inertial speed (i.e. groundspeed)
This is true for aircraft physically connected to earth surface or using antigravity instead of aerodynamic lift. Earth is just another non-privileged inertial sistem. Once you slip its surly bonds you wouldn't care a bit about its position if it wasn't for the gravity.
Now everyone who thinks that airplane "knows" its speed and position relative to ground and its airspeed is threfore affected by turning downwind - think harder. If you think hard enough, you'll soon find the way of simply distinguishing gravitational and inertial force acting on airborne aircraft. Patent it as soon as you find it, there are gigabucks to be made by making entire gyroscope industry obsolete!
I apologise if I used some incorrect terms, but my science classes were not in english and neither was my ATPL theory.