OK so you want to look at this with vehicle body as reference.
See the 3 examples below and let me know if you think any of the equations are wrong.
(Attachment Link)
That is a nice diagram, although there are some errors in the equations.
For the purposes of this thought experiment, the vehicle is assumed to be travelling at a constant velocity. The question being asked is whether there is any net power remaining that
could be used to accelerate the vehicle.
The first equation error is that F
M should be equal to -F
G. The two forces must be of equal magnitude but opposite direction, if the vehicle is to remain at a constant velocity.
The second equation error is the inclusion of the P
in term in the equation for P
out. The motor knows nothing about P
in other than the force F
G that it has to compensate for.
The third equation error is the sign of P
net. The net power remaining is equal to the power provided by the generator, less the power consumed by the motor, not the other way around.
With these corrections, P
out will be negative in case A), the below wind speed case. In other words the motor will be acting as a generator, as would be expected.
In case C), the above wind speed case, P
out will now be 4W. So a power P
net = 6W will remain and could be utilised to accelerate the vehicle.