More high CHT's etc. Some test results

[Kevin Horton]

The standard correction for CHT testing is found in FAA Advisory Circular 23-8C, section 23.1047 Cooling Test Procedures for Reciprocating Engine-Powered Airplanes.

The test data is corrected to determine predicted CHTs on a day with 100deg F at sea level, and normal lapse rate from sea level upwards, as follows:

Predicted CHT = Test CHT + (100 - 0.0036 * test pressure altitude - test OAT).

Example:
Test CHT = 400 deg F
Test pressure altitude = 4000 ft
Test OAT = 75 deg F

Predicted CHT on a day where the temperature is 100 deg F at sea level is 400 + (100 - 0.0036 * 4000 - 75) = 411 deg F.

In other words, the standard correction assumes that if the OAT increases 10 deg, the CHT would also increase 10 deg. This AC suggests the same correction would apply to oil temperature. For cylinder barrels, they apply a 0.7 factor to the correction - i.e. the cylinder barrel temperature is predicted to increase 7 deg F for a 10 deg F increase in OAT.

[jjconstant]

Thank you H?kan and Kevin...I love the expertise available on this site! This is very timely and useful information, as I'm in the process of finishing up the EZ Cool cowl flap installation and it will be great to have more valid numbers.

[az_gila]

Originally Posted by Kevin Horton

.......
In other words, the standard correction assumes that if the OAT increases 10 deg, the CHT would also increase 10 deg. This AC suggests the same correction would apply to oil temperature. For cylinder barrels, they apply a 0.7 factor to the correction - i.e. the cylinder barrel temperature is predicted to increase 7 deg F for a 10 deg F increase in OAT.

That is for calculation, not for measurements, correct?

If a cylinder barrel is less affected by OAT (30% less by those numbers) than a cylinder head, doesn't it then imply that in real life the measured CHT would probably lie somewhere between the two numbers?

Are our Lycoming cylinder barrels that much better cooled, and thermally isolated, than the aluminum cylinder heads?

Is this what I see when I notice that my measured CHT is not quite following the OAT in real life?

On a hot day here in AZ that 30% difference between barrel and head could easily be 20 F to 25 F

[DanH]

Originally Posted by hlangebro

Drilled the Power jet from a #42 to a #40. This really helped both CHT's and EGT's.
I average the CHT's dropped 10-12F

Sure. You reduced power.

I am an aerospace engineer.

Good. Measure upper plenum pressure. Measure the same in a stock Vans cowl. Compare, then consider the shape of your right inlet in the context of converting dynamic pressure to static pressure without separation losses. Right side view, from your build log:

[flyingriki]

Originally Posted by DanH

Sure. You reduced power.

Dan, how does more fuel reduce power? Have Ly-Con ported heads and they want to open up the jets too - why bad?
Thanks!

[DanH]

Originally Posted by flyingriki

Dan, how does more fuel reduce power? Have Ly-Con ported heads and they want to open up the jets too - why bad?
Thanks!

Best power mixture is ballpark 0.45~0.52 BSFC, maybe a little more. Richer or leaner results in less power.

From Haken's first post:
Climb, cowl flap full open, full rich, 115kias:
cht: #1 393, #2 408, #3 391, #4 379
137hp, 85%, fuel flow 12.3gal/hr. Fuel flow seems low....

Actually, if those numbers are accurate, BSFC is already at the upper limits for best power. A Lycoming at 25 degrees and WOT seems to ring in at 0.51~0.52:

(12.3 x 6)/137 = 0.53

Consider this Continental chart, used here because of its clarity. Peak power (at this MP and RPM) is 250 at about 108 lbs/hr, or 0.44 BSFC. This chart only goes to 120 lbs.hr, at which HP is down to 245 or so, about 0.49 BSFC. Of course CHT is dropping, here about 18 degrees between 0.44 and 0.49, and since it is dropping faster than Hp, some might consider it a good deal.



Of course, one might also argue that a better approach is to learn how to cool the engine at best power. No additional fuel cost, and you get to use all the engine you already bought. Failing that, retarding the ignition timing will also drop CHT, and again, it's free.

[prkaye]

I just finished finally resolving my high CHTs this summer. I reamed my main jet up from #42 to #40, AND I added a pair of lower cowl louvers (the ones sold by Spruce). I did both mods and then did some experimenting flying with the louvers covered and uncovered on the same day (to get meaningful comparisons). The results confirmed that the louvers made a much more significant difference in CHTs than enlarging the jet. Enlarging the jet alone only seemed to make a marginal difference in CHT (it did increase WOT fuel flow from 11.2GPH to 12.4GPH) but the louvers made a very significant difference.
I haven't done careful flight testing to determine the resulting loss in speed, but it doesn't seem to be more than 2 or 3 knots.
With the louvers, on a hot day (ground OAT 30 C, or 86 F) I was able to take off and climb at 1000ft/min continuously (to 8000ft) with hottest cylinder not getting above 395 F. In typical cruise all CHTs are sitting between 330 and 350. I'm very happy with these results. Before the mods on a hot day like that I would be up over 400 within the first 1000 feet of climb.
I luckily had some leftover blue paint from when my plane was painted, so I was able to paint the louvers to match the lower cowl (not that anyone looks under the plane on the ground anyway).
RV-9A, O-320, carb.

[Kevin Horton]

Originally Posted by az_gila

That is for calculation, not for measurements, correct?

Type certificated light aircraft are supposed to have enough cooling to operate at up to at least 100 deg F at sea level. It can be tough to find a day at exactly that temperature to conduct the test though. The intent of the correction described in AC 23-8C is to take test data at a day somewhat cooler than 100 deg F, and use the results to determine if the cooling would be adequate at 100 deg F. The correction is probably somewhat conservative - i.e. it probably predicts a CHT at 100 deg F that is slightly warmer than what you would actually find if you could do the test on a 100 deg F day.

If a cylinder barrel is less affected by OAT (30% less by those numbers) than a cylinder head, doesn't it then imply that in real life the measured CHT would probably lie somewhere between the two numbers?

The CHT wells are embedded in the cylinder heads, so I'd hope that the observed CHT was a good measure of actual CHT.

Are our Lycoming cylinder barrels that much better cooled, and thermally isolated, than the aluminum cylinder heads?

I suspect there are two factors at play:

1. the bottom of the cylinder barrel is bolted to the crankcase, which is a lot cooler than the cylinder head. This must provide some conductive cooling to the lower portion of the cylinder barrel.
2. the highest temperatures occur during the combustion when the piston is near the top of the stroke. The gas temperatures will be falling due to expansion as the piston moves toward the bottom of the stroke. Thus the cylinder barrels are subjected to average internal gas temperatures that are lower than seen by the cylinder heads.

Is this what I see when I notice that my measured CHT is not quite following the OAT in real life?

On a hot day here in AZ that 30% difference between barrel and head could easily be 20 F to 25 F

I think you are seeing that the standard correction is a bit conservative, as described above. I.e. it overpredicts hot day CHTs, to ensure that testing at cooler day does not allow a manufacturer to claim his design had adequate cooling, when in fact there would be a problem if you could find the correct temperatures for the testing.