I also noticed that when designing a plane, some planes appear as ready (can be produced immediately) while others need to be 'researched' (designed) first. This looks like a bug
And I think there is a bug in speed calculations - about 2/3 of the planes I design have the same 0 / 5000 / 10000 altitude speeds. These speeds shouldn't be equal, so this is likely another bug
I also find it hard to design airplanes with high maneuverability. The only way to increase it is to increase the wing area, and that tanks the range. So I don't know how to build high range high maneuverability planes. Any hints here?