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

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Kleinstein:
Going to 0.5 m/s in only 2 ms  sounds like a lot of acceleration - actually 250 m/s² and thus about 25 G, which for a 10 kg vehicle would need 2500 N of force. Something in this calculation does not sound right !  :-//

bdunham7:

--- Quote from: electrodacus on December 14, 2021, 10:29:35 pm ---No motion of the vehicle means zero power can be generated at the wheel.

--- End quote ---

Let's stick with that for a moment.

So you say that for any non-zero speed, the power is (whatever your formula is) or about 60N * (wind speed - vehicle speed), but right at zero the power goes to zero?

So if I'm sitting there and the vehicle is stopped with brakes, there's no energy being dissipated by the brake, but if I allow the vehicle to creep forward at 1mm/s, now the brakes are dissipating almost 600W?

electrodacus:

--- Quote from: bdunham7 on December 14, 2021, 10:44:28 pm ---
--- Quote from: electrodacus on December 14, 2021, 10:29:35 pm ---No motion of the vehicle means zero power can be generated at the wheel.

--- End quote ---

Let's stick with that for a moment.

So you say that for any non-zero speed, the power is (whatever your formula is) or about 60N * (wind speed - vehicle speed), but right at zero the power goes to zero?

So if I'm sitting there and the vehicle is stopped with brakes, there's no energy being dissipated by the brake, but if I allow the vehicle to creep forward at 1mm/s, now the brakes are dissipating almost 600W?

--- End quote ---

Yes in theory if you want to keep the vehicle at 1mm/s brakes will need to dissipate 600W
In real world there is elasticity and plasticity if needed as everything is made up of atoms.
There is also a stick slip hysteresis that will prevent you to move to slow it will be a fairly sudden transition from moving to stopped.
A wind turbine is only about 40% efficient and one fixed to ground and about 1m^2 swept area will produce around 240W in 10m/s wind.
Sails are just so much more efficient but you do not usually want your wind turbine to move.
For accelerating a vehicle you can not beat a sail so even if you want to use energy storage you will still select a sail and not a propeller.
The propeller/fan just has that effect that it can store energy in the compressible medium and use that to accelerate latter.
Nice thing about that is that stored energy being in material will not take any space/weight on the vehicle.

Similar thing will be a vehicle with wheels only having one wheel on a solid road and one wheel on a very long moving conveyor belt but that conveyor surface will be some super stretchy rubbery material so if you are stopped a lot of potential energy is stored in the long rubbery surface of the conveyor belt then if that wheel was just with brake on and not connected to other wheel it will be like a sail vehicle impossible to exceed conveyor speed.
But if the wheel on the solid road is connected with some gear ratio to the wheel on the flexible conveyor belt taking some of the power from accelerating the vehicle and putting it back in stretching that rubbery material even more than vehicle can exceed conveyor speed for some limited amount of time.   

 
   

bdunham7:

--- Quote from: electrodacus on December 14, 2021, 11:01:40 pm ---Yes in theory if you want to keep the vehicle at 1mm/s brakes will need to dissipate 600W

--- End quote ---

One of us has a fundamental misunderstanding of a very basic principle of physics.  I think the brakes will dissipate approximately 60mW under those conditions. 

electrodacus:

--- Quote from: Kleinstein on December 14, 2021, 10:40:50 pm ---Going to 0.5 m/s in only 2 ms  sounds like a lot of acceleration - actually 250 m/s² and thus about 25 G, which for a 10 kg vehicle would need 2500 N of force. Something in this calculation does not sound right !  :-//

--- End quote ---

See my video at minute 2 https://youtu.be/4Hol57vTIkE?t=121  that graph there where you can see both the speed (blue) and power (green) calculated with exactly this formula and similar 1m^2 sail just lower wind speed 6m/s and a 10kg vehicle and the result perfectly match reality if you where to also add the small friction losses as that example is for an ideal vehicle with no losses.

And so for that ideal vehicle it takes 154ms to get to a quarter of the wind speed 1.5m/s then 1.39seconds to get to half wind speed 3m/s and 12.5 seconds to get to 3 quarters of the wind speed 4.5m/s and of course it will take forever to get to wind speed.

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