The airframe you are mounting the engine on has as much bearing on what prop you choose than the engine itself. The guys on the Sundowner thread were "gearing" the engine for top speed at the expense of acceleration and vertical penetration.

The Sundowner in the thread you linked to is a racer. Race pilots aren't known for try to keep their stuff long, they just want as much speed as possible, and if they burn up an engine in the process... oh well. (Nascar engines last for one race before being completely rebuilt, right?)

As far as "Practical RPM" goes, yes, you can get 16,000 RPM out of those engines when they are turning a ducted fan (Which is onlt a diametr of about 3 or 4 inches) and yes, you WILL be replacing them often.

They will not, and SHOULD NOT turn a propeller anywhere near that speed. At least, not if you still want to use it in a year or two from now.

BTW, this is exactly why I tell beginners NOT to buy a tach. I've seen too many people ruin an engine buy trying to get 16,000 RPM out of one because "the Specs says it should do it"

There is one thing I teach all of my students. Tune the engine so that you see a slight trail of smoke behind it as you fly. When you do this it's running slightly rich. If you tune your engines to this point it's a good bet that the engines will experience a very long a productive life. It only takes one lean run to absolutely cook and engine and ruin it. I've seen guys on the line chasing that elusive "perfect" tune, only to get an engine too lean and burn it up in the air. As Mike pointed out, the racing guys know that their engines have a short lifespan, but they accept that as part of the sport. You may lose a few hundred RPM, but keeping the engine a tad bit rich will keep that engine running for a long time.

Ken

Well, I'm hoping my 12-6 prop isn't too small for this engine. I'm using it to pull my Hangar-9 Tango, and it seems like it has plenty of torque. Most of the time, I'm at 1/2 throttle, usually opening it up when I'm pulling out of a hover or going vertical on takeoff. I haven't had any problem through the last two gallons of fuel, and I only get smoke when I open it up. I tried to set the needle so it'll run rich, almost four-stroking and smoking constantly, but it always blows unburned fuel out the muffler and shuts down when I do that. So, I just set it so it runs and starts with good reliability, smoke or no smoke. I did the same with a 46AX, and it's on its second year and 8th gallon running an 11-6 without issues. Am I doing the right thing? Both engines are warm to the touch after each flight, so I don't think they're running too hot.

NorfolkSouthern

Norfolk,
That prop should be ok. It's a little lower pitch than OS recommends for the engine, but I think you'll be ok. Here's what I do to keep my engines running a bit rich. Tune it for peak RPM, and then richen it 2-3 clicks. If you do this the engine should leave just a slight smoke trail in the sky, and will be rich enough to protect the engine. As I said, you may loose a few hundred RPM. But the engine will last a lot longer doing it this way.

Ken

That's what I've been doing, Ken: Backing off a couple clicks to try getting as much smoke as I can. As for the prop, I may switch to a 12-7 or 12-8. I have the 12-6 because I'm trying to go for torque. Perhaps the higher pitch will do the same? I doubt the Tango has enough ground clearance for a 13, and I'm thinking that the added torque may cause it to cork-screw when ever I open up the throttle. Any opinions?

NorfolkSouthern

Lower pitch = more torque
higher pitch = more speed

Try a 12x7 and a 12x6 and see which one you like better. I doubt that they 12x6 is going to harm the engine. But an 11 inch prop is too small for the 55 AX.

Ken

Norfolsouthern...

Try a 13x4 on your Tango, you may like it. It will stay in the optimum RPM range and give a lot of "pull". You lose to end speed but you don't want that type of plane going too fast anyhow. I ran that prop on a 3D profile and it was a nearly perfect match. If ground clearance is an issue just put bigger wheels on the mains.

Like RCKen said, dial it for maximum RPM then back it off 2-3 clicks or until you get a 200-300 RPM drop and it will be fine.

I flew a 13x5 on the 55AX today; I really think a 12x6 will be fine. I probably should have been using one too.

Dear All,

recently broke the connrod on my 55 ax. Shards of the rod probably seized the piston. Repairing will cost more than 60% engine price so bought new one. Now that the engine is gone, I want to know of any mistakes we made. I am posting the pictures (two months out of use, so may be a bit "brown"). Can you guys make a diagnosis?

AMEYAM

pix2

ameyam, Try posting these pictures in the Engines forum:

http://www.rcuniverse.com/forum/forumid_114/tt.htm

Go there and start a new thread by clicking the "New Post" button

Mike, he really needs to show that to Bill but I really wanted to know what he is using for fuel and how much run time is on that engine. Just an FYI thing, I have glow engines that are years and years old with I don't know how much fuel run through them that don't look anything at all like that. Just an inquiring mind wanting to know??
Gene

I'm curious too. Not enough oil? Running too lean or too fast?

He did.

Yeah I thought I posted here but I guess I didn't hit 'ok' but I've got a long season and 12+ gallons through my 46AX and it is nowhere near that black on the piston. My guess is the engine was too hot, too lean, and improperly lubricated due to fuel starvation and kicked the bucket.

"Rode hard and put away wet."

Oly

what kind of fuel were you using?

This thread has been moved to the Engines Forum, so I am closing this one.