RE: Cox .049 max RPM?
APC usually doesn't make a "specialty" prop that doesn't have any real world application.
The 4.2x4 doesn't really fit into competition anywhere, except for maybe some 1/2A control line racing event.
It's not enough prop for AMA 1/2A pylon planes.
It's just about the perfect prop if you build small, thin & light enough for RC speed. I hope these props never go away, but I sure don't see much evidence that they're even getting used much by the guys at this forum.
Jeremy Chinn is the guy who built the most serious and "pure" 1/2A Cox TD.049 speed project that I know of.
He called it "Caliente" and it did something like 123 mph. It had a high aspect wing with swoopy tips IIRC...might have been a V-tail or T-tail too..?
IIRC it was piped....[well, it HAD to be piped to be able to hit 123 mph if I had to guess how he did it].
That's all I remember about it...but that effort has to be a prime example of what mick1404 was looking for in this thread.
I think you could block off 1 of the exhaust ports and turn it into a transfer port. Raise the other exhaust port to acheive 180 degrees of exhaust duration and adapt a G&Z .061 full wave pipe.
Begin testing with a cut down 4.2x4 prop and 30% oil and the lowest compression that will allow it to run and hit resonance without getting hot. Gradually increase compression and load until you see severe plug damage, pitting on the piston and head ....of course you'll probably blow out moving parts regardless of how careful you try to be..but that's how I'll wager you get a Cox TD engine to survive as a piped speed engine long enough to make some recorded passes.
Jeremy would be the guy to ask about how he did it though.