Hi full wings
As I mentioned, the state of energy of the airplane remains constant. This does not mean that there are no energy transfers ocurring between the airplane and the air.
They occur. The airplane gives mechanical energy to the air. The airplane moves the air about it, creating a circulation around the wings, the reaction of which is Lift, Drag and aerodyinamic moment. Lift helps to maintain the state of energy. Drag does not. It would reduce it, but the Thrust from the engines counteracting Drag helps to maintain airspeed and thus the energy state constant.
The air is not giving energy to the airplane in the form of Lift. Lift is just a force (a much more misterious magnitude than energy, in my opinion). It gives energy to the airplane in the form of heat, vibration and others, but not in the form of potential or kinetic energy. These remain constant.
In the case of the glider in an updraft flying at constant altitude and speed, the air isn't giving potential or kinetic energy either. But it supplies the energy wasted by drag.
My conclusion is that an external supply of energy is needed for sustained flight. It can be energy from fuel or energy from rising air, or from a towing airplane, or any other. The amount of such energy is proportional to Drag.