The myth of shock cooling?

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SaraLima
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Re: The myth of shock cooling?

Unread post by SaraLima » Sat Jan 13, 2018 7:47 am

Good post Lood.. Well expressed. :smt023

The other thing I would like to mention is that the Americans can afford to be a bit "cavalier" about treating their engines properly because of the Big Mac "basket of goodies" formula. This formula is used by most international organisations to evaluate the real purchasing value of a currency to decide how much to pay expatriates.
I'm not going to detail the formula here.. why should I when Auntie Google can do it for you? but effectively the REAL purchasing rate of Rand vs US$ is about R4.6 to $1 - So based on that, an engine reman of an IO-360 costs the average yank about R140K in real SA terms - related to how it directly impacts his pocket.
While still pricey, its nowhere near the real cost to us of half a bar plus. So its a lot easier to get yourself into the news once again and "go all controversial" about good engine management practises if the "out of pocket" risk is less than a quarter of that facing South African aviators. :shock:

I guess I am trying to say that yanks can afford to take the risk, we, unfortunately cannot. 8-[
And I fully endorse your comment about good airmanship. :D
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Re: The myth of shock cooling?

Unread post by jimdavis » Sat Jan 13, 2018 7:56 am

Lood wrote:I didn’t read the whole thread, just spotted a post here and, but I got the drift. This lead me to a a question:

Have you replaced a certified aircraft engine recently? Not on a commercial aircraft, or on one that is used in a flight school or other business, but the engine on your own, personal pride and joy where you, and only you, have to pay the bill?
And I’m not asking the possible few amongst us who are in the fortunate position where the price of an aircraft engine is small change.

It happened to me a few years ago and believe me, be it OWT’s, myths or whatever, forking out that type of money hurts extremely. Not that I did anything different before, but that experience left me treating my engine with even more care and respect and I heed to most caution on anything that might stress my engine.

The last time, I was sitiing behind an IO360, but now it’s an IO520. A good friend replaced his 520 last year, with a factory reman, and the price for the engine alone was north of R800k.

If my engine fails, for whatever reason, so be it, but I’m making damn sure that it’s not as a result of how I treat it. To me, it all boils down to good airmanship.
I agree with you 100% Lood. Your very last sentence says it all - but only up to a point.

Good airmanship is based on experience and knowledge. And knowledge, unfortunately is not always knowledge, it may emirate from perceived knowledge passed on to us by our instructors, or from engineers, or from others who claim to have the knowledge themselves. I suspect most of us don't have the capacity to always separate perceived knowledge from actual knowledge.

This being so - the most reliable knowledge about engine handling has to come from the manufacturers and the POH. Any departure from this is questionable and possibly damaging to our engines.

I think of it this way. The manufacturers HAVE to give us the best possible instructions on engine handling. simply to protect their own reputations.

Jim
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Re: The myth of shock cooling?

Unread post by HJK 414 » Sat Jan 13, 2018 11:22 am

Chalkie wrote:........I feel I have to reply (to follow up on my reply to your PM) I really feel by introducing large, geared, turbo-charged engines to the discussion, all this has done is muddied the water. Perhaps GL can clear up the original question: Was your post referring to GA training-type aircraft or all piston powered aircraft, no matter size, complexity or altitude?

Let us look at the two paragraphs quoted above:

No I do not advocate pulling your C421 engines to idle at FL220, for various reasons one being airmanship, another POH SOP's and more importantly how do you expect to descend (pump up) your cabin to get it to destination altitude if the turbo is derived of the heat energy it needs to turn the compressor? This would be one reason why the POH would require a power on descent, with power reductions as the aircraft enters denser air and (of course) the engine must drive the propeller to avoid harmonic vibration.

Jan, the 80% of the 12% is a good argument on this thread, but is it a fact? Somehow, I think not.

My personal take on the question GL posted, is for GA training (type) aircraft practicing glide approaches, forced lobs, and just plain bad management of a descent on a cross-country resulting in the throttle(s) being pulled to idle and the nose shoved down for a few minutes; 2, 3 or 4, but not 20. This will not damage your engine.

Again, I will quote the facts about the original engines of ZS-KHO (my first twin conversion.) This aircraft was used extensively (still is) for training including all the single engine training associated with this type of training. Both the engines on this aircraft did 2400 hours without a top overhaul.

Those engines were "shock cooled" many times and survived.
Chalkie - we actually agree........

You are right - The big difference is that training aircraft often fly low (warmer air) and rather slow - in comparison to an AC 685 - C421 or similar coming out of the levels with an OAT of -25 and doing well over 200 knots through that air - especially in the initial phase of the descent.

I did not try to muddy the waters by scaling it up to GTSIO's and or pressurized GA birds - I was simply trying to demonstrate / warn / make a note - that the general term that "shock cooling is a myth" - in my opinion is too simplistic. (I found it too generic a statement)

As you correctly stated - pulling the taps back to idle on a GTSIO (or very low RPM) will get the prop to drive the engine and your may have pressurization issues. Yet a C414 or a Duke is not geared and you still would not wish to pull those taps back too far coming out of the levels ....in my view it is bad engine management apart from all the other side effects.

As for the 80% statement - no science behind it - I used it as an example as I can imagine that in very cold air at 200 knots + / you are shedding an enormous amount of heat through those front pots ..... If it is incorrect - my bad.

Finally - I am with just about every other poster here that adheres to the Manufacturers Recommendation - POH.
You ignore it at your own peril - and as Lood correctly stated - it costs real money to replace those engines - take care of them - even if you err on the ultimate safe side of caution.

Cheers
And thx for the input.

JK
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Re: The myth of shock cooling?

Unread post by SandPiper » Sat Jan 13, 2018 12:57 pm

HJK 414 wrote:

"Lets assume you are happily cruising at 17.000 Ft and your left engine throws that rod - you end up with immediate loss of oil pressure and you will not be able to feather that prop. ......... and that means max drag on that side."

AFAIK the prop will feather with loss of oil pressure, whether you like it or not...

Just saying.
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Re: The myth of shock cooling?

Unread post by Mauler » Sat Jan 13, 2018 1:53 pm

jimdavis wrote:Good airmanship is based on experience and knowledge. And knowledge, unfortunately is not always knowledge, it may emirate from perceived knowledge passed on to us by our instructors, or from engineers, or from others who claim to have the knowledge themselves. I suspect most of us don't have the capacity to always separate perceived knowledge from actual knowledge.
Quite right.

But I loathe that term "airmanship". Not only is it a repository of good things like skills, experience and knowledge, but it also is often the refuge of the scoundrel since it also contains a witches brew of old wives tales, prejudices and downright lies!
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Re: The myth of shock cooling?

Unread post by SaraLima » Sat Jan 13, 2018 2:05 pm

Mauler wrote: But I loathe that term "airmanship". Not only is it a repository of good things like skills, experience and knowledge, but it also is often the refuge of the scoundrel since it also contains a witches brew of old wives tales, prejudices and downright lies!
I'm ready for another good fight, Why not start a new thread? :wink: :lol:

Title: The Myth of Airmanship? #-o :mrgreen:
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Re: The myth of shock cooling?

Unread post by cage » Sat Jan 13, 2018 2:09 pm

Mauler wrote: But I loathe that term "airmanship". Not only is it a repository of good things like skills, experience and knowledge, but it also is often the refuge of the scoundrel since it also contains a witches brew of old wives tales, prejudices and downright lies!
Let us not forget it's also a great stick with which to beat others when they come up short of someone's own, inner standards :evil:

Ooh, you're going to get the popcorn brigade going now ;)
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Re: The myth of shock cooling?

Unread post by Mauler » Sat Jan 13, 2018 2:13 pm

SaraLima wrote:I'm ready for another good fight, Why not start a new thread? :wink: :lol:

Title: The Myth of Airmanship? #-o :mrgreen:
Nah. I just dislike the misuse of it.

I'ts ok to say something like "he exhibited good airmanship", since it is a general term referring to qualities that make one a competent pilot.

But to take your own twisted beliefs and then say that applying them is good airmanship immediately places you in my "highly suspicious rascal" category. :lol:
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Re: The myth of shock cooling?

Unread post by okflyer » Sat Jan 13, 2018 3:24 pm

I am following the thread from the beginning and I was waiting that a much more competent Avcomer than me will focus a little bit more on material part of the whole issue. First of all I am an IT guy by trade and I do not claim to be an engine engineer, but I started my career developing quality assurance systems for the car industry. I was always interested in metal work and the aspects of hardening, quenching and heat treatment of for specific alloys. There is a specific reason why a lot of engine manufactures in the car industry still use casted iron and non aluminium alloys for engine parts (even in the high performance sector).
Aluminium/Al-Alloy has very limited temp envelope before it uses strength in a progressive manner .
The problem is also that aluminium has a memory. Unlike iron based alloys every stress leads to tiny cracks in the atom matrix structure (like glass). More cracks, more chance that those tiny crack connect. That is why metal structures in commercial planes will be exchanged after specific time in use and not only based on inspection.

If heated over a longer period under load that effect gets much worse. Due to weight and heat exchange reasons air cooled aviation engines are made of this metal for cylinders, piston and engine housing. Like a lot of old recommendations regarding leaning, information about this aspect is outdated due to modern research.

This is an extract from an AVWEB online Articel by Mike Bush https://www.avweb.com/news/savvyaviator ... 162-1.html
--------
Both TCM and Lycoming specify CHT limits (460°F and 500°F, respectively) that should be considered emergency limits, not operational limits. Allowing your CHT to get anywhere close to those values for significant periods of time will most likely result in premature exhaust-valve problems and increased incidence of cylinder-head fatigue cracking. I do not like to see CHT above about 400°F, which is the temperature at which the aluminium alloy from which your cylinder head is made loses one-half its tensile strength. (The strength decreases rapidly as the temperature rises above 400°F.) For legacy aircraft, I recommend a maximum target CHT of about 380°F just to provide a little extra cushion, and consider any CHT above 400°F to be grounds for "doing something right now" to get it down
---------
Although, the article is focused on leaning and CHT temp, it shows that overheating (and this starts much earlier than the manufacturers state as being critical) is the real problem with Aluminium and not the shock cooling, as mentioned before, it takes a long time to reduce the temps to a critical value, but easy to overheat which will alter the strength of the overheated part for ever.
Last edited by okflyer on Sat Jan 13, 2018 4:26 pm, edited 1 time in total.
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Re: The myth of shock cooling?

Unread post by jimdavis » Sat Jan 13, 2018 3:48 pm

okflyer wrote:it shows that overheating (and this starts much earlier than the manufacturers state as being critical) is the real problem with Aluminum and not the shock cooling, as mentioned before, it takes a long time to reduce the temps to a critical value, but easy to overheat which will alter the strength of the overheated part for ever.
Thanks okflyer, that's a very interesting angle on the discussion.

Jim
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Re: The myth of shock cooling?

Unread post by C Africa » Sat Jan 13, 2018 5:10 pm

Mauler wrote:
SaraLima wrote:I'm ready for another good fight, Why not start a new thread? :wink: :lol:

Title: The Myth of Airmanship? #-o :mrgreen:
Nah. I just dislike the misuse of it.

I'ts ok to say something like "he exhibited good airmanship", since it is a general term referring to qualities that make one a competent pilot.

But to take your own twisted beliefs and then say that applying them is good airmanship immediately places you in my "highly suspicious rascal" category. :lol:

I must say I agree with this. In particular there is usually some smart @$$ at every airport who takes it as his duty to instruct all and sundry about what they SHOULD do.

I've had the irritating guy tell me that I "broke the law" because I failed to "overfly" the field before joining the circuit at an unmanned field. I told him to do himself a favour and go read what CAA published and to stop annoying people with incorrect admonishments.


C
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Re: The myth of shock cooling?

Unread post by kudu177 » Tue Jan 16, 2018 12:05 pm

Richard Smit wrote:LFD,

I think you’re may just be the best post I’ve ever read on any forum, all of which seem to be ego-issue magnets.

Nicely said!
+1 =D>

Also, he can write and spell which, for a hack like me who spends too much time correcting other people's written English, is rare pleasure.
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Re: The myth of shock cooling?

Unread post by savas » Tue Jan 16, 2018 5:09 pm

Interesting informative thread.
Okfkyer’s post makes so much sense as i often wondered whether cooling is worse or not than heating?

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