OK465, what wilyflier was saying, in essence, is that the perfect circle relative to the airmass does not change the forces upon the aircraft so it maintains its altitude. If you try to fly a perfect circle relative to the ground that's a different story.
Consider the forces acting upon the plane as well as whatever frame of reference you are using. (And if the airmass is in some way changing the inertia of the aircraft must be considered in your estimates of what is happening.)