As proven in practice ca. 1935.
Michael wrote:
You asked for my opinion, and it’s based on 20+ years of first-hand GA maintenance and thousands of articles, discussions and text-books consumed over those years.
I’m not trying to be ornery, but this is just how OWTs get started and end up being so persistent in GA – stuff related in articles that were written non-scientifically (has anyone investigated ring flutter in a typical NA GA engine on a test stand?) and then articles based off other non-scientific articles (i.e. it becomes an echo chamber) and when these things are said right next to phrases like “you should never push pistons” when a couple of seconds of thought will reveal that a reciprocating engine cannot work unless you push the pistons in every single upstroke, the OWT detector ends up getting pegged so hard it bends the needle against the right hand case of the instrument.
Thousands of articles notwithstanding, is there any actual primary data about ring flutter in GA engines produced in a scientific, controlled manner – not by people saying “hmm, I think it’s probably ring flutter”? Not opinion, or articles that don’t cite their sources, but actual credible primary data produced on, say, an engine test stand? GAMI blew away a ton of OWTs by actually doing controlled tests on engines and generated real data so it’s entirely possible that this data exists…but my google-fu just isn’t finding any actual primary data. Remember: the plural of anecdote is not data. Also despite many of the OWTs being falsified by controlled tests, they still often persist and are still often promulgated by highly competent aviation engineers with whom I’d trust my life – this is why your appeal to authority here isn’t very satisfying unless there’s a citation to an actual controlled test providing primary data.
achimha wrote:
Car engines are durable under constant high load as evidenced by the test stands.
And as demonstrated by Thielert/Austro.
Peter wrote:
Volume manufacturing is all about making things that work in practice.
Indeed. That Japanese learned that from the Americans 60-70 years ago but with generally a bit less creativity and more refinement. German engineering continues to reach for the sky (which is OK) and fall short on reliability despite high cost (which is not). I think simplicity is a better philosophy for GA, where the faults can’t be hidden and fixed at the dealer and the buyer is apt to be more hard nosed than a car buyer.
Don’t compare air cooled engines with water cooled engines. Car engines, being water cooler have way tighter tolerances because the temperature variation is slight. Aircraft and other air cooled engines require slacker tolerances so are more susceptible to problems.
Just a note to clarify that the words ‘clearance’ and ‘tolerance’ are not synonymous: Where operating temperature variation is wider, as in an air cooled cylinder, it may be necessary to specify a larger clearance at manufacture and accept that the running clearance will vary with changes in operating temperature. That is not the same thing as allowing a wide range of manufacturing tolerances, or manufacturing less consistently, with clearance that varies between examples at manufacture.
Silvaire wrote:
Where operating temperature variation is wider, as in an air cooled cylinder, it may be necessary to specify a larger clearance at manufacture and accept that the running clearance will vary with changes in operating temperature.
Remains me of the SR-71. On the ground the fuel pours out of it, but in the air at operating alt and temperature at MACH 3, it is all tight (or so the story goes). Anyway, huge clearances and close to zero tolerances.
I put this Q to a highly respected US engine shop, to a guy I know personally:
Have you ever seen engines with damaged top ring grooves, as if the top ring was wobbling around and widening the groove?
Answer:
Silvaire, you are right. I should have used the term clearance not tolerance. Without doubt clearances need to be bigger in air cooled engines over water cooled engines.
This The dynamics of second ring flutter and collapse in modern diesel engines appears to be relevant. The summary says:
“Second ring fluttering and radial ring collapse are recognized as having significant influences on engine blowby and oil consumption. […] this paper based on a modern heavy-duty diesel engine. However, the principles described are equally applicable to all engines.”
However, they want $25 to purchase the content.
There are enough google hits mentioning drag racing, etc. While I know nothing about that, I am prepared to believe it’s not a complete myth. But nothing shows up that’s applicable to any of our engines.
