On 17 Mar, 18:33, Mike the Strike wrote:
All of the energy to maintain a glider in steady flight through still
air is derived from gravity - there is no other source of energy.

And how often do we fly in still air? For that matter, how often do we
build gliders on the top of hills, fly them to the bottom and then
abandon them. A very little energy may be stored as gravitational
potential, but gravity certainly isn't the source of the energy!

Ian