Author Topic: Mess with your minds: A wind powered craft going faster than a tail wind speed.  (Read 147385 times)

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Offline electrodacus

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Here is the key - there IS an additional energy source, but it is not as obvious as wind on a sail (which is why "intuition" is a really bad influence - really bad.).  It is, however, fully contained within the system consisting of only the Blackbird, the wind and the ground.  It is not external to that system and it is not an energy storage mechanism.

If you cannot OPENLY consider such a possibility and work through the relevant calculations, you will never understand.

Such closed-mindedness would have kept the study of physics from progressing any further than Newton.

Maybe additional energy source was a bad choice of words on my part. Normally I will say an external energy source as in a on board battery or fuel based engine.
There are no hidden energy sources meaning there is an energy storage device.
Examples of energy storage devices will be
Linear kinetic energy.
Rotational kinetic energy. (flywheel)
Pressure differential.

All the above are present on the Blackbird but:
Linear kinetic energy can not be used to exceed wind speed in this particular case, and even a simple sail vehicle has that and for direct down wind it can not be used to exceed wind speed.
Rotational kinetic energy available again in any vehicle on wheels as the wheels are a flywheel but also in case of blackbird the massive propeller can store energy in that way and if propeller pitch if variable then that energy alone can be used to exceed wind speed.
But the pressure differential alone as demonstrated on the treadmill model is sufficient, allowing to exceed wind speed.

As I mentioned without energy storage no direct downwind vehicle powered only by wind can exceed wind speed.
And you may think why is that so relevant. It is because vehicle will slow down below wind speed once stored energy is used up.

Online bdunham7

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If you look back at earlier example I stated that it takes around 100ms to get to 1m/s and the vehicle kinetic energy at 1m/s is 50Ws
So according to your theory it will take more like one full second for this vehicle to get to 1m/s instead of just 100ms.

I don't know what numbers you are using for mass, etc, but:

Where v = speed (m/s), a = acceleration (m/s2), m = mass (kg), t = time (s), F = force (N or kg * m/s2))

F = ma => a = F/m

v = at = t*F/m

t = v/a = mv/F

Pro tip for physics beginners when confused:  solve for t where possible

Edit: I forgot to write that this can only be used as an approximation at the slower speeds where F is nearly constant.  As the speed increases and F decreases, you need to integrate.

« Last Edit: December 17, 2021, 02:33:18 am by bdunham7 »
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Offline electrodacus

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If you look back at earlier example I stated that it takes around 100ms to get to 1m/s and the vehicle kinetic energy at 1m/s is 50Ws
So according to your theory it will take more like one full second for this vehicle to get to 1m/s instead of just 100ms.

I don't know what numbers you are using for mass, etc, but:

Where v = speed (m/s), a = acceleration (m/s2), m = mass (kg), t = time (s), F = force (N or kg * m/s2))

F = ma => a = F/m

v = at = t*F/m

t = v/a = mv/F

Pro tip for physics beginners when confused:  solve for t where possible

So provide the time it takes for this vehicle to get to 1m/s and 5m/s
I say 103ms and 8.3s

Online bdunham7

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So provide the time it takes for this vehicle to get to 1m/s and 5m/s
I say 103ms and 8.3s

I don't know and I don't know the mass you are using, but in any event how does this help resolve anything?  What would knowing the initial acceleration do as far as understanding faster-then-wind travel?  Or is there another point?
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Offline electrodacus

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So provide the time it takes for this vehicle to get to 1m/s and 5m/s
I say 103ms and 8.3s

I don't know and I don't know the mass you are using, but in any event how does this help resolve anything?  What would knowing the initial acceleration do as far as understanding faster-then-wind travel?  Or is there another point?

I do not know if this exercise will be helpful or not. Like I mentioned I'm fairly bad at explaining things.
In any case this vehicle is 100kg wind speed is 10m/s, sail area is 1m^2 and air density 1.2kg/m^3

While a sail is more efficient than a propeller a sail even ideal sail vehicle can not exceed wind speed direct downwind.
The reason a sail vehicle can not exceed wind speed while traveling directly downwind has to do with the fact that a sail has no energy storage device to be able to do that while a propeller used as a fan can create a pressure differential and that is a way of storing energy.

Blackbird will accelerate much slower than an equivalent area sail but Blackbird is using that extra time below wind speed to store energy so that it can then exceed wind speed for some limited amount of time.
I can add a generator to the wheel of a sail vehicle and store that energy in a supercapacitor. This will make the sail vehicle also accelerate slower as I take energy from the wheel but it will also allow the sail vehicle to do the same and exceed wind speed if I then use the energy stored in the supercapacitor putting that in an electric motor that will move one of the wheels.
This supercapacitor sail version will be more efficient than blackbird meaning it can exceed wind speed by even higher margins but eventually both will need to slow down below wind speed as stored energy is being used up.

Probably nobody will be impressed by a sail vehicle with a supercapacitor for energy storage but since blackbirds energy storage is sort of invisible it looks more like magic.

Online bdunham7

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I say 103ms and 8.3s

In any case this vehicle is 100kg wind speed is 10m/s, sail area is 1m^2 and air density 1.2kg/m^3

Accelerating a vehicle from a stop to 1m/s in 100ms is an acceleration of 10m/s2 (slightly over 1g) and would take 1000 newtons of force.  What were the forces calculated earlier, 53N at zero speed and 46.8 at 1m/s?  So maybe an average of about 50N?  So I'd say that is going to take more like 2 seconds.

Quote
Blackbird will accelerate much slower than an equivalent area sail but Blackbird is using that extra time below wind speed to store energy so that it can then exceed wind speed for some limited amount of time.

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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Offline electrodacus

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Accelerating a vehicle from a stop to 1m/s in 100ms is an acceleration of 10m/s2 (slightly over 1g) and would take 1000 newtons of force.  What were the forces calculated earlier, 53N at zero speed and 46.8 at 1m/s?  So maybe an average of about 50N?  So I'd say that is going to take more like 2 seconds.

It will be 60N at zero vehicle speed
Vehicle at 1m/s has a kinetic energy of 0.5 * 100kg * 1m/s^2 = 50Ws (same as 50 Joules)
So since average was around 500W available over that period about 100ms (0.1 seconds) are needed to transfer that amount of energy from wind to vehicle kinetic energy.
This is ideal case no friction in real world with some friction added it will be a bit more than 100ms but not by much.
I know we take very different routes in calculating the same thing but results needs to be the same   
In 2 seconds vehicle will be at over 3m/s



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.

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.

People just imagine that there is a much larger energy involved when that is not the case.
The treadmill model is even more ridiculous as for the vehicle to accelerate to 1m/s (way more than it was demonstrated in the video) it will have needed just 0.5* 0.5kg * 1m/s^2 = 0.25Ws that is 0.07mW and so a cell phone battery could do that test over 200000 times with power at the wheel and 100000 times with a 50% efficient propeller. There are no typos there is just how small the amount of energy needed is for that 500g or so model.

Online bdunham7

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It will be 60N at zero vehicle speed
Vehicle at 1m/s has a kinetic energy of 0.5 * 100kg * 1m/s^2 = 50Ws (same as 50 Joules)
So since average was around 500W available over that period about 100ms (0.1 seconds) are needed to transfer that amount of energy from wind to vehicle kinetic energy.
I know we take very different routes in calculating the same thing but results needs to be the same

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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Offline PlainName

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Quote
so a cell phone battery could do that test over 200000 times with power at the wheel and 100000 times with a 50% efficient propeller

But the fact is it did not. It also didn't use super-caps or any other storage device.

The treadmill model also didn't store wind as it got up to speed, because it started already at speed. One moment it is going nowhere - it is at wind speed with no power input at all. The next it is accelerating up the treadmill - that is, going faster than the wind - with only the treadmill power. Nothing has had time to charge up, stretch, whatever you want to intimate. At least, if it has, the time has been so short the thing wouldn't last long enough to appear to go anywhere and would slow down again straight away instead of running off the top of the treadmill.
 

Online BrianHGTopic starter

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How many here would like to see that happen?

I'd like a ticket for this, please. 

Are you building something?  Just don't use a rubber-band instead of gears or chain-drive.  We wouldn't want any of that "stick-slip hysteresis energy storage" to cloud the discussion.
Nope, just words...
 

Online BrianHGTopic starter

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Not according to this: https://www.omnicalculator.com/sports/cycling-wattage
Its ~10W at 1km/h vs ~300W at 36km/h

That's a nice calculator. A screen grab is attached below.
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.
« Last Edit: December 17, 2021, 11:05:06 am by BrianHG »
 
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Offline Kleinstein

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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.

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.

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.

 

Offline Labrat101

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

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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.

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.

Online bdunham7

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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.

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.

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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Offline electrodacus

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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.

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.

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.

Have you looked at the link I provided. The same very high pressure the closer you are to the sail / wall.
To me all seems to line up and make sense both calculations if done properly will have the same result I provided. For me the one involving power and energy works better (simpler).
Blackbird propeller fan will act as a wall with variable area.
There will be a significant pressure differential on upwind and downwind of the disc shape created by the blade swept area.
« Last Edit: December 17, 2021, 08:34:49 pm by electrodacus »
 

Offline Kleinstein

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To me all seems to line up and make sense both calculations if done properly will have the same result I provided. For me the one involving power and energy works better (simpler).
The calculation with only energy and avoiding forces at all costs gets quite complicated and error prone if there are elements (like a sail) that don't have 100% efficiency. It may look easier to calculate with energy only, but I am afraid this is only an easy way to the wrong result. A sail is not 100% energy efficient - with low speed it is actually rather low efficiency.

Even if you think you got the energy based calculation done, please also have a look at the way with forces, without taking the result from the other calculation for granted. The force based calculation is not that complicated and the way it is usually done.
 

Offline electrodacus

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To me all seems to line up and make sense both calculations if done properly will have the same result I provided. For me the one involving power and energy works better (simpler).
The calculation with only energy and avoiding forces at all costs gets quite complicated and error prone if there are elements (like a sail) that don't have 100% efficiency. It may look easier to calculate with energy only, but I am afraid this is only an easy way to the wrong result. A sail is not 100% energy efficient - with low speed it is actually rather low efficiency.

Even if you think you got the energy based calculation done, please also have a look at the way with forces, without taking the result from the other calculation for granted. The force based calculation is not that complicated and the way it is usually done.

All this calculations are done for ideal case so to get the best case and show that even with best case (ideal) no vehicle can exceed wind speed direct downwind without energy storage.
But in terms of efficiency (converting wind power to kinetic energy) the sail is by far the most efficient.
Propeller may peak at around 70% efficiency so nowhere near a sail and the way wind interacts with any vehicle will be as an equivalent sail. It needs a surface area in order to push the vehicle.
That is why is just impossible for wind to power a vehicle that is driving directly downwind at or above wind speed.
This seems to me so obvious that it is frustrating to see people trying to find way a vehicle can still be powered by wind when no air molecules can push forward the vehicle. Above wind speed there is an apparent wind but the direction opposed the vehicle travel direction thus it can not be used to accelerate the vehicle.
Same applies for a direct upwind vehicle and while fairly different as wind power is always available it requires energy storage as the wind direction opposes the vehicle travel direction. And for direct upwind I even provided video of a toy vehicle showing exactly how energy is stored and released multiple times a second.
So despite all tests being fully supported by my theory (not actually mine as I have not come up with anything new) people seems to just prefer to "believe" the current wrong explanation.
The current explanation is not even close to describing what is seen in the test results and the claim is in violation of energy conservation.
When you take any energy from the wheel while above wind speed all that is taken out of the vehicle kinetic energy and you can only put back at best of fraction of that back as propeller is at best 70% efficient but even if ideal 100% efficient it will not be able to accelerate.

Offline Kleinstein

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All this calculations are done for ideal case so to get the best case and show that even with best case (ideal) no vehicle can exceed wind speed direct downwind without energy storage.
But in terms of efficiency (converting wind power to kinetic energy) the sail is by far the most efficient.
.....
We have spend quite some time with the low speed case to show you that the formula provided is wrong. They produce a diverging force / acceleration (e.g. accelaration to 0.1 m/s in less than 1 ms) near zero speed and are thus obviously wrong.  :horse:

A sail is not the most efficient way to harness the wind. It is actually very low effciency at low speed. Assuming 100% efficiency for the sail is one cause of getting the rediclulous fast acceleration. At zero vehicle speed the sail has zero energy efficiency - that is a very simple fact. A Wind turbine has a efficiency better than zero and is thus higher effciency than the sail (proven here for the case when the windmill is not moving).

With just a sail in a 1 D world (straight downwind) one can not reach a speed higher than the wind.
However the sail is not the only option and with a more intelligent way (e.g. the prop drive like in the Blackbird) it is possible to get more energy from the wind than the sail and move faster than the wind.
 

Offline electrodacus

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All this calculations are done for ideal case so to get the best case and show that even with best case (ideal) no vehicle can exceed wind speed direct downwind without energy storage.
But in terms of efficiency (converting wind power to kinetic energy) the sail is by far the most efficient.
.....
We have spend quite some time with the low speed case to show you that the formula provided is wrong. They produce a diverging force / acceleration (e.g. accelaration to 0.1 m/s in less than 1 ms) near zero speed and are thus obviously wrong.  :horse:

A sail is not the most efficient way to harness the wind. It is actually very low effciency at low speed. Assuming 100% efficiency for the sail is one cause of getting the rediclulous fast acceleration. At zero vehicle speed the sail has zero energy efficiency - that is a very simple fact. A Wind turbine has a efficiency better than zero and is thus higher effciency than the sail (proven here for the case when the windmill is not moving).

With just a sail in a 1 D world (straight downwind) one can not reach a speed higher than the wind.
However the sail is not the only option and with a more intelligent way (e.g. the prop drive like in the Blackbird) it is possible to get more energy from the wind than the sail and move faster than the wind.

Please see this link https://eurocodeapplied.com/design/en1991/wind-pressure-freestanding-wall
Play with some numbers and you will understand why it is not wrong to have high acceleration rates at the start.
The way pressure varies at the back of a sail / wall is exponential and that is perfectly matching that decreasing rate of acceleration shown using power and energy. Not to mention they perfectly match any experimental test.
You just think sail is low efficiency because you use incorrect formula. The reality is that you can not have a more efficient wind power conversion to kinetic energy than a sail.

Also my point is that no vehicle can exceed wind speed directly downwind unless it has an energy storage device.
That energy storage device will be charged during the initial acceleration phase thus a sail vehicle will accelerate much faster than an equivalent area blackbird vehicle since Blackbird is not only less efficient but it needs to charge energy in to the storage device so that latter can exceed wind speed.
Not considering energy storage will get you in this sort of wrong conclusion that will violate the conservation of energy (even if you do not get that is the case).
« Last Edit: December 17, 2021, 10:02:08 pm by electrodacus »
 

Online bdunham7

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is in violation of energy conservation.

What's that?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline electrodacus

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is in violation of energy conservation.

What's that?

Energy cannot be created or destroyed just converted from one form to another.
Vehicle (any wind only powered vehicle) has no access to wind power when above wind speed directly downwind.
All tested vehicles the blackbird and the treadmill model show decrease acceleration rate after exceeding wind speed meaning they will get to top speed and start to decelerate after that but none of the tests run for long enough to show that so people proposed wrong theories about what happens.
 

Offline PlainName

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Quote
That energy storage device will be charged during the initial acceleration phase

Where is the acceleration phase on the treadmill?
 

Online bdunham7

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Energy cannot be created or destroyed just converted from one form to another.

How would that be violated by a sail dissipating all or most of the wind energy and only transferring a small portion of that energy to the vehicle?

Quote
The way pressure varies at the back of a sail / wall is exponential and that is perfectly matching that decreasing rate of acceleration shown using power and energy. Not to mention they perfectly match any experimental test.
You just think sail is low efficiency because you use incorrect formula.

The formula used earlier that seems to be widely accepted as correct (the drag equation) is also exponential and doesn't diverge to infinity as the vehicle speed approaches zero.  In fact, it doesn't matter whether the vehicle is stationary or moving beyond how that changes the relative speed of the wind and vehicle. 

So now you are positing some new drag or sail formula that would give you an extremely high pressure and thus an extremely high initial acceleration for a 10m/s wind and a stationary vehicle.  So what would the pressure and acceleration be if you had an 11m/s wind and a vehicle moving at 1m/s?  Would that be different?

What experimental tests are you referring to? 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline electrodacus

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Quote
That energy storage device will be charged during the initial acceleration phase

Where is the acceleration phase on the treadmill?

I think I explained that before here but it is when you put the vehicle on the treadmill.


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