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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


F2B: ST G21/46 diamond lap of piston ring

By Joe Supercool

Readers of this site may recall some problems I have experienced with my ST G21/46 F2B stunt motor. Nobody else seems to have suffered the indignities that have plagued me with this fine motor. All reports read along the lines “model X performing to F2B standard with a ST46 now purring its way around”. I assume ST means Super Tigre and not Stalker! So what form did my problems take?

Most common symptom was the motor starting off beautifully, then leaning out and refusing to break back rich, eventually leaning out to give a 9 minute run with the motor so lean that the model lands under power! Next symptom was the motor running nicely, but losing power whenever the nose was put up, as in a loop: a fatal problem for vertical eights and hourglass. Another symptom was the motor going too rich, then too lean, and so on, in a generally temperamental and frustrating way.

In response, I pulled out the tank 3 times, pulled it apart looking for blockages, changed the uniflow vent position, etc, etc. Tried new fuel, old fuel, very oily fuel, more nitro, new plugs: in short, nothing worked.

Now being something of a loser for some time now, I have been watching for a word that describes this problem: it is INTRANSIGENT. Once I reach the need to use this word, I am finally on track. It means that the solutions I have tried are not connected to the problem! How is that for intellectual honesty!

So now I am left with blaming the motor. Problem is, there isn’t much in an ST46 to go wrong: there are only 3 moving parts. I couldn’t really blame the conrod or the crankshaft, so it came down to the piston.

The ST46 has an aluminium piston with a single iron ring. I know nothing about piston rings, but assume they are made of some form of cast iron like meehanite. They are the very devil to remove, as they are likely to snap or take a permanent set in either of which case they are ruined. The original ST ring was actually found to be broken, as assembled by the factory. This was rather distracting, and sent me off trying rings from 3 other manufacturers.

Suffice to say, none of these gave me a good running motor, certainly not one that “purred its way around”. It appeared there were different ways of tempering the ring, and of making it truly circular. What shape it assumed after opening it out to drop in the ring groove, seemed to be an entirely separate matter.

Now to progress this diatribe, with the second last ring I tried, I thought I detected an improvement the more the motor was run: ie, it appeared that perhaps the ring required a lot of running in. So out of the model and onto my test stand for 5 hours running at the Bindoon light aircraft field.

Every 45 minutes I peeped in the exhaust port and noted the condition of the ring. There appeared to be a bright rub mark about 3mm long at the top of the ring, which moved around a bit during each run. Now when I say “at the top of the ring”, you need to note that the ring is only about 1mm thick, so that only .3mm depth of the ring was actually rubbing. OK, well Supercool is nothing if not tenacious, so I ignored this and continued to the end of 5 hours running.

Then into the model, to find nothing had been gained; the run was as bad as ever. Well, with the State Champs only a month away, and with another defeat at the hands of Peter White looming, something had to be done.

I recalled purchasing some diamond lapping paste about 4 years previously, at the Claremont Timber and Woodworking show. It was used for sharpening woodworking chisels, and at the time I thought it might come in handy for setting up my combat G15’s. It was cheap, Chinese, but mostly, it was there. Later I was to learn from Stan Pilgrim that it comes in various grades, right down to fine enough to act as a polish. I had no idea what grade my purchase was, but then ignorance was always my greatest strength.

So I stripped the poor Tigre down for what may easily have been its last overhaul before becoming a boat anchor. With the piston out, I was able to examine the ring a little more scientifically. The previously mentioned rub mark was present, plus two others, on either side of the ring gap. But that was all. The rest was black with carbon, suggesting that there was no contact with the cylinder wall at all. Not good; it’s not supposed to be like that.


Click to enlarge.
Photos by Grant Lucas...8MP Canon digital camera

Could this have been the reason for my poor motor runs? Only one way to find out. I took out a brand new ring, and replaced the bad one on the piston. I then smeared some diamond paste on the ring, and on the bore of an old ST46 cylinder, which was fortunately hoarded in my motor junk box.

With only a very few rubs of the piston up and down, the lap marks showed that this ring also was neither round nor square: ie, it was crap also. So I kept lapping, with piston rotation to help make the ring round. Within only a very few minutes, I produced a very nice, uniform pattern on the ring surface, which suggested that the ring was both round and square. Hopefully I had not damaged the ring groove with the diamond paste, but there was no guarantee of that.

Examination of the cylinder bore showed where the previous ring had polished high spots in the chrome. So in with the piston and some more diamond paste, and within seconds these spots were gone as well. The piston was then washed, with the intent of removing the diamond paste, and the motor re-assembled.

I gave it 30 minutes on the test stand in my backyard, and hoped the lady next door did not enquire as to why her washing smelt of castor oil. At this point Ian Thompson wandered in to pick up some team race props, and took a few giant lungfuls of the aromatic blue fumes to make his day.

Then into the model and off to the flying field at Whiteman park for some test flying. Does this story have a happy ending? Well, so far, the motor has been just superb, running the same way everybody else’s ST46’s run. At the moment, I am using my own 12X6 and 15% nitro to give me plenty of power thru the wind, with lap times of 5.7 on 65’ lines. Slow compared to the top guy, who seems happy at 4.6’s, but I am old and feeble, and if I can get away with slow, then I am content with that.

Now if I am not an orphan in respect of these problems, here is what I suggest. Rings appear to be very hard to make, and need to be lapped in by hand if they are to work properly. For a mere $50, I will lap in your ST46 piston ring for you. This will save you $10 for buying your own diamond paste. I will also drill a hole in the front of the crankcase, so you can get (expletive deleted) the gudgeon pin out easily.

Now, lets see how the State Champs shape up this time!

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