General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
Kleinstein:
--- Quote from: electrodacus on August 30, 2021, 02:36:46 am ---
It can be any gear ratio the result will be the same as far as vehicle direction of travel.
2:1 will mean G wheel turns 2x in the same time M will turn only once but since also the force on G is half of that on M (again ideal case) Power at M and G wheel will be the same just opposite direction so vehicle can not move as with no delta in power you can not change the vehicle kinetic energy.
0.5:1 (1:2) means M wheel can spin 2x faster than G wheel but then force at M is half that at G thus again Power applied by both wheels is equal and so no change in vehicle kinetic energy thus vehicle id not moved.
All of the above is based on ideal case in real case power at wheel M will always be slightly lower than at wheel G due to friction and so vehicle will move from right to left if M wheel slips else is G wheel slips then vehicle will stay where it is and energy balance will all be wasted as heat at the interface between the treadmill surface and the wheel.
So if wheel M has perfect grip vehicle remains stationary while energy delta will end up as heat at the G wheel interface with treadmill moving surface.
--- End quote ---
The power is the same (actually opposite sign) on both sides only in the ideal stationary case. In the ideal case there is no extra power needed to change the kinetic energy as the speeds are constant. In the non ideal case the power on both side does not have to be the same. If needed to overcome friction or to accelerate the vehicle the power on both sides can be different (e.g. more power going in to accelecate, or more power coming out to slow down). The system has plenty of power (force exchanged through the vehicles times paltform speed) - so there is no real arguing about not enough power to increase the kinetic energy. The vehicle could move and still produce extra power, e.g. for lights.
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To calculate the stationary speed we don't even need to look at forces or power, just look at the velocities. The calculation was shown in a similar way before and is still correct:
The gear ratio (k) connects the speeds of the two wheels and thus the speed of the vehicle relative to the 2 platforms (V1 and V2). So as an equation V2 = k * V1. The 2 plattforms move relative to each other at a speed V0. So V2 = V1 + V0.
Do the math with the 2 equations, and one gets V1 = V0 / (k-1). This gives a valid solution for all gear ratios except k = 1. So the 1:1 gear ratio does not work as we have seen before.
With a suitable value for the gear ratio you can have V1 at nearly any speed you want: both directions ( sign of V1) and also faster or slower than the relative morement of the platforms. In a reall life situation very high speed ratios may not work well due to friction, but gear rations of -1, 0.5, 1.5 or 3 are not a problem. This would give you half and twice the relative speed of the platforms in both directions.
The equations show the the vehicle could move at the calculated speed to avoid slip or tearing the vehicle appart. The platforms will provide the necessary forces and power to make the vehicle move this way, as this is the path of least restance (only friction).
PlainName:
--- Quote ---Just make a test.
--- End quote ---
You posted a video of a test, which shows the exact opposite of what you say would happen. Why is that video not actually reality?
Come to that, have you tested your hypothesis? Where did you post the video?
Alex Eisenhut:
"Braking", not "breaking". Please. I beg you. Make it stop.
Brumby:
--- Quote from: Alex Eisenhut on August 30, 2021, 11:55:35 am ---"Braking", not "breaking". Please. I beg you. Make it stop.
--- End quote ---
That grates on me also ... but that is the least of the transgressions.
Brumby:
.... and for Pete's sake, can we please abandon these absurd, asinine, daft, foolish, futile, harebrained, idiotic, illogical, imbecilic, laughable, meaningless, mindless, pointless, puerile, ridiculous, senseless, silly and worthless discussions about two wheels and motor/generator function on the vehicle. There is absolutely NO relevance between these discussions and the mechanism which operates between a wheel and a propeller.
The propeller is essential to the mechanism and ANY discussion which excludes it is simply a waste of time.
Sheesh :palm:
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