Oil temp / Cowl Flaps

[TIP_Fytr]

Hello all! I’ve had a 1964 Mooney M20E for two months and have made an observation about oil temperature (as read from UBG-16 Engine Analyzer) that I wanted to share and get insight on to see what the forum thinks: 

1. Oil temperature during 120mph climb is 175-180F with cowl flaps full open.

2. After level off I keep it WOT, set 2450 RPM, close the cowl flaps as speed approaches 150mph, and lean the aircraft via LEAN function on engine analyzer. 
 

3. Oil temp slowly increases over the course of 15-20 minutes to 220-225 deg F, at which point I will slightly trail the cowl flaps and the temp decreases to 175-180 within a few minutes. 

I run 100 deg F ROP and this temp increase with cowl flaps closed occurs at all altitudes and the highest I’ve flown is 10,000 MSL under these observations. Any suggestions as to why this is occurring, if trailing the cowl flaps is a viable solution, or other insight would be very helpful. 
 

Below are some photos of the engine compartment.

 

 

Edited July 31, 2023 by TIP_Fytr
Wrong Picture

[A64Pilot]

 

At 100 ROP, your running her hard, middle of Summer I’d suspect that even if everything’s perfect that run that hard that full closed cowl flaps will cause both cyl head temps and oil temp to run higher than desired. Cowl flaps control airflow through the cooler too.

In my opinion place the cowl flaps in whatever position that’s required to keep oil and cyl head temps in the middle of the green. They aren’t like wing flaps, you don’t have to cruise with them closed.

I don’t know where the thought that cowl flaps should always be fully closed in cruise comes from, certainly in cold Wx, but in the dog days of Summer and running at best power mixture probably not. Put the cowl flaps in whatever position is required to keep temps in the middle of the green, there is no reason I can come up with to run towards the upper green with the flaps closed.

Perhaps someone can explain if I’m wrong. This time of year even running at 60ish percent LOP my cowl flaps are in trail. I’m in a J but I don’t think that matters.

If I’m at higher altitudes I will close them for descents with the goal of keeping temps in the middle of the green, often though of course with higher airspeed and lower power it cools off of course.

‘There is no speed loss with them slightly open and even if there was I think keeping the engine running cool is more important

Edited July 31, 2023 by A64Pilot

[TIP_Fytr]

5 minutes ago, A64Pilot said:

 

At 100 ROP, your running her hard, middle of Summer I’d suspect that even if everything’s perfect that run that hard that full closed cowl flaps will cause both cyl head temps and oil temp to run higher than desired. Cowl flaps control airflow through the cooler too.

In my opinion place the cowl flaps in whatever position that’s required to keep oil and cyl head temps in the middle of the green. They aren’t like wing flaps, you don’t have to cruise with them closed.

I don’t know where the thought that cowl flaps should always be fully closed in cruise comes from, certainly in cold Wx, but in the dog days of Summer and running at best power mixture probably not. Put the cowl flaps in whatever position is required to keep temps in the middle of the green, there is no reason I can come up with to run towards the upper green with the flaps closed.

Perhaps someone can explain if I’m wrong. This time of year even running at 60ish percent LOP my cowl flaps are in trail. I’m in a J but I don’t think that matters

Thanks for the reply!  At 10,000MSL the air is much cooler and thought I should be seeing cooler oil temps without trailing the cowl flaps.

Cylinder temps are all 380 deg F or less I don’t notice any increase in cylinder temps like I do the oil temp. The oil cooler can be seen in the pictures I posted.

[A64Pilot]

The higher you go, the cooler the air gets, normally 3F per 1,000 ft, but also the thinner it gets too, so as non intuitive as it seems but often the thinner airs ability to cool outweighs its cooler temp. I know that doesn’t sound right, but trust me  Down at lower altitudes it’s not as significant as it is much higher up and we non turbo guys fly lower of course so it’s there, just not as pronounced as it is much higher. Think how much lower your indicated airspeed is at 10,000 ft, well cooling ability goes down with the indicated airspeed.

380 is higher than I like, in my opinion it’s the max I’ll allow in a climb, I like what I think is closer to 350, but as I only have factory instrumentation that’s a guess. I can actually keep mine at the same place as in cruise in a climb by climbing at an airspeed tgat gives me 500 FPM. That starts out as 130 indicated and decreases of course with altitude.

I keep mine at the F mark on the factory gauge, just barely below the mid point.

There is a body of evidence that running that high a cyl head temp decreases the life of the cylinder, now it may actually be higher power as much as or more than higher temp, who knows.

‘I think there may be an STC to mount your cooler on the firewall like on a J, but if you can keep it at 180 with trailing cowl flaps I wouldn’t change anything I’ve always heard that 180 to 200 as ideal oil temps.

On edit, I can think of no reason at all to not run with cowl flaps not fully closed, especially in record setting heat like we are having now.

Many people will change the flap rigging in Summer so that fully closed is at least 1” open. I don’t see the point in that, they are adjustable for a reason, adjust them. If Mooney wanted them only full open or full closed, then we would only have two positions, heck many fine aircraft don’t even have cowl flaps, they essentially run with open ones all the time, because they have to have good cooling for the climb.

Edited July 31, 2023 by A64Pilot

[TIP_Fytr]

I usually only see 380 in the climb then the CHT stabilizes in cruise between 340-360 across all 4 cylinders.

What you said about altitude and temps makes sense thinking about it! 

[A64Pilot]

If your running 350 ish in cruise at full throttle and 100 ROP, your baffling must be doing its job, I think many wish they could run that cool with closed flaps or trailing.

If I were you I’d just run them trailing, in a couple of months you won’t have to, and depending on where you are this Winter you may struggle with getting temps warm enough, many have to partially block off airflow to the cooler.

Our aircraft just don’t have the precise cooling our cars do, they run warm in Summer and cool in Winter, there just isn’t really a thermostat, there is an oil Vernatherm but it doesn’t work like many suppose and doesn’t have the authority your cars thermostat does.

[M20F]

1 hour ago, TIP_Fytr said:

Thanks for the reply!  At 10,000MSL the air is much cooler and thought I should be seeing cooler oil temps without trailing the cowl flaps.

Cylinder temps are all 380 deg F or less I don’t notice any increase in cylinder temps like I do the oil temp. The oil cooler can be seen in the pictures I posted.

It is colder but what matters is +/- ISA.  I just looked at KLZU where I am based.  -4C at 18000 (cold).  It is though +17C ISA 18K feet. It is ISA +14C at 3K feet (23C).  In a perfect math world my oil temp should be 3C lower at 3K then 18K even though ambient is 27C different between the two. 
 
Certainly your baffling, vernatherm, etc.  can impact the results. But ambient doesn’t have the effect we think it should.  

[The Other Red Baron]

We have an E as well with the exact same oil temperature issue.  Just have to keep the cowl flaps open until winter (if you have a winter.)

If you decide to throw money at it and find a solution please let me know.

[Pinecone]

Standard temp, like standard pressure is just a number picked as a baseline.   The same temp is the same temp, reference to ISA has no bearing.

Actually, cowl flaps should be rigged to 1" open for lowest drag according to Mooney test pilot.

Max of 400 CHT for Lycoming and 380 for Continental is what Mike Busch recommends.   I set my JPI to alarm at 380 (TSIO-360) and just set things to keep it below that mark.

[A64Pilot]

16 hours ago, The Other Red Baron said:

We have an E as well with the exact same oil temperature issue.  Just have to keep the cowl flaps open until winter (if you have a winter.)

If you decide to throw money at it and find a solution please let me know.

I think, but am not certain that an STC exists that mounts the oil cooler like it is in a J, which takes air from the backside of the engine enclosure and dumps it into the lower engine compt.

I think that would cool the oil better, but if all it takes is to partially open the cowl flaps, what’s wrong with doing that?

I’m not even sure the STC even exists much less the cost, perhaps someone else will comment.

Quick look found this, maybe this is it? I wouldn’t hold my breath expecting a speed increase though.

https://lasar.com/stc-kits/laskit126

[The Other Red Baron]

5 hours ago, A64Pilot said:

I think, but am not certain that an STC exists that mounts the oil cooler like it is in a J, which takes air from the backside of the engine enclosure and dumps it into the lower engine compt.

I think that would cool the oil better, but if all it takes is to partially open the cowl flaps, what’s wrong with doing that?

I’m not even sure the STC even exists much less the cost, perhaps someone else will comment.

Quick look found this, maybe this is it? I wouldn’t hold my breath expecting a speed increase though.

https://lasar.com/stc-kits/laskit126

Yep I looked into that but unfortunately we don't have the correct serial number.  I think we have the "doghouse."

[AdamJD]

I have an F model.  Same issue.  My CHTs are btw 350 and 360 in the summer depending on how hard I'm running the engine, but the oil temps are between 210 and 220.  My oil cooler is mounted on the front.  I had the oil cooler flushed.  I even had the thermostatic oil bypass valve tested in boiling water and it opened just fine.  I would even touch the oil cooler each time I landed to make sure it was hot, and it has been.

Then I noticed many of the fins on my oil cooler are bent.  OSH is the only grass or dicey area I taxi or takeoff on, so I'm not sure what did the damage.  The location of the oil cooler means anything hitting your plane, including possibly moderate rain, can bend those thin delicate aluminum fins. 

So I suspect air is not making it through my oil cooler efficiently.  I talked to Pacific Oil Cooler at OSH and the guy said to carefully take thin duckbilled pliers and straighten out the fins.  I haven't looked for the tool yet, so I haven't done it.  I'll report back if it works when I do.  

[The Other Red Baron]

1 hour ago, AdamJD said:

I have an F model.  Same issue.  My CHTs are btw 350 and 360 in the summer depending on how hard I'm running the engine, but the oil temps are between 210 and 220.  My oil cooler is mounted on the front.  I had the oil cooler flushed.  I even had the thermostatic oil bypass valve tested in boiling water and it opened just fine.  I would even touch the oil cooler each time I landed to make sure it was hot, and it has been.

Then I noticed many of the fins on my oil cooler are bent.  OSH is the only grass or dicey area I taxi or takeoff on, so I'm not sure what did the damage.  The location of the oil cooler means anything hitting your plane, including possibly moderate rain, can bend those thin delicate aluminum fins. 

So I suspect air is not making it through my oil cooler efficiently.  I talked to Pacific Oil Cooler at OSH and the guy said to carefully take thin duckbilled pliers and straighten out the fins.  I haven't looked for the tool yet, so I haven't done it.  I'll report back if it works when I do.  

Our fins were beat up pretty good so we asked our local shop if they could comb the fins out but it made no noticeable difference in oil temperature.

[cbarry]

Cowl flaps should be renamed “heat dumps” and then maybe pilots would use them as such.  With the “heat dumps” closed on a model with the oil cooler mounted forward on the lower cowl, I’m convinced there’s actually back pressure on the engine side of the cooler that restricts airflow through the cooler.  (You can actually observe the cowling airflow back pressure by simply watching the oil filler access door slightly bow upward as the cowl flaps are closing.) Opening the heat dumps just a little can make a lot of difference.

[A64Pilot]

10 hours ago, AdamJD said:

I have an F model.  Same issue.  My CHTs are btw 350 and 360 in the summer depending on how hard I'm running the engine, but the oil temps are between 210 and 220.  My oil cooler is mounted on the front.  I had the oil cooler flushed.  I even had the thermostatic oil bypass valve tested in boiling water and it opened just fine.  I would even touch the oil cooler each time I landed to make sure it was hot, and it has been.

Then I noticed many of the fins on my oil cooler are bent.  OSH is the only grass or dicey area I taxi or takeoff on, so I'm not sure what did the damage.  The location of the oil cooler means anything hitting your plane, including possibly moderate rain, can bend those thin delicate aluminum fins. 

So I suspect air is not making it through my oil cooler efficiently.  I talked to Pacific Oil Cooler at OSH and the guy said to carefully take thin duckbilled pliers and straighten out the fins.  I haven't looked for the tool yet, so I haven't done it.  I'll report back if it works when I do.  

You can buy plastic “combs” at a HVAC place, I’m sure Amazon has them, usually it’s four combs in a square and you match the comb to your cooler, made for airconditioners of course.

Found one like I’m familiar with but search airconditioner fin straightener and you’ll see many.

I think though you’re going to find out what you have is normal. Years ago when I put 29” tires on my Maule I’d hit 220 in a climb and run 200ish in cruise. I couldn’t find anything wrong so I called Exxon to ask if I should shorten oil change interval. He cautioned me to follow the POH of course but not to worry at all about the oil, that 220 isn’t even anywhere near its limit and actually they like to see their oil run at 200 or over as it stays cleaner and actually lasts longer.

So I suspect you should of course straighten the fins, and maybe fly with the flaps open a bit in hot Wx, but that you may not have a problem that really needs fixing.

Realize of course this is just my opinion, but if it’s been flying like that for longer than some here have been alive without issue, it may not be a problem.

https://www.amazon.com/Straightener-Conditioner-Conditioning-Automotive-Refrigeration/dp/B092ZF7S1R/ref=sr_1_29?crid=DPBLJ5WU071V&keywords=air+conditioner+fin+straightener&qid=1690894832&sprefix=airconditioner+fin+straightner%2Caps%2C279&sr=8-29

Edited August 1, 2023 by A64Pilot

[Mooney-Shiner]

Maybe check the dates on the oil lines and get them replaced in case that they are slowing the oil flow? Keep the oil lines away from the exhaust stacks?

[67 m20F chump]

My 67’ F runs hot oil temps when I’m running hard.  The oil pressure also falls off at 215*.  CHT’s are always good but I don’t have the dog house.  I have the aft mounted oil cooler.  I’m thinking about adjusting the cowl flaps a turn or so more open when in the closed position.  So far I just power back or open the cowl flaps.

[A64Pilot]

Just open the flaps as needed. Stay within the allowable range of opening in accordance with the maintenance manual. Those limits are there for some reason, it might be several things like hanging up if adjusted too open if fully opened or even possible they can be opened too much that they don’t work as well or who knows.

I think my J’s limits are 1/4 to 1/2” open when fully closed but that’s from memory, point is the MM has limits.

I’ve come to the conclusion that especially in hot weather that trailing is normal and desired, that fully closed is a cold Wx thing, and many say you gain a kt or two with them open an inch or so which is trailing for me. It seems to me that trailing is the preferred position, not full closed?

[nevadabandit]

Should your oil lines be that close to the exhaust pipes?  Our M20E oil lines exit the top of cooler and run along next to the block, way above the exhaust.

Just a thought.

[TIP_Fytr]

2 hours ago, nevadabandit said:

Should your oil lines be that close to the exhaust pipes?  Our M20E oil lines exit the top of cooler and run along next to the block, way above the exhaust.

Just a thought.

After speaking with my mechanic who specializes in Mooney’s the touting of the oil lines is good and shouldn’t create heating issues. 
 

Thanks for all the info from the forum! Sounds like trailed cowl flaps are the solution during summer operations!

[nevadabandit]

Good Good

[AdamJD]

Dumb question.  I have a Pacific Oil Cooler mounted on the nose on my M20F.  It looks reversible.  The fins on the inside, facing the engine, are mostly unbent.  The fins on the outside, facing the impact air, are fairly bent and beat up.  Looks like it has mounting holes on both sides?  Is the oil cooler reversible?  Can it be turned around?  Does the direction of oil flow through the cooler matter?

[N201MKTurbo]

3 hours ago, AdamJD said:

Dumb question.  I have a Pacific Oil Cooler mounted on the nose on my M20F.  It looks reversible.  The fins on the inside, facing the engine, are mostly unbent.  The fins on the outside, facing the impact air, are fairly bent and beat up.  Looks like it has mounting holes on both sides?  Is the oil cooler reversible?  Can it be turned around?  Does the direction of oil flow through the cooler matter?

When I owned an F, I would go out to the plane every few years and sit on a stool in front of the oil cooler with some 2C tweezers and spend about 1/2 hour straightening out the fins. I could make them look like brand new. Way easier than trying to flip the cooler. Besides, if you flip the cooler in short order both sides will be beat up. I tried different radiator combs, but the tweezers worked the best.

You can keep the tweezers together and shove them between the fins, the rounded back of the arms will push them mostly into place if you run them back in forth. If there are any kinks in the fins, you can open them up and grab the fin and straighten it out.

[Pasturepilot]

Have you verified your vernatherm is fully working? I had similar oil temps and the cooler was basically ambient temp. Hit it with an infrared thermometer after a flight and see if it is as hot as the rest of the oil. 

[takair]

For what it’s worth, my 1964 E model typically runs oil temp between 180 and 200F…even in Georgia (when we were there)summers with cowl flaps closed in cruise. Early in ownership I saw some slightly higher numbers.  As other suggested, be sure the fins are not bent over.  I did replace the cooler, but it was only marginally better.  I also sealed up around the perimeter of the cooler.  As I recall, this made the biggest improvement.  I notice in the OPs post, the left side cylinder baffles are warped and may be leaking.  While one would think the biggest impact is on CHT, any extra air going the wrong way can alter the pressure gradient in the cowl.  I would suggest looking at the dog house in an effort to help Oil temp, even if not intuitive.  

One reason owners may be compelled to close the cowl flaps in cruise is because it is a TCDS required placard for many models.  The placard in mine is hidden next to the cowl flap handle and is brittle, so may be missing in some aircraft.  I suspect it is because early cowl flaps were rather flexible sheet metal with the link off center, so they likely flex at speed.  Here is the reference for those who don’t want to look it up.

"COWL FLAP - PULL TO OPEN - DO NOT OPEN ABOVE 150 MPH." (Not applicable for M20C S/N 680001 and ON or M20G, M20J or M20K).