Since I was a kid, I've just run my Cox engines with whatever seemed to get me the best RPM in flight.  But I was just reading a Sceptre Flight review on the Black Widow by AeroModeller from 8/74, and they were talking about propping to get the best horsepower in flight, saying that propping to get ~15.5K is the better technique, as the Black Widow makes its peak horsepower at 15.5K per the charts.  So that would be propping for about 14K static.  The article states that a 6x4 would be the optimal prop for control line.  This is totally different from what I've always thought, as I run the 5x3 Cox props mostly, and occasionally break out into the 6" APC props.

What say ye?

Mark

My dad believed in 6x4 props so that’s what he bought when I was a kid.  I gravitate to 6x3 or 5x3 for cox engines depending on use. I noticed that APC 6x3 seem heavier than cox or master airscrew. APC 5.7x3 seems really light.

Heavier APC seemed to do the trick for an old OK cub I have.

Issue is that best power in what flight configuration?  In racing or scale or free flight , prop for level flight.  If you want stunt performance, setting it up for best power in level flight would overload it in maneuvers.  So in level flight propping below the max power point is important.