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

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electrodacus:

--- Quote from: Brumby on December 17, 2021, 01:52:39 am ---
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.

--- End quote ---

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.

bdunham7:

--- Quote from: electrodacus on December 17, 2021, 01:49:57 am ---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.

--- End quote ---

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.

electrodacus:

--- Quote from: bdunham7 on December 17, 2021, 02:12:57 am ---
--- Quote from: electrodacus on December 17, 2021, 01:49:57 am ---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.

--- End quote ---

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

--- End quote ---

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

bdunham7:

--- Quote from: electrodacus on December 17, 2021, 02:34:04 am ---So provide the time it takes for this vehicle to get to 1m/s and 5m/s
I say 103ms and 8.3s

--- End quote ---

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?

electrodacus:

--- Quote from: bdunham7 on December 17, 2021, 02:39:15 am ---
--- Quote from: electrodacus on December 17, 2021, 02:34:04 am ---So provide the time it takes for this vehicle to get to 1m/s and 5m/s
I say 103ms and 8.3s

--- End quote ---

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?

--- End quote ---

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.

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