It's a good idea, in principle. But according to the sources I could find, to get enough gravitational potential energy to equal the electrical capacity of Zeph

It's a good idea, in principle. But according to the sources I could find, to get enough gravitational potential energy to equal the electrical capacity of Zephyr's batteries, you'd have to lift the entire craft by about 70km. https://www.aviationpros.com/engines-components/aircraft-airframe-accessories/batteries/press-release/12438400/amprius-amprius-silicon-nanowire-lithium-ion-batteries-power-airbus-zephyr-s-haps-solar-aircraft https://www.aviationpros.com/engines-components/aircraft-air... https://en.wikipedia.org/wiki/Airbus_Zephyr https://en.wikipedia.org/wiki/Airbus_Zephyr I can imagine you'd want to take advantage of gravity to a certain extent, but it seems a bit tricky to estimate how much. Presumably there's a particular altitude at which the aircraft's overall efficiency is at a maximum. Deviating from this altitude allows you to store a bit of extra energy (which means you can get away with a smaller battery) but you don't want to deviate too far, or you'll lose more energy to various inefficiencies than you're saving. And you probably need to keep some electrical reserve power anyway, in order to be able to actively navigate away from unfavorable winds.