RE: Thunder Tiger 120 Pro question.
Swill,
Your engines RPM number indicate the optimum prop to be an APC 16.5x11 (APC 16.5x12 -> repitched to 11). Another consideration would be an APC 17x10N, the prop chart shows it to be at the high end of the load, but still capable of 8400 rpm.
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Schpankme