Long-term flights demand I don't carry fuel to expend; that I fly an electric Cessna. Let's fly it just hard enough against the Venerean equatorial wind to stay aloft. That means, my plane will drift a bit - back into the shadow.
Say I fly so the wind against me is only 50 ms-1. My "450" constant thus becomes 112.5. My 300 m2 plane shall experience 506.25 N of drag; demanding only about ten kW to push against this wind. This wattage can use solar in daylight. As for night - I will be spending some nighttime for every four day stretches. Up to forty-eight hours, if it's a maintenance weekend.
Koski & Grcevich (p. 83) note that this looks like a ticket to a "blistering death". My Cessna would need batteries to keep up through the long night. Perhaps a fallback into the clouds, to raise the air-pressure.
The worse news: my plane even at thrust can carry only 16875 kN. This amounts to almost two tonnes up here. Increasing wingspan improves that but also raises drag, so energy-cost. Say 2100 m2; I am prepared to go up to 70 kW, and carry fourteen tonnes.
At the final pinch: hydrogen air-bags (note: these cannot be trusted to be HOT air, at night). Here we bring in Philip Kramer the biomedical expert in Koboldt's book (and before him, Neal Stephenson). They would warn against "enclosed ecosystems" against overheat - even without sunlight.
The flotilla's ambience up in 70 km is cold. But this air is also thin. Convection won't work like it would on the surface - or in the more human-friendly cloud-layer. 'Tis more like space.
If I am radiating the heat from within, I expect to be running that heat into conductive (metallic) plates on the underside of the plane. Retracting the wings for night-fly mode will reduce radiation. But at night, that's a feature and not a bug.
I recommend the drift option to anyone with days to burn around the day of the city-visit. A plane so designed should work for a minor-offence gaol. Also telescopes. And we shouldn't rule out lower-income permanent-resident flotilla employees who don't need to be using the central reactor's fuel, themselves, all the time - that is, commuters. [3/25 - 5/6 2020] Put 'em on hibernation.
More seriously, there might be some emergency preventing full power to the non-nuclear planes. An unladen plane can work at lower thrust and its own power for as long as mechanics need to work on it. Here, then: The Backup Generator.
BOOST 12/21 - originally posted last night 5:15 PMish; figured I could think this one over on this slow weekend day. ADDITION 12/30 - retracting wings at night is good for other reasons.
TEST FLIGHT 8/6/2020: the battery Cessna, May 2020.
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