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)

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!

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
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(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:Once you accept this, the idea of carb heat increasing the power on an over-leaned engine makes sense, as per my earlier post on this thread.

FFF
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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