Inertia can be thought of as a resistance to change the speed of an object.
To change the kinetic energy you have to apply a force and that force is due to inertia...

And yes this is a more complicated way to talk about ground speed, but it is not incorrect as was suggested earlier. When turning downwind the model has to accelerate in order to keep a correct airspeed.
I think everyone is talking about the same thing here, except maybe for goirish who almost had 70 years without being wrong