91FX gives even more than 2.4bhp, but it is more essential to check the thurst I believe.
If you use the right pipe & propeller, which in other words means to settle with the right RPM
which is not maybe that high as 15000, after all 91FX is not that high-rpm machine as some
other .91 are. I heavily hesitate the sense of using propeller on .91FX or .91Heli which is smaller
than 13". Surely the engine will screem like the End-of-the-World is comming at 16-18 k RPM
but I wander how much this is uselful on this Particular Airframe; after all it might even not take
off easily, or even if takes off the induced drag of the frame might be too big for
such small diameter propeller so the speed-and-flight will be quite likely disspointing
because the thurst even at 16000 might be not enough; i.e. we are just talking again
and again abut how the Engine+Propeller match the Airframe...
As for .91FX here is what I get steadily repeated performance (nearly 3.1 bhp)
(a copy from
Engines: two OS.91FX just broken in, and two because preparing for a fast Twin.
Fuel: 11% Nitro, 3% Aerosave oil(synth, Fuchs), 17% Castor oil, rest methanol,
Temperature 24 deg.C, 45% hummidity. Static thurst by Pe's Calculator and
by Fish-Scale were giving very much same results (+/- 50 g). I used the
fish-scale because wandered what coefficient to place for G-sonic and it
seems that 1.2 as for ordinary APC is the right one. The RPMs are the
max-possible measured continuously within couple minutes while playing
with the needle.
1. OPEN EXHAUST:
APC 13x10 gives 10750 rpm, Static thurst = 5.7kg
G-Sonic 13x10 gives 11300 rpm. Static thurst = 6.4kg
2. RESONANCE PIPE(Macs) after being tuned for each(!) propeller:
APC 13x10 gives 11200 rpm. Static thurst = 6.2kg
G-Sonic 13x10 gives 12050 rpm. Static thurst = 7.15kg
The difference between the engines was in the range of less
than 50 rpm on the same propeller and varied while
playing with the carb needle to get to the max RPM possible.