New Cylinder AD's released by FAA

[Old Akro]

However, let’s also assume that one engine is always run in the cruise at a mixture setting that produces internal cylinder pressures of around 600 PSI, and the other is always run in the cruise at a mixture setting that produces internal cylinder pressures of around 1100 PSI.

Its not that easy. It depends on how close this is to yield strength.

[Creampuff]

I beg to differ: It is easy.

Detenation which has the most extreme px will damage the bearing first

I'm pretty sure pre-ignition causes more extreme pressures than detonation.

[andrewr]

Which of those two engines is more likely to have a cylinder failure first?

It’s not a hard question, and it won’t hurt you to answer it.

Your hypothesis is that reduced internal cylinder pressure reduces the likelihood of cylinder failure.

It actually IS a hard question to answer. You might assume that it is true, but you can't provide a definite answer without running a LOT of cylinders to failure. A proper answer for the question should include a p-value.

[No Hoper]

Creampuff
All else being equal one would surmise that the lower pressure would be more benign environment, but I suspect all else isn't equal

[Creampuff]

The answer is in this thread.

Please read Mr Atkinson's post at #68, twice.

[Weheka]

yr right, in my experience operating skydive aircraft, 470s and 520s, many, many thousands of hours, I never had a cracked cylinder. To be fair nearly all the engines were factory remans. Always carried power in the descent i.e. 2300 rpm 23" when down to certain height, straight in approach reducing power as required.

If people are cracking cylinders in skydive ops then they need to look at how they are managing the engine.

[andrewr]

Please read Mr Atkinson's post at #68, twice.

I have. He outlines which you would expect to fail earlier, but do the actual results agree?

What is the p-value?

[Creampuff]

Do the actual results agree?

Yes. They are all piled up in yr right’s hangar.

What is the p-value?

I have NFI but, given the assumptions of the question, it makes NFD.

[Weheka]

You can drag a horse kicking and screaming to water....but

[yr right]

As I've said before there are pilots and there are operators. If you abuse the engine it will not last. The myth and that's all it is that lop is a magic fix for all is just that. If it wasn't no engine would make o/h at all with execption to a lop engine and this is clearly not the facts. Cly press is not the cause of a head failure at all. As I said before. Learn se basic engine knowlege on any hangar floor. Yes but a lot of meat bombs don't do a power decent. By adding power you are not getting shock cooling across your engine. And factory o/ h engines have what. New cly as standard.

[Weheka]

Well as far as I was concerned heat was my enemy, so on operations, hard climbs and descents, up to 27 per day, 1000 + hours per year I used a lot of fuel. With the TO rpm at 2700 (noise abatement) full throttle I liked at least 26gph, rest of climb always on the rich side of rich, didn't like cylinder head temps anything over 380 and that seemed to work well for me.

However when I was ferrying or cross country I ran LOP, was amazed how low the cylinder temps were while burning a lot less fuel. My speed was also reduced so maybe I didn't save much at all in the scheme of things, probably needed more coaching in LOP operations. Anyway I liked it, smooth running engine, low cylinder head temps, great!

[43Inches]

I have never seen a cracked cylinder due shock cooling on a training aircraft. We had a few cracked pots, all were due to metallurgical, manufacturing or installation defects, granted high temp variations can aggravate these issues. All our aircraft were treated with large high to low power variations during engine failure practice, from small 100hp engines to 350hp turbo engines. It definitly was not good for the engine life, but I disagree it causes cracking, otherwise just shutting an engine down after each flight would crack it.

If cylinders are coming off engines it's generally not something the pilot instigates unless he's doing something really exceptionally bad.

[Jabawocky]

43"

Exactly

[Aussie Bob]

I am thinking that shock cooling is a myth in some aircraft. With an engine monitor I see similar temps on descent with throttle closed that I see with a good dose of descent power.

That equates to a drop from around 370 to around 300 regardless of power used, excepting remaining at cruise. The biggest shock cool is always at engine shutdown.

[43Inches]

I think it's the opposite maltreatment of engines, from low to high power when the engine is too cool that causes the most damage. Applying full power from the end of a long glide or having done the incremental long cool down of your Navajo, then suddenly applying full power such as in a go-round. It's better to keep the temps up and cool down taxiing in once on the ground prior to shut down.

The other thing that will do a lot of damage is rapid throttle/rpm changes that throw the engine balance out. I would add excessive vibration to temp and pressure as engine killers.

[OZBUSDRIVER]

By 1918 Heron and Gibson had designed, manufactured, and tested cylinders that consisted of open-ended machined steel barrels with an external thread on a portion of the upper end and a mounting flange on the lower end. The finned cast aluminium head, which was fitted with valve seat inserts and valve guides, was internally threaded. The pitch diameter of the internal thread on the head was slightly smaller than that of the external thread on the cylinder barrel so that the head had to be heated in order to allow assembly. This resulted in a joint that was mechanically secure at the cylinder's operating temperature and provided the best escape path for waste heat. In concept, if not in exact detail, the modem air-cooled cylinder had arrived, but not everyone was ready to accept it, possibly because of the “not invented here” syndrome prevalent in some companies.

Modern technology!

Still interested in finding a link to actual assembly of a modern cylinder. I dare say, little would have changed. From this snippet it would appear the entire barrel enclosed by the casting becomes an interference fit...including the threaded portion.

It may be a long, long time ago for me since I graced the halls of the Mechanical Engineering department of the old QIT but I still get curious about how things work.

[cockney steve]

Which brings us back to my intuition-led hypothesis.
the thread acts as a physical restraint on the head, during the low-power warming-phase....we've already established the alloy head expands faster, but when the barrell is also up to the sama temp. the interference- fit is restored (within the plastic and elastic limits of the alloy, it being the softer, weaker material)

The wall- thickness of the cylinder, is effectively reduced to the thickness at the root of the thread...the thread is strongest at this point,being the base of an essentially triangular section....again, during differential expansion,the "overlap" of the threads, male and female, will diminish....in extremis, the alloy thread would ride up the flanks of the steel thread and either slip off, or the crests would shear each other off...essentially a broaching-action.
No doubt the size of the interference-fit collar has ben determined to give sufficient frictional grip to make the threads redundant under normal operating conditions.

I am an afficionado of Scott motorcycles....this unique 2-stroke twin has a central-flywheel with overhung cranks in 2 separate crankcase compartments.....the flywheel is furnished with a tapered bore each side...the crankshafts are pushed in, each having a matching taper...the assembly is pulled tight with a long draw-bolt.
These engines run sucessfully and dismantling often shows the tapers have cold-welded to each other...do not discount the frictional grip of metal against metal!

Aviation alert! The Scott engine was an option to power the Flying Flea (Pou de Ciel )

[Creampuff]

This thread is at risk of descending into rational objectivity. Shame on you, OzB, 43, AB, J, Weheka, Cockney S et al!

My understanding is that the most wear and tear on a bog standard piston aero engine occurs at …

Startup. (This is consistent with yr right’s point about the number of ‘cycles’.)

Cold engine and high RPM too soon after start = badness.

So, it’s probably smart to maintain very low RPMs after start until the engine is sufficiently warm.

However, my understanding is also that if you want to do even more damage, you should…

Set 25/25 in the climb (if it’s an injected engine).

Disabling the enrichment function and moving PPP closer to TDC = worseness.

So, it’s probably smart to climb at wide open throttle and higher RPM.

But let’s ignore this silliness and get back to reality.

Thrash cold engines on the ground and climb at 25/25! And if you want to really ‘take care’ of your engine, set 40 degF ROP, in accordance with the POH. That will put you in the Pantheon: “I’ve been doing this for 20,000 hours / 40 years and therefore my opinions about engine management / maintenance are objective truths.”

We had a few cracked pots, all were due to metallurgical, manufacturing or installation defects.

That’s crazy talk! All engine problems are surely due to ‘excessive leaning’.

[Jabawocky]

Creamie.....those Peroni's see to make you a genius

[Walter Atkinson]

This oughta stir the pot:

Shock cooling is a myth (unless your CHTs start at redline and you chop the throttle and descend in the yellow airspeed arc--in that case, you *might* shock cool it).

If you are concerned about shock cooling you'd better NEVER fly into rain. Think on that.

I can explain where the myth originated, but I don't have the time right now. Maybe later.