The real small ARF --PBY's are a good place for 3 blades because efficient 2 blades do not clear everything.
Some scale 4 engine bombers force under sized props.
Cheat a little and spread the engines till the right props fit.
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Super Marine Spitfires went to 5 blades for carrier landing clearence and then finally counter-rotating props because of carrier landings and their special needs.