i know you meant APC lol , but when i said use a 11x4 on the 39 i meant Master, and i will defend the 11x4 Master as a good hanging prop on the Cougar against anyone . the APC 10x4 is fine too, but its all revs, the Master 11x4 gives way more thrust, and coz its got less bite than a toothless granny you can use 11" not 10" if you were to use the APC.
for me the Master ONLY works when your hanging/post stall, as soon as you get moving the APC is better. i always use the 11x4 unless im doing climb and glide or plie-in race.
Masters are also good for combat coz they are cheap and last longer.

for scale, add one to an Extra or something and you'll get the realistic buzz LOL