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Building Tips and technical articles. => 1/2 A building. => Topic started by: Mark Mc on September 17, 2020, 06:04:24 AM
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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
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1: I've never run a Black Widow, but I've run a lot of reed-valve 049 engines. In general a 6x4 is way too much prop. Starting from a 6x3, planes go faster with a 5 1/4 x 3, not with a 6x4.
2: Experiment! The engine, plane, and fuel all feed into the "best prop" equation. The best prop for a scale Stearman with all the flying wires in place will be different than the best prop for a speed plane or mouse racer.
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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.
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Prop for max line tension during those overhead manuevers.
If you make a change, any change to the prop, the fuel, or the compression that forces you to open the needle...then you have just made a change that is developing more HP.
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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.
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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.
+1
Except for a competition-like free flight climb you'd want to prop it for that 7-second blast to the sky, not level flight.