I have an OS .55 (broke in with 9x6) currently running an apc 11x7. I tried to run a 12x6 today, and the rpm's just didnt seem to come up to normal. Am I trying to put too large of a prop on this thing, or is the 11x7 the best it's going to get? My general understanding is if you up the diameter, decrease the pitch by the same amount you increased the diameter. Im just trying to get a little more torque for takeoff, im not really too concerned about the top end speed. The aircraft is a 4* 40, and im running 15% nitro 18% oil Byron fuel, with an os #8 plug. Any input would help as far as the airframe goes also, the best prop for the airframe..... yadda yadda yadda.......

Hey Spit,
I have a well used 4*40 that had an OS55 on for engine break in. I used an APC 11x5. At 4 3/4 pound it was fast and had unlimited verticle. It seems many are running this engine with huge props, but I don't get it. I tried a 12x5 and verticle and speed were both down from the 11x5 so I quit playing with props.

When I had the ST51 on, it flew best with the 11x5 also. The 12.25x3.75 was slow, but the torque was incredible.

Have fun, MikeB

Don't get wrapped around the axle worrying about what RPM's you should be turning. First of all, the specs that they quote are not real world performance. So don't even try to get to what you "think" it should be doing. Second, when you put a bigger prop on you are going to lose a few RPM's. It's basic, your swinging a bigger propeller and it's going to take some away from the engine.

Just tune it to run well, and then richen it up a click or two, and you'll be fine with it.

Ken

I'd just run the 11-7

forget running a 12-6 on a 55AX

Wow!! RCKen has a point here.

Noise does not equate to power! Jeech!! I say that because this engine will perform very well and more than you ever expected it to if you work with it. Set it so the RPM are no more than 11 - 12 K and that even sounds high. It has so much power at 10 - 11 K that properly set up, it will do more than you ever expected it to.

I have an OS 75 AX on a Goldberg Protege as well as a Goldberg Skylark 70. The Protege, a high wing aerobatic trainer, has so much power with a 13.5 x 8 Bolly prop that I almost cried when I broke that prop in a crappy landing (my fault). The Skylark 70 has a 14-6 and flys like it was on rails and does absolutely everything I expected it to do, and then some.

As Ken said, don't get to concerned about the spec RPM and power levels at that RPM. Prop it so that it runs well, and in that case, I would not exceed 11K or so RPM because, as I said, it will make lots of noise, but won't be doing what it was designed to do. Prop it up a tad and check it's performance.

Over time, I have collected a box filled with props. When I get to the point where I feel that I should expect more, I swap props. This will go on for several short flights until I get back to the prop that I selected in the first place, and that was one that did not get overly agressive in the RPM range, but was quiet, and powerful.

Head on over to your LHS (Local Hobby Shop) and pick up several props of different sizes, and work on it. Swap, fly, land. Swap, fly, land. Swap, fly, land. Sooner or later you will find one that works great and I would be willing to bet that the one you select produces adequate (if not more) power, at a lower RPM, and make quite a bit less noise.

CGr.;

I have swung an APC 13x4w on my .55 ax on a 40 size profile and it was balistic. So as CGR and Ken say do not worry about your RPM or the noise it's making. Tune it properly and fly it. Then play with props to get the performance you want.

Hi!
Suitable props for real world flying is a 11x6, 11x7, 11,5x6, 12x6, 13x4...All APC.
A 9x6 is way way to small for anything this engine is built for. Why on earth did you choose that prop???

I've ran a 12x7 on the .55-AX and it was great. I've also ran a 13x4. My favorite though was an APC 11x8 on my WM Rambler-45.

Yea, but if he wears a hat, you don't notice it

But he's right (as usual), while a bigger prop might reduce the rpm, you're pushing more air. So don't worry about the RPM, see how the plane preforms with that prop and use it or discard it accordingly.

PS, this is one of the reasons I tell people not to bother with a tachometer. I actually knew a guy who burned out a good engine by running it too lean - He was trying to get the 11,500 RPM out of a 12x6 that he was getting with an 11x7.

In a way, your prop is a personal thing. Try some different ones and use the one you like best.

I run my engines with different props and decided what I thought ran good etc only to see a guy at the field with the same type engine using some prop I considered to be no good. His plane flew fine.