ORIGINAL: DarZeelon
Spinning a 9x7 APC prop at 25,000 RPM would, according to the PropPower calculator, require 4.26 HP.
Can you please name the high performance .91 engines that you were referring to, which can actually make this HP AND be capable of spinning at this RPM?
Not that I have actually tried it, but a 1.05 cu.in. ROSSI 93R105 ducted fan engine is supposed to deliver 6.97 bhp at 23.000 rpm.
Rossi usually delivers what they promise, so it does not seem unreasonable that 4.26 bhp from a similarly built .91 engine should be possible.
/Red B.