RE: What is the max prop RPM?
That pretty much blows up the myth about 1/2A props going super sonic.
A Nelson FAI .40 turns a 7.2x8.6 prop over 30,000 which puts the prop about 650 mph. What was that again about props going over 75% of mach being no good? There might be diminishing returns, but this is the only way you'll go 200 with a .40 powered engine that we know of.