Pretty sure Maynard Hill cracked the problem on this. I know he set an official altitude record with an IC engine that was around 8k feet and made flights over 10k ft.
Actually Maynard Hill set a record altitude of 26,919 feet 42 years ago. It was that project that he invented the electrostatic wing leveler that he also used to maintain a fixed climb angle through the flight. That system measured the voltage of both wingtips, and used the difference in voltage to level the wings. Another sensor mounted to the tail was used to control pitch. The technology was going to be sold by World Engines, was advertised once and never mentioned again. Turns out that the US government thought it would be too easy to weaponize into cheap drones.
The sensor exploited the fact that there is a voltage gradient of about 100 Volts per meter of altitude. However to detect it you must first ionize the air and use a very high input impedance amplifier to sense it. This is easily done with a small amount of radioactive isotopes, in fact you most likely have 3 or 4 in your home right now. They look like smoke detectors and sell for ten bucks at Walmart.