General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
Kleinstein:
--- Quote from: electrodacus on December 27, 2021, 10:44:54 pm ---
--- Quote from: gnuarm on December 27, 2021, 10:37:01 pm ---I see what bdunham7 meant...
Ok, how about a 20 km/h wind? I don't even need to work. I just have to keep pressure on one pedal which I can do easily without doing work on the bicycle. No motion, no work. Do you agree with that?
I debated with myself over the stationary vs. infinitesimal speed issue realizing that either way you would find some inane point to argue about. 230 km/h head wind is pretty much reductio ad absurdum.
--- End quote ---
Yes 230km/h is absurd that is why I selected that speed. I set 1km/h for the bike so it is low speed but possible while keeping your balance then increased the head wind in the calculator until power required was equal with 300W as 300W can be done by a fit person.
Obviously power needed is not 300W else you will see electric bikes with 300W motor and 230km/h top speed.
20km/h (5.55m/s) head wind is no challenge as power need to slyly move forward will be
0.5 * 1.225 * 0.408 * (5.55)^3 = 42W so basically a breeze.
And yes you can just stay still by leaving your body weight on the pedal at just 20km/h head wind.
--- End quote ---
A very high wind speed will give very high forces - there is nothing ridiculous about that. The problem is with the wrong equation it gets ridiculous as it predicts too high a power at low speeds, like 20 km/h wind and 0.001 m/s movement relative to ground. 42 W of power don't looks so bad, but at the snails pace that would be still 42 kN if you calculate the power as force times speed.
I know the snails speed may not be so common, but things always start slow.
electrodacus:
--- Quote from: gnuarm on December 27, 2021, 11:16:30 pm ---
Ok, so if the user is stationary in a 20 km/h head wind, how much power is required to maintain this position? If you come up with any answer other than zero, you can't explain how a brake works. The brake can maintain this position into the 20 km/h wind while dissipating no power. A rider can do the same thing by simply standing on the pedal preventing it from rising up. One foot on the ground for balance, one foot on the pedal to maintain position. No power transfer. ZERO
You seem to have already agreed that this is correct. In that case you must agree there is no power transfer at the wheels for a stationary bike/car/blackbird and that the equation that predicts power at the wheels based solely on the wind relative speed must be wrong.
--- End quote ---
If there are no sort of brakes it will require around 42W. A brake will anchor the vehicle to the ground thus no work is done on the vehicle.
A rider with sufficient weight standing on the pedal will still be a form of brake. It is a gravitational based one but still a brake.
There is potential wind power that can not be used because of the brake but there are 42W available at 20km/h wind speed and that 0.408m^2 equivalent area.
The equilibrium state will be the bike being pushed at 20km/h relative to ground so that there is no more force on the bike and no potential energy relative to air but there is now a potential energy relative to ground based on vehicle weight and speed relative to ground.
Without any energy storage the bike can be between zero speed relative to ground if anchored to ground and wind speed if there is no friction loss so not anchored to ground.
The vehicle can be anywhere between this two speed directly down wind at wind speed relative to ground and zero speed relative to ground and at this ends it will have potential energy storage relative to ground or to air.
This kinetic energy while it is a form of energy storage can not help the vehicle get outside this speed limits if it travels directly down wind at all times.
This kinetic energy can be used if vehicle travels at an angle to the wind direction that is how a sail vehicle can exceed wind speed.
In case of blackbird direct downwind version pressure differential is used to exceed wind speed for a limited amount of time.
In case of blackbird direct upwind version elastic and or gravitational energy storage in combination with stick slip hysteresis is what is used to drive at any speed (limited by frictional losses) for any amount of time.
electrodacus:
--- Quote from: Kleinstein on December 27, 2021, 11:25:45 pm ---A very high wind speed will give very high forces - there is nothing ridiculous about that. The problem is with the wrong equation it gets ridiculous as it predicts too high a power at low speeds, like 20 km/h wind and 0.001 m/s movement relative to ground. 42 W of power don't looks so bad, but at the snails pace that would be still 42 kN if you calculate the power as force times speed.
I know the snails speed may not be so common, but things always start slow.
--- End quote ---
The 42W are not for that 0.001m/s but for the effect of drag due to a fluid traveling in the opposite direction at 20km/h.
The 42kN are valid just in theoretical world in reality you will not be able to drive at 0.001m/s as the wheels will slip due to this huge force so speed will become higher very fast so fast that your brain (or mine) will not be able to see the transition.
That is like saying no vehicle can ever leave from standing still as the force needed to move that vehicle will be to high.
Wheel will have a force at with it will start to slip and that will be the max force you will be able to provide at the wheel.
You can see the effect when you try to accelerate a vehicle to fast and wheel will spin.
Kleinstein:
--- Quote from: electrodacus on December 27, 2021, 11:41:12 pm ---
The 42W are not for that 0.001m/s but for the effect of drag due to a fluid traveling in the opposite direction at 20km/h.
The 42kN are valid just in theoretical world in reality you will not be able to drive at 0.001m/s as the wheels will slip due to this huge force so speed will become higher very fast so fast that your brain (or mine) will not be able to see the transition.
That is like saying no vehicle can ever leave from standing still as the force needed to move that vehicle will be to high.
Wheel will have a force at with it will start to slip and that will be the max force you will be able to provide at the wheel.
You can see the effect when you try to accelerate a vehicle to fast and wheel will spin.
--- End quote ---
You claimed the power would be the power needed to drive against the wind - I (and most others here) were in doubt of that. That is your theory that gives the wrong result ! I just enteres a very small speed and used the very basic force = power / speed formula for a mechanical movement.
IanB:
--- Quote from: electrodacus on December 27, 2021, 11:18:56 pm ---
--- Quote from: IanB on December 27, 2021, 11:14:40 pm ---What would you say if someone showed you a wind powered vehicle that moves directly upwind without energy storage?
--- End quote ---
That is not possible so I will show him where energy storage is. Maybe for this particular case I will ask him to take a video with a high speed camera to notice how the movement is not constant but it fluctuates as energy storage is charged and discharged.
--- End quote ---
That is the wrong answer. Maybe you would like to try again?
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version