There's been plenty of discussion on here about the Blackbird as featured in this video. I get that it harvests energy from the relative velocity of air to ground. But I still can't wrap my head around it in the reference frame of the vehicle.
In that frame, the wind speed is already zero or backwards. Let's say we're at wind speed, so it's zero. So we have to give the air some net backward momentum. But the vehicle is still speeding up in this frame, hence gaining energy, so the energy of the air has to decrease to compensate.
If the air were a rigid body, we couldn't increase its momentum without increasing its energy. But since it's a fluid, we can decrease the pressure of a fluid element while increasing its average velocity, and if the pressure decrease is enough, its total energy decreases. Voila, we get both thrust and energy from the air. No problem there.
But my issue is that, if you just look at the propeller-air system, it's indistinguishable from a fan running in a room, because the free field is stationary in this frame. We know a room fan takes energy to run — sure, part of that is just due to internal friction, but I have to think it would run much easier in vacuum, from which I deduce that the fan is transferring energy to the air, not vice versa. And if this airflow is no different from that of the Blackbird, the air would also be gaining energy in the latter case.
So where's the gap in my reasoning?
Best Answer
That depends on your frame of reference. In the frame where the fan is at rest, that is correct. In a frame where the fan is moving forward, it is not. There the fan is slowing the air.
The direction of energy transfer depends on the frame you are examining. In the case of the blackbird, both the ground energy transfer and the air energy transfer are affected by this choice of frame, but the net energy transfer will not be affected.