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Shock Cooling

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I am a believer in shock cooling doing damage to engines and was amazed to see the myth factor

 

From Wikipedia http://en.wikipedia.org/wiki/Shock_cooling_(engines).

 

Shock cooling (engines)

 

From Wikipedia, the free encyclopedia

 

Shock cooling refers to the theory that damage to engines (particularly air-cooled aviation piston engines) may occur because of an excessively rapid decrease in temperature.

 

The situation where rapid cooling arises is on descent from altitude. In this condition, less power is demanded of the engine (it is throttled back) so it is developing much less heat. In a descent, the plane's airspeed increases, simultaneously increasing the cooling rate of the engine. As metals expand and contract under temperature changes, dimensional changes in the engine may exceed tolerance limits.

 

Contents

 

[hi

 

Manifestation[edit]

 

Damage from shock cooling is most commonly believed to manifest itself as stuck valves[1] and cracked cylinders.

 

Analysis[edit]

 

While the subject is controversial and hotly debated, some believe shock cooling, as commonly explained, is nothing but a myth. This position is supported by the fact twin engine planes commonly experience ideal conditions for shock cooling during simulated, single engine failures, yet statistically show no difference in wear or damage distribution between engines. Equally, it has been pointed out the rate cylinder head temperatures drop off after a normal engine shutdown is often much faster than the usual rates deemed to present a shock cooling risk. Furthermore, others believe[citation needed] damage usually associated with shock cooling is actually caused by rapid throttle changes where fuel, which has been supercooled during high altitude flight, is introduced into a very hot engine cylinder during descent, where rich of peak (as opposed to lean of peak[2]) operation is considered the norm, thus causing higher operating temperatures. It is well established[citation needed], high operating temperatures in of themselves, can contribute to excessive component wear and damage, which is typically associated with "shock cooling". Given the available data, it strongly suggests "shock cooling" is nothing but a myth, at least in the context as commonly explained. Nonetheless, the topic will remain highly controversial and surely continue to be hotly debated well into the future.

 

Kas Thomas,[3] a respected aviation engine expert and author believes, "shock-cooling is not a major contributor to cylinder head cracking".

 

Detection And Prevention[edit]

 

A single cylinder head temperature (CHT) sensor, or in more sophisticated installations, an array of sensors, one for each cylinder, may be employed to monitor the temperature and cooling rate of the engine. Usually a simple analog gauge or a more advanced graphical bar-graph display[4](see external links below for an image) is used to present information to pilots. Spoilers on the wings or thrust reversal may also be deployed to lose lift without having to reduce engine power substantially, slowing the rate of engine cooling.

 

 

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There's a lot of this crap out there. If an engine has enough fins to keep the temps controlled when the engine is flat out at a lower climb speed. It will cool rapidly When idle power or feather is done, and it is descending at a higher airspeed.

 

Anyone who has done welding will know what care has to be taken when cooling the job out. It's done in a furnace slowly or placed in dry lime.

 

When an engine is feathered the cowl gills are closed fully straight away, and partially so on descent. A Chieftain is descended with power ON .

 

Jet engines suffer damage when shut down, suddenly in flight. They scour out the casings by the still hot turbine blades so tip clearances are excessive . If you use high power on approach the idle time before shutdown is a specified minimum.

 

With large radials the engines may have to be cooled by idling on the ground before shutting down as the heat from the hot parts migrates to the cases and seals etc are damaged. Glider tug and meat bombing operations cause cracking of heads on air cooled engines. It's about the worst thing you can do to an engine. Climb at low forward speed and follow by a power off descent. Head temps can go to 230 Celsius (which is pretty warm). Not a good thing to stop heating it and expose it to a high speed very cool breeze, suddenly, all day long. Nev

 

 

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Makes sense, Nev. Our little rec. aircraft are made light and simple, and cowl flaps would introduce more complexity to flying and maintaining them. I added butterfly-type cowl flaps and use them whenever on descent.

 

Just have to remember to open them again. The joys of air-cooling.

 

Co-incidentally today I'm just about to investigate why my CHT's have been playing up; only one gives a reading at present. Presumably loose connections.

 

 

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I believe in shock cooling with air cooled engines . That is why i recommend the liquid cooled Rotax 912. 012_thumb_up.gif.cb3bc51429685855e5e23c55d661406e.gif

 

 

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from some of the data i have seen, shock cooling is far less a problem than most make out. as cylinder head temps have more to do with internal cylinder pressures than exhaust gas temps, or the amount of fuel burnt. at idle, fuel is still being combusted, and still generating heat. low throttle settings, mean less cylinder pressures, less cylinder temps.. but when you look at the changes in temperature ranges that could be considered shock cooling, they are relatively small and of no real consequence when it comes to metals failing. full power, with blocked cooling inlets, then flying into a near frozen lake might do something that could be described as shock cooling.. but consider that most aircraft metals are hardened at 500 deg C, then dunked into room temp water within the allowed time limit of 10 seconds Max!, a small change of a 50 to 100 deg wont worry it.

 

a nice little graph derived from real time engine testing with live data feeds, notice the ICP, internal cylinder pressures, and CHT, are almost perfectly matched, regardless of the EGT.

 

pp77-landmarks.jpg

 

 

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There are a lot of Pawnee glider tug drivers who would nod their heads in agreement with the claim that shock cooling causes cylinder cracking.

 

GCV (I think it is a still the largest gliding club in Australia) has a Pawnee tug fitted with a Chev engine just because they got sick of rebuilding the 0-235. It sounds great and is giving very good service.

 

The Pawnees came out in 2 models, the PA25 (above) and the later PA26 which had a 260hp Lycoming at first and eventually a 285hp Continental (horrendously expensive to rebuild).

 

Kaz

 

 

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OMG...Where did you get the bit about "most aircraft metals are hardened by heating to 500c and dunked in water"

 

 

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GCV ... has a Pawnee tug fitted with a Chev engine just because they got sick of rebuilding the 0-235. It sounds great and is giving very good service.

I have heard of another straight six being put in one up in Qld. Love to see how a big iron car engine was adapted into an aircraft.

 

 

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actually, aluminium, most specifically 7075 series (used quite a lot in older aircraft)is heat treated from o condition to T6 hardness by heating to 493degC then quenching. then age hardening at 121 deg for 24 hours.

 

at approx 400deF, aluminium will start to loose a portion of its structural strength, and starts molecular changes in the metal, is the reason why 400DegF is the crucial number when it comes to CHT's and shortening engine life.

 

If shock cooling as most think of it, that being idle power for a period of time, then why dont we see the same damage with almost every twin engines aircraft that are used for training?

 

cooling of the metals at the rates seen in aircraft, does not cause significant changes in relation to excessive heating(dunking in large volume of water from high temps will) . now a glider towing aircraft, sky dive aircraft,etc the engine will be working hard with low forward speed, generating a lot of heat, what are the CHT's of these aircraft? how accurate are the CHT monitors, if they have any? im sure climbing all day with CHT's over the 400def, or up to even 500deg as seen on some aircraft on hot days will be doing far more damage than the cooling on descent..

 

there is a CHT data on the web taken from a IO540 engine in a single engine RV10, from top of climb at 13,000ft, to sea level in 10NM. idle power from long cruise. the CHT temperature trace dropped, but at a far slower rate that could possibly do any damage.. as in cruise, the engine temp should be relatively cool already(if all is normal)

 

now rapid heating from ground taxi speeds, full power to height and under load will generate a lot of heat, and effectively age hardening the materials, causing increase in the metals brittleness. eg, cracks..

 

 

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I know of a V8 in a Fletcher topdresser. It has 8 straight exhausts pointing straight down which look good & makes a real grunty noise. I've not seen or heard of any others so it may not have been that successful but at least it would not have had shock cooling problems. I assume it was an alloy engine but with the radiator & all the other paraphernalia it would have been pretty heavy.

 

 

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Ultralights . Twin engined aircraft used for training DO have engine problems, moreso than those used for longer trips.. CD6 b aircraft in Australia similarly on the shorter legs. You are right about the hardening of various metals but there is a fair bit of warping and cracking associated with hardening aluminium castings. Various sections of an engine operate at largely varying temps. Ie exhaust valves . When a red hot (or hotter) Valve stops and is sitting on it's seat the rim cools rapidly. Cracks form from the outer edge at right angles to the surface. Many engines require a period of ground idling if they have been using high power prior to landing, even jets, ( as I said earlier). before shutting down. Nev

 

 

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Current opinion of the self proclaimed experts is that shock cooling is not the big problem it was always made out to be. It is far easier to wreck an engine by running too lean at high power. You can watch the CHT rise when running at full throttle at under about 5000' and pulling the mixture back slowly. It can result in detonation and pre ignition, which can destroy the engine in seconds.

 

 

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I don't recall shock cooling being made out to be a great problem but there were rules of engine handling which you were taught which prolonged engine life and reliability. You didn't take off without reaching the minimum temps and didn't exceed rev limits, didn't use TO power for more than 5 minutes, and didn't do power off descents without some consideration of what it was doing to the engine.. You didn't operate in prohibited RPM settings and you used "balanced power" with Radial engines. The worst thing you can do to a Radial is operate at high RPM with throttle closed, for instance. This is more a problem with in flight adjustable props. Nev

 

 

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You didn't fly oversquare. that has been shown to be an OWT and so may some of the other truths we were tought.

 

 

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If you read my post carefully, I compared the slower full power climb speed with a HIGHER descent speed. The descent speed is usually the same as cruise unless there is a speed restriction applying, ( like below 10,000 alt @ 250 knots.etc) OR you are engine out in a single.. Also often the potential energy of being high can be converted into a time saving faster descent, if you are in the position to use it. Nev

 

 

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What seems to be being overlooked here, is the effect of metal fatigue from repeated rapid cooling. Glider tugs commonly do up to about 12 tows per hour - that's about 2.5 minutes at full power, at climb speed (around 60 knots) followed by 2.5 minutes of descent. That's 12,000 thermal cycles per 1000 engine hours. The various hot parts do not all heat up and cool down at the same rate, and there have to be temperature gradients in order to move the heat from inside to the fins. A cowboy in a glider tug can do thousands of dollars worth of damage in the form of cylinder head cracking, cylinder barrel cracking etc. The issue is NOT overplayed; but it's more severe in the larger barrels on Lycomings etc than on the smaller ones on RAA aircraft

 

 

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There are a lot of Pawnee glider tug drivers who would nod their heads in agreement with the claim that shock cooling causes cylinder cracking.

GCV (I think it is a still the largest gliding club in Australia) has a Pawnee tug fitted with a Chev engine just because they got sick of rebuilding the 0-235. It sounds great and is giving very good service.

 

The Pawnees came out in 2 models, the PA25 (above) and the later PA26 which had a 260hp Lycoming at first and eventually a 285hp Continental (horrendously expensive to rebuild).

 

Kaz

There never was a Pawnee with an 0-235.

 

The first Pawnee was the PA 25 150 (150 hp).

 

Next was the PA 25 235, with an 0-540 of 235 horsepower.

 

Then they added the PA 25 260, with a constant speed prop.

 

The PA 36 Pawnee Brave is a totally different design.

 

It was at first supplied with the Continental 'Tiara' engine, soon replaced with the IO-540. This is the 300 Brave.

 

Soon aftermarket firms were fitting the IO-720, so the factory followed. These are 375 or 400 Braves.

 

 

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I would assume he meant the Pawnee with the 235 HP low-compression 0-540; that's the version most commonly used as a glider tug.

True, sorry.

 

 

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Have read that shock cooling can be a prob with Rotax water cooled engines when doing circuits. On approach, low eng rpm, water circulating slowly,engine cooling, crack throttle for go- around, colder water in rad hits cylinders, cylinders suddenly cooling(contracting), piston suddenly heating(expanding) due to fuel burn, engine stops. Emergency landing, pilot checks engine and good chance it will start but will not develope full power.

 

 

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Have read that shock cooling can be a prob with Rotax water cooled engines when doing circuits. On approach, low eng rpm, water circulating slowly,engine cooling, crack throttle for go- around, colder water in rad hits cylinders, cylinders suddenly cooling(contracting), piston suddenly heating(expanding) due to fuel burn, engine stops. Emergency landing, pilot checks engine and good chance it will start but will not develope full power.

Yep, heard about this - it's known as a "Cold seizure".

 

I'm always wary, especially at this time of the year, during any decent or in the circuit, that I adequately maintain engine temperature.

 

Pud

 

 

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Do you have a thermostat in the cooling water circuit?

If you are asking me - yes I do, and it works just fine when tested out of the engine.

 

Pud

 

 

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Yep, heard about this - it's known as a "Cold seizure".

I'm always wary, especially at this time of the year, during any decent or in the circuit, that I adequately maintain engine temperature.

 

Pud

Close cowl flaps, apply carby heat.

 

 

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