Wet Heads

[Lumps]

Thanks Jabawocky, I don't own a Mooney M20M, was just a hangar talk curiosity thing. Here is the POH excerpt - they allow max continuous power (89%) at peak TIT. Red box be damned it seems.

[A Squared]

Originally Posted by Lumps

Also, if it was such a problem with this engine why don't Chieftans etc also have wet heads, the article seems to indicate not.

Nobody seems to have responded to this particular part of your question, so I'll have a shot at it:

The TLS engine is unlike the Navajo engine, the TLS engine is a "parallel valve" engine, and the Navajo engine is an "Angle valve" engine. The parallel valve engines are much lighter, and have significantly different cylinder heads. The angle valve cylinder heads have better cooling at the valves; the valve guides are longer (more transfer area) the valves are further apart so have more mass between them to transfer heat, and have greater cooling fin area. So, with a different head construction, different compression ratios and different operating parameters, it doesn't follow that the Navajo engine should need oil cooled valve guides like the TLS engine does.

[Jabawocky]

AxA, good pickup. I had forgotten that was even raised.

And Lumps, you hit the neail on the head. The use of the expression "best power" is not necessarly "Best" as in "The best" spot to operate. A concept lost on many folk.

Just for your entertainment and educational pleasure, and for any of your hangar flying mates, go to the website and play this interactive "Red Box" demo, and then you can manipulate the one at the bottom of the page.

Another free give away courtesy of Advanced Pilot Seminars. Advanced Pilot

And we must thank Noel and Felicity Schultz at Chinta Air in South Australia for asking us to put that n the website. Of course it could have been the effects of Felicity and her over zealous force feeding some of us red grape juice......but thats another story best left alone, because I bet she is watching this

[Lumps]

Asqrd thank you! At last

Running 89% power at peak TIT with the lowered heat transfer ability in the FLs probably didn't help CHTs either.

Jaba - the M20M POH comes under fire by one of the engineering test pilots that helped write it - the article is in a certain folder under 'cooling down turbos' you might recognise it!

[Old Akro]

Red box be damned it seems.

Walter, John & Jaba all contributed generously to my thread about the TSIO 360. Peak TIT is not necessarily in the "red box".

You should also note that TIT is a different curve than peak EGT. TIT peaks later than EGT, so peak TIT will almost certainly be LOP EGT.

An interesting question for Walter et al. is: If below 65% power is about the threshold safetly go LOP - or just about any other mixture condition - in a de-reated engine such as this, is the 65% taken from the original power rating or the derated power rating?

[Jabawocky]

Akro, I am not sure I can agree with the different curve bit, the TIT is always a higher number but as for lagging significantly behind relative to fuel flow left to right, not so much. The TIT is directly related to all 6 inputs, and is a collector average.

You do raise an excellent point however about what do you consider the base line for 100%. In a derated engine say the 235HP in the C182, this is achieved by nobbling the RPM, thus you start out at 90% of the original 260HP engine and the ICPs are the real number to look at. They are actually higher per cycle but with fewer cycles per minute there is less HP.

Remember the red box concept has fuzzy edges and is conservative.

So the Parallel Valve turbo engine is more closely related to say the IO540D4A5 Comanche engine, except that the CR is 8:1 midway between the 8.5:1 of standard CR's and the Turbo's normally 7.2/3:1 in many planes. So I would not view this as a derated engine at all.

So to get a rough guess on this I would say all the variables are about even. The buffer of the fuzzy edges of the red box will cover it.

The point here to remember the Red Box is actually based on a nominal cylinder pressure limit that the Three Amigo's determined was a good safe number to use that would give longevity and some wriggle room for the variations you are talking about. You are trying to split hairs but in reality we just need to be within a few cm of the deal. If that makes sense.

Add to that the conservative nature the ICP boundary for the red box is in the order of maybe 5-10% in terms of ICP. So you could push the edges in a bit, but when you do the longevity is compromised if all other things are not optimal, namely cooling airflow distribution.

So in a non exact science answer what would I call the TIO540AF1A relative to the red box? I would call 270=100% but no room for squeezing the edges as much as say the IO540D4A5.

One thing we show in the class is how the edges of the red box were defined during its development. Almost impossible to describe here in a post as you need graphics moving on a screen, but there is a science behind it.

Hope that helps and if JD or Walter have any tidying up to do, I am sure they will give me a quick

[Old Akro]

Akro, I am not sure I can agree with the different curve bit, the TIT is always a higher number but as for lagging significantly behind relative to fuel flow left to right, not so much.

I've had a fair bit of trouble getting real data on this. There is a bit in the Bosch Automotive handbook and I've found a couple of old NASA papers. But none of it is definitive. For engine calibrators & researchers, TIT and EGT are crude measures. Its only pilots who are stuck trying to operate with them!

We should really be moving toward installing lambda gauges. Then we could measure the A/F ratio rather than use EGT as a proxy for it. If I had an experimental aircraft that's what I'd do. For my certified twin, I'd rather have a go at reconciling the Russians & Chinese than go through the STC process.

TIT is higher than EGT because is a more constant flow of hot air. The EGT probe is allowed a fraction of cooling in between pulses It measures temperature a bit like measuring RMS AC voltage.

TIT appears to peak 25 - 50 degF after EGT. But I'm not sure why. I can imagine that the temperature in the exhaust pipe would be a complex thing that behaves differently ROP & LOP and at different gas temperatures & flow rates. Some ROP conditions will have continued burning of the charge in the exhaust piper for example.

I believe the TIT curve will be a bit "flatter"and less "peaky"than EGT. But its at the margin really.

Which is an opening to talk about gauges. A stock analogue gauge using a k type thermocouple is lucky to be with 100degF and its calibration curve will be non-linear. I calibrated our stock Alcor ones and they are within about 10 degF in the operating range after fettling, but outside that it very quickly deteriorates. The digital gauges are a lot better, but not all are cold junction compensated and I challenge you to discover which are! Without cold junction compensation they can vary by the change in ambient temperature from the calibration temperature - in other words up to probably 50 degF. The trends count more than the absolute readings.

This impinges on the TIT debate. There is no point in being hung about about being degree perfect on an instrument that will probably have 50 degF error.

I think manufacturers install TIT gauges to protect the turbo, then they get used as a proxy for EGT for the purpose of leaning. The proper thing is to install an engine analyser and use EGT for leaning and the TIT to monitor turbine temperatures to protect the impeller & housing.

[Lumps]

Peak TIT is not necessarily in the "red box".

Quite. But to put it back in context:

Running 89% power at peak TIT

What you say about TIT peaking 25-50dF after EGT is interesting though, could it be that the clever heads at Mooney meant the engine to be run LOP!?

(I'd say there'd still be a few pots with uncomfortably high ICPs, at 89% power on a factory engine)

[Jabawocky]

Howdy Akro,

I have been studying some dyno runs to see if I can spot what you are talking about and I can't, and it does not make sense in a conforming engine.

You say the TIT will be flatter at the top, yes that makes perfect sense as some start peaking and falling the others are depending on the close F/A ratio matching going to peak at different times, so there will be some individual EGT's dropping while others are still rising.

I am sure you have a good grip on that, so I really do not see a mathematical average differing greatly from what the collector average (TIT) is measuring actually happening. The trend graphs on the GAMI dyno all track each other, accurately and even on an engine with unusual exhaust plumbing.

I think we are looking for microns when measuring for cm's here.

If on the other hand you had an engine with vast spreads of F/A ratio's then I can see a measurable effect, but that would not be a conforming engine would it?

JD or WA might have some views to add when they wake up over there.

[Walter Atkinson]

Jabba has it correct.

TIT and EGT track very closely in a conforming engine. Based on our evaluations, the raw values of the EGT/TIT probes are quite close to reality. The TIT probe sees six pulses of hot gas instead of one as in EGT every 720 degrees of crank rotation. In MOST applications, this results in TIT being a higher value than EGT--even though the gases are actually cooler at the TIT probe since it is further down the plumbing. There are a few installations where TIT is lower--like the C-421.

[Old Akro]

Walter, thanks.

I don't have primary data like you. So, I'll defer to your advice.

My belief that EGT & TIT curves is part based on a conversation I had with a mate who is an ex FEV engine calibrator and some old NASA papers. His hand drawn graphs are still on my office whiteboard.

I haven't looked back in detail, but one of the NASA papers I liked best was titled " ultra lean combustion in aviation piston engines" by a guy named Chirivella. His TIT curves are a bit different from his tabled EGT curves. The peak is less defined and the slope up to peak less steep.

There was truly some amazing work done by NASA, the AIAA and a couple of the universities in the sixties & seventies. Its a pity it didn't filter through more. I'm astounded and grateful that NASA gives full & free access to it all via the Langley library. One of my favorites (and please don't ask me to find it) is a project done by a university who designed and installed a new wing on a Seneca using a laminar airfoil, full span flaps and spoilers for roll control. But that's a diversion.

I forget why I think TIT peaks about 25 degF after EGT. Chirivella does not put EGT & TIT on the same graphs. I may have transposed one to the other using BMEP as a reference. I also fancied John Deakin may have referred to this in the thread on leaning by reference to TIT in BeechTalk. But, I don't recall clearly.

[Jabawocky]

Akro,

I can only imagine the hand drawn curves are possibly accurate, and if drawn on a whiteboard by me, possibly not , but in any case the flatter peak etc on any given day would be as a result of variation from cylinder to cylinder.

I have rerun some dyno runs which are clearly showing no offset. That is not to say you can't have some as I suggested above from a larger GAMI spread, or other external influences but really we are measuring microns when cm would do I think.

If you can find something on this that we can all learn from it might make for intellectual entertainment but from a piloting perspective it may not be significant. But the more I know the more I realise how much more there is to know.

JD said he had nothing to add by the way. He has a word quota per day I think. Some would say I should have one and a lot smaller too

[jdeakin]

Jaba said:

JD said he had nothing to add by the way. He has a word quota per day I think. Some would say I should have one and a lot smaller too

Naw, just lazy. I figure if someone says pretty much what I'd say, leave it alone.

Best...
John Deakin

[Old Akro]

I have rerun some dyno runs which are clearly showing no offset. That is not to say you can't have some as I suggested above from a larger GAMI spread, or other external influences but really we are measuring microns when cm would do I think.

Walters comment about "conforming engines" may be the missing link.

[Jabawocky]

Akro.....

That was my post not Walters, but anyway what missing link?

Either of us...or JD whoever gets to it first, will most likely answer you, but I don't get it.

I mentioned about the conforming engines. But still do not follow you.

Howdy Akro,

I have been studying some dyno runs to see if I can spot what you are talking about and I can't, and it does not make sense in a conforming engine.

You say the TIT will be flatter at the top, yes that makes perfect sense as some start peaking and falling the others are depending on the close F/A ratio matching going to peak at different times, so there will be some individual EGT's dropping while others are still rising.

I am sure you have a good grip on that, so I really do not see a mathematical average differing greatly from what the collector average (TIT) is measuring actually happening. The trend graphs on the GAMI dyno all track each other, accurately and even on an engine with unusual exhaust plumbing.

I think we are looking for microns when measuring for cm's here.

If on the other hand you had an engine with vast spreads of F/A ratio's then I can see a measurable effect, but that would not be a conforming engine would it?

[Lumps]

From an article written in 1998:

Current owners of wet-head TLSs and potential customers for the Bravo might be asking the same question: Does the oil-cooling scheme make the turbo Lycoming worthy of its 2,000-hour TBO? Too early to tell, but the initial indications are promising. The question remaining to be answered is this: Now that the exhaust valve guides are cooler, what is the next-weakest link in the chain? Bravo (and wet-head TLS) owners then face a dilemma square-on: Do you throttle back a bit--to, say, 75 percent of maximum for cruise--and sacrifice about 10 knots for the possibility of greater engine longevity? Or, do you run at maximum cruise--because, as one TLS owner told me, "I didn't buy the airplane to go slow"--and hope the additional oil cooling does the trick?

Would love to know from any Mooney drivers or maintainers if this engine proved capable of such abuse with the addition of wet heads.

From a Mooney test pilot:

With all that said and considered, this former test pilot flies turbocharged Mooneys at 50 degrees rich of peak TIT. Like the normally aspirated Mooneys, it is my professional opinion that this mixture setting is a perfect compromise between best power (100 degrees rich of peak) and best economy (peak) mixture settings. I have never seen an engine over-temp in level cruise using 50 degrees rich of peak, even the notoriously hot running -GB engines in the early 231's. I might be using a little more fuel at 50 degrees rich of peak TIT than at actual peak, but come on-- what's an extra gallon an hour of burn cost compared to that $60,000 engine I'm flying behind? Saving a few dollars of extra fuel burn by operating at peak TIT or even lean of peak TIT turbocharged engine just isn't worth it to me. Replacing a turbocharged engine costs too much.

[Walter Atkinson]

1) the oil-cooled valves do have good service records.

2) It's a shame that that Mooney "test pilot" knows so little about the laws of physics or engine management. His statements are simply at odds with the laws of physics or the known data.

[Jabawocky]

Hey Walter...ya think that bloke might have written a certain Lycoming publication?

amazing!

[Walter Atkinson]

***ya think that bloke might have written a certain Lycoming publication? ***

Little chance. The Mooney guy is at least a pilot.