By this argument the earth cannot be a reference frame either, since it is doing "turns around a point" and is therefore undergoing nearly constant acceleration (elliptical orbit) - not talking about coriolis. The acceleration the earth undergoes is calculable and is analogous to the acceleration the glider undergoes.

Again, I think we mostly agree, I just want to make the important point that kinetic energy (as is motion) is purely relative. The glider can have simultaneously 100 knots worth of kinetic energy relative to the earth, and zero kinetic energy relative to another glider flying in formation with it.

A turn does require an increase in energy. An increase in total energy applied to the atmosphere is required to turn at a constant speed, which can be measured by the increase in load factor (1/cos(theta)). Induced drag increases, thrust required increases, more fuel is burned, etc.

Reference frames for the purpose of calculating kinetic energy or relative velocities are arbitrary and based on our interest at the moment.

Suggested reading: Relativity, by Albert Einstein, or any of a large number of books on the subject by other authors.