Would it be a good idea to have a go at dynamic balance by inserting weights in the prop driver, to offset the counterweight end ?

That would defeat the purpose of the counterweight, plus its two inches in front of the counterweight mass.

The spinner / propnut, two or three bladed prop, drive washer, shaft with counterweight, drive washer and reciprocating piston and rod need to be taken into account as well.

You have two rotating ends of different masses joined and separated by several inches of crankshaft of non uniform density.

I'd guess thats another reason why it's very difficult to make the counterweight effective, besides the difficulty with a single cylinder engine.

I vaguely remember the idea of making the prop-driver balance-able being tried with the 1960's OS 15 Max 3 glow.

OS didn't further pursue the idea.

Ray

They typically have a geared counter balancer mounted right next to the crankweb on the engine's crankshaft.
The problem with out model airplanes is having a prop and spinner on the front. every time you change the prop out , the positions is a little different. Plus the weights of the prop and spinner change too. so it would be most difficult to keep it in balance.
Then there is the nature of a single cylinder engine too. Everything is against you achieving a perfect balance.

The main culprit is the heavy cast iron piston. It is a reciprocating part. A counterbalance shaft can at best add rotary mass to help the rotary counterbalance. This can never balance out more than 50% of the reciprocating weight.

Also, the counter balance shaft runs twice crankshaft speed and absorbs a considerable amount of power.

These shafts are for secondary balance issue elimination. I thought this was not the case here.

I can see the merit of placing a heavy blade of a prop on the same angle as the piston at TDC but this is because of the moment arm or distance that the blade length can provide will mean that any amount of offset weight will be at its most effective.

With a prop driver the small amount distance or moment arm length will provide very little meaningful gain no matter what amount of weight you could apply.

The point with single cylinder engine balance is to try and cancel out out the linear travel of the piston and con rod with the circular motion of the crank and although you 'can' achieve 100% primary balance (at TDC and BDC or 0º and 180&ordm with just a standard crank web, but the secondary balance (90º and 270&ordm will be shot to ribbons by doing so - you will double the secondaries by achieving a perfect primary that way. Then there is the combustion pulse to consider that only exists at TDC, so the best most designs can aim at is something under 60% but even that lives only in text books.

The 'heavy blade' idea could be visualized like an inline three cylinder engine with the both the left (heavy blade) and right pistons (the real piston here) rising and falling together and the middle piston (the crank web) providing the missing 'balance' but all of that really needs the moving parts to have exactly the same weight, height, distance apart, acceleration etc and this is never going to happen if the moving parts all vary greatly as in a single cylinder model engine.

The fact that the pistons travel is not in the same plane as the spinning crank web will give rise to a rocking couple, and this same mechanism of rocking is far worse if you then view how great in comparison the prop driver is from the piston - get the prop end balanced wrongly and things can get pear shaped rather rapidly.

Iwould theorize that you could indeed get your 'three cylinder' balanced almost perfectly but it would only apply for one engine speed due to the differences mentioned before and probably why engine manufacturers have simply gone for more of an average across the usable rev range.

I don't have anything to say here other than it's been done before. Metkemeijer brother's MB40 F3D engine, some versions of Nelson F3D engine to name a couple. Keep in mind that these are 27+kRPM engines, making 2+kW. There are all sorts of balancing trickery out there, asymmetric, tungsten carbide slugs. The RC car engines have fairly hefty counterbalances typically, then you send it off to get tuned and it comes back with half the counterweight ground away and they run up to 35kRPM.

If you have a problem in the RPM range you are operating that can't be tuned out by increasing stiffness of the airframe/mount or selecting a prop of different weight/geometry, it might be worth a trial and error approach to improving engine balance. But, I would bother unless you spent a lot of time operating at the trouble spot.

Hi Greg,
A lot if those 'racing' techniques involve removing the offending imbalance rather than trying to add a counter balance.

A lot of the smoother running engines that I have heard about aim to reduce piston and wrist pin weight.

I have seen some rubber RC mounts that actually make the problem worse when their natural resonance matches that of the running engine!

Anyone for a flat twin Saito here?

Depends which one.

If it has a single throw crankshaft then you are trying to balance two pistons and rods with the counterweight, surely a single piston and rod of a similar capacity engine would be lighter.

Also the firing impulses are at 180 and then 540 instead of every 360 degrees. Still, they run fine.

Now if you are talking about the two throw crankshaft models then the pistons and rods balance each other, the slight rocking couple due to the cylinders being slightly off set can be mostly balanced by a counterweight, and they fire every 360. Beaughtiful.

Back to the OP.

The primary out of balance mode for a single cylinder engine is at engine speed and in the opposite direction. This is due to the counterweight not quite balancing the weight of the piston and rod at TDC and BDC, but being too heavy to balance the big end of the rod half way between.

To balance the primary out of balance moment would require a second shaft, rotating the opposite direction with a (smaller) counterweight beside, below or behind the main counterweight. Even then there would still be a secondary rocking moment.

All in all far too much trouble considering that our model engines run surprisingly smoothly all things considered.

I forgot to write that these specifically used counter balanced prop drivers.