Oh, we can already build planes that use high-efficiency propellers (with efficient and lightweight gear-less axial flux motors) and soar above the clouds 24/7, using height instead of batteries and ultra-light back-ground solar cells laminated into a giant flying wing (or at least much more wing than fuselage, because the motors should be distributed to reduce demands on wing stiffness). A weight of less than 2.5kg/m^2 in the wing should be feasible with large, monolithic carbon fiber sections, joined by narrow engine segments. Overall it should be possible to stay below a wing loading of 5kg/m^2, so a 100m span, 1m chord 500kg plane gets about 18h out of 10km height, see https://www.rcgroups.com/forums/showthread.php?2601411-Is-wi... .