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| Mess with your minds: A wind powered craft going faster than a tail wind speed. |
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| PlainName:
Edit: Sorry - removed a post after considering I might be being sucked into something I know little about. |
| electrodacus:
--- Quote from: Psi on December 10, 2021, 12:07:56 pm ---Whenever I look at attempts to disprove that blackbird can travel continuously downwind faster than the wind. I always come back to the fact that, if a sailboat can do it then how can it suddenly become impossible on land. Regardless of the design or implementation of blackbird, if it's possible on water, it should be possible on land. And if it is possible on land, how would it be done? I think the core of the issue is going back to first principles. What the difference between a vehicle going from point A to point B faster than the wind on water vs on land. Because it's accepted fact that you can do it on water without breaking any physics --- End quote --- If a boat can do this it is done the same way with using energy storage. sailing at an angle and not directly down wind allows you to increase the speed above wind speed and if friction losses are small enough like zero in case of ideal vehicle then you can maintain that speed even after you stop zigzagging. Blackbird drives directly down wind and that means it can not increase speed above wind speed same way as no sail vehicle land or water can exceed wind speed directly downwind. When vehicle speed equal wind speed there is zero wind power available and the only way to exceed that speed while only moving directly down wind is by storing energy before that point and using that stored energy to do so. Same is valid for directly upwind tho there it works fairly different still impossible without energy storage. You can not do this on water or on land (directly down wind no direction change) without energy storage. I do not claim that blackbird can not exceed wind speed significantly because it can but is only for a limited amount of time (as long as the stored energy will last). This is an electronics forum so if you know how an inductor works then propeller is a very good analogy since inductor is also an energy storage device storing the energy in the magnetic field around same as propeller stores energy in pressure differential that it creates. To be clear my claim is not that blackbird is not exceeding wind speed (that was demonstrated clearly in many forms) the claim I make is that energy storage is used to do so meaning that it can maintain above wind speed only for a limited amount of time maybe a few minutes depending on design and how low are the friction losses. |
| electrodacus:
--- Quote from: Kleinstein on December 10, 2021, 12:57:03 pm ---I can absolutely understand that one would have trouble with the explaination base on the sail going around the shaft. I think this is even wrong. --- End quote --- Yes that explanation is very wrong as the propeller is connected to vehicle and moves at the same speed so once at wind speed or above the propeller can not see any force from the wind. --- Quote from: Kleinstein on December 10, 2021, 12:57:03 pm ---However the case with driving the prop from the wheels is relatively easy - just get the difference between forces and power. This way one can avoid al the complication from aerodynamics and wing profiles. --- End quote --- The thing is that power out (propeller) can not be higher than power in (wheels). Above wind speed that power in (wheels) is provided by the vehicle kinetic energy so is like braking the speed will be reduced if propeller is not able to put all that power back. So ideal case where propeller is 100% ideal and there is no friction this vehicle will not be able to accelerate when above wind speed at best (ideal case) just maintain the speed. The stored energy in pressure differential is what actually accelerates the blackbird when above wind speed and so that means when that is used up it will slow down below wind speed. Propeller is nothing more than the wheel equivalent for traveling trough a medium as opposed to traveling on the surface of a medium for regular wheel. What helps in this case store energy is the fact that air is a compressible fluid. That is why you can not demonstrate this faster than wind direct down wind with wheels only on solid non elastic surfaces. All the wheels only demonstrations including the one in Derek's video are the equivalent of directly upwind and presented wrongly as directly downwind since people confuse the input with the output. I demonstrated fairly well (I think) in my video that a direct upwind vehicle still uses energy storage but is very different not needing air but energy storage device is part of the vehicle mechanism as in my example the belt was the energy storage device and that is charged and discharged multiple times per second. So any of those devices will show the same jerky movement due to multiple charge discharge cycles and with a high speed camera this is super obvious. Without energy storage and stick slip hysteresis none of those vehicles will be able to move upwind as they are locked gearboxes. |
| Kleinstein:
--- Quote from: electrodacus on December 10, 2021, 04:59:07 pm ---When vehicle speed equal wind speed there is zero wind power available and the only way to exceed that speed while only moving directly down wind is by storing energy before that point and using that stored energy to do so. --- End quote --- We are running in circles here. The video showed that it works (2 experiments) and give an explaination how, it just is not so easy to understand, and thus the title. The point is, that there is power from the wind available, even when moving with the wind at exactly the speed of the wind or somewhat faster. This does not work with a passive sail, but it does work with the fan. The energy storage idea to explain the experiment is just not working: both the large vehicle driving outside and the small one on the treadmill have quite some friction. There is nowhere enough stored energy to drive the vehicle for the time they where going faster that the wind or against the treadmill. The kinetic energy does not count here as it is not available to accelerate or keep the speed. |
| electrodacus:
--- Quote from: dunkemhigh on December 10, 2021, 04:40:03 pm ---Edit: Sorry - removed a post after considering I might be being sucked into something I know little about. --- End quote --- This is just made up to look complicated because people have little experience with propellers and air (compressible fluids). To make this work with math they just modified a formula. They used vehicle speed minus wind speed instead of the other way around and only looked at the case when vehicle is above wind speed. If they have tried to use that wrong modified formula below wind speed they will have realized they have a problem. By considering the energy storage problem is super simple but air is invisible and propeller is a bit magic (it is not). An ideal sail vehicle is 100% efficient meaning all available wind power can be converted immediately to kinetic energy and everybody agrees it can not exceed wind speed directly down wind and can also not travel at any speed directly upwind. To claim that without energy storage you can travel directly upwind or directly down wind at faster then wind speed is silly. Analogy in electronics will be a linear converter than can either get a higher voltage or current than the input source. You need an energy storage device either an inductor or capacitor to do that or both but can not be done with just resistor dividers. |
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