I've seen questions regarding propeller aircraft and altitude and their interaction. But I'm wondering what the relationship between just the propeller and altitude (changing air density) is. How does the efficiency and ability of a propeller scale with altitude?
Real-life scenario which inspired this question: a two-bladed two-engine propeller system is being used to propel a payload on a balloon platform (i.e lift is handled by helium buoyancy, no minimum speed required to stay afloat). The propeller is only used to move horizontally slowly (< 5m/s). This occurs at 65,000 feet and the propellers were analyzed at these conditions. Now, the new altitude is going to be 85,000 feet. How does that affect the scaling and re-design of the propulsion system? The Reynolds number will of course be different for the same desired airspeed. Additionally, the system drag will be lower so less thrust will be required at the same airspeed. But if the required thrust needed was the same, how does that and other factors affect the efficiency? And knowing that the required thrust will be lower scaled to air density, what is the expected change in propeller requirements?
Addendum: can the same 65,000 feet propellers be used at 85,000 feet, or is a new design required?