General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
bdunham7:
--- Quote from: gnuarm on December 30, 2021, 11:04:48 pm ---The wind resistance is low compared to the force generated by the propeller driving the wheels because the angle of the blades is nearly parallel with the wind. The lift of the airfoil is what provides the force that rotates the propeller and therefore the wheels.
It is a very simple process once it is properly understood.
--- End quote ---
It is fairly simple, but you have it backwards. The propeller is driven by the wheels. If you posit the wind driving the propeller somehow, ED's theories would be right since the interaction between the propeller and the wind is dependent on the windspeed relative to the vehicle. The way it works when the vehicle speed is the same as the windspeed is the propeller reacts with apparently still air, neglecting a bit of turbulence, and that allows it to continue to push the vehicle even when it already at or above windspeed.
fourfathom:
Stored pressure-difference energy? I was looking up explosive shock-wave overpressure propagation, and even at only 1 PSI, the pressure-wave is moving about 70 MPH (about 700 MPH at much higher pressures). The idea that wind-pressure builds up behind a surface (and a surface that is slowly accelerating towards windspeed), and somehow this accumulated open-space pressure will persist for minutes, or even seconds, is pretty ridiculous.
gnuarm:
--- Quote from: Kleinstein on December 30, 2021, 10:13:43 am ---When going at a very low speed, that is low RPM on the pedeal on will not be able to get 300 W. The driver has the same problem as the vehicle: it is hard to get the same power, as the forces have to go up.
The problem why going 1 km/h again a 40 km/h head wind with a normal bicyle is not possible is not because the 300 W are needed, but because it is hard to produce even 30 W at such a low speed, as this needs very high force.
The is just not enough force to produce 100 W at a very low speed. So if the magic (..)³ expression would be true one would never be able to start from a stand still aginst any headwind. One allways starts with infinitesimal low speed and thus infintesimal small (essentially zero) mechanical power.
There is no need to repeat the supposed formula for the maximum available wind power for the moving vehicle. Without a good source or maybe an acceptable explaination this is worthless like repeating "The earth is flat.".
The (w-v)³ type equation is not only withput a good source, but in addition also proven wrong (see above).
--- End quote ---
Your assumption that power is required to accelerate the vehicle is not correct. At a stand still the force can be anything, but with velocity zero the power is zero. f = a • m, p = f • v
If the force applied equals the force from the wind, the net acceleration is zero and the velocity remains at zero so no power.
No fancy math required, just a clear understanding of how to apply the science and math.
gnuarm:
--- Quote from: fourfathom on December 30, 2021, 06:57:42 pm ---
--- Quote from: electrodacus on December 30, 2021, 06:43:45 pm ---It is clearly seen form the equation that I constantly post here.
--- End quote ---
Unfortunately, regardless of your claims your equation does not apply to the Blackbird-type vehicles, where there is a propeller connected to wheels rolling on the ground.
--- End quote ---
Mostly because it is the WRONG equation for figuring the power generated at the wheels. He can't understand how to think about the problem and refuses to learn because he doesn't want to have to say, "I was wrong". Rather he makes the rest of the universe, including real world tests, wrong.
IanB:
--- Quote from: gnuarm on December 30, 2021, 11:34:13 pm ---Your assumption that power is required to accelerate the vehicle is not correct. At a stand still the force can be anything, but with velocity zero the power is zero. f = a • m, p = f • v
If the force applied equals the force from the wind, the net acceleration is zero and the velocity remains at zero so no power.
No fancy math required, just a clear understanding of how to apply the science and math.
--- End quote ---
I really don't think that Kleinstein was saying that huge power is actually required to accelerate the vehicle. To the contrary, Kleinstein was pointing out how unreasonable such an idea would be.
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