Hi Bob,
I'm new to both Gas and Glow so take my recommendations accordingly... I haven't flown the 15x8 it's just a break in prop. I'll tach it and see what we get. I have about another hour to put on it before it will see the air. It still has a rough spot in the mid range that is smoothing out with the more time I allow it to break in. My DLE20 likes to be loaded for the mid range to smooth out so I'm expecting the FG-21 to like the load of a 16x8 to smooth out the mid range also...
If you're looking for more rpm I might try some 30%. But that's the extent of my real knowledge.
I'm still learning gas/glow and they are much different than the 1/2a pen baddlered GLH's we flew so many years ago.. lol
I did some more scale work on the P-47 and detailed the ordnance.
Here's the real WWII graffiti and how mine came out.
Joe