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.
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.
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
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
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.
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?
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.
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.
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?