Keith,

I purchased a 120SP last year and have run a gallon of Omega 10% through it on the bench, breaking in per the attached instructions. It idles nicely within the recommended 1400-1600 rpm range and I'm currently getting 5500-5600 rpm at full throttle with a Master Airscrew 18x12 Classic prop. Here's the puzzling part. I switched to a custom 17.5" 3 blade prop with adjustable pitch. I set the pitch @ 12, expecting to loose around 500 rpm. (I didn't have a 20x12 2 blade for comparison purposes, but expected the custom 3 blade to reach near the same rpm as has been advertized with the 20x12.) I can only turn it at 4400 rpm! I switched to a Zinger 18x12 3 blade, thinking that the custom prop might be a little heavy. I can only turn the Zinger at 4300 rpm. Am I missing something in the engine setup? For comparison purposes, what rpm should be expected from the 120SP turning the recommended APC 15.75x13 3 blade prop? It just seems like the engine should spin these props a bit faster.

My plane is heavy for a .60 size because I added all the goodies (i.e. rotating retracts, retractable tailwheel, glass and paint, etc.) plus the extra weight of the 120SP. It weighs in at near 13 lbs. I really want to use a 3 blade prop but, at this point, I'm worried about generating enough airspeed to safely fly it. Any assistance would be greatly appreciated.

Sincerely,
Joe