General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
fourfathom:
--- Quote from: IanB on December 31, 2021, 09:14:44 pm ---Right, but the key thing is the motor the doesn't need any input power if the vehicle is stationary, because the vehicle is held in place by the gears that can't turn backwards. So from that stationary starting position, any tiny bit of power applied to the motor can make it turn forwards. There is no need for any "holding torque" on the motor to waste power as heat.
--- End quote ---
Yes and no.
My thought experiment posits a motor directly connected to the wheel, via gears if you like, but in a completely linear fashion where the wind can cause an un-powered motor to rotate in either direction.
But even with your one-way gearing, you will need more than "any tiny bit of power" to get the motor to turn. The motor needs to generate sufficient torque to overcome the (head wind) wind-force. Whatever your gear-ratio, there will be some minimum amount of power needed to make the motor spin. Otherwise the motor is stalled and power turns into heat.
PlainName:
--- Quote ---you will need more than "any tiny bit of power" to get the motor to turn
--- End quote ---
I think you may be picturing this in your mind, seeing pretty big gears or pulleys, and figuring you need a substantial motor even so. But perhaps those big gears are several orders of magnitude too small, and the stress may break gear teeth in reality but for the purpose of this exercise it's not going to happen.
thm_w:
--- Quote from: electrodacus on December 30, 2021, 06:33:30 pm ---Why can you put 300W and go at low speed when climbing a hill ?
Why do you think driving in head wind is any different in that regard?
The bicycle speed will be low but your gear ration can be set so that your legs move very fast thus even with low force needed at the pedals you can still produce 300W
There is a limit based on when the traction wheel will start to slip but 300W even at 1km/h is possible on a bicycle.
--- End quote ---
This was already explained to you, 1km/h is not possible at maximum power (~60rpm) on any common bicycle. Use the gear speed calculator link I provided to understand.
You can put hill gradient into the watt calculator as well if you are curious: https://www.omnicalculator.com/sports/cycling-wattage
Just please don't come back and say "100% gradient can't be done on a bike, the calculator is wrong!"
https://www.cyclist.co.uk/in-depth/682/how-steep-is-too-steep-when-cycling-uphill
thm_w:
--- Quote from: dunkemhigh on December 31, 2021, 10:27:24 pm ---
--- Quote ---you will need more than "any tiny bit of power" to get the motor to turn
--- End quote ---
I think you may be picturing this in your mind, seeing pretty big gears or pulleys, and figuring you need a substantial motor even so. But perhaps those big gears are several orders of magnitude too small, and the stress may break gear teeth in reality but for the purpose of this exercise it's not going to happen.
--- End quote ---
Their point is correct though, I believe.
You still need some minimum amount of torque. But the thing with gearing is, if you have a large gear ratio, then this amount of torque would be incredibly low. Which means, required motor and power input can be very small.
50km/h wind acting against a bicycle (0.5m2), wind load is 60N. 10000:1 gear would bring that down to 0.006N. Of course this is theoretical, no real system uses gears that extreme. The largest worm gear I see on mcmaster is 120:1
IanB:
--- Quote from: fourfathom on December 31, 2021, 10:09:20 pm ---Yes and no.
My thought experiment posits a motor directly connected to the wheel, via gears if you like, but in a completely linear fashion where the wind can cause an un-powered motor to rotate in either direction.
But even with your one-way gearing, you will need more than "any tiny bit of power" to get the motor to turn. The motor needs to generate sufficient torque to overcome the (head wind) wind-force. Whatever your gear-ratio, there will be some minimum amount of power needed to make the motor spin. Otherwise the motor is stalled and power turns into heat.
--- End quote ---
The distinction between torque (rotation force) and power is where this thread was coming unstuck before.
Suppose, for a moment, that we have ideal, frictionless gears. It's not going to happen in the real world, but suppose we have Teflon gears and roller bearings and whatever.
Now, suppose we wish to move our vehicle forward at 0.1 m/s against a 100 N force of headwind. We can calculate the required power as 0.1 x 100 = 10 W. So, if there are no losses in our ideal gear train, then the motor needs to output 10 W to achieve this rate of forward progress.
If, maybe, we only have a 1 W motor, then we cannot go this fast. However, we could go at 0.01 m/s, since 0.01 x 100 = 1 W.
How big the motor is determines how fast we can go, but if we just want to go at any speed at all, then we can introduce ludicrous gear ratios and make the motor as tiny as we like.
This is why I say "any tiny bit of power". In the real world, of course, some power is required to overcome the friction in the gears, and more gears will have more friction, so there is a law of diminishing returns. However, in principle, a wind up clock mechanism could make a vehicle move against a gale force headwind, albeit at a glacial pace.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version