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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: IanB on December 30, 2021, 04:21:15 am ---Not necessarily. I could just put my feet on the pedals and stop them from turning. No need for brakes. --- End quote --- It will depend in what gear you are and your weight but if it can not lift you then that is a sort of gravitational brake not that different from friction brake. It will also an energy storage device since if wind power can lift the pedal up when it will come down it will release all that stored energy so your standing on the pedal will have no real effect. --- Quote from: IanB on December 30, 2021, 04:21:15 am ---Why do you introduce absurdly irrelevant things into the discussion? It is a tricycle. It does not fall over when stationary. --- End quote --- I see you did not get my analogy. The analogy was referring to being able to keep the speed at zero like trying to swim in a river against the current and maintain zero speed relative to ground. You may average around zero but it will me more or less that that at any one time. --- Quote from: IanB on December 30, 2021, 04:21:15 am ---This is against all experimental evidence. For this experiment we have an appropriately low gear ratio. One turn of the pedals moves the bike forwards 30 cm. It will be effortless to move the bike against the headwind. Nothing like 300 W required. Not even breaking a sweat. (Note: 30 cm/s = 1 km/h) --- End quote --- You did not do this experiment else you will know that it will be about 300W and not sure in what physical shape you are but I'm fairly certain you will be sweating. --- Quote from: IanB on December 30, 2021, 04:21:15 am ---Again, no experiment shows this. If it were true, the brakes would get hot and start smoking. But they don't. --- End quote --- Most brakes will handle 300W. I have not see any smoking brakes when going down hill either and there you also need to brake with about the same power level unless you like extreme speed. |
| electrodacus:
--- Quote from: Brumby on December 30, 2021, 04:52:09 am --- --- Quote from: electrodacus on December 28, 2021, 09:44:37 pm ---"extra thrust" ? You get that vehicle is only powered by wind ? There is no extra thrust and all the thrust as in the case of a sail is provided by the wind. --- End quote --- But the propeller is spinning. A spinning propeller pushes air. This is called thrust and it must exist in the local region around the propeller. Surely this thrust has an effect when applied to the surrounding air. --- End quote --- If propeller has no input power or if the input power is taken from the output power (always smaller than input) then vehicle will just slow down and not accelerate. Let me put this in a different way. You have a high power LED connected to a solar panel. While there is light from the sun the LED is nice and bright then you say I can continue to have light even when there is no longer any sun light reaching the solar panel because the power LED produces light (propeller) and that LED is powered by the solar panel (wheel on the ground). |
| lordium:
--- Quote from: electrodacus on December 30, 2021, 01:38:58 am --- --- Quote from: lordium on December 30, 2021, 12:32:45 am --- --- Quote from: electrodacus on December 29, 2021, 06:31:48 pm --- --- Quote from: lordium on December 29, 2021, 08:53:18 am ---... --- End quote --- ... --- End quote --- ... --- End quote --- The propeller is a wheel for traveling trough a medium instead of traveling on top surface of a medium like a regular wheel. Propeller is powered by wind and or pressure differential depending on vehicle speed. Since output power of the propeller is lower than input power from the wheel there could not be any net gain. The role of the propeller is to compress air creating that pressure differential where energy is stored to allow vehicle to exceed wind speed for a limited amount of time. ... --- End quote --- I draw a picture to help here... So there are 2 cases: A) the fan is NOT spinning (but wheel is, clutch?) B) fan is being spun from the wheel through some gearbox or similar so in case A the wind is pushing the vehicle, but then we turn the gearbox on and now the fan is spinning... The wind is still pushing the same, and now the fan also adds some to the vehicle. You say the power needed to turn the fan from the wheel and generate thrust is higher than just having the fan locked and not spinning (like a sail)? |
| electrodacus:
--- Quote from: lordium on December 30, 2021, 06:10:12 am ---I draw a picture to help here... So there are 2 cases: A) the fan is NOT spinning (but wheel is, clutch?) B) fan is being spun from the wheel through some gearbox or similar so in case A the wind is pushing the vehicle, but then we turn the gearbox on and now the fan is spinning... The wind is still pushing the same, and now the fan also adds some to the vehicle. You say the power needed to turn the fan from the wheel and generate thrust is higher than just having the fan locked and not spinning (like a sail)? --- End quote --- Do you agree that a propeller is less efficient than a wheel ? Propeller maybe a realistic 70% while a wheel can easily be 95% efficient. If you agree then why not just take the power from wind pushing the vehicle from the back wheel and outing that in to driving the front wheel ? Do you see the problem? The difference is that road is a solid thus not compressible and you can not store any energy. The only reason the propeller works is because you can store energy by increasing the pressure differential. If you where to replace the air (compressible fluid) with water (incomprehensible fluid) the it will be the same useless stuff as getting the power from back wheels and putting in to front wheels. So neither a gear box nor a propeller are magic devices they can only output less power than you put in so the thing that helps here is the air (compressible fluid). So all you do when you take part of the wind power from the wheel and putting it in to propeller is storing energy. |
| lordium:
--- Quote from: electrodacus on December 30, 2021, 06:39:55 am --- --- Quote from: lordium on December 30, 2021, 06:10:12 am ---... --- End quote --- Do you agree that a propeller is less efficient than a wheel ? Propeller maybe a realistic 70% while a wheel can easily be 95% efficient. If you agree then why not just take the power from wind pushing the vehicle from the back wheel and outing that in to driving the front wheel ? Do you see the problem? The difference is that road is a solid thus not compressible and you can not store any energy. The only reason the propeller works is because you can store energy by increasing the pressure differential. If you where to replace the air (compressible fluid) with water (incomprehensible fluid) the it will be the same useless stuff as getting the power from back wheels and putting in to front wheels. So neither a gear box nor a propeller are magic devices they can only output less power than you put in so the thing that helps here is the air (compressible fluid). So all you do when you take part of the wind power from the wheel and putting it in to propeller is storing energy. --- End quote --- We lose some power to spin the propeller, but we gain it (some?) back because it is now easier to move forward. like we decrease drag, so we can go a little faster using same power. I see it like we move energy around in the system, but the total is still pretty much the same (minus some loss here and here). We make something less efficient (spinning the thing isn't free) but make something else more efficient (lower pressure infront of us, woho here we come). so the total energy is the same for the 2 cases, but the speed isn't. How much difference? Now is where all the %'s come in. But I still see it as "possible" to go faster without "cheating". Or where did my thoughts go wrong? |
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