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| Veritasium wrong about faster than wind direct down wind. |
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| electrodacus:
--- Quote from: Kleinstein on December 10, 2021, 05:50:18 pm ---The fan blades going aroung in circles are a bit like the zig zaging or a sail boat. Doing it this way may be possible, but would require really good aerodynamics. It may work in theory, but not sure it actually works with all technical limitations included. The way it is done in the video is way easier and does not need a really high performance porp / air / foil: --- End quote --- When blackbird is at say at same speed as the wind all air molecules around the propeller (ignoring the pressure differential) will have same speed as the propeller thus they can not interact with the vehicle in any way. When above wind speed things are even worse as now air moves relative to propeller but from the opposite direction of travel so not only that can not help but it will be counter productive. --- Quote from: Kleinstein on December 10, 2021, 05:50:18 pm ---Even if the vehicle is morving at the speed of the wind, it can still slow down the wind and this way extract energie from the wind. The trick is using the fan. With no external wind a fan can produce air movement and this way a force on the fan. This is how a propeller an a plane works. On the vehicle moving with the wind the fan is used to push the vehicle forward. As the fan sees only a relatively low relative velocity to the wind, this needs relatively low power. The wheels are used to power the fan. As the speed is relatively high (about the speed of the wind), only a faction of the force available from the fan can produce enough power to drive the fan. The rest is available to overcome friction and speed up the vehicle. An alterntive way to look at it is from the point of view of the vehicle or in the treadmill experiments: the ground is moving in one direction and provides the power to drive the fan to push forward against the moving ground with the fan. This is similar to the case of wind turbine to drive a vehincle against the wind, just with the role of wheels and turbine switched. --- End quote --- Taking energy from the wheels means taking energy from vehicle kinetic energy that is directly proportional with vehicle speed. Before exceeding wind speed wind pushes the vehicle same as it pushes a sail vehicle so some of the power provided by the wind that way can be used for accelerating the vehicle and some can be used to rotate the propeller. If air was not compressible then powering the propeller will just be a waste of energy but since air is compressible energy can be stored this way by increasing the difference in pressure between the upwind and downwind side of the propeller. This increase in pressure differential can power the vehicle and allow it to exceed wind speed. The proof is simple just leave the vehicle drive until it gets to peak speed and the see how it starts to decelerate. Another proof can be made by going out in a day with no wind or large indoor space and push a vehicle to 6m/s or whatever the design speed is and the releasing it showing how vehicle will accelerate and so exceed the speed it was pushed at but then slow down as energy is used up. I will say this was sort of already demonstrated with the treadmill model. Ground or treadmill can not provide power to that model after the vehicle starts moving in the opposite direction. The pressure differential that was created by the propeller powered by treadmill when vehicle was held in place with the hand is what pushes the vehicle in the opposite direction that treadmill is moving. |
| Labrat101:
Take one extremely Hot Curry . 4 Large Guinness I guarantee you . Your underwear will travel faster than the Wind . :popcorn: |
| PlainName:
--- Quote ---When blackbird is at say at same speed as the wind all air molecules around the propeller (ignoring the pressure differential) will have same speed as the propeller thus they can not interact with the vehicle in any way. --- End quote --- This is what you are missing. The propeller is turning, so there is lateral movement relative to the air. Since there is movement relative to air, there is potential energy to be tapped. Thus, even when the vehicle is going at wind speed, the prop is STILL moving relative to the air and can transfer power. If an airplane is standing on a runway, how can it take off if there is no wind? That's exactly the same situation where the vehicle is moving at windspeed but the prop is turning. |
| Alex Eisenhut:
--- Quote from: dunkemhigh on December 10, 2021, 11:46:12 pm --- --- Quote ---When blackbird is at say at same speed as the wind all air molecules around the propeller (ignoring the pressure differential) will have same speed as the propeller thus they can not interact with the vehicle in any way. --- End quote --- This is what you are missing. The propeller is turning, so there is lateral movement relative to the air. Since there is movement relative to air, there is potential energy to be tapped. Thus, even when the vehicle is going at wind speed, the prop is STILL moving relative to the air and can transfer power. If an airplane is standing on a runway, how can it take off if there is no wind? That's exactly the same situation where the vehicle is moving at windspeed but the prop is turning. --- End quote --- I was about to ask how a desk fan can make wind since the propeller is not moving relative to the desk? Or for that matter, why does it take less time flying to Germany from Canada than the other way? Or if you build a Veritasium vehicle but reverse the gearing so the prop blows forward. Will it run slower than the wind or not? |
| electrodacus:
--- Quote from: Alex Eisenhut on December 10, 2021, 11:50:27 pm --- I was about to ask how a desk fan can make wind since the propeller is not moving relative to the desk? Or for that matter, why does it take less time flying to Germany from Canada than the other way? Or if you build a Veritasium vehicle but reverse the gearing so the prop blows forward. Will it run slower than the wind or not? --- End quote --- Big difference is that desk fan is not powered by that wind it creates. It has an external energy source. |
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