General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
Microdoser:
It did take some head scratching before I got it, but I can totally see how it works.
It does require the surface it rides on to be moving towards the rear of the vehicle faster than the air though, which is where it gets the energy to move upwind. You don't get free energy.
This is why it works on a treadmill as well as with a tailwind. I imagine it would need a push start too because stationary props don't provide any thrust.
You do need a really really low friction gearing setup too.
electrodacus:
--- Quote from: gnuarm on December 21, 2021, 02:41:13 pm ---
Lol
I didn't read the whole thread. How long has electrodacus been insisting the car that goes down wind faster than the wind doesn't? How long has he insisted energy involves time and power doesn't?
I find his arguments mildly entertaining. The diagram posted seems to me to be a lot of fluff. Especially amusing is his stored energy argument to explain "impossible actions" when it is just poor construction at work. Then he says a better constructed model has "micro storage" of energy or something.
If the wheels are geared 2:1, applying a force to the 1 wheel by moving the belt it is riding on will exert say 1 unit of force in the direction of the belt. The gearing will result in twice the movement in the 2 wheel and half the force. The two forces are opposite, so the vehicle has a half unit of force pushing it in the direction of the moving belt with the movement relative to the ground being twice the velocity of the movement of the belt.
I'm willing to say I'm wrong when someone shows that to me. Happy to, in fact. But someone who doesn't understand that "rate of change" in regards to values like power and energy fundamentally involves time can't possibly construct a rational argument. While rate of change can refer to some other variable, in the case of power, it is the rate of change of energy wrt time. Power involves time. Energy does not.
This is such an insane argument! I won't argue with the guy because his arguments are so specious, yet I want so badly to help him understand. But I'm not going to chase impossible dreams. They guy clearly does not want to understand, so no one can help him.
--- End quote ---
I have never claimed blackbird is not going faster than the wind.
What I was saying is that it can only do so using energy storage and in case of blackbird that energy storage is the pressure differential created by the propeller.
A gear can not amplify power thus output power will always be lower than the input.
electrodacus:
--- Quote from: Microdoser on December 21, 2021, 05:43:27 pm ---It did take some head scratching before I got it, but I can totally see how it works.
It does require the surface it rides on to be moving towards the rear of the vehicle faster than the air though, which is where it gets the energy to move upwind. You don't get free energy.
This is why it works on a treadmill as well as with a tailwind. I imagine it would need a push start too because stationary props don't provide any thrust.
You do need a really really low friction gearing setup too.
--- End quote ---
There is no need for push start. The way it works is that it stores energy while below wind speed and uses that stored energy to exceed wind speed for a limited amount of time.
For blackbird including the small treadmill model the energy is stored in the pressure differential created by the propeller and only works because air is a compressible fluid.
If you where to replace air with say water (non compressible fluid) then it will no longer work.
I can build a vehicle that uses a sail add to it a energy storage device say a super capacitor and a generator/motor and have it perform the same (actually better) than blackbird or the small treadmill model.
electrodacus:
I need to bring back the discussion to what is relay important.
What is the available wind power to direct down wind vehicle.
I say that available wind power is defined by this equation 0.5 * air density * area * (wind speed - vehicle speed)^3
If anyone disagree with that please provide the correct equation so we can compare the predictions.
Kleinstein:
One may call the pressure that drives the prop forward stored energy, if one really wants too. However this a energy in a dynamic balance: loosing energy to drive the vehicle and to the less than perfect aerodynamics and getting new energy from the rear wheels.
The point is that the driven prop can get you more drag than a sail. It can do it a zero speed (e.g. no wind and not moving and thus zero drag for the sail) - this case is the abvious one I think. Similar it can do it when the vehicle is driving at the speed of the wind and thus zero relativ speed between the vehicle and the wind.
When starting at low speed the slow moving prop would still have some drag, not much, but it should be enough to get it starting. The prop would need not extra power and at low speed the prop would actually generate extra forward power and act as a windmill. It would need some minimum wind speed to get it going.
I don't think they needed the adjustable pitch during driven. This is likely just a thing of finding the best setting to get the highest speed. Once set right it should be OK to keep it there. The prop is not very heavy and not going very fast, as it is optimized to work at a relatively low relativ air speed. Remember it only sees the difference, so normally not the full wind speed. So the kinetic energy in the prop is low compared to the kinetic energy in the main vehicle. The model on the treadmil works with a fixed prop.
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