On 17 Mar, 16:16, Darryl Ramm wrote:

Gravitational potential energy gained by soaring (climbing in lift) or
by winching/towing/motorglider engine is the power source for gliding
flight.

So what's the power source when climbing in a thermal, when
gravitational potential enegry is decreasing?

If you take a 500m launch, you have 4,905kJ/kgof potential energy to
play with - no more and no less. As we all know, that's about enough
energy to keep a glider flying for ten minutes or so. For flights of
any longer, another power source is needed, and though gravitational
potential may be used as a store from time to time, it is only the
store, not the source.

Ian