Never seen this question before, talked to an old timer this evening that hasn't flown in about 20 years (well, maybe like once or twice a year each year since he was big into it 20 years ago) we were talking about balancing vs not balancing props, putting two prop nuts on four strokes (never heard that one before either, is that common?) and then came this new piece of advice, mount the prop to where it is coming up while the engine is approaching top dead center!? that is what he said that he remembered but wasn't sure if that was the way or not.. anyone know about this?

also the double prop nut came up when i told him about a mishap on monday where my prop came off of my .61 os on my yak mid flight! i had a safety nut on it, i guess that is what it is called, the thing that looks like a small aluminum nose cone? anyway that fell off and i had to basically dead stick it with the engine running, floated in really nice for such a hot flying plane! i have had so many mishaps with this yak it is one of the chinese made cy models or whatever you call them, but its not that big of a deal for what a good price i got it for, just a little more glue and some triangle stock and this thing is a superb plane. first the cowl cracked and fell into the prop and i almost lost it because of that, but the prop finally chewed it up enough for it to break and fall off and i flew it in fine, then the canopy came off and knocked one side of the elevator completely off! now that was a hard one to bring in! then the prop fell off, and today the firewall came unglued before i even got it in the air, thank the Lord that i noticed it while i was taxiing out onto the runway! other than a few small weak spots i have been very pleased with this plane! it is the same plane that nitroplanes sells but i got it cheaper at texasrcplanes.com! anyway, kinda chased a rabbit here talking about safety nuts and props falling off etc. i really would like to know what position to mount the prop in for optimal performance!

I try to mount the prop so that it will be horizontal when the piston starts coming up against compression. The idea is that, when the engine quits, the compression will stop the prop in the horizontal position and help prevent broken props in case of a bad dead stick landing. It may also protect the engine crankshaft not to mention making flip starting easier and safer.

Most 4-strokes have a double prop nut. It's just like a jam nut you find in lots of other applications. Usually the second nut is tapered and fits into a recess in the first or something similar to help them really lock good. The first nut can't loosen up because of the pressure created by the second nut. 4-strokes can backfire from time to time, especially if they aren't tuned well, and the extra security helps prevent the prop from flying off if this happens. My new OS .75-AX 2-stroke even has a tapered jam nut. It's probably the simplest, best all around way to lock a prop nut. You wouldn't want to use lock-tite since the nut has to be removed pretty often and things like safety wire or clips would be a pain too.

Another mistake I see a lot is not getting the prop nut tight enough. The crankshaft threads, the nut and the washer are all hardened so there is no reason not to put some real torque on it. After all, it's holding a spinning knife at 14,000 RPM.

so it will not help performance to mount the prop in any certain position?

I like to mount the prop so compression starts at about the one O'clock position. I tend to flip my engines by hand and its just easier to flip across the top and slightly downward.

When I flew 1/2 A airplanes, little two channel planes with no throttle, (you just fly til it runs out of gas), then yes, we did mount the prop so it would windmill to the horizontal. These planes had no landing gear and were landed on their belly. It was easy to break a prop if you didn't mount you prop that way.

As far as I know, there is no specific position to mount the prop to counter any vibration of the spinning crank or to counter any micro surge in rpm at the time of combustion. It would take someone way smarter than me to prove the math on that one.

Aerodynamically, No

The thory is that one should get a prop balancer and find the heavy blade and mark it, now when you mount your prop you bring the piston to TDC and the prop goes on so that it is parllel to the piston travel ... (heavy side down)

Im just going to try and scetch somthing up...

So the idea is that the heazy blade balance out the piston and there always opposit to each other if it works well thats an opinion becasue some say that when the prop is horozentil it makes it worse and it spends half it time horozonetle.. (i just balance my props...)

So with piston at TDC the heavy blade is the one with the red rectagle around it, I dont use this but i do knwo about the theory...

Steven

I have seen and heard references to mounting unbalanced props in a certain orientation to help with balancing the piston/rod. Most crankshaft counterweights really are not big enough. I'm guessing you would put the heavy side of the prop in line with the crankshaft counterweight, but that may not be so. I balance my props and mount them horizontally against compression like ChuckW suggests. That makes it easiest to hand flip start the engine too.
ChuckS

yeah i hand flip as well so i always mount mine to where it is almost vertical (probably one o'clock as well) so that i can flip through the compression easily.

I also mount at 1 o'clock

Just trying to explain the theory...

Steven

I like to hand-start mine too, but I place the prop closer to 2:17 position.

Don't mean to hi-jack, but I'm new to 4-strokes. Can they be hand started? My OS book says to run it up with a starter to get fuel into the engine and then apply the heater and fire away....

Any different / better techniques? I now have (3) 4-strokes that will be needing break-in time.

Thanks!