Engine damage

[tigisfat]

You asked about CHT theory:

cylinder head temp is just a way to measure the temperature at which the engine is running. Many aviation aircraft are air-cooled, so you can't measure the coolant temp. For the most part, it's affected the same way as any other engine temperature metric; that is to say, anything that would make the engine run warmer will increase CHT.

Making the mixture more lean will increase CHT.
less airflow will increase the CHT, just like a lack of coolant in a car.
Making the engine run harder and for longer periods of time will increase CHT
Increased ambient temps will increase CHT.
A lack of engine lubrication will raise EGT, after engine lube temps rise.

 

[Snave]

Cylinder Head Temperature is not just a gauge reading by itself, it's also used as an indicator to the overall `health` of an engine. The CHT is a simple screw-in device that measures - unsurprisingly - the latent temperature in what is the hottest part of any engine. Too hot and detonation can occur - the fuel/air mix ignites before the engine wants it to. Also, too hot and the metal in the engine starts to lose structural integrity. This would be an extreme example.
Too cold and the engine can suffer from `shock cooling` where too-rapid cooling of the cylinder head shortens the life of the engine considerably, and in extreme cases can crack the head itself - differential expansion is a beeatch! So CHT is one to watch!

Others have expanded on the readings and how aspects of flight can affect but I can tell one other aspect that receives little coverage, but is actually also a small contributory factor.
First, let's emphasise that most AIR-cooled aircraft engines are the ones that need to have a weather eye kept on the CHT - liquid-cooled engines do not usually suffer from overheating in the air (but can on the ground). As with cars, radiators and oil coolers can manage to keep the engine and cylinders within limits under most conditions, most of the time. The early Spitfire, for example, had only a single underwing radiator and this was partly masked by the undercarriage leg. Even idling the Spit cuold - and did - overheat.

However, the cylinder cooling on any aircraft is a compromise - it must deal with the full gamut of temperatures encountered in the real world, it must deal with the highest demand on cooling - high power and low airspeed - and yet must be closely-cowled enough not to cause excessive drag, disrupt airflow, allow for the best possible visibility, and other factors. It follows that airflow optimisation can be done using flow pattern analysis and wind tunnel experiments to discern the optimum shape, size and location for `normal` use, but never for every conceivable one...

But I once experienced strangely high CHT readings in flight in a Piper Archer which defied attempts to understand until a fast jet jockey who was also a member of the club explained it - I had been flying at normal cruise (in fact at lower than normal cruise as there was a really strong tailwind that day and my groundspeed was over 140 knots - at 55% power! In an Archer!) but noticed that CHT was climbing slowly and inexorably. It wasn't a major cause for concern as I was closing on the destination field anyway, but even after I reduced power for descent, the CHT continued to climb! This obviously had me worried, and my thoughts started running through the possibilities - engine trouble, blocked air intake, detonation, even a faulty sensor, but the engine ran sweetly with no other indication of a problem and, thankfully as I turned base and reduced power still further, the temps turned downwards and declined slowly and steadily right back to normal levels. I landed normally, parked up, noted the log and had a word with the engineer. He decided to run it up on the ground and check how long it took to reach normal temperature, and found it totally normal.

We discussed the possibilities in the clubhouse for quite some time and even got some rather esoteric suggestions such as bird strike, ice blockage and dodgy fuel to add to the list until the fast jet guy piped up:
"Was there a lot of wind that day..?"
When I pointed out the groundspeed indication he nodded and said simply:
"What you probably had was diminished pass-through effect of the cooling air caused by a pressure build up to the rear of the engine - where the heat normally escapes... that `reversed` pressure, even if it's only a few knots relative, can upset the pattern of flow and disrupt the exhaust of the hot air."

It then became obvious why the temps had stopped rising and returned to normal - I'd descended from the high speed, then turned base, presenting a completely different resolved angle of airflow to the vents and inlets, and `normal` airflow had resumed, carrying away the trapped hot air from the cowling. I believe that this is much more of a problem for `pusher` aircraft.

Apparently the same effect has been seen in fast military jets up in the jetstream where the strong tailwind can delay by a fraction the jet efflux, causing localised `hot spots` in the tailpipe.

Hope this adds to the information bank!

 

[Ken Stallings]

You asked about CHT theory:

cylinder head temp is just a way to measure the temperature at which the engine is running. Many aviation aircraft are air-cooled, so you can't measure the coolant temp. For the most part, it's affected the same way as any other engine temperature metric; that is to say, anything that would make the engine run warmer will increase CHT.

Making the mixture more lean will increase CHT.
less airflow will increase the CHT, just like a lack of coolant in a car.
Making the engine run harder and for longer periods of time will increase CHT
Increased ambient temps will increase CHT.
A lack of engine lubrication will raise EGT, after engine lube temps rise.

Overall a very accurate and comprehensive summary. However, lack of oil lubrication can also increase CHT even more than EGT. Exhaust gas temperature can increase if the engine block is hotter, but more likely lack of lubrication will cause increased friction in the engine block, especially the pistons, and therefore increase CHT.

The two most common causes of CHT increase is too lean a fuel mixture for the conditions, especially during climb on a hot day. This is followed closely by high angle of attack climbs even when the mixture is fully enriched. This is why pilots are advised to enrich the mixture to full during a full power climb and to only lean it as the reduction in CHT allows.

There are a couple of methods to increase quality of cool airflow through the engine. A prime one is opening up cowl flaps as this increases the avenue of air to escape out the back of the engine, creating more of a vacuum up front to assist in drawing in more cooling air at the intake. Another is to simply decrease the climb rate, increase climb airspeed, and therefore the influx of cooling air in the intake.

Fuel acts as a liquid coolant in a piston engine. I know this might sound strange, but it is very true. This is why enriching the fuel mixture can reduce CHT very quickly.

One of the best reasons to have a digital engine monitor is to keep up with CHT for each cylinder. Then, soon as you notice the CHT is climbing too high, you can immediately take steps to remedy the problem.

The most immediate problem with high CHT is predetonation of fuel in the cylinders. The CHT reaches a high enough temperature so that the fuel sprayed into the cylinder detonates before the full compression stroke is completed. This causes the piston to be slammed down before the crankshaft makes its natural rotation and is aligned properly for a smooth power stroke.

This can cause multiple catastrophic problems rapidly. It can shear the crankshaft, destroy the piston attachments on the crankshaft, crack a cylinder head, or a piston or piston shaft, and even cause fuel to spray out and ignite in the engine bay causing a fire.

You can feel and hear the predetonation and I've been told it is a very uncomfortable sound to hear! If it isn't stopped immediately, it can lead to vastly more uncomfortable sounds and ultimate an extremely uncomfortable silence!

Ken

 

[Brett_Henderson]

I'm gonna have to respectfully refute the tailwind theory.. The airplane (or its engine) has no idea what the winds aloft are. Now, that a pilot might choose to fly at a slower TAS because of a tailwind, that lesser TAS could lower the cooling effect.. but there is no reverse pressure to the cooling system from a tailwind. The only time that could happen, is right after a wind-shift, and it would quickly equalize. For a jet, at jet-stream speed, that might be a problem when they transition a shear.., but for light GA, it's a non-issue. A tailwind simply increases ground-speed.

Remember.. wind is a ground-based phenom.. to the airplane in flight, there is no wind.

 

[Ken Stallings]

I'm gonna have to respectfully refute the tailwind theory.. The airplane (or its engine) has no idea what the winds aloft are. Now, that a pilot might choose to fly at a slower TAS because of a tailwind, that lesser TAS could lower the cooling effect.. but there is no reverse pressure to the cooling system from a tailwind. The only time that could happen, is right after a wind-shift, and it would quickly equalize. For a jet, at jet-stream speed, that might be a problem when they transition a shear.., but for light GA, it's a non-issue. A tailwind simply increases ground-speed.

Remember.. wind is a ground-based phenom.. to the airplane in flight, there is no wind.

I agree. Airspeed, especially indicated airspeed, is entirely relative to the force of air coming across the airframe. That is why indicated airspeed is, by definition, difference between static air pressure vice pitot tube ram air pressure. Therefore, by definition, if you are flying at 100 KIAS then it doesn't matter what your tailwind is, you have the same differential in air pressures as you would at 100 KIAS with the same headwind component.

I think you are precisely correct with regard to wind shears.

This is precisely why microbursts can be so very dangerous because the rapid shift of relative wind in speed and/or direction can happen so fast, that the momentum of the aircraft cannot adjust to keep the indicated airspeed consistent. Ultimately this could in extreme conditions lead to immediate stall or overspeeds, a situation especially dangerous during an instrument approach due to being at slow approach speed and being so close to the ground.

However, if the changes in wind happen in their more normal gradual rate, the aircraft's inherent momentum will adjust and the airspeed quickly restore itself to its previous value. If a tailwind shifts to a headwind, then the reduction in groundspeed translates into a loss of forward momentum of the aircraft and that allows the indicated airspeed to remain the same because that loss of momentum matches the increased forward force of air mass. In the reverse, where a headwind turns into a tailwind, the aircraft gains forward momentum to match the increase in tailwind force, and therefore again, the airspeed remains constant. In both cases, under normal conditions, the changes in airspeed are slight and very brief. And during that transition, you experience turbulence on the airframe.

Cheers,

Ken

 

[fliger747]

The cub never ever comes even close to getting too hot, but it is an airplane with a pretty open cowl, only four cylinders and a low airspeed range. My partners Cessna 180, which I flew back from Anchorage the other day has pretty sophisticated egt/cht etc instrumentation. Cylinders will vary, in that plane it's #6 that you have to keep an eye on. Not a real problem compared to a large radial with say 18 cylinders and closely cowled to minimize cooling drag. None of the GA planes I have flown from 185's to Seneca's had much of a cooling issue with sensible use of the cowl flaps...open on the ground and takeoff initial climb and otherwise mostly shut...

CHT is probably the most poorly modeled parameter in FS. They should have coupled it to EGT.....

Cheers: T

 

[teson1]

fliger, Brett, pilottj, azflyboy, tigisfat, Snave, Ken,

Wow, thanks to all of you for the great feedback!

I have a much better understanding of the real-world impact of CHT now, and the conditions that may cause concerns.

So, in some planes, the pilot will really want to pay close attention to CHT!

Thanks again! :salute:

Gunter

 

[Snave]

I'm gonna have to respectfully refute the tailwind theory.. The airplane (or its engine) has no idea what the winds aloft are. Now, that a pilot might choose to fly at a slower TAS because of a tailwind, that lesser TAS could lower the cooling effect.. but there is no reverse pressure to the cooling system from a tailwind. The only time that could happen, is right after a wind-shift, and it would quickly equalize. For a jet, at jet-stream speed, that might be a problem when they transition a shear.., but for light GA, it's a non-issue. A tailwind simply increases ground-speed.

Remember.. wind is a ground-based phenom.. to the airplane in flight, there is no wind.

Respectfully, wind is a relative variable in the aviating world like any other. Although the aircraft is flying at a speed within the airflow, if it is not travelling directly in the path of a particularly strong wind then there is a relative velocity vector component to the air mass movement around the aircraft which is exactly the same as the crosswind impact and which can `blank` intakes, exhausts and ducts in the same way as a T-tailed aircraft can be blanked at high AOA by wings and fuselage - and given the relative stability of even GA, there will always be some lag between wind vector and velocity change and the aircraft or pilot responding. At extremes that variance can impact on localised surfaces not directly dynamic through control of airflow like wings and empennage.

The key indicator for me, on that day, was that the CHT continued to increase AFTER a power reduction. There was an outside factor influencing the cooling of the cylinder head, and it was eliminated by changing the relative wind, and descending. I think on balance, it is still the likeliest explanation, although I readily accept that I might also have strayed into a pocket of volcanically heated air and ash that partially blocked the inlets. But I don't think there were any eruptions around that time!

 

[Brett_Henderson]

You've definately got a head-scratcher here, as to why the high CHT... I'd suggest that you go fly around at that same IAS, and see if that's the problem. Whatever IAS it was, it sounds like there might be a resonant sweet (sour) spot for cooling airflow, at that IAS.

Although the aircraft is flying at a speed within the airflow, if it is not travelling directly in the path of a particularly strong wind then there is a relative velocity vector component to the air mass movement around the aircraft which is exactly the same as the crosswind impact and which can `blank` intakes, exhausts and ducts in the same way as a T-tailed aircraft can be blanked at high AOA by wings and fuselage - and given the relative stability of even GA, there will always be some lag between wind vector and velocity change and the aircraft or pilot responding. At extremes that variance can impact on localised surfaces not directly dynamic through control of airflow like wings and empennage.

There is no relative vector.. there is no in-flight, X-wind blanking (except during a slip.. and that has nothing to do with the wind.. it's a function of "fighting" the wind relative to the ground.. like trying to stay lined up with a runway..and could be replicated, in no wind at all).

An airplane doesn't know that it's "crabbing" to stay on course.. Wings and control surfaces and air-intakes/outlets have no reference to the ground.. They're all the proverbial fish in a giant aquarium. That the aquarium might be moving is of no concern to his fins or gills as he swims around inside of it..

 

[Brett_Henderson]

Now..let's say that it's a glass-bottomed aquarium.. When the fish tries to not only swim toward a "runway" underneath the aquarium, but also keep his body lined up with the runway as the aqurium keeps moving.. that "slipping" will have one of his gills seeing a different water-flow than the other gill.. but not if he's just crabbing toward the runway.. Make sense ?

 

[rvn817j]

Just a Couple of Quick Notes - Real World

CHTs - In my RV-8 I experience most CHT issues on 'hot'/humid days (e.g., 80 degrees f and above) at high power settings and/or higher angles of attack (air flow over the cylinders is reduced). So, for example, shortly following takeoff, I have to be looking at CHTs and many times I will reduce power slightly to keep CHTs in the normal range. Sometimes, at lower altitudes (2000 - 3500 feet) at higher power settings in straight and level flight, I also experience CHTs inexcess of normal. Again, a slight power reduction will solve the problem.

As you climb, standard temperature deviation is -2 degrees f per 1000 feet altitude (or you have an outside air temperature probe installed to determine it). Cool air helps greatly with CHTs. But, you don't want it too cool either because that could cause other issues.

Google Lycoming O-360, for example, to get engine operating parameters.

 

[teson1]

Jay, thanks for the input.
The hands-on description of real-world handling you and the others have contributed is exactly what I was looking for. Great!

 

[Sunny9850]

I think the others have already covered most of the basics better than most textbooks do it.

A few more examples to what can affect the CHTs in one particular airplane.

CG...because it will alter the overall pitch of the aircraft at a given airspeed and power setting. This in turn changes how the air enters the cowling and how effective it will move through the cowl....and therefor cooling the cylinders.

Air Density...less dense air simple means there are less molecules in a given space to interact with the hot engine. And most likely you are going to ask the engine to work harder to achieve the desired performance.

The Baffling in the cowling is extremely crucial. Even a relatively small "crack" between the flexible baffling and the cowling can change how hot the engine gets.
The design of the Prop, cowling openings and such of course are very important as well.
If I had the $9000.00 I would love to swap the standard Harzell prop on my Saratoga for one of their Scimitar models. Not only does that new prop run smoother and quieter, provide more thrust in almost all flight regimes it also shuffles much more air into the cowl openings. And while "Sara" usually doesn't have CHT issues despite living in SoCal...a few degrees less would not hurt anything.

Speaking of cowlings...my 1982 Saratoga has the relatively large dual nostrils in the upper half of the nose.
Piper has updated that a while back with to round, smaller openings (LoPresti has a very similar looking design available) which actually improve both cooling and reduce drag.
And of course there was the hideous looking Turbo Lance with a single large opening below the prop....very very efficient in both cooling and drag.............but not even a mother could possibly love that face

As for the tailwind causing a hotter engine....don't think so. Unless the airplane was literally sitting on top of an even faster moving flow which might then indeed cause a bit of a "blockage" of the outflow opening on the bottom of the cowl.

The highest GS I ever got with the Archer I flew before Sara was 172 kts. SoCal ATC actually came back and one time after checking in with a new sector and asked me to confirm that I was indeed flying a PA28-180.
The highest recorded GS in the Saratoga so far is 202 kts....recorded in her GNS 530 flying back from Castle last year.

Stefan

 

[fliger747]

Best sustaimed ground speed to date.... 709 knots. CHT not much of an issue with the turbines at 35,000'.

Cheers: T

 

[Snave]

Now..let's say that it's a glass-bottomed aquarium.. When the fish tries to not only swim toward a "runway" underneath the aquarium, but also keep his body lined up with the runway as the aqurium keeps moving.. that "slipping" will have one of his gills seeing a different water-flow than the other gill.. but not if he's just crabbing toward the runway.. Make sense ?

Ah but if it's a 4-knot fish and the water in the aquarium water is quartering at 8 knots then there is a component of cross-flow. assuming the fish can't go 4 knots at a vector from straight ahead. He may be able to maintain relative alignment with the runway - and his perceived velocity vector stays the same over the runway centrelines - but he's actually swimming in a cross flow - same effect as ground speed v. indicated air speed. He's converted his 4 knots into a lateral and forward motion the cumulative effect of which is to retain position in one plane (sic).

I could quote some strange anomalies of similar ilk from my submarine days, involving some undersea currents in certain parts of the world, but I'm not sure I'm allowed to...:salute:

I must admit I furrowed my brow at the explanation, and think it was simplistic for the hard of understanding like me, but all craft of any weight (and fish) have an inertia when inserted into a fluid medium (and one doesn't normally argue with a Tornado pilot with Amraams...)

What I think I might have experienced was akin to response lag or possibly some kind of pressure differential inside the cowling. I know and comprehend what you are saying about the aircrafts perception of its own trajectory in the medium, but as we all know, you don't apply 180hp from a standing start and immediately go flying. Speed builds gradually and velocity changes are not instantaneous. Our concoctions of metals and plastics, woods and fabric take time to respond to change. And with a higher mean speed as a tailwind, the proportional change in percentage terms is a greater velocity in absolute terms.

Possibly, it is this rate of change that may have been the cause.

To this day I still cannot come up with a better alternative explanation - and at the time I did consult one of my aviating mentors - my other `alfs sisters hubby with over 12,000 commercial and military hours including rotary wing on rig and SAR duties and he couldn't think of anything else that explained it. The other thing that persuades me is that my subsequent career has involved spending long hours in wind tunnels - where wind speed can be changed violently and very quickly - and have noted similar changes in internal combustion engines where the platform has been rotated relative to the airflow at the same time as the velocity has been altered even when the engines are liquid-cooled so should dampen the effect substantially.

 

[Brett_Henderson]

Unless the 4knot fish is consciously swimming partially sideways (ala slipping to not only approach the runway on runway centerline(crabbing), but also be "pointing" runway centerline(slipping)), it wouldn't matter if the aquarium were moving 100knots.. there is no crossflow to him, his fins, or gills.


ANYway.. you seem to have a good understanding of fluid dynamics.. and do seem to see the merit to the reason why it's sound argument to say that there's no way a tailwind means anything to the cooling flow. But you've got a mental block that won't let go of the idea that it could..

Your points about inertia/momentum are very accurate.. in that a wind CHANGE could have a temporary effect on the cooling flow, but unless it's constantly changing (to the point that you'd be beaten alive by the turblence effect), it doesn't take but a few seconds for it to equalize... not nearly long enough to affect cooling in any meaningful way. And if it's a sustained tailwind (like we're talking about), there's absolutely no effect on cooling, at all. This isn't me liking to argue.. this is aerodynamic fact.

I'll admit to having trouble forcing my self to "see" it too. Like.. my instinct is that if an airplane is flying a perfect circle in a steady wind, that the airplane would "feel" the wind differently, during different parts of the circle... but it does not.

 

[fliger747]

Given the relation of aircraft speed and the magnitude of any reasonable wind change, and the rate at which an airplane can change direction, any effects are very transitory and not liable to effect engine cooling which has it's own significant thermal inertia.

Light aircraft adjust more quickly to a changing local dynamic environment, and large ones more slowly. In a very large aircraft such as the 747 this can actually work to our advantage. This is especially true in cross wind landings......

Cheers: T

 

[Sunny9850]

Best sustaimed ground speed to date.... 709 knots. CHT not much of an issue with the turbines at 35,000'.

Cheers: T

LOL Woopdidu.....seat 7B Heathrow to NY JFK the digital display in a certain British-French aerial machine showed much more than that but I wasn't flying it
But I have seen much more than those speeds I mentioned in the left seat of C-441, Be-58G and AC550... but none of those were Piper Archers like the one in the story and I did not mention it to brag that my Archer was faster....simply to note that I understand the notion of immense pride when a usually rather slow steed suddenly develops "wings"

Stefan

 

[teson1]

Hi,
I finally uploaded RealEngine v1.1 to SOH and flightsim.

Please do not use v1.0 uploaded yesterday! I made a stupid mistake - uploaded an old zip version with same name from another folder... possibly partly non-functional.
I did test and retest everything was working fine prior to releasing, I swear. Tested the zip. And then this stupid error...

My sincere appologies to those who have downloaded v1.0. :redf:

Gunter

And now I need some flying.

 

[Brett_Henderson]

Someone uploaded it to Simviation, too (I'm assuming you).. and a thread has commenced... I posted suggesting a devoted, RealEngine Q&A thread.. I'll monitor it and answer questions, best I can..