The Reynolds number at very high altitude is very low. Here is an article about airfoil study for 60000 ft altitude flight. My previous airfoils are not very suitable in a small aircraft at 60000 ft, they require longer chord to be efficient. I made series of new airfoils for short chord and high altitude and ended up with the KS415/14.3.
altitude = 20000 m
velocity = 80 m/s
wing chord = 0.8 m (80 cm)
Re = 396331.94
M = 0.2711
Therefore it is beneficial that the airfoil used in this kind of aircraft is such that provides maximum L/D at low Re, here around 400000.
Here are some simulations:
Then some airfoils that I created:
More simulation at low Re, two conditions: 80 m/s at 600000 ft and 111 m/s (400 km/h) at 60000 ft:
Added case 154 m/2 (300 kts) at 60000 ft:
Of these, the KS415 exhibits the lowest drag. Here is the geometry of the KS415:
Here is a smoothed version of KS415/14.3:
And simulation for a Reynolds number range: