my_wan
Graduate Poster
- Joined
- Feb 24, 2007
- Messages
- 1,074
You say: "Now if this wind is what turned the propeller the ejected air would necessarily be in the same direction as the air turning it, but it's not."
I agree that the air is not ejected backwards. But: That doesn't mean the wind isn't turning the propeller. The propeller slows down the wind flowing through it and that gives the cart energy to move forward. Right?
Yes, except that the wind is not what's turning the prop. If the prop is spinning against the wind so that it slows the wind down where does the prop get the energy to actively slow that wind down? From the wheels, not the turning prop. The wheels get the energy from forward motion of the whole craft, not from the turning prop. So yes, the wind is trying to slow down the prop, plus it's pushing the whole cart, which gives the cart forward motion. The forward motion turns the wheels. The wheels give the prop the power to turn the prop against what the wind is trying to make it turn. Therefore the wind cannot be what is turning the prop.
Once the craft exceeds ground wind speed then the wheels no longer have to force the prop against the wind because the wind relative the to prop is now working in the same direction. The continued force through the wheels now only has to add motion to the wind instead of working to slow it down.
If you were right and the wind would drive the cart to opposite direction: Wouldn't the cart initially accelerate backward instead of forward?
No, because the wind is pushing the whole cart, not just the prop rotation. The motion of the whole cart give the wheel torque which then provides the torque to turn the prop in the opposite rotation of what the wind alone would turn it. Once faster than the ground wind speed it no longer has to work against the wind to continue turning the prop that same direction.