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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: thm_w on December 30, 2021, 01:31:06 am ---Hey guys, isn't it absurd to think that one could move into 500km/h winds with only 1W of power at a speed of 0.000001km/h? That is clearly WRONG. The equation is WRONG. Here is a video of 500km/h winds destroying a car to prove my point. :palm: Its like you haven't heard of gear reduction or understand that taking equations to their limit makes the numbers seem "wrong" (as pointed out already). A racing bike shown is not capable of riding at 1km/h at cadence, the gear ratio simply does not allow it. --- End quote --- I think 1km/h on a bike should be possible if you have good enough balance. It is about 30cm per second so 1ft per second. But yes getting things to extreme is a good way to test if an equation provide correct results. The correct equation will not provide this insane results for 1km/h bicycle speed. If all you have is around 300W max power available then my equation correctly predicts that you can drive at 1km/h in head winds of 35km/h 35km/h + 1km/h is 36km/h round number as it is 10m/s So 0.5 * 1.2 * 0.408 * 103 = 244.8W will say that the rest to 300W is friction and rolling resistance. So if you have 300W available you can drive at 36km/h or a max headwind of 36km/h so (bicycle speed + wind speed) < 36km/h Nothing as absurd as 230km/h head wind and can be tested. |
| thm_w:
--- Quote from: electrodacus on December 30, 2021, 01:49:37 am ---I think 1km/h on a bike should be possible if you have good enough balance. It is about 30cm per second so 1ft per second. But yes getting things to extreme is a good way to test if an equation provide correct results. The correct equation will not provide this insane results for 1km/h bicycle speed. If all you have is around 300W max power available then my equation correctly predicts that you can drive at 1km/h in head winds of 35km/h 35km/h + 1km/h is 36km/h round number as it is 10m/s So 0.5 * 1.2 * 0.408 * 103 = 244.8W will say that the rest to 300W is friction and rolling resistance. So if you have 300W available you can drive at 36km/h or a max headwind of 36km/h so (bicycle speed + wind speed) < 36km/h Nothing as absurd as 230km/h head wind and can be tested. --- End quote --- It is 100% not possible on a racing bike with wind blowing in gusts. The highest gear ratio (53/39 + 25 cassette + 60rpm) puts you at 12km/hr. Try to pedal at 6rpm (10s per rotation) with any power. If you were crazy enough to build a high enough gear ratio, and have guides preventing the bike from falling over, it should work. Keep in mind, as pointed out to you already, moving 0 km/h takes ZERO power. So its a matter of interpolating after that point. 0.001km/h might take 1W say. It sounds "wrong" but its not. |
| electrodacus:
--- Quote from: thm_w on December 30, 2021, 02:13:29 am --- It is 100% not possible on a racing bike with wind blowing in gusts. The highest gear ratio (53/39 + 25 cassette + 60rpm) puts you at 12km/hr. Try to pedal at 6rpm (10s per rotation) with any power. If you were crazy enough to build a high enough gear ratio, and have guides preventing the bike from falling over, it should work. Keep in mind, as pointed out to you already, moving 0 km/h takes ZERO power. So its a matter of interpolating after that point. 0.001km/h might take 1W say. It sounds "wrong" but its not. --- End quote --- Not quite sure you understand. I provide you with a bike that has no brakes and put you in a head wind of 36km/h. If you input no power your bike will accelerate in theory up to wind speed 36km/h in practice it will be less than that due to friction. So in order for you to keep some speed around 0km/h relative to ground (exactly 0 will not be practical same as balancing something on the edge of a knife) but some arbitrary low speed 1km/h or 2km/h against the head wind will require 300W. In the other direction direct down wind with friction brakes enabled to maintain 1 or 2km/h the friction brakes will need to be capable to dissipate around 300W as heat. A bike that is not moving because is anchored to the ground has nothing to do with this problem as you basically become one with the earth and so that power accelerates the earth rotation witch considering the earth mass is just ridiculously low and nobody will ever care about that and so earth is just considered stationary. I provided this example before but if you want to stay stationary in a river you will need to output significant amount of power to swim against the current and maintain approx zero speed relative to ground. The lowest energy state is your speed relative to ground being the same speed as the current or air speed. |
| IanB:
--- Quote from: electrodacus on December 30, 2021, 03:08:32 am ---I provide you with a bike that has no brakes and put you in a head wind of 36km/h. If you input no power your bike will accelerate in theory up to wind speed 36km/h in practice it will be less than that due to friction. --- End quote --- Not necessarily. I could just put my feet on the pedals and stop them from turning. No need for brakes. --- Quote ---So in order for you to keep some speed around 0km/h relative to ground (exactly 0 will not be practical same as balancing something on the edge of a knife)... --- End quote --- Why do you introduce absurdly irrelevant things into the discussion? It is a tricycle. It does not fall over when stationary. --- Quote ---...but some arbitrary low speed 1km/h or 2km/h against the head wind will require 300W. --- End quote --- 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) --- Quote ---In the other direction direct down wind with friction brakes enabled to maintain 1 or 2km/h the friction brakes will need to be capable to dissipate around 300W as heat. --- End quote --- Again, no experiment shows this. If it were true, the brakes would get hot and start smoking. But they don't. |
| Brumby:
--- 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. |
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