate to 28 or 30mph even without the initial help from the wind.
So I did looked at all the numbers to make sure they all fit correctly.

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We don't care how much energy a super cap or cell phone battery can store, as neither the backbird not the small one on the tread-mill uses such a device.
There is some energy in the pressure flield around the vehicle - there is quite some volume, but not much pressure. However energy storrage in the free air around a prop is an extremely leaky form of energy storage. One could argue not to call it storrage at all. Even if you don't use the energy, it will dissipate very fast - litterally at the speed of sound. So even of there would be a lot of energy to start with it is gone after 1 second.

The other problem with the energy in the air is, that the ernergy would go up when the vehicle and prop go faster. So if it works at all, to use that energy one would have to slow down the vehicle, just like with the kinetic energy.

So I don't see any way to explain the experiments with some form of unintentional hidden energy storrage. Especially in the small tread-mill vehicle it would be very hard to hide a motor, batteries and controls.

As the experiments are real, this is not so much about wether it is possible to go faster than the wind, but about the how. Once you understand how it is possible, it also gets obvious that it is possible.

The calculations that show how the Blackbird vehicle works do not include energy storage as they calculate the steady state.

Labrat101:

--- Quote from: BrianHG on December 17, 2021, 11:02:55 am ---     If I finish my other work by Sunday, it will be time to tackle 'electrodacus' on the wind powered craft.

--- End quote ---
electrodacus:

--- Quote from: bdunham7 on December 17, 2021, 04:23:40 am ---
The green statement is true, the red one is not.

As you've stated, the maximum force is 60N and the mass is 100kg.  So at what point in the following do you disagree?

a)  F=ma => a = F/m.

b)  v = at => t = v/a

c)  t = mv/F

d)  substituting in 100kg for m, 1m/s for v and 60N for F (60N is the maximum force, thus the result will not be perfectly accurate, but an upper bound for acceleration and thus a lower bound for time)
we get t = 100kg * 1m/s / 60N.

e)  Solving we get:  t = 100/60 kg m s-1 N-1 or 5/3s (1.66666666s).  Integrating F over the the interval would give us a somewhat lower average F and thus a longer t, but our differences here are large enough not to worry about this yet.

To resolve this, you need to point out exactly what is wrong with my assertions, not repeat your own.  In the case of your statement (in red) I would specifically point to the error being your belief that the power transmitted to the vehicle is 500W or so, when in fact this more accurately represents the power not transmitted to the car (dissipated).  In fact at the very beginning when the speed is zero, the power transmitted to the car is ~60N * vehicle speed, or nil at zero, a small amount in the beginning stages and then more as the speed picks up--to a point.

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Fair points so I took a look on what pressure will you have on a flat 1m^2 wall in 10m/s wind.
Seems calculation is fairly complicated and here is an online calculator

https://eurocodeapplied.com/design/en1991/wind-pressure-freestanding-wall
I used:
10m/s
1m for both lenght and hight of the wall so we have 1m^2
hbase = 0
Icorner =0
Solidity ratio = 1
Air density = 1.2

The result is a net pressure for the first 0.3m behind the sail / wall of 0.196kN/m^2 but that is average for this volume with way higher just on the wall
Then for the next volume that starts at 0.3m and ends at 1m is 0.120kN/m^2
So I will say this is a good explanation of what happens as there will be super high air density just next to the wall but I think my way of calculating what happens is way easier and accurate.

Maybe I'm just biased but I'm confident my results are correct as they match any real world experiment.

I do not see why it will be a difference between a flat wall traveling at 1m/s with a headwind of 9m/s and a flat wall traveling when there is no wind at 10m/s
Same amount of power should be required to overcome drag since as far as that wall is concerned the same thing happens.
bdunham7:

--- Quote from: electrodacus on December 17, 2021, 06:44:39 pm ---Fair points so I took a look on what pressure will you have on a flat 1m^2 wall in 10m/s wind.

So I will say this is a good explanation of what happens as there will be super high air density just next to the wall but I think my way of calculating what happens is way easier and accurate.

Maybe I'm just biased but I'm confident my results are correct as they match any real world experiment.

--- End quote ---

You appear to have realized that your energy transfer model would require the initial acceleration to be very, very high--diverging to infinity as the speed is zero.  And so it won't work.

Keeping in mind that the Blackbird is not a wall or a sail, you are going to have to stipulate what the equations or formula or calculator or whatever-have-you are going to produce for the force on the sail, if we're using sails to demonstrate something.  I thought we had that all worked out with formulas that you and IanB agreed on and the result was 60N at zero vehicle speed and 53N at 1m/s vehicle speed.  If you want to change that, fine--but justify it somehow. 

And perhaps this wouldn't be so hard to verify experimentally, and perhaps someone else already has and such results can be googled.
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