Originally Posted by eflightray
You have to stop thinking of standing still and watching a plane in a wind.
I think that is the key right there. We have to stop thinking that way, even though we actually are standing still watching a plane in the wind. From our stationary viewpoint, we are presented with the illusion that it slows down as it goes upwind and speeds up when going downwind. It's more than just an illusion. It really is slowing down and speeding up, but only if referenced from a stationary position.
If you were sitting in the plane, doing circles on a windy day, you wouldn't feel that speeding up and slowing down at all. It would just feel like you were doing circles on a calm day.