It sounds like you are churning a lot of air, with very little thrust.

At the high RPM range you have that engine, it tends to produce little power.

Try trading some of those RPM's for thrust instead.

Try a 10x6 or a 10x7 prop. I'd also try a 9x8 for speed.

Note however with the 9x8 the plane will seem very slow and heavy when it takes off, but if you do a diving pass it will pick up a LOT of speed.


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I fly my Falcon 25 with a 600-700 watt electric motor @ 14.8v being pushed right to it's thermal limits.
My little plane is pretty fast this way and seems to have plenty of power.

As it is my small plane is pretty heavy already, so it lands really hot and I must use spoilerons for landings.