Originally Posted by
unmanned_droid
It would likely change altitude looking for more favourable winds rather than use a motor.
Yes but:
It should not need to "look" ....... global upper wind actuals and forecast are stunningly accurate [most Met. stuff is similarly accurate if H2O is not involved]. As early as 1970 a good friend earned an OBE for pioneering hitherto undreampt of excellence. There are several global models [including UK] which have been supremely good for 40 years. The last time there was a shortfall was Falklands time, when the model was essentially northern hemisphere, but it was soon extended. "Use the winds you found on the way here" not unknown!
In the upper air, a rise or fall of several thousand feet is not going to help much unless there is a great deal of shear, which in itself causes problems to lighter-than-air vehicles.