This is a question about the proper use of the word "thrust." Suppose a freewheeling prop is turning in the wind. Is the air coming out the back (downwind) side properly called thrust?
I understand the basic physics. I just don't know if thrust is the proper term to use for the air coming out the back.
I'm not a native English speaker and not an expert on aviation, but here's my ideas on this.
In case of a plane propeller, I think "thrust" refers to net force of the air on the rotating propeller, including the propelling forces (pushing the plane in the designed direction) and the drag forces (acting in the opposite direction). It does not refer to just the useful part of the force, it does not exclude the drag on the propeller.
When the thrust is in the opposite direction to the flow of air, it is positive. Propeller achieves this, in a simplified model, by increasing the incoming air speed, so the air speed behind the propeller is higher than the air speed in front of it. (When the thrust is in direction of the flow of the air, it is negative.)
A wind turbine is not designed to create positive thrust, since we do not intend to fly it. Instead it is designed to extract as much energy as possible from the moving air. The air behind the turbine is slower than the air in front of it. So the drag force is stronger than the propelling force, and net thrust is negative. In the frame of the moving air, the wind turbine moves through static air, and creates a wake of air that starts moving in direction of the turbine. This is like a plane propeller which experiences negative thrust, that acts to slow it down.
