During a long testing session for RPM and thrust (7 different props in the 10" diameter range) with a ringed OS 50 I observed and made notes:

"All props lose a few hundred RPM as the engine gets warm."

In the 1 to 2 minute range the engine will lose a few hundred RPM which might be a reason engines are sometimes set lean. Normally I always set the high speed needle and back off 3 to 5 hundred and then fly. Usually I will fly a long up line and adjust as necessary. After the long up line adjustment the needle is almost never touched. I thought it unusual that the engine would lose RPM after it ran for a minute or two and have not noticed the same condition on any non ringed engines.

Incidentally I settled on a 10 1/2" X 6" APC prop at 5 pound 8 oz. thrust.

Bill