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Piston ring flutter - applicable to our aircraft engines?

A pilot friend drew my attention to this.

It is related to the recent high oil consumption thread

A google on piston ring flutter digs out many links – example.

Specific to Lycoming, I found this one but that talks about it in a different context: damaging the engine during a descent under a very low power.

This Lycoming PDF (which is their current service letter on the break-in of new/rebuilt engines) mentions it on page 4, again in the same context

I can completely understand that the top ring expands during the combustion cycle (due to the gap pressure) and in low MP situations won’t expand as much and will let the burning gases past it, to bung up the lower ring(s), eventually leading to an elevated oil consumption.

But that is a different idea from what the non-Lycoming links suggest e.g.

  • you get an elevated oil consumption immediately
  • you can damage the engine purely as a result of low power operation

On the former, that’s not the case for sure because I have done FL200 flights during which no high oil loss was found.
On the latter, I can’t see Lyco would authorise low power operation at all if it damaged the engine. There are some warnings about minimum 15" MP e.g. here

Also I was doing high altitude flights 2005-2008 and the engine was rebuilt in 2008 and no damage or any piston ring fouling was found, so it can’t be that simple.

Is anyone familiar with ring flutter in general and whether the conditions are applicable to our engines? It does seem a very old thing on which there must be a lot of knowledge.

That’s if it exists at all… there is no pressure on the ring except during the combustion cycle, and there is negative pressure on it when it is on its way back down e.g. the intake cycle. So the whole thing can be a complete myth, and looking at some aviation related online discussions from years ago I am not the only one who thinks this.

Which leaves us with only one remaining plausible idea: during low MPs, the muck in the burning mixture gets past the top ring and gradually clogs up the oil control rings. That would at least match my own data. It would also suggest following up a very high altitude flight with a topoff with some “detergent” oil.

Administrator
Shoreham EGKA, United Kingdom
I´d say ring flutter is the main reason for ring breakage and mostly experienced at “high revs”. This was not rare in motorcycle engines getting revved a lot decades ago. Clapped out ring slots could be the reason for breakage at high revs. So for sports engines the makers had to find better material for rings, especially the top rings, chromed for cast iron liners, or even molybdenum alloyed. High revs in aviation might mean 2500 rpm or more with piston sizes of 5 " . In this regard I wonder how turbocharged Diesel truck engines stand continuous hard operation on motorways and doing many, many thousand hours at that with no problems, with even bigger bores and extreme cylinder pressures. No lead mixed into Diesel fuel, by the way, to protect valve seats so Lyco should look here how they do it. Diesel engines are a species with only the very best of material used in valve seats and valves. And for detergent motor oil a Diesel type may be even better than an Aero W 100 or synthetic oil to keep the diesel – and aero – engine clean. Vic
vic
EDME

Top ring sealing against cylinder pressure is not done by pressure in the gap, it’s a self servo situation in which the cylinder pressure being sealed gets behind the ring around its entire circumference and pushes the ring into the cylinder wall. A static seal is also formed between the ring and the lower ring land.

I found this article interesting in relation to ring flutter and the differences in function between top, second and oil rings:

https://www.motorstate.com/TheGreatPistonRingControversy.htm

Last Edited by Silvaire at 09 Nov 03:06

Peter wrote:

Which leaves us with only one remaining plausible idea: during low MPs, the muck in the burning mixture gets past the top ring and gradually clogs up the oil control rings. That would at least match my own data. It would also suggest following up a very high altitude flight with a topoff with some “detergent” oil.

+ 1

FAA A&P/IA
LFPN

+ 1

So what do you think of the piston ring flutter theory, @Michael?

The MP at say FL200 is about 15" I think, and indeed one is doing max revs which for me is 2575.

But other versions of the IO540 go up to 2700 (certified) or 3000+ (homebuilts).

Administrator
Shoreham EGKA, United Kingdom

You NEVER want to be pushing the pistons and that’s basically what’s happening in a fast, steep power off descent.

Once again, this is all about maintaining proper POSITIVE pressure on the cylinder rings to ensure proper seating and sealing.

FAA A&P/IA
LFPN

You NEVER want to be pushing the pistons

Why?

The crankshaft and conrods are stressed both ways in normal operation.

The combustion chamber pressure is of the order of 100psi.

maintaining proper POSITIVE pressure on the cylinder rings

What happens on the intake cycle?

Administrator
Shoreham EGKA, United Kingdom

Michael wrote:

You NEVER want to be pushing the pistons and that’s basically what’s happening in a fast, steep power off descent.

But you can’t avoid pushing the pistons. Every compression stroke whether you’re at WOT or the throttle is closed will push the piston, as will every exhaust stroke.

Andreas IOM

alioth wrote:

But you can’t avoid pushing the pistons. Every compression stroke whether you’re at WOT or the throttle is closed will push the piston, as will every exhaust stroke.

I guess I wasn’t explicit enough : you do not want the crankshaft to be driving the pistons, ever (except start of course) .

Obviously, you need the cranks inertia to push it through the non-power strokes, but you still need enough pressure in the combustion chamber to be driving the piston and seating the rings.

When you’re descending fast with little or no throttle and/or gas you are in effect using the engine compression as a brake, just like big rig trucks going down hill.

Last Edited by Michael at 09 Nov 13:01
FAA A&P/IA
LFPN

you do not want the crankshaft to be driving the pistons,

What is the mechanical reason for this?

If the parts are strong enough to work one way, why aren’t they strong enough to work the other way?

Administrator
Shoreham EGKA, United Kingdom
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