Hi all.

I'm questioning myself about moving on from a large prop to a small one on an already broken-in "taped bore" engine (maybe 1 or 2 hour running time). According to some local RCU experts, (it also sounds logical to me) that would dramatically increase max RPM and operating temperature, thus requiring a different piston / sleeve fit. Piston and sleeve would expand more, and therefore would create a bigger piston/sleeve "gap" as running-in has been carried out at low temperature (it wore more piston/sleeve material), thus harming top RPMs.
Do you think that that kind of engine wouldn't be able to reach the same RPM as if it had been broken-in first with a small prop ?

Thanks for your input !

You will not be able to notice the difference between the two.

If you want to run a smaller propeller then try it and see if the plane flies better or worse.

....even if the RPM's are higher, it will probably run cooler because the load is less.

FBD.

Yess, exactly what Flyboy Dave said
Bigger prop (more load), lower rpm, and the engine struggles hard and develops more heat. That 36 loves to rev.
I think it will be happier on an 8-8 than that 11something (if it weren`t an 11-4 that would also allow relatively high revs)

OK no problem then. I'll report figures asap /

I have to agree that you won't find any difference. I think too much is made of this "squeezy tight taper fit" because, no matter how the engine is run in, what's the next thing that's done? Bring it back to idle and play with the idle mix. Put it in a model and start it up and let it sit there at idle, taxi nice and slow out to the strip etc. In other words, a lot of an engine's life is spent at low throttle settings so running much cooler than where it was run in. Not to mention running it in winter and summer.

Hey downunder, that's true, but I know some guys who just seem to have forgotten the throttle channel right after take off
I personnaly run my engines WOT right from the start to make them reach normal operating temperatures asap...
We'll see what it gives in a few weeks [:-]

A lot of factors come into play with changing prop sizes to determine the best power output. For example, you have some engines where more output is achieved at lower RPMs. Then, there are the characteristics of the props themselves.

Generally speaking about props, it is true that a larger diameter prop is more efficient than smaller diameters. Looking at the bore and stroke of the engine could give some idea of whether it may like larger or smaller props. In the end, it always comes down to a fly off between your props, engine, and the airframe. Some people forget about the cowl or cheek area directly behind the prop, and that figures into prop performance.

Almost nothing can be learned while taching the engines on the ground as this has nothing to do with the environment of flying at some airspeed. You can measure hover power this way though.

I want to go fast, thus high revs are mandatory, even if efficiency is low...

It doesn't always work that way with high revs. You sometimes go slower that way.

One of the reasons for the high revs is to get more power out of the engine. If you get beyond the hp peak, things will go south in a hurry.

Everything has to be designed and coordinated together for speed.

I break in my glow engines at SCREAMING RPMs with very small props. I want my motor running at the manufacturers MAX rated RPM while it's breaking in. I also want it plenty rich. Not rich enough to 4-stroke, but rich enough that it's belching a lot of smoke and oil out the exhaust pipe. It takes a small prop to get those kinds of RPMs and still be rich.

I use 4 or 5 props as I break in an engine. Start with the smallest one and work my way up to the big one. Lean a few clicks per tank and keep the RPMs screaming.

I think it works. I once broke in a 60 size motor with the prop I intended to fly on it. I was a newbie to the hobby and thats what the guys at the field told me to do. That caused a LOT of load and heat on a brand new engine. That engine never did run very good. It started and ran, but it was no racehorse. I even went down to a 10-6 prop on that motor later on just to see if it would pick up RPM. It never would turn over 8800RPM, no matter WHAT prop you put on it.

Break em in screaming and rich. They'll always be powerful. Your mileage may vary.

Flame suit on.

Rcpilet ....

....people that have raced two stroke engines, like myself....know what you are talking about.

The others....people who read "the engines forum" in magazines will believe what they read.

That's just the way it is.

FBD.