At 1000 rpm or so, my airplane will taxi and get up to, what, 15-20
kts? But at double the rpm it will fly at 80-90 kts, though it would
take a long time to take off. Surely double the rpm produces more
than double the propellor thrust...or does it? Anyway, it seems very
nonlinear, that is, double the rpm and I get much more than double the
performance. Why is that?