ORIGINAL: rsmith0525
Hey guys, I maiden my SD 50 os55. I wasn't satisfied with the speed. Used a 13 x 8 then 12 x 8 prop, things looked a little more promising but my satisfaction level hasn't been peeked. The engine has only had 8 tanks put through it so I'm still in the breaking in mode. Thinking about putting a larger engine in her and using the os55 in the viper 500 when it gets off backorder. Any ideas for the SD for me?
Man, that is a whopping prop load for that poor engine! 12x8 is a big load even for a "big block" .60 - typical numbers for a good muffled sport .60/.61 on a 12-
6 are 12,500-13,000. Yeah I know the OS specs list it as a candidate.. which means it can turn it without overheating and run nice and quiet like people want these days and what engine manufacturers like to offer. But it doesn't mean you get maximum horsepower from those sizes.
My SWAG would point to something in the order of 10x7 - 10x9, maybe 11-7 or 11-8 tops if you want to keep the rpm down. Just because the .55 is known as a "torquey" doesn't mean it can swing .60-.90 size props and generate all its horsepower. Torque alone does not equal speed - but horsepower does. As a partaker of this forum I trust you are looking for speed, and if so you want to prop the engine closer to maximum horsepower in the air. Somewhere on this forum I bet there is more data to be found, but I can't find anything useful on the 'net in a quick look. I am going to presume the peak horsepower point for this guy is in the 14,000-15,000 range, someone please jump in and correct me if I am out of kilter.
Some numbers I saw quoted for the .55AX:
10-8, stock muffler less baffle, 13,600
10-9, ditto , 12,900
10-7, Ultra Thrust muffler , 15,500
None of these rpm numbers are excessive or even "high" so I see no harm in testing down to these sizes for the best candidate. Prop, engine, and airframe are a system, and IMHO you are overpropping the engine and hoping for speed but the prop is holding it back.
MJD