C model CHT's

[Johnnybgoode]

CHT's ... a C-model mod story

Background... I bought a local '65 C last fall for a good price that had sat for a couple of years, flying only enough to keep it in annual. It had new cylinders, after a brief stint with chrome cylinders which "didn't break in properly" (and bladders and a panel-mount GPS.) Thinking the undisclosed high-CHT problem (on 3 and 4 cylinders only) was primarily with the JPI grounds I relocated the instrument ground and lowered CHT's about 30 degrees, but not enough for me, or below my target 400 degrees.

I also purchased a Lasar cowl closure, thinking that reverse airflow out the guppy mouth wasn't a good thing. After dropping it off for mod I received a call from the A&P indicating that the mod couldn't be completed due to engine sag. Lord mounts and spacers and the mod completed later I'm happy to report the results. Takeoff today around 90*F indicated temps resulted in just over 400 in climbout, remedied by IAS increase - prior to mod CHT's would reach 450! Cruise temps were maintained below 400* for the first time in ownership.

The Lord mounts allowing the engine to sag resulted in the lower-cowling forward baffling strip not remaining in the channel, allowing my cooling air to reach the area below the cylinders, and robbing me of its cooling power.

If you experience high CHT's on a C check the placement of that baffling strip. It might just give you a hint as to where to begin.

Lots of C's with high CHT's - just one more data point to check.

Patrick

[Oscar Avalle]

Interesting! I apologize for my ignorance what baffling strip are you referring too?

Sent from my iPhone using Tapatalk

[rbridges]

that's awesome and much better than 450.  The mooney guys typically recommend keeping CHTs under 380 on the lycoming.  what cruise temps are you getting?  I typically climb out at 100+ knots to keep my nose lower.  The other day, ATC asked me for best rate of climb, and my oil temp climbed pretty quick from the increased angle.

[Johnnybgoode]

Oscar,

There's about a 3' piece of rubber baffling attached to the engine side on the inside of the lower cowling - it is supposed to ride in a channel on the aft side of the front cowling. In my case the worn mounts allowed it to slip below the channel and create an air path to the the lower side of the pressure area. It bypassed the cylinders and resulted in the higher CHT's. Raising the motor kept it in the channel and fixed the cooling/airflow issues.

rbridges, I climbed out at around 130 mph today - same as pre-mod, having read up on the IAS's that are common for a C.

Patrick

[Seanhoya]

Johnny,

 

High CHTs are a problem for vintage carbuerated Mooneys, and the problem is more confounding because not everybody is afflicted by the problem.  Look - at max gross weight and above 90 degrees, you are going to be hot.  The best remedy is to fly early in the morning and climb at the max IAS allowable.The fact that your airframe sat for a few years and had the cylinders rebuilt (to me) is problematic, and especially so because it is a 1965.  Is this always an issue?  Absolutely not, but you clearly are experiencing high CHTs.  Make sure you are at full rich on the mixture (helps with cooling), and also basic things - when was the last time you cleaned your oil cooler filter?  When was your last oil change?  Are the fins in your oil cooler still intact?

 

This is a hard, probably unsolvable problem which many vintage Mooney pilots deal with (I did the Lasar cowling mod, replaced my oil cooler and carbuerator, and still have high CHTs from time to time).  But if you still have high CHTs, I would verify your compressions, order an oil analysis, and really look at your engine health.

 

In any situation, you are still probably cruising at 140 KIAS on 9 GPH...not many can claim that!

 

Clear Skies!

Sean 

1968 M20G N6907N

[carusoam]

How rigid is that piece of rubber? Mine had paint on it from really early on in it's life.

How much effort would be required to renew that piece of rubber?

Best regards,

-a-

[jamesm]

I  wish that was my problem with High CHT's  ....  After watching the Pipistrel  Panthera video several  about cooling drag

it totally made sense to me ..... 

I have a Lasar  cowl enclosure  so one day I decided to  measure cowl inlet versus cowl flap exit air outlet cooling.

I was lazy and didn't want  to calculate the area of all  the air inlet and outlets weird shapes,  so I figured I would use  inlet shapes perimeter  dimensions instead. I have no real bases on my findings. I was using one those  flexible  tape measures  that seamstress  use in the fabric stores to cut fabric to measure the perimeter measurements .

I measured my perimeter of  the right side ( Cyl #1 & #3 ) cowl inlet  roughly 25" which   sometimes get above 450 dof  versus the  left side inlet 27" rough perimeter measurement for  (Cyl #2 & #4)  which I can rarely can keep it below 420 dof on climb out .The cowl flap outlet rough perimeter measurement (~6.5" width  x 1" opening x 2 top & bottom = ~ 13.5" perimeter measurement)  x 2 cowl flaps.      my disclaimer ......  I may be way  way off base on this .... 

total  cooling air inlet perimeter 52" measurement  versus  

total cowl flap cooling air outlet perimeter 27" measurement

which is actually less since I did not account for  exhaust  pipe circumference.

From what I have heard  the  cooling outlet (cowl flaps, exit air)  is suppose to much  larger  than  your inlet cooling.. Mooney has it is ass backwards on C model Mooneys . It is  no wonder there are cooling problems .... 

Many of us,  C model owners with various  makes  and models of  engine monitors  installed  and various thermocouple types depending make of engine monitors and some  installations  have the OEM single probe CHT's installation from the factory. CHT/EGT probes calibration which probably varies from manufactures. Engines that are different stages of life.  baffling  condition questionable depending on people's prospective. then you have aircraft weight OAT air density  and  on on  that has effect engine cooling. Lot of variables to consider.

I  can't seem to find common method on consistently  keeping the CHT cool on climb out.

  One thing for certain ... It seems to be on going problem here is a report that has been posted several times about Mooney guppy mouth cowling

http://www.mooneypilots.com/mapalog/M20C%20Evaluation/M20C_Evaluation_Report.html

Sometimes  I think the CHT's  values have some truths to them but  part of me say we  are flying behind random number generators. Unfortunately  we can't legally experiment engine cooling without  mound of  paper work. If the FAA and Mooney had done their jobs we wouldn't be have this discussion so often.  

James 67'C

[carusoam]

I think Al owes ya one there James...

They really improved the long bodied version of the C.

Improved a dozen other things as well.

Unfortunately, the price increases at a faster rate than the improvements...

Best regards,

-a-

[Oscar Avalle]

What you guys are saying makes a lot of sense. I will look at the baffles and I am pretty sure that in my case they are not in good shape and are not performing their job.

 

Oscar

 

http://mooneyspace.com/files/file/54-baffles-for-m20c/

[ryoder]

Mine runs warm in Tampa so I climb with the cht as part of my scan. I bet we all do.

[mike20papa]

As far as the engine "sag"  Are you aware you can shim the lower mounts with alumn. sheet shims.  make them out of .063 and use up to 2 in the lower mount.  I think Maxwell the infamous EasTex Mooney mechanic has a article on his web site.  JOe

[Johnnybgoode]

Thanks Joe,

I am aware of the shims and the article. In my case the mounts needed changed out and shims alone would have been a band-aid. Then when I changed them, later, I could have faced alignment issues with the already installed cowl closure. I opted to bite the bullet now rather than work the alignment issues later.

Patrick

[HRM]

CHT's ... a C-model mod story

The Lord mounts allowing the engine to sag resulted in the lower-cowling forward baffling strip not remaining in the channel, allowing my cooling air to reach the area below the cylinders, and robbing me of its cooling power.

Patrick

 

I don't think this is confined to C models. I have had increasing CHT's and have been wits end with it. Having exhausted nearly everything else I am looking at engine sag and thinking that may be it. At annual next month I am going to have my shop check it out.

[Andy95W]

I'm really looking forward to seeing how Sabremech's new cowling enclosure affects CHTs.

[Andy95W]

Can you post a pic of your lower cowl baffling?

From inside my guppy mouth, with LASAR cowl mod. Orange rubber is the baffle material, red painted metal is the lip of the original cowling and top part of the channel.

From below, looking just above the oil cooler. Again, orange material is the rubber baffling, and you can see the lower portion of the channel.

Dave- if you have a DTW overnight coming up, I could use a hand re-installing my landing gear. Bring work clothes.

[bonal]

I was watching a news story on climate change and they were siting the usual indicators but then it caught my attention when they mentioned a noticeable increase in CHT on several Mooney aircraft over the past years.  Now I'm convinced.

 

Seriously I know I need to do some work on the dog house an plan to near future.  at a 120mph climb I am seeing just shy of 400 at 2700 25 plus. Once in cruise I leave cowls open for a time after reconfiguring but on plus 80 degree OAT will usually have to leave them there and I notice oil pressure to go a little closer to the yellow.  I will then go 2450 21mp and I see around 360 and my pressure comes back to the middle of the green. At least I know that there is controllable air flow through my cylinders.

 

Andy yours looks nicely squared away.  good luck on the gear

[Andy95W]

I was watching a news story on climate change and they were siting the usual indicators but then it caught my attention when they mentioned a noticeable increase in CHT on several Mooney aircraft over the past years. Now I'm convinced.

Seriously I know I need to do some work on the dog house an plan to near future. at a 120mph climb I am seeing just shy of 400 at 2700 25 plus. Once in cruise I leave cowls open for a time after reconfiguring but on plus 80 degree OAT will usually have to leave them there and I notice oil pressure to go a little closer to the yellow. I will then go 2450 21mp and I see around 360 and my pressure comes back to the middle of the green. At least I know that there is controllable air flow through my cylinders.

Andy yours looks nicely squared away. good luck on the gear

Your CHT description matches mine to a 'T'. I think I could devote a solid week to my baffling and only drop about 10°, so at some point it is what it is.

My gear isn't a problem. My irrational desire to clean everything with mineral spirits and a toothbrush is. (Although I am replacing all hardware and bushings)

[bonal]

My gear isn't a problem. My irrational desire to clean everything with mineral spirits and a toothbrush is. (Although I am replacing all hardware and bushings)

 I Know mine represents pretty well but I sure have seen some outstanding Mooney's here on MS good luck on brushing your gear cavities.  WAIT! I thought regular brushing is supposed to prevent cavities

[Andy95W]

HAH! LOL

[Johnnybgoode]

N1395W,

That's exactly the baffle I was referring to in my original post. Yours looks quite effective.

Patrick

[jetdriven]

As far as the engine "sag"  Are you aware you can shim the lower mounts with alumn. sheet shims.  make them out of .063 and use up to 2 in the lower mount.  I think Maxwell the infamous EasTex Mooney mechanic has a article on his web site.  JOe

The shims are steel, and either 1/8" or 1/16".  They are actually a Lycoming part IIRC.

[Johnnybgoode]

Indeed they are, and sourced from Lasar in my case. We ended up using them as well as new mounts to line everything up where it needed to be.

Patrick

[Taracka]

Hello,

  I have been thinking about this high cht issue for a while and here are my thoughts.  

  First, I still have yet to hear from anyone with a "c" model fitted with a 4 channel digital engine analyzer who isn't seeing high cht's (greater than 400) on best rate climbout.  My C model has been thoroughly gone through (baffles perfect, etc.) and will produce 450 deg. cht's if I let it by not shallowing climb to get greater than 120 mph.   If I do this I can keep cht's around 400 then about 360 in cruise.  I have read a lot about cowl design and I can tell you Mooney really screwed up on the "C" model.  The inlet holes are too large even with the cowl closure which mine has, the upper plenum volume is too small, the cowl flaps are too small, and the oil cooler airflow raises the pressure in the lower cowl making airflow even less efficient.  

  Do a Google search on  "M20C evaluation report".  Here you will find a series of tests conducted several years ago with a well maintained 1965 M20C.  Included in the article is a chart and in this chart you will notice CHT's as high as 440 during the climb.  Also, there is all this talk parroted by many (I think started by Mike Busch) about not letting you cht's get over 400 no matter what because the cylinder head aluminum loses half of it's strength at that temp.  What they don't indicate and I'd be curious to know is how much strength that aluminum needs to lose before it poses a problem to cylinder integrity, is it 50 percent or much greater.  If the cylinder is five times stronger than it needs to be then losing 50% of it's strength is a non-issue.   Lycomings engineers were pretty capable guys and they must've done tests involving lots of math and metallurgical knowledge to determine that the green arc was good to 450 with a redline of 500.  Furthermore, my "C" models's engine has made it to TBO twice with high cylinder head temps and zero cylinder problems.  Also, in the "Lycoming flyer, Operations", it says "For maximum service life, maintain the following recommended limits for CONTINUOUS operation.  Cylinder head temperature 400 or below."  It also states to observe redline temperature and not exceed this value (500df) for climb and high power cruise operations.  I would think with how much Lycoming gets sued they'd have changed that info if they felt it posed a risk to cylinder integrity.

  Nevertheless, it would be good to find a solution to reduce cht's on this model.  I suspect it will involve a complete cowl redesign.  Perhaps the new one currently being tested by the member on this forum will show positive results.  

   So again, if someone out there with a 4 channel digital engine analyzer on a "C" model can get temps below 400 during a 100 mph climbout on an average day with temps in the 40-80 deg range. I'd like to hear about it.  Also, this has to be done with a warm engine, in other words, second takeoff.  On mine the first takeoff shows noticeably cooler cylinder head temps due to the oil temp being lower. 

     Rich Taracka A&P IA

[ArtVandelay]

In regards to Lycoming recommendations, not that I don't trust their engineering, but from a business perspective, they don't want their engines lasting too long, otherwise you won't have any repeat customers. So I would error on the keep it under 400 if at all possible.

[Andy95W]

I get that the POH says best rate is right around 100 mph.  In real world flying, I don't see much actual difference in feet per minute between 100 mph and 125.  Maybe the Mooney wing likes the extra airflow over the wing and you don't lose much relative efficiency.  I see 1000 fpm by myself or 800 fpm with 2 people

In a mountainous environment I'd do the 100 mph just for the better angle of climb, but anywhere else my C seems to like 125.

I agree with your observation about making TBO reliably even with poor instrumentation and above 400° CHTs.  40 years ago people did it all the time.  I guess I'm just hoping I can nicely exceed TBO with the better instrumentation and keeping my CHTs as low as possible.  And who know, maybe it'll help with preventing valve sticking.

My C is like yours- baffling in good shape, CHTs about 395-400 at 125 mph.  I do have the LASAR cowl closure which actually seems to help.