Keeping Your Cool

[Bob - S50]

I've got about 65 hours in the Mooney now and I'm starting to notice that it is more difficult to keep the CHT's down, the oil temp down, and the oil pressure up as I fly at higher altitudes.  Just wondering if everybody else sees the same thing in their J's.

 

When I fly at 8000' or below, keeping everything where I want it is easy.  65% power, 8.7 GPH (LOP), cowl flaps closed I usually use 2400 RPM.  CHT's are usually below 380, oil temp just below 200, and oil pressure around 65 to 70 psi.  I usually use WOT unless at lower altitudes.  In that case I usually use about 20" MP and LOP.

 

However, when I climb to 10,000' or above that is not the case.  The other day we were at 11,500' for a couple hours over northern CA to avoid turbulence.  I had to increase RPM to 2600 with WOT and leaned to about 8.6 GPH.  That gave me a hottest CHT's between 380 and 390, oil temp about 205 or 210, and oil pressure of about 61 or 62 psi.  OAT was about 15C.  I had increased the RPM so peak pressure would occur further past TDC to reduce CHT temps.

 

I'm guessing it is because, even though the higher altitude should be cooler air, the indicated airspeed drops off faster than the temperature.  Indicated airspeed is a pretty much directly related to air mass flow over the cylinders.  So even though the power and true airspeed are about the same, there is less cooling air.  That makes the CHT's and oil temp go up.  The hotter oil makes the oil pressure lower.

 

Much higher and I might have to pop the cowl flaps open to the trailing position to keep things cool.

 

Agree?  Disagree?

 

Bob

[Super Dave]

As an alternative to trailing the cowl flaps, how about going a touch further LOP?  Might be less airspeed loss than opening the cowl flaps?

[carusoam]

Tell us more about how far LOP you were, since LOP starts 1° after peak. On some engines, LOP goes as low as 100°F before the engine shuts off...

Are you using a JPI for your data collection?

Why is your OAT reporting °C?

Did you try trailing the cowl flaps to see an improvement?

Best regards,

-a-

[Bolter]

Someone else should check my math and physics, but...   It seems that your fuel flow is essentially 65% power based on the standard formula of ~15HP per gal/hr.  Agreed.  However, LOP power at the high altitude conditions you describe may not be 65% power anymore.  Your fuel flow may no longer be LOP in your higher altitude examples, and you are operating at a high temperature 50°F ROP operating point.  Follow this to the point of ridiculous extension...if you were at 20k feet, would 8.5 GPH still be 65% power from the engine?  At some point you need to lean to peak below 8.5 GPH.

 

-dan

[aaronk25]

Your J has adjustable cowl flap linkages. When the cowl flaps are fully closed they should have 1/4" -1/2" of trail, if you don't want to run hot.

Try that first. Running more than 30 degrees LOP, to try to control CHTs is a inefficient way of reducing CHTs. Our MPG starts to suffer if the engine is leaned more than 30 degrees LOP. Your better off closing the throttle to a given MP then lean to 15-25 LOP.

Try it your self set up 23" for example and run 20LOP. Note your IAS. Then throttle up to 25" but reduce the fuel flow to whatever GPH you had at 23" and check your IAS, it will be lower than what you indicated at 23".

RUNNING 50 degrees LOP, is inefficient, but cool.

[Piloto]

One thing that really helps on cooling is having the cowl flaps adjusted to trailing 1/2" open when in the closed position. It will not affect your speed but for sure it will help cool down your engine even if you forget to open the cowl flaps on take off. 

 

At 15,000 feet I get 155kts TAS at 7.8gph. About 30F LOP.

 

José

[Lood]

I don't think this is really that important, but for what it's worth, during my last oil change, I went back to Shell W100 and my engine is running around 10 deg cooler - both the oil temperature and CHT's are down.

I don't want to highjack the thread with an oil debate, but this is evident in my engine. I changed over from Phillips AD100 and something else that I've noticed, is that my engine burns less Shell than it did Phillips. It just seems to like the Shell oil better.

[Bob - S50]

Thanks to all...

 

Carusoam,

 

1.  We do not have GAMI injectors but the stock injectors are good enough to run LOP.  I generally lean until the richest cylinder is about 10F LOP.  That puts the leanest cylinder about 30 or 40F LOP.  If I lean much more than that the engine starts to run a bit rough.

 

2.  Yes, we use a JPI EDM 730.

 

3.  We use C rather than F because all aviation weather is reported in C.

 

4.  Did not trail the cowl flaps because I was able to keep everything in limits without needing to do that... barely.

 

Super Dave,

 

Any further LOP and it starts to run rough.  Plus my goal is to set 65% power and cruise at 150k because that is what I flight plan and file at.  However, at lower altitudes, if the engine permits, I do in fact lean a bit more if I need to in order to keep the CHT's down.

 

tomcullen,

 

I was in fact LOP at 11,500'.  I checked by enrichening the mixture just a bit and watched all the EGT's go up.  I then leaned a bit again and watched all the EGT's go down.  The altitude I was at is just about exactly the highest you can go and get 65% power.  Anything higher and there is not enough air to develop 65%, so yes, 8.6GPH much above that altitude would in fact be ROP.

 

aaronk25 and Piloto,

 

Very interesting.  I did not know that about the cowl flaps.  I'll have to discuss that with my partners and see if we want to do that.  I also agree about LOP.  From what I've read, about 30 LOP is pretty close to the most efficient power setting you can get.  Leaner or richer than that and your economy starts to suffer.

[Bartman]

I say check the engine baffles. I have never seen anything near 380 in my J, except once time when I tried to run at peak or slightly ROP. I tend to run 2,400 and 15 LOP at 10,000 and see about 8 - 8.3 range on fuel flow. I think 8.7 at that altitude would put me at peak or slightly ROP, but may be a calibration difference between our two instruments. My flaps are about 1/2 inch in trail.

[aaronk25]

The difference between tightly closed cowl flaps and 3/8 of a inch open, via the linkage adjustment is worth 40 degrees CHT.

Try the cheap option first.....

[Guest]

Bob,

The cowl flap adjustment is in the manual, but is very easy. You will need a 7/16" wrench, with the cowl flaps fully closed they should be flush with the cowl. Open the flaps fully, disconnect the quick coupling, loosen the jamb nut on the cowl rod top and bottom, adjust the rod length until in the closed position the flaps are open 1/4-3/8". Tighten the jamb nuts and oil the ends of the rods. It is best that both ends remain engaged by at least one rod diameter or 14/".

Clarence

[Bob - S50]

Bob,

The cowl flap adjustment is in the manual, but is very easy. You will need a 7/16" wrench, with the cowl flaps fully closed they should be flush with the cowl. Open the flaps fully, disconnect the quick coupling, loosen the jamb nut on the cowl rod top and bottom, adjust the rod length until in the closed position the flaps are open 1/4-3/8". Tighten the jamb nuts and oil the ends of the rods. It is best that both ends remain engaged by at least one rod diameter or 14/".

Clarence

 

Thanks.  After the suggestion I looked in the manual and found it.  However, it did not say how to make the adjustment, just that you could open them a max of .25".  Now I know how easy it will be.

 

Bob

[FloridaMan]

I'd make sure your baffling is in check as well. I remember years ago some car magazine bought an old Camaro and did a bunch of bolt-on mods. The biggest performance increase came from the initial tune-up to factory specs. Proper baffling and sealing is incredibly important. On tightly cowled airplanes like ours, cooling is achieved by a high pressure differential between the inlet and exit of the cooling air. The air passing through the tight fins of our cylinders needs to pass through quickly and there's a lot of resistance in those fins. If there's a gap anywhere in the baffling, that air would much rather go there, thus reducing the high pressure side, and likely also bleeding into the low pressure side, further reducing airflow. This is why it is advised to not run engines like ours for very long with no cowling as they will overheat very quickly. 

[Bartman]

I would check or have someone check the baffling. 380 seems a bit hot for LOP in cruise. Just doesn't add up.

[smccray]

I've got about 65 hours in the Mooney now and I'm starting to notice that it is more difficult to keep the CHT's down, the oil temp down, and the oil pressure up as I fly at higher altitudes.  Just wondering if everybody else sees the same thing in their J's.

 

 

I've run into difficulty keeping the CHTs low, but at the altitudes and temps you reference above I haven't had that problem.  I would check your baffles.

 

 I generally run at peak EGT above 8000 ft with cowl flaps fully closed with peak CHTs at about 360.  I did have a problem in July at 10K ft MSL with an OAT at about 25 C flying from Denver to Dallas.  I tried leaving cowl flaps a little open and going a little LOP- I found that TAS was about the same with either condition so I saved fuel.  When I crossed into OK, OATs dropped to 18C and I was able to run at peak all the way home.

[Releew]

I switched from 15W-50 to Shell 100+ and the temperature dropped 10 Degrees plus on the CHT.  Oil runs cooler as well.  Engine seems to maintain oil level better at 6 quarts.  When I noticed the CHT getting up around 400F I just crack the cowl flaps .5" and the temp will run consistently at 375F on the warmest of days.

 

Rick