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

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bdunham7:

--- Quote from: electrodacus on December 17, 2021, 02:52:40 am ---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

--- End quote ---

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.

--- End quote ---

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. 

electrodacus:

--- Quote from: bdunham7 on December 17, 2021, 03:20:22 am ---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.

--- End quote ---

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



--- 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.

--- End quote ---

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.

bdunham7:

--- Quote from: electrodacus on December 17, 2021, 03:46:43 am ---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

--- End quote ---

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. 

PlainName:

--- 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
--- End quote ---

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.

BrianHG:

--- Quote from: fourfathom on December 16, 2021, 05:05:32 pm ---
--- Quote from: BrianHG on December 16, 2021, 11:33:54 am ---How many here would like to see that happen?
--- End quote ---

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.

--- End quote ---
Nope, just words...

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