I know I'm supposed to write about sailplanes and soaring. However, I spent a lot of my flying career driving single engine light planes just about everywhere and in just about every weather it's possible to do so. I also read all the blogs on Lets Go Flying which led me to think this subject might interest some readers.
This is about piston engines - you know, those noisy things on airplanes that blow cool air over the pilot affecting his emotional well being. Most young folks today, having better things to do, aren't spending their weekends taking car engines apart and putting them back together like I did in my misspent youth. This episode in my life taught me that piston engines are basically transducers which efficiently convert money into noise, heat and noxious fumes. Fortunately, there's a useful 'side effect' called horsepower.
Why is this important? Well, a piston airplane engine converts money into noise pretty much the same way a 1950's hot rod did. They work on the same 4-stroke "suck-squeeze-bang-blow" principal.
This is about the squeeze-bang part. To squeeze (compress) the air/fuel mixture properly, and get the power (bang) we expect, the pistons have to seal to the cylinders well enough to prevent leaks. If they leak excessively, your mechanic will wag his finger at you and your wallet will get lighter.
To help keep things air tight, pistons have "compression rings". (And one called an "oil ring" but that's not important here.) They really are metal rings about 5 1/2 inches in diameter and very thin. Two of them fit very precisely into grooves cut into each of the aluminum pistons. There is a roughly 1/8th inch gap in each ring so a mechanic can expand them in order to slip them onto a piston and to allow room for thermal expansion. Keep these gaps in mind.
Rings are designed to rotate in their grooves once every few minutes or so to keep them and the cylinder bore wearing evenly. We know they rotate because cylinders do, in fact, wear more or less evenly. Unfortunately, top and bottom compression rings don't necessarily rotate at the same rate which creates a problem. Not a serious problem mind you, just a low level psychological warfare kind of thing inanimate objects are wont to indulge in.
It seems that every so often ring gaps on one piston will line up and your normally smooth running engine will run noticeably less smoothly. Eventually, the ring gaps will rotate past each other and, with full squeeze-bang restored, the engine will again run smoothly - as if by magic. Each set of rings has its own mischief schedule so the frequency and duration are unpredictable - sort of like slot machine symbols. To be fair, light engine roughness can be caused my many things, but the cryptic "come and go" kind is often caused by rotating ring gaps.
It always happens at night, over water or any time you are beyond gliding range of an airport with nowhere to land. In short, anytime a pilot is really paying attention to the engine. It will always end just as you cross a shore line, the sun rises or flat Kansas wheat fields appear.
Generations of aviators are familiar with the phenomena and even gave it a name, "Automatic Rough". When the AR gremlin does its thing, idle conversations stop and pilots eyes will nervously scan engine gauges - which, naturally, will say nothing is wrong. Nonetheless, maps will come out and nearby airports will be identified. The course may be altered to pass near them.
A really nervous pilot may land and consult a mechanic who will run the engine then shrug while looking at the pilot suspiciously - AR gremlins hide when a mechanic is present. This pilot will go on his way a little less trusting in piston engines - which some will say is not necessarily a bad thing.
So, back to my usual subject, flying gliders - intentionally. If you know how to land without an engine, "Automatic Rough" will seem a little less threatening and you may deal with it more confidently - and mechanics may stop looking at you sideways.