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Shock cooling is a very serious consideration on air cooled engines.
Maybe you should read the engine handling cautions on the Wright 1820 for starters. Chuck |
On a massive great '40s radial hanging out in the breeze, I reckon it would be a problem, I'm talking about on (slightly!) more modern enclosed lycomings and continentals as fitted to the majority of light a/c. These don't operate anywhere near the power levels of the big radials and the engine handling characteristics are very different I assume, also they usually operate at much lower speeds, so the cooling effect would be much less.
I wonder whether the idea of 'shock' cooling comes from the days of the radials where it was a problem, but like many aviation myths continues to this day. |
The Gipsy engine as fitted to the Chipmunk has 'warm' and 'cold' air positions. It is actually the 'warm' that is filtered, and 'cold' that is unfiltered, and normal practice is to run the engine continuosly in 'warm'. The 'warm' air is drawn from alongside the cylinders, and the 'cold' from an outside air scoop.
RAF Chippys were permanently wired in 'warm', but because we often operate from short strips we like to be able to use 'cold' for take off for those few extra bhp. Once established in the climb, it's into 'warm' and there it stays until the next take off. Go-arounds in 'warm' are not a problem. SSD |
I think you'll find the old big radials operated at no more revs than most horizonally opposed types ... in fact usually less. They also tend to give better performance than their horsepower implies as they normally turn a nice big diameter prop at a slower speed than your typical lyc or cont.
IM |
True, but big horse power implies big heating. Lots of cylinders and multiple rows, high alts(therefore v.cold temps) and higher speeds does seem like a recipe for problems.
RPM isn't really relevant in this, rather the specific output of the engine and the relative thermal inefficiency of all piston engines shows that the bigger and more powerful the engine, the larger the heating effect. Combining the two things (high engine temps and a colder ambient temp due to the issues above) I can understand that radials will suffer the problem of shock cooling but I don't think that the relatively tiny lycomings and conti's will suffer compared to the different animal that the radial is due to their different design and utilisation. |
Well lets examine cylinder head temps.
What difference in cyl heads temps will you find between all aircooled aircraft engines? If you own the airplane you are free to ignore shock cooling with large power changes, actually the proper description is thermal shock and it works both ways, ie. adding power and reducing power. But hey, have at it and ignore physics, but I would not allow anyone to operate any engine of mine without consideration for thermal shock among other things. Anyone who thinks there is a quicker thermal change after a normal shut down than when reducing power to say idle at flying speed may well be advised to do a little research on the topic, thermal shock is not a myth it is a fact of physics. Then that is only my opinion and everyone is welcome to their own way of doing things. :ok: Chuck |
Ok look at it this way,
Power is limited by the ability of the engine to get rid of heat. Take any size engine and stick it in a poorly designed cowl and you will find that you can only sustain high power settings for so long before the temps hit red and power has to be reduced. So you could say that the cowl or cooling ability of the engine itself (fins, water cooled? ect) are what limits the sustained power an engine can deliver. Now take your big radial. Sure, it may put out more power than a "typical" cont or lyc, but it's cooling will be designed to take this into account. Assuming it is cowled properly it will run no hotter than a properly cowled cont/lyc so that aurgument is dead! sorry Now take any aircooled engine and consider where most heat is generated ... the cylinders and heads. The case and oil system are designed to keep internal heat at a controlled level as best as possible and generally do ... but this is much harder for the aforementioned sticky out bits. Therefore we try to either stick them out in the airflow with deflector baffles or within a pressure cowl. On the ground with no airflow through the cowl or down through the fins, the whole engine will cool at a similar rate. OK so the cylinders/heads will be hotter, but they will cool at a "similar" rate (cylinders may cool slightly quicker I guess as they will have a greater surface area). However the cooling differential will be nowhere near as great as when they have a good airflow over them. It is this cooling differential that causes the problem. I suppose ( but don't know for sure ) that a/c like YAKs with closeable fins to reduce airflow through the cowls might have addressed this to a certain extent, but normally this doesn't apply. So, when you have really heated the engine in a high power climb (re glider tugging) and you suddenly reduce power and dive, the engine case will not cool anything like as quick as the exposed heads, and unfortunately, due often to poor cowl design (in many cases) nor will the heads. The cast iron cylinders between the two shed heat very quickly in the middle but the areas near the two ends more slowly due to proximity of ally case and head and also poorer airflow. Anybody who has ever had anything to do with cast iron wil know this uneven heat/heat loss will eventually cause a crack! As I said before, In most "normal" operations Say again Slowly may be correct that this is not a terrible problem. But in any extremes like tugging, para dropping ect it is very real. Also as I have said, it is something you should gaurd against whatever, so it pays to practice good proceedure anyway and be sympathetic to your engine ... be it in cooling, leaning, use of carb heat ... whatever! :ok: IM |
Chaps,
Surely a hard worked club trainer would be subjected to potential shock cooling problems when hacking endlessly around the curcuit, think how many EFATOs and PFLs these aircraft do too. Yet their engines often get to TBO due to regular use and good maintenence... Just food for thought. Kingy |
Well Chuck has about 29,500 more hours than me ;) , but I'd like to make three small observations about shock cooling from experience.....
1 - There are engines that are not big radials that require gentle treatment, e.g. Lycoming and Continental TSIO as used on light aircraft. 2 - I had a share in a light aircraft where we were gentle with the engine and it ran well past TBO and was still in good condition when we sold it 3 - If engines are handled in a cautious manner from early in training, there is less risk of 'regression' with potentially catastrophic results later on ...... I nearly 'chopped' the throttles on a turbo engined Navajo at 10,000' when flying for the first time ... thank goodness that the instructor was alert and had strong forearm which stopped me :mad: otherwise I hate to think of the consequences |
Thank you short stripper, that was the point I was trying to put across, in normal ops with a 'normal' flat four aircooled engine, I don't see any issues, but in the examples outlined above, I can very much understand how it could be a problem.
What I meant about the difference with large radials is not that there operating temps are different, rather that when the power is reduced, because of the very way they have to able to get vast amounts of air through them to keep their operating temps sensible, that same ability would cause them to cool faster than a well cowled flat engine. My only experience of radial engines is in a Yak 52/18 and even then it is limited, but the cowling fins had a great amount of importance put onto them. Despite my comments about whether shock cooling is really an issue in normal light a/c, I do operate them as if there was a problem. Since I'm not really convinced one way or the other because of no actual test data, (I'm still sure about the drop off rates in a recently shut-down engine, but I can't find the results of an experiment from university that I was part of. A slightly different thing really, as it was measuring temperature differentials across an water-cooled car engine) I operate with due caution and teach the same. |
Porsche use air-cooled engines in many of their cars, don't they? Does anyone know if they have any special precautions to avoid shock cooling?
I have no idea what the answer to this is, by the way, having never driven a Porsche. Not sure whether it's a good comparison, though, since car engines aren't subjected to high airflows at low power settings, as aeroplane engines are in the descent. FFF ------------- |
Not any more, they have to be water cooled due to the enormous heat generated by such high power outputs. An air cooled donk would just melt!!
My sister's VW camper, is definately air cooled!! I don't see any potential probs as the engine is mounted at the back in an enclosed space without a huge amount of cooling airflow, but as it is such a low powered engine she has never had any over heating issues with it. |
Good morning again:
I have a few spare moments so will make a few short comments regarding engine handling. I have worked all my career as a pilot / mechanic and during the past fifty years have made many observations and learned many lessons by making mistakes. When operating any aircraft engine you should first read the manufacturers recommendations for that engine and model number. Three main factors contribute to wear in an engine. (1) Piston travel (2) Thermal spikes and or overheat (3) Bearing and moving parts reverse load changes.. So when operating the engine controls you should strive to move the throttle as smoothly and slowly as practible for any given power change. As well remember that the lower the OAT the faster the temperature drop will be when reducing power. That is about as simple as I can describe engine handling. :ok: P.S. Note number one piston travel, remember that one when advancing prop RPM especially during the approach and landing. I leave the RPM at cruise setting during the approach and landing, when the throttle / s are closed for the touch down is when the prop control is moved to full fine. Chuck |
Kingy think of the sequence for a typical training circuit.
1. Full power (2500 rpm ish) for take off and climbout. 2. Reduce power to 2300 rpm (ish) for the downwind 3. Reduce power to 1900 rpm (ish) to attain Vfe 4. Adjust power on base to reach the runway 5. Power off for landing 6. Taxi in at around 1000rpm All the changes are relatively gradual and the power and temps have a chance to stabilise between changes. This is why a good flying school should realise the TBO on their engines. As far as I am aware, hotrods like Bonanzas etc need a bit of decent speed management when coming out of the cruise to join the circuit. It's not just slowing down in time, it has to be done in a manner that doesn't over cool the engine. |
At the flying schools I teach at we have had no problems with a/c not reaching their TBO's, depite the (relatively) rough handling they are subjected to; lots of power changes, PFL practices, start up and shut downs at least 6X a day. Compare this to most of the private operators at the airfields, who seem to have no end of problems. I feel this is down to under-use rather than the way the a/c are operated.
Private a/c have an easier life as they are generally operated for longer flights, have less take offs and landings, less hard landings(though this may not always be the case!!) just as much maintenance support and are just generally treated better than you average club hack. Despite this I know of at least 6 a/c at my home field that didn't get anywhere near their TBO's. I know the guy's who fly them and they are not muppets who handle the engine badly, and yet they have had major problems. I always think that we over emphasise on certain areas such as 'shock' cooling and ignore many of the more serious issues, such as corrosion due to under use, that affect far more people than a relatively unproven 'problem' that is shock cooling on a light a/c used 'normally.' |
What is "used normally" mean to you as far as power changes related to OAT mean Say Again Slowly?
Maybe I just have not understood your methods of teaching engine handling. With all due respect shock cooling is not a relatively unproven problem, it is a fact. Chuck |
Normal use to me Chuck is tavelling from A to B generally below 8000ft and often in +ve temps. Smooth power changes, not rattling the throttles around constantly, lowish speeds (certainly below 130kts) with maybe the occasional PFL.
It's probably easier to describe conditions that I would call abnormal in respect to most people who read this particular forum. Paradropping/Meatbombing. Aero's. High altitudes. Instructing, especially exercises 4-10(JAA syllabus) Lots of power changes and climbs and descents. Any aircraft with radial engines. The vast majority of private pilots in the U.K do not own their own aircraft and due to the way he syllabus is often taught, they don't tend to use the mixture control so CHT's are often relatively low, because they don't care about hom much fuel they use or often understand the whole issue of leaning the mixture correctly. Personally I'm not sure what you mean about power changes related to OAT |
Thanks for the explination on what / how you teach engine handling.
I had problems with your thoughts on thermal shock, by OAT I am refeering to the lower the OAT the slower you reduce power to a given setting that will ensure the engine does not cool to low. Do you teach the relationship of carb heat to ensure that the mixture is not to lean? Chuck |
Just a few things to note ...
Lots of Porsche engines are aircooled ... the 358 is basically a beefed up VW with better heads. Well used engines often make TBO where under used ones often don't. This isn't so much to do with engine handling as it is to do with internal corrosion. A well run, well used engine will outlive a well used or little used but well run one. Make sense? Well heat will cause the breakdown of oil and cause certain acids to be released ... this is bad. Lack of use alows any water vapour to remain within the engine ... this is bad, also certain parts such as cam followers, pushrods ect are held tight to other parts in one position for extended periods ... this is bad. Poorly run engines suffer more wear by misuse than well run ones ... this is bad. However, accept in catastrophic circumstances this may not be as bad as a a little used engine that isn't properly inhibited or at least run regularly. Hopefully that makes sense? Any more contentious myths? ... or none myths?;) IM |
I have to say Chuck that I don't, Since the vast majority of flying here is done in a smallish temperature range, neither very high nor very low (also due to the fact most people stay below 5000' the vast majority of the time) It has never been an issue, but it is something I will certainly brief my students on, so if they do end up flying in different places, they may have a chance to get it right.
We certainly do teach about the change in mixture due to carb heat, it could be catastrophic if you didn't! if somebody descended from high alt without richening the mixture and with carb heat applied.... Detonation city!! We generally teach as much as possible about the workings of engines and aircraft, I find it makes a much safer pilot, to be honest every one that learns to fly should be made to strip an engine down and descibe the parts of it and the problems that can arise before they get their licences issued!! |
S A S :
Do you teach your students to use carb heat to check for to lean a mixture, when leaning the mixture? Chuck |
I've been away for a couple of days and yes I seem to have started something off :O
As the owner of a very expensive IO540-engined plane I have read everything I can find on engine management generally and shock cooling specifically, and have not found anything other than the same old stories being regurgitated by everyone, with no supporting data whatsoever. Nobody has any data to support a CORRELATION between shock cooling and cracked cylinders - never mind an explanation of what exact mechanism would cause it which accounts for anomalies like the supposed lack of problems caused by shock HEATING on takeoff for example, and the fact that most schools don't get cracked heads despite doing PFLs all the time. Except one: a year or so ago I read an article by a commercial glider towing pilot, who over 20 years or so has worn out or damaged many engines. He found that they used to get cracked heads regularly, until they modified their operating procedures so that instead of cutting power abruptly after releasing the glider they allowed the CHT to stabilise at its CRUISE value (which doesn't take all that long, a minute or less), and after that they could cut power all the way back (and descend quickly) without any problems. So, based on this bit of data, it would appear that shock cooling does not happen unless the CHT is above a certain point to start with. For someone who wants a bit of reading, see http://www.avweb.com/news/columns/182146-1.html for some relatively respected stuff on engine management. |
No I don't Chuck, we generally teach (depending on whether the a/c is fitted with anEGT gauge) to bring mixture back until you get rough running and then richen by 1-3cm depending on type. Or go to peak EGT and richen until temp drops by 25 deg F.
IO540, my point exactly. |
How many cylinders are probed to read EGT?
If after leaning the mixture by either method that you use what would you do if as a final check you applied carb heat and saw an increase in power? Chuck |
Probably be worried something was wrong!!
Seriously Chuck, I've never even thought about this, I assume you are implying that the mixture would be too lean. We never teach people to leave the mixture at it's most efficient, as it would be too easy for inexperienced chaps and chapesses to make an expensive error. We definately teach for people to err on the safe side with regard to leaning. Especially as most never go above 3000ft anyway!! It's a good technique and I'll try it out tomorrow. Can't believe I'd never heard of it before! |
Chuck
May I join in? I'd enrichen the mixture a little, on the basis that hot air is less dense than cold air and the fuel air mixture is therefore slightly richer with carb heat 'on.' |
I too think that Chuck is checking for a weak mixture by applying carb heat, which gives a lower charge of air. A weak mix is far worse for the engine than too rich, hence that check.
There was a problem with cracked Lycoming cylinder heads during my time instructing on RAF Bulldogs, put down to pilots not giving due consideration to the effects of rapid cooling during the descent. |
ShyT, was it ever proved? Or was it a convenient scape-goat. Did you continue to have these probs when procedures were changed?? (I'm assuming they were)
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The use of carb heat as a check for to lean a mixture is the poor mans multi function mixture check and it is dependable...
If you find that you get no drop in power or an increase in power when you apply carb heat, that is a bullet proof indication the thing was to lean...hot air is less dense therefore hot air enrichens the mixture. Normally you get a drop in power with carb heat due to the hot air enrichening the mixture. So if the mixture is to lean carb heat will correct the to lean mixture back to normal...thus the increase in power.. Poor mans check and works every time to ensure you definately are not to lean. See how nice I am sharing all these simple things with everyone. Its usually the simple things that get us. :O That and thinking with the wrong head. :ok: Chuck |
Thanks for that Chuck, have you any other gems that I can use to try and make myself look intelligent in front of students!!:p I need all the help I can get most of the time!
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SAS :
No problem. Any time I see a topic that may need some suggestions I am more than happy to share little things. Chuck |
Say again slowly,
We were regularly operating up to 10,000ft and so leaning the mixture was taught from an early stage of training. Our type of flying was in the abnormal category according to your earlier classification but to be honest I've never heard it called that before. Procedures weren't changed as such. Pilots were reminded of the importance of complying with the manufacturer's advice already in place. It was important that QFIs were fully informed and fully understood the importance of engine warming etc, as many military fliers of my era had been trained entirely on jet aircraft and only converted to piston engines later on in their careers. :ooh: |
Interesting way of doing it!!
It isn't really 'abnormal' flying in any context other than maybe in way the 'normal' pilots who frequent this forum generally operate their a/c. Did the reminders actually do anything about the number of cracked cylinder heads? |
Say again Slowly
I don't know but then I was a pilot - you would have to ask an engineering stats person. ;) |
Chuck
If you find that you get no drop in power or an increase in power when you apply carb heat, that is a bullet proof indication the thing was to lean...hot air is less dense therefore hot air enrichens the mixture. Is this really true? The reason carb heat application reduces the engine power (shown by an RPM drop on a non-CS prop engine) is that the air is warmer, which reduces the efficiency of the engine. The operation of the carb, or the fuel injection unit, should reduce the fuel flow approximately correctly to maintain the air/fuel ratio. (I say "approximately" because both metering systems measure a mixture (no pun intended) of volume flow and mass flow when it comes to air, but they do measure mass flow accurately when it comes to the fuel.) |
IO540,
There's another thread about carb heat, in which I explain that the fuel flow does not decrease when you apply carb heat (or at least, not appreciably). However, I think that when Chuck says: "Normally you get a drop in power with carb heat due to the hot air enrichening the mixture" he is over-simplifying things slightly. The application of carb heat reduces the amount of air going into the engine, which (ordinarilly) reduces the power output. Because the fuel flow doesn't change, you also get a richer mixture. Now, if the mixture was too lean to begin with then the lean mixture would cause a lack of power (as well as engine damage). In this case, when you apply carb heat, not only will you get a reduction in power because of the lower air flow, you will also get an increase in power because of the more efficient fuel burn due to the more appropriate mixture - and the increase in power may well be more than the decrease in power, resulting in a net increase. At least, I think that's what Chuck is getting at. FFF --------------- (PS - Chuck, hope you don't mind me jumping in. Explaining it myself usually helps me get it clear in my own head, though.) |
FFF
I don't believe your explanation is correct (but I don't know for sure) The normal operation of a carb (or a fuel metering unit) is that it measures the flow of air (actually it measures a parameter which is a combination of volume and mass flow) and dispenses the fuel accordingly. So if you reduce the airflow, the fuel flow must also reduce. But do you actually reduce the airflow with carb heat? I would bet that whatever air flow parameter is measured by the carb does not reduce - what merely happens is that the air is warmer which reduces the combustion efficiency. In the present thread context, the question is why the application of carb heat fails to drop the RPM (assuming non-CS prop) if the engine has been leaned "excessively". I don't know the answer to this. I suppose that if the engine was leaned to (or past) peak EGT, and if applying carb heat really made the mixture richer, then the RPM would either not charge or actually increase a little. Can one get carb heat to INCREASE the RPM on an engine which has been leaned far enough? So it's an interesting question. Re leaning: contrary to very common belief, leaning does not cause engine damage unless it is run at high power, say 75% or higher. Lots of people are mortified of leaning during taxi for example - but it is completely and utterly impossible to do engine damage at 10-20% power or whatever it is. Lycoming authorise peak-EGT operation (the worst temperature case actually) up to 75% power. Detonation occurs before peak-EGT (about 50F rich of peak), right where people are likely to be cruising if using the carb heat method to check for correct leaning! But fortunately detonation cannot occur at power settings below 75% typically. Leaning past peak EGT is no danger because all the temperatures actually fall again. It is fine to lean way past peak EGT on the ground for example. |
IO540,
From my post on the other thread that I referred to: The fuel flow into the carb is dependant on the volume of air passing through it. The volume of air, for any given throttle setting and engine speed, will not change if you apply carb heat. Therefore, the fuel flow will be identical. What does change, though, is the density of the air. Warm air is less dense, therefore there is less mass of air going into the engine (or, to put it another way, fewer molecules). If there are fewer molecules of air, then you can't produce as much power, since you need the air for the fuel to burn. FFF ------------- |
IO540,
I don't think this statement is correct: "The normal operation of a carb (or a fuel metering unit) is that it measures the flow of air (actually it measures a parameter which is a combination of volume and mass flow) and dispenses the fuel accordingly". I don't think a carb is so clever as to measure mass flow. Fuel is drawn through a jet or series of jets, depending on the barometric depression in the venturi. The depression is merely proportional to airflow velocity. Also, you said that detonation cannot occur below 75% power. What basis of fact do you have for that? In my experience, piston engines can suffer from detonation at much lower power settings, depending on the load on the engine. Hence the requirement to remember "RPM max, mixture fully rich, carb heat cold". |
My reference to using carb heat to check for to lean a mixture was meant for use in small bug smasher type airplanes such as normally used in flight training.
Hot air is less dense than cold air...therefore hot air will make the mixture more rich... Chuck |
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