Quote:
Originally Posted by RedDevil
Oh boy. Bear with me. Deep dive!!!!
Air driven? You miss the essential part.
it has absolutely NOTHING to do with the speed of the air!
The only thing that matters is the difference between the wind and ground speed.
It should work on a treadmill in still air just the same as with wind on solid ground, or salt, or whatever.
The only thing that matters is a difference between wind and ground speed.The air resistance of the body is negligible in comparison to the pressure on the propeller. When the car is not at speed the wind pushes against the body (which does give way by rolling away) and the propeller blades (which start to turn against the wind, so they give way less quickly)Bear in mind that the drag on the propeller has a diffeerent dynamic than the drag on the rest of the car, because it is virtually moving slower (by slowly turning against the wind) and because its surface is much bigger.The second law of thermodynamics is not what you think it is, I fear:
In other words, the second law describes entropy in an isolated system. Here we have two entities that move relative to one another; these provide energy to the system, so it is far removed from an isolated system.
The only thing the ground needs to do is provide enough grip for the wheels to roll and be flat enough to not cause too much friction. Be it dirt, salt, ice, whatever. Tarmac and salt should do fine.
It has to, otherwise it could not push the wheels over whatever they are resting on.What about 'equal'? The wheels touch the ground, the propeller faces the wind; there are no other points on the car that endure outside forces.Ground velocity? OK, we're looking at the car now.
The prop does NOT push the air back faster than the ground moves.
It needs to move the air back SLOWER than the ground, otherwise the wind cannot push it forward as then it would require more energy to turn the prop than the wheels provide.
Because it is moving slower than the ground, it requires less energy to produce the same force the wheels get over a bigger distance.In a 1/2 ratio you can lose half the energy to all kinds of friction and still move at wind speed. That's enough margin for me.Not relevant Any decent prop will do. Blackbird's officially goes 2.8 times the wind speed; there's enough margin whatsoever.Again not relevant but it is directly opposed and equal to the force on the wheels. Pick a reasonable force to work with and do the math from there.Indeed it does nothing when the wind does not blow and the ground does not move.But the main aerodynamic forces don't apply to the car, but the propeller (which is turning backwards slower than the ground speed so still moving forward related to the ground, and still enduring pressure from the wind.You forget about the propeller turning backwards, which is needed to maintain pressure to move the car forward.The ground endures the wind at full speed, the propeller at maybe half speed, the car faces a headwind.Replace car with propeller and then you've nailed it!Yes, the wind and the ground are the two main entities and they move relative to one another.No! Then it does not work.
If the propeller endures zero wind speed there is no pressure on the propeller, no force to drive the car forwards, to push the wheels over the ground and to turn the propeller. It would come to a halt.
The propeller needs to move the air passing through at a speed just shy of the speed of the car. Then the wind can still put pressure on it, forcing the car forwards.
Yes, the wind and ground form a dynamic duo moving relative to each other.There's a margin to compensate for any bad effect from that (apparently).Fair enough, will do.
Assuming friction on the wheels, gears and car body is relatively low compared to the load on the propeller, there is very little power needed to move the car other than overcoming the resistance from the propeller being driven against the wind.
Not so much just drag but load on the blades of the propeller, and not so much friction as the force that friction provides to the wheels to make them turn.It obviously cannot provide more power to the propeller than the wheels provide. But then it does not need to, as it pushes the air backwards at a slower speed than the car is moving relative to the ground.
Power is force times distance, and the propeller endures the same force over a smaller distance. 
I too see a shortcoming, but not in the video.
Funny how easy it is to spot the shortcomings in others and how hard it is to see your own. A man cannot look beyond his own horizon, but easily see what behind the horizon of others.  |
1) the blue-wheeled car is 'mechanically' coupled to 2X4.
2) Blackbird is aerodynamically coupled to the impulse which pushes it, as well as the aerodynamic coupling between the prop. and the surrounding air mass.
3) there is aerodynamic slippage at both couplings, something not present in the blue-wheeled car/ 2X4.
That's what I was referring to.
While all energy from the 2X4 will transfer to the blue car, only a fraction of the wind energy will transfer to Blackbird, and from Blackbird's propeller to the air.
This puts the two vehicles into two separate worlds.
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The mass of Blackbird, with driver, will exhibit rolling resistance challenges not experienced by the low-mass model on a treadmill. Blackbird is already challenged with respect to energy capture through a viscous-shear fluid coupling, then we pile on the resistance of tires rolling on dirt.
Entropy is any irreversible low grade heat generated within a system which cannot be harvested for useful work.( it will show on an infrared thermograph ). The lakebed, car, and atmosphere is the system. Just like any car.
Until the ground velocity matches air velocity, Blackbird's entire body is a sail. It's a non-dimpled golf ball at sub-critical Reynolds number, with high separation, high drag. All of it.
You might be amazed at the increase in rolling resistance on dirt. And this is robbing energy from the system.( not represented on a treadmill ).
For Blackbird to go more than two times faster than the air it's in, the propeller will have to move air very fast to provide that kind of thrust.
We can't quantify anything yet, as to 'equal' or anything, until we get more data.
The propeller diameter determines the 'disc' area and mass of air associated with the prop., as a function of rpm, and pitch of the 'screw' on its helical pathway, as it pulls air through it faster than the air outside the disc.
Other forces the car experiences are cannibalistic losses at capture, every step of power transfer, and finally at thrust.
I can't address how much energy we can lose, as we can't so far quantify anything about the car.
My book on propellers is pretty clear that the choice of a propeller is an enormous decision and made only after much consideration.
The propeller needs to transfer a net amount of horsepower to the air which exceeds the road load horsepower of the car at the point where ground speed equals wind speed.
It would be good to establish an airspeed and a ground speed for Blackbird. It's very difficult leaving this open-ended. My training doesn't allow for it.
And remember, power is force, distance, and time, like lb-ft/second.