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| Mess with your minds: A wind powered craft going faster than a tail wind speed. |
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| Labrat101:
It's Christmas 🎄 maybe the answer is under the tree . Ask Santa if he can travel faster than tail wind ! Eat drink & think of something else . :popcorn: :popcorn: |
| Kleinstein:
Asking how fast Santa has to ride his sled to get all around the world in 48 hours is a nasty one: https://www.popularmechanics.com/military/aviation/a30317765/air-force-redesign-santa-sleigh/ |
| Domagoj T:
Here's a very rough design of a wind powered yoyo toy with an infinite string that uses the exact same principle as the previously described yoyo experiment. Note that the teal batman wings are connected to the lower side of the purple string and they magically* fold like an umbrella when they reach the rear axle and unfold once they go past the front axle. The wings travel front to back in regards to vehicle, but forwards in regards to the ground, this means that the wings always travel slower than the vehicle, so there is a sweetspot where vvehicle > vwind> vwings. That sweetspot depends on combined friction of the system and wing efficiency, but it most certainly is there, if friction is minimal. *that's a problem for the engineering department |
| electrodacus:
--- Quote from: dunkemhigh on December 24, 2021, 08:23:12 am ---I think the big problem for electrodacus is that at wind speed (and faster) there seems to be no relative wind to power anything. The only way to go faster than what's pushing is via some stored energy, ergo there must be some stored energy somewhere. Where? The only possible place, given that the prop is quite important, is the prop wash. Which seems reasonable if you haven't seen this thing work, and all the explanations so far seem to run up against the problem that at wind speed there is no push (or even pull), so there is no obvious way this could be working. So... how about an alternative explanation without the maths that no-one can agree on... ISTM the most important thing to bear in mind is the prop is rotating and pushing air back. For a given rotational speed, the air will flow backwards at some derived speed - the faster the prop goes the faster the air flows back. You could use this to push the vehicle at a given speed by altering the speed of the prop, so we could say that the prop turning at xrpm is equivalent to airflow of ym/s. It works in reverse, of course - push the vehicle and the airflow will cause the prop to turn proportionally to the vehicle speed through the air. So, back to wind speed and no push. But there is a push - the prop wash is effectively a wall (albeit quite flimsy!) moving at a negative speed relative to the vehicle and tied to the vehicle. If the vehicle is moving at wind speed, the prop wash is moving backwards at some speed relative to the vehicle and thus the wind. The wind can push against the prop wash to continue pushing the vehicle a little faster. In effect, the prop wash is an infinite length of the string we see with that yo-yo. By what powers the prop? It is still the wind because it is able to be pushing the vehicle still. With the prop wash moving backwards and effectively part of the vehicle, the vehicle is not at maximum speed relative to the wind, so the wind can keep pushing it. --- End quote --- What you are describing is the offset due to constant wind. Do you agree air is a compressible gas ? Else this will just not work. To remove air from equation you can just take the power from back wheel and apply that however you want to the front wheel of the vehicle so no propeller and more importantly no compressible fluid (air). Now try to imagine wind pushes the vehicle you take part of that wind power from the back wheel and apply it to the front wheel. What happens when you get at or above wind speed ? What powers your back wheel to be able to power the front wheel ? Same thing if you use a propeller under water so imagine Blackbird type vehicle driving at the bottom of a river so all is the same except air (compressible fluid) is replaced with water (incomprehensible fluid). Again it will not work so that vehicle will never be able to exceed stream speed powered only by the water stream. |
| electrodacus:
--- Quote from: Brumby on December 24, 2021, 11:22:34 am --- --- Quote from: electrodacus on December 24, 2021, 12:15:56 am --- --- Quote from: Brumby on December 23, 2021, 11:33:30 pm ---I draw attention to my earlier question: --- Quote ---Are you saying that energy is stored across the entire swept area of the propeller? --- End quote --- --- End quote --- Yes the entire swept area of the propeller acts as a barrier keeping the low and high pressure volumes separate. --- End quote --- How does that work? (I think this needs a diagram.) --- End quote --- Here it is. I posted this several times but people try to ignore it or say it is incorrect. Propeller disc is the red one in the diagram. P2 is the pressure immediately after the disk down wind and is higher than PA that is ambient pressure while P1 is the opposite so lower than ambient pressure thus the exact mechanism of pressure differential energy storage. Increase this pressure differential and you increased the amount of stored energy. https://en.wikipedia.org/wiki/Axial_fan_design |
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