Minimum CHT

[201er]

Someone mentioned being sure to stay above bottom of green arc for CHT. Also my mechanic cautioned me against flying at low power settings (below 65%) due to low CHTs.

1) What would the bottom of the green arc for CHTs be numerically? My factory CHT gauge is unreliable but I have an EDM 700. What numbers are you looking for on low end?

2) Is it really bad for the engine to be doing something like 45% power or whatever minimal power would be for maintaining altitude? You have to admit the fuel flow at low power settings is incredible. Could be going 20kts faster than a skyhawk for several GPH less. When you're just hanging local/sightseeing, what's the lowest power setting to use and what's wrong with using low power settings?

3) If low CHTs are bad, is it a good idea to lean for highest possible EGT or CHT during low power setting? My guess around 50ROP should yield the highest possible CHTs for that setting?

[KSMooniac]

I think any power setting that yields level flight at any speed above MCA is fine for CHTs... there isn't such a thing as shock cooling, and there is really no reason to worry about keeping CHTs above a minimum level.

[carusoam]

201er,

[1] Check the limitations section of your POH...

An example of the IO-550G (O2 POH)  There is a section on Power Plant Instrument Markings... Green Normal range= 250 - 460F.  My cyls are often in the 280 - 320 range.

According to my IO-360 POH the green range is 300 - 450F.

Your engine may vary, but your POH should state what the range is, including oil pressure and temperature ranges that you want to also maintain.

[2] Lowest power settings that will keep you afloat will probably be near (50% bhp) or something in the 16.5" - 17.5".  Any lower than that and landing will be assured.....

[3] These are fun experiments that I ran in my M20C.  The C had a huge yellow arc that takes away much of the low rpm range for extended periods.

[4] My mechanic had "warned" me about picking up additional lead deposits while operating at low rpm and cool temperatures.  My machine was highly un-instrumented, and I never found any issues because of my experiments.  Overall, I did not fly slowly for many flights.  Could be an OWT.  YMMV....

[5] You might also find the limits of your governor while experimenting in the low rpm range.  you might bump into the region where the gear pump is running too slowly to maintain the oil pressure to the prop and some oscillation of rpm may occur.  My M20C POH gives its lowest range for an O-360 as 1950rpm, @ 20" MP, 48.5% bhp, 6.1 gph

[6] An interesting resource you might be able to find is the thread on the gentleman who flew his mooney across the US recently.

http://www.mooneyspace.com/index.cfm?page=2&mainaction=posts&forumid=2&threadid=1050

 

http://razzolink.com/~jonathan/DreamAchieved.pdf

Best regards,

-a-

[201er]

POH states a maximum CHT and a CHT operating range but no minimum. The max is 475F and the oparting range is 300-475F. I'm kind of surprised that 280F could be considered worse than 460F.

[Piloto]

Below the green arc temperatures cylinder wear may accelerate for a new cylinder due to insuficient lubrication and tight rings gap tolerances.

José

 

[carusoam]

Jonathan's dream flight excerpts...

2. My power settings were 1950 RPM, 17 inches MP, leaned to 5.2 gph. At 13,000 feet my TAS was about 115 knots, indicating 95-97.

 

Best regards,

-a-

[takair]

Quote: 201er

Someone mentioned being sure to stay above bottom of green arc for CHT. Also my mechanic cautioned me against flying at low power settings (below 65%) due to low CHTs.

1) What would the bottom of the green arc for CHTs be numerically? My factory CHT gauge is unreliable but I have an EDM 700. What numbers are you looking for on low end?

2) Is it really bad for the engine to be doing something like 45% power or whatever minimal power would be for maintaining altitude? You have to admit the fuel flow at low power settings is incredible. Could be going 20kts faster than a skyhawk for several GPH less. When you're just hanging local/sightseeing, what's the lowest power setting to use and what's wrong with using low power settings?

3) If low CHTs are bad, is it a good idea to lean for highest possible EGT or CHT during low power setting? My guess around 50ROP should yield the highest possible CHTs for that setting?

[201er]

No no. Mid time engine. Mechanic was just talking about general practice. He said engine is better off running 65/75% than at lower power settings.