Michael wrote:
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.
I still don’t understand.
Whether the throttle is at WOT or at idle, the crank still must push the piston through the compression and exhaust stroke. The only difference from the point of view of the piston is that if you’ve got the throttle at idle, there’s going to be a lot less force pushing down on the piston during the ignition phase, and slightly less work to do during the compression stroke. If anything, if the throttle is closed you’re pushing less hard on the piston during the compression stroke! And what about the exhaust stroke? There’s very little pressure in the cylinder, yet you’re inevitably pushing the piston. If having low cylinder pressures were so bad, then the exhaust stroke would wreck our engines in no time (piston being pushed, with very little pressure in the cylinder).
I’m sorry it just sounds like an old wives’ tale to me, especially given that so many training aircraft manage to go to TBO without piston ring issues.
They may go to TBO but maybe at the expense of 1 qt of oil every 3 hrs.
Aviathor wrote:
They may go to TBO but maybe at the expense of 1 qt of oil every 3 hrs.
But they don’t. For example, the C172 I learned to fly in at Houston Gulf flew 100+ hours a month – nearly all of it training flights with things like practise forced landings, always made TBO (except the one time that the camshaft spalled), and didn’t consume excessive oil, it was a few years ago but from memory 1 qt every 10 hours was typical. This was the notorious O-320-H2AD as well (where AD seems to mean Airworthiness Directive).
My own aircraft regularly flies final at idle power (in fact most fabric and tube vintage tailwheel types will do especially the ones that don’t have flaps), is used to tow gliders (which is a pretty harsh operating regime with lots of short flights) compressions are excellent, and I’ve put only 1 qt of oil in it since the permit revalidation in May (and I’m just getting ready to do the 6 month check and oil change with 35 hrs flight time since the permit was done.
I’ve never seen a 172 training flight that lasted > 4 hours @ FL180 ! 
Maintaining adequate cylinder pressure is a very real and well know phenomena as witnessed by the cylinder braek-in procedures that is called out by both Lycoming and Conti, ie. run it hard until the rings seat .
alioth wrote:
My own aircraft regularly flies final at idle power
Were talking pronlonged descents and/or flights @ >50% power
the cylinder braek-in procedures that is called out by both Lycoming and Conti, ie. run it hard until the rings seat .
Isn’t that a totally separate issue?
Were talking pronlonged descents and/or flights @ >50% power
Why would prolonged flights cause a “flutter” related problem, and not the many short periods e.g. when landing? That’s assuming there is a flutter issue at all…
Personally, I have know way of knowing if/when “flutter” is occurring, just that the phenomena is well enough understood by Lycoming for them to warn against operating conditions that allow it to happen.
My concern is very low cylinder pressure over prolonged periods .
The glider towing and 172 trainer see just as much WOT full power as they do reduced power descents.
alioth wrote:
But they don’t.
Maybe because they spend as much time at 100% BHP as with the throttle closed?
Michael wrote:
Personally, I have know way of knowing if/when “flutter” is occurring, just that the phenomena is well enough understood by Lycoming for them to warn against operating conditions that allow it to happen.
But is it? Do they have any actual hard data for this (outside of the run-in period) – can we even show their article is more than just conjecture? It seems like the manufacturers are just as bad at promulgating OWTs and pseudoscience as anyone else.
Why would it take a prolonged low power operation to cause ring flutter but not a short period? If low cylinder pressure during the combustion stroke is what causes the problems (combustion products leaking past) then surely it’s going to happen whether you operate with the throttle closed for 10 seconds or 10 minutes.
Outside of Lyco & Conti, the most experienced recip experts are the guys over at GAMI, George Braly and John Deakin.
George Braly signs his posts with: “It’s not how hard you run your engine, it’s how you run it hard” . I agree with that statement 100% .
Very low power ops and excessive “babying” of our recip engines does them no good and done over long periods bugger them up just as bad as running them over the top.
