Which is better? 12x6 prop pulling 11,100rpm or 13x6 prop at 8,900rpm? My OS 56 alpha is spec for both. OS website specs 1.0hp at 10,000rpm and practical rpm range 2,400rpm to 13,000rpm. Using it on a Fliton Votec 351. Plane seems to be faster with the 12x6prop (but that could be my imagination cos its noisier too). But its so cool flying with the 13x6, I can see more trail of smoke from the plane! Comments appreciated.

There is no perfect answer to your question. You are within an acceptable range with either prop. Probably it is a little faster with the faster rpm of the 12x6. Theoretically, the plane moves forward 6 inches with each rotation of the prop (this is the second number on the prop). Since both of the props you tried end with "x6", then the one that spins faster will go a little faster through the air.

Change em out depending on your mood, that's what I do. I got an 11x6 for normal fast flying and a 12.25x3.75 for super vertical and a little hovering, and thats on my trainer.

Look for a 13x5 and try that too.

Different brands also work differently. Try other brands because they just might work better. They've got different blade area, airfoil, etc. and sometimes suit your airplane a bit better. Try the same size to start with. Different brands of 12x6 will work differently. Its the different areas etc that make it worth testing.

There is no magic brand or shape. But your engine/airplane combination WILL have a favorite.

I tried a 12-6 and a 13-6 on my Sea Fury w a OS 61 engine. I had shorter take off runs with the 13-6 prop but for everything else the plane performed better with the 12-6 prop. Since then I discovered an APC 12.5-6 prop which gives the best of both worlds on not just my OS 61 engine but also my Tower Hobbies 75 engines.

I would use the 13 x 6 prop. I do believe that it has more torque than the 12 x 6 that could be loosing performance at a high rev. number. A quicky plane could be better with the 12 x 6 but your Fliton plane is not designed for speed so the 12 x 6 prop will not give a sustantive gain. Fliton creates more aerobatic planes, then you will need a bigger mass of air in your surfaces so the 13 x 6 could be the best.

That seems like a big RPM difference to me between the two props, but since those are the numbers you provided I threw them in an online thrust calculator http://www.lcrcc.net/thrust_calc.htm and it spit out these numbers 12x6@11100=7.23 lbs of thrust and the 13x6@8900=6.4 lbs of thrust
Just looking at this it seems like the 12x6 is a no brainer, plane will go faster and have better vertical.

Which is better? What's your idea of better? They'll both fly the plane. Which ever one YOU like the best is better.
Paul

Prop selections depends mostly on your model, flying style and expectations. Changing props is like driving a standard shift in different gears. First gear gives snappy acceleration and fast response but low top-end speed. Third gear is poor acceleration but faster top-end. Props offer the same choices. You'll never make a Lazy Bee a racer no matter the prop and you'll never 3-D a Quickie 500, but there is a best prop for each for any pilot. Experiment.

Thanks for all your replies. The thrust calculator is really great, and proves the 12x6 is the best for power! But I still love the 13x6 prop, the plane is so quiet, not screaming like the 12x6. And somehow even after tuning for max rpm, the 13x6 leaves a nice smoke trail behind, hehe, must be incomplete combustion. Many thanks again.

p.s. Here's an on-board video of my plane using 13x6prop. Enjoy!

http://www.youtube.com/watch?v=PF1awEIot2A