I was just reading Model Aviation magazine, and there's a pic of a Decathlon. The caption talks about the setup, and says the owner is swinging a 12x6 Master Airscrew on an OS 46AX.

Today I tried experimenting with a Master Airscrew 12x5 on my 46AX. There was a VERY noticeable drop in RPM's (no tach today, but I get 13000peak RPM with an 11x5) from my Master Airscrew 11x5. I had to keep the throttle at or above 3/4 for decent flying capability. I burned through a tank of fuel in just a few minutes (I did re-tune for the larger prop), and my engine was HOT when I landed. Now I actually really enjoyed the extra thrust, but can not or will not put my (can't think of one good enough word to describe my AX) engine through that again, as much as I'd LOVE to fly my Dolphin with a 12x5 (Maybe for a few weeks with my new 55AX before the Decathlon is assembled; although I don't want to get spoiled!).

Am I missing something here? I'm running 10% Nitro, but I can't imagine 15% would make THAT much of a difference...

A 12x5 would be at the outer limits for the 46AX. Mine all seem to be very happy with an 11x6 MAS or APC. I prefer the MAS for the sound. But on my funfliers I do use a 12.25x 3.75 for the extra grunt and thrust for pullouts. It does drop the rpms but my throttle is constantly being played with on the funfliers anyway. I run 5% nitro in all my 2 strokers (.46-.91) and they seem to like it.

Curt

Beat me to it. I was gonna say 11-6. A 12- 4 might be fine for 3D??? An 11- 4 is good for 3D on a 40 soooooo?

Thanks; maybe I'll try a 12x4. It's pretty amazing at how much of a change one inch in diameter, or one 'degree' of pitch can make.

I really enjoyed how my plane climbed and looped with the larger diameter prop. Maybe I'll get one of those aluminum nuts instead of a spinner so I can make frequent prop changes; I have a flying buddy that likes to play cat'n'mouse at the end of a day, and I need at least a 5 or 6 pitch prop for that

I can't imagine that engine pulling a 12x6 very well. Sure, it may turn it but performance wouldn't be great.

The only 12" prop that works on that engine that I know of is the APC 12.25x3.75 or similar. That is mainly for fun-fly/3D plane such as a profile.

The .46 really works well with 11x5 and 11x6 props for most types of planes and types of flying. The engine really seems to love those. APC 11x6 is ideal in my opinion. I've seen smaller diamter, higher pitch props used for racing but that's about it.

I too think a 12 is too big for your 46....I'm not a big fan of Master Airscrew.....Stick with APC I think you'll like the perfprmance.

They must have really made a gutless design with the AX. I've had the 46 FX for 5 years now, and right now it swings a 14X4 APC, and it has plenty of grunt.

Edit - 11X6 Scimitar on that picture. But that's the plane it is currently on.

Try that 12-5 with a lower nitro fuel or a colder plug. The fact that it is hot tells me that that when lugging that prop the timing is too far advanced. Lowering the nitro or using a colder plug should give you more RPM and a cooler engine with that prop.

OK, I'll try it. I'm actually using a 'hot' plug, with 10% nitro (it's the same model plug that came with the engine, though not the exact same plug).

It should swing a 12x4 or a 12x5 OK. I'm flying a 13x4 on my OS 50SX.

A MA12x4 is a good 3d prop on a 46. The APC12.25x3.75 is another good choice but cost 3X the price.

David

What is an acceptible range of RPM to be considered 'OK'? I'll try to use a tach next time I'm at the field. It definitely 'swung' the prop; I could feel the difference when tuning as the plane was pulling pretty hard, and it was climbing, and LOOPING, very well. There was just a serious drop in RPM, and again, tremendous fuel consumption and extra heat, unless I should be flying much slower with a bigger prop? I normally get 13000rpm at peak, then tune down to about 12,600 by richening the needle valve. And I normally fly at about 3/4 throttle, full throttle for climbs, with my usual 11x5 prop, but the plane flies A LOT faster. Maybe I don't need to fly as fast with the bigger prop.

No... not at all. The .46-Ax is an EXCELLENT running engine with tons of power. I was just looking through the manual and 12x6-7 is actually one of the recommended props. I did not realize that. Still, my experience tells me that APC 11x5 or 11x6 is really, really good for most sport flying.

Wow I just read that too. I've been using an 11x5 'cause that was the size that came with my Nexstar, and it seemed to work really well. Maybe I've just been really easy on the motor, so I noticed a big change with the 12x5. If the manual says 12x6-7, I should be fine. Wierd, but I liked having the RPM's and the engine running cooler.

Don't give up on that 11x5 by any means and don;t hesitate to try an 11x6. Both are great all around sport props for that engine. My only recommendation would be to try an APC once someone is confident they are through breaking props on a regular basis. It will perform a lot better than a Master Airscrew. I also recently bought a few Graupner props after seeing Minnflyer and RCKen talk about them several times. I have one on my Skybolt right now and so far, so good. I would say they are on par with the APC's, just not as readily available.

It is good to experiment with various prop sizes, pitches and deigns. What works for one plane and flying style may not work for another. Just because the manual says you can run a 12x5-6, 11x6, 10x7 or whatever, doesn't mean you have too. Run what works good for your plane and they way you like to fly. Don't forget about ground clearance either. Sometimes that will determine what prop you choose.

Yeah I'll try some things this week, including some Top Flite wood props; I heard they worked well also.

Do you know the difference between the MAS K series and the G3 series? I wonder how much of a difference that is.

I just tried a 12x4 on my 46FX last night and had the same problems with the engine getting hot. My instructor suggested I stick with the 11-5 or his recommendation 11-6 on my engine. I did like the torque of the 12x4 but you are right, you have to give it more gas to overcome the lower rpms.

GaRCfield: Are you really gaining any benefit from operating that engine at 13,000 RPM besides noise? Even though engines are rated at a certain HP at a certain RPM, (1.6 HP @ 16,000 RPM for instance) that is not a practical RPM range for most engines. Yes, I know, you are not reaching 16K, however, I've found that there are some that just begin to operate well at that RPM, but the general use engines that we use, OS engines for instance, normally like between 11,000 and 12,000 RPM and will run on forever with those numbers.

I prop'ed my smaller planes with that RPM in mind and never had a problem with performance.

Just wondering.

CGr.

Like I said I use the 11x5 prop because it's what came with the Nexstar (which would have come with an OS 46 if I got the RTF) and I thought it has been working well. Also I've read here that people seem to think the OS 46AX and an 11x5 prop is a good match. As you figured, I knew I was well under that 16000rpm mark, so thought I was fine (I get 12,600 actually, just under 13000). I didn't know that 11000 to 12000 was a better mark to shoot for. I'll try a different prop with a tach and see what happens.

For people that have never drag raced or had an engine on A dyno this may get some gray cells moving.
An engine may be able to pull 10,000 rpm but the dyno may show you that it's time to shift at 8,000 where the peak torque is and you will get better times.
Just A thought about RPMs.
Just for grins toss on an APC 11X4 and see how you like it.
For racing the 10X8 APC seems to be the number one choice for that engine at my old club. For sport flying I have always used the APC 11X4 to 11X8 on that and other .46 engines with the 11X6 or 5 my number one choice.
My advanced instructor was A man that liked to find that perfect prop for each of his planes and it sort of stuck to me too. I spend A lot of time changing props on A new plane until it does what I like.

???

Im confused. I was always taught a .46 (40 sized) engine should have a 10x6, or 11x5 or at most a 12x4 (Presumably 9x8 if you want to go the other way).

Dont mean to hijack the thread, but am I running too small a prop on my .46FX?? (10x6)

To qualify this, I get PLENTY of speed out of my trainer with a 10x6, and vertical is GREAT!! (Wont hover, but it will climb almost unlimited in about 75 degree up angle, and it is a trainer after all..)

No, you are not running too small of a prop. Yes you could experiment with other sizes but the 10x6 is within the range of your engine.


If you are happy and comfortable with the flight characteristics then everything is just fine. As you advance in your flying, you may want to experiment with various props to see if there is anything you like better but there's no need to be concerned at all about the one you are using now.

OK so my running an 11x5 at 13000rpm on the ground is just fine then... right? Some people, both here and at my field, think that's a little fast. It's all I know however. I used a 12x5 and didn't like it (well, liked the climb, didn't like the lower rpm/speed). Should I try a 12x6 for more speed? It would bog the engine down even more though. Sorry if this is dragging on, I feel like I"ve heard different things now and am confused.

It isn't a problem at all. In my experience that is the "sweet spot" for the .46-AX. It likely unloads once in the air and is turning a little faster than what you get on the ground. RPM is good for tuning but ultimately the way the plane flys is what's most important. I know racing and other stuff gets a little more complicated but we're talking trainers and sport planes.

I do recommend experimenting with props at some point after a pilot advances though. Even if you don't find anything you like better, you learn how shape, diameter and pitch affect flight characteristics. This will help with prop selection on planes down the road.

Actually this is exactly what I was doing today. I like the 11x5, but was seeing what different characteristics I get from different props. I decided today that I'm not crazy about the 12x5, but I was playing with it anyway to see what it did differently. Flew with both; didn't have a prop reamer or drill bit to get my 11x6 on with though, and want to try a 12x6 as well.

Glad to hear 13000rpm is ok though. It was suggested I richen the needle a bit because of that (people said they saw a lack of smoke from my exhaust, only on high speed low passes when I climbed away. I do the pinch test though, and make sure there is plenty of an rpm spike when I pinch.