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Mess with your minds: A wind powered craft going faster than a tail wind speed.
Kleinstein:
--- Quote from: electrodacus on December 25, 2021, 07:54:59 pm ---
--- Quote from: fourfathom on December 25, 2021, 07:46:46 pm ---
I believe we can analyze, or at least demonstrate this with a fixed-pitch, fixed-gearing propeller. That's how the treadmill model is built. I don't know if this model can start from a full-stop, but it does accelerate once past windspeed.
--- End quote ---
That model will be able to start from a full stop assuming high enough wind speed so that power based on vehicle equivalent back area can overcome vehicle friction.
The thing you (all) seems to be ignoring is that air speed relative to vehicle is what can power the vehicle and so when air speed / wind is equal with vehicle speed in exact same direction then there is no way for the air to push the vehicle.
And somehow you imagine that propeller can push air backwards but that will require an energy source and wind is no longer there to help when vehicle is at or above wind speed.
You just think that you can take energy from the wheel but the problem with that is that when above wind speed that wheel no longer takes energy from the wind but will need to take energy from the vehicle kinetic energy and so vehicle will just slow down if you ignore a energy storage device like the pressure differential witch continues to push the vehicle basically taking the role of the wind for a few seconds or minutes depending on the design.
--- End quote ---
Yes the propeller to push back against the air and thus the wind does require some power and an energy source. So this sounds like you agree that with a given power (from a battery or whatever source) the prop can drive the vehicle a little faster than the wind speed or at exactly windspeed (this is the case that is a bit easier to calculate).
Now comes the slightly tricky question: how much power is needed for the prop to create a given thrust (force) to overcome friction ?
electrodacus:
--- Quote from: Kleinstein on December 25, 2021, 08:09:30 pm ---
Yes the propeller to push back against the air and thus the wind does require some power and an energy source. So this sounds like you agree that with a given power (from a battery or whatever source) the prop can drive the vehicle a little faster than the wind speed or at exactly windspeed (this is the case that is a bit easier to calculate).
Now comes the slightly tricky question: how much power is needed for the prop to create a given thrust (force) to overcome friction ?
--- End quote ---
This sounds like a familiar question. Why is important how much power is needed to overcome friction (not much for such a low friction vehicle like blackbird or even the treadmill model). The important fact is that some power is needed and since wind can not do that when vehicle is above wind speed the only remaining option is the pressure differential energy storage.
And because friction is so low the vehicle will not start to slow down sooner and is able to be powered with that relatively small amount of energy for 2 or 3 minutes.
Kleinstein:
The amount of power is important, as this decides if a movement at that speed is possible.
A suitable approximation for the power needed, when the vehicle goes at the speed of the wind would be a prop power proportional to the trust (may need a larger prop for more thust). This would be independent of the wind speed, as the prop does not see the wind speed.
Alex Eisenhut:
--- Quote from: electrodacus on December 25, 2021, 07:54:59 pm ---
The thing you (all) seems to be ignoring is that air speed relative to vehicle is what can power the vehicle and so when air speed / wind is equal with vehicle speed in exact same direction then there is no way for the air to push the vehicle.
--- End quote ---
The thing you seem to be ignoring is that the propeller is part of the vehicle and is blowing its own wind backwards. The effective wind speed is both.
Do you understand 2+2? Then why don't you understand 2 - (-2)?
fourfathom:
--- Quote from: Labrat101 on December 25, 2021, 08:03:59 pm --- [... fixed-pitch propellers ...]
I am not sure how this can be done as both these propellers have totally different properties .Your know about boats . It would be like saying a life raft is the same as a boat or worse still a Ship . They both travel or go in water . (spot the difference)
--- End quote ---
I'm not sure what you're saying here. The fixed-pitch propeller drove the simple vehicle against the treadmill belt, faster than the (still air) wind. Why can't this be used to analyze the principles?
https://youtu.be/kWSan2CMgos?t=30
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