cherokeedriver
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I'm reopening a discussion regarding the mysteries of the best use of the Constant Speed Propeller. I own a 1969 Pa-32-300B, with a Lycoming IO-540-K1A5 engine and a Hartzell Prop Mod # HC-C2YK-1BF hub and F8475D-4 blades.
I just had the engine overhauled and it has caused me to carefully review my operations of the aircraft, including reviewing the Piper Owner's Handbook (POHl) as well as the Lycoming Owner's Manual 1976 (LOM) (PDF attached for those interested), which covers a variety of Lycoming O-540 motors, with specific references broken down by engine model number.
Using the table on page 37 of the Owner's Manual and data collected from LOM, I created a new and improved Power Settings Table (attached - use at your own risk). In particular I added the Fuel Burn info derived from LOM Figure 3-320. as well as some leaning instructions.
When looking at the new table, I noticed that when selecting a particular power profile (is 55%, 65%, 75%) that the Fuel Burn varies by about 1 GPH depending on the RPM selected. (ie: look at 55% at level at the 55% 165HP settings, at 2100 RPM the fuel burn is 11.2 GPM but at 2400 the GPH is 12.2 GPH. This example holds true across the board.
This data begs the following questions
(1) Why not always use the lowest available RPM settings when in cruise?
(2) The Piper Owner's Handbook (POH) page 24 states "When selecting cruise settings below 2300 RPMs, limiting manifold pressure for continuous operation, as specified by the Lycoming Operator's Manual should be observed". I have review the LOM carefully and find not specific comment relative the IO-540-K series engine on this point. (The only comment I see is at page 3-13 "Limiting manifold pressure for continuous operation of IO-540-C4b5, C4C5 and J4A5 with Hartzell Propeller HCE2Y type hub and 8465-7R blades. Do not exceed 27 inches manifold pressure below 2300 RPM.) SO -- does anyone know what the comment on page 24 of the Piper Owner's Handbook is referring to? (BTW my engine and prop hub and blades are stock). Can you refer to any specific document to support your answer?
(3) Can someone explain to me why, when using the Power Settings Table, you get different results in determining a given MP setting when you (A) calculate the MP, adjusted for nons-standard inlet air temp. using the footnote to Power Setting Table as compared to (B) first adjusting for the Inlet Temp. (OAT) for Density Altitude and then determining the MP setting?
EXAMPLE: A. Assume a desired 65% Power at 2400 RPMs at 6000 Pressure Alt at 100 degrees OAT. Using the table this results in 21.7 MP. PLUS 100-38 = 62 degrees = .62 * .18 = 1.1. 21.7 + 1.1 = adjusted MP of 22.8 MP.
EXAMPLE B: Same assumptions as above. The Pressure Alt. of 6000 Feet at 100 degrees F = Density Altitude of 9,000 FT. AT 9000 Ft at 65% Power at 2400 RPMs the MP setting is 21.0 inches.
Should not the results be the same? What am I missing? Why not adjust the Pressure Alt for DA first, before making the calculations? The engine doesn't know the Pressure Alt or the OAT, but responds to the control settings and the DA, no?
I also note (and was surprised to see this in the LOM) that when leaning below 75% power for economy to LEAN TO PEAK EGT. (Not rich of peak as I had traditionally been taught - and - obviously not LOP - as currently advocated by many. ) Referring to the table on 3-7 - the CHTs are LOWER at the peak EGT as compared to being slightly rich of peak EGT! I think I've been running rich of LOP for too long. This ins interesting and counter-intuitive. I normally equate higher EGT with higher CHT but this turns out not to be true at reduced power settings and peak EGT. I tried this on a flight last night - monitoring the EGT carefully on my GEM Engine Monitor - and it does work. At 65% and Peak EGT the CHTs were a comfortable 360 on all cylinders. FYI - I am not going to bother LOP operations till our engine is entirely broken in.
View attachment Lycoming_O-540_Operators Manual.pdf
View attachment PA-32-300 CHEROKEE SIX Power Settings Table (POH page 37) with disclaimer.pdf
I just had the engine overhauled and it has caused me to carefully review my operations of the aircraft, including reviewing the Piper Owner's Handbook (POHl) as well as the Lycoming Owner's Manual 1976 (LOM) (PDF attached for those interested), which covers a variety of Lycoming O-540 motors, with specific references broken down by engine model number.
Using the table on page 37 of the Owner's Manual and data collected from LOM, I created a new and improved Power Settings Table (attached - use at your own risk). In particular I added the Fuel Burn info derived from LOM Figure 3-320. as well as some leaning instructions.
When looking at the new table, I noticed that when selecting a particular power profile (is 55%, 65%, 75%) that the Fuel Burn varies by about 1 GPH depending on the RPM selected. (ie: look at 55% at level at the 55% 165HP settings, at 2100 RPM the fuel burn is 11.2 GPM but at 2400 the GPH is 12.2 GPH. This example holds true across the board.
This data begs the following questions
(1) Why not always use the lowest available RPM settings when in cruise?
(2) The Piper Owner's Handbook (POH) page 24 states "When selecting cruise settings below 2300 RPMs, limiting manifold pressure for continuous operation, as specified by the Lycoming Operator's Manual should be observed". I have review the LOM carefully and find not specific comment relative the IO-540-K series engine on this point. (The only comment I see is at page 3-13 "Limiting manifold pressure for continuous operation of IO-540-C4b5, C4C5 and J4A5 with Hartzell Propeller HCE2Y type hub and 8465-7R blades. Do not exceed 27 inches manifold pressure below 2300 RPM.) SO -- does anyone know what the comment on page 24 of the Piper Owner's Handbook is referring to? (BTW my engine and prop hub and blades are stock). Can you refer to any specific document to support your answer?
(3) Can someone explain to me why, when using the Power Settings Table, you get different results in determining a given MP setting when you (A) calculate the MP, adjusted for nons-standard inlet air temp. using the footnote to Power Setting Table as compared to (B) first adjusting for the Inlet Temp. (OAT) for Density Altitude and then determining the MP setting?
EXAMPLE: A. Assume a desired 65% Power at 2400 RPMs at 6000 Pressure Alt at 100 degrees OAT. Using the table this results in 21.7 MP. PLUS 100-38 = 62 degrees = .62 * .18 = 1.1. 21.7 + 1.1 = adjusted MP of 22.8 MP.
EXAMPLE B: Same assumptions as above. The Pressure Alt. of 6000 Feet at 100 degrees F = Density Altitude of 9,000 FT. AT 9000 Ft at 65% Power at 2400 RPMs the MP setting is 21.0 inches.
Should not the results be the same? What am I missing? Why not adjust the Pressure Alt for DA first, before making the calculations? The engine doesn't know the Pressure Alt or the OAT, but responds to the control settings and the DA, no?
I also note (and was surprised to see this in the LOM) that when leaning below 75% power for economy to LEAN TO PEAK EGT. (Not rich of peak as I had traditionally been taught - and - obviously not LOP - as currently advocated by many. ) Referring to the table on 3-7 - the CHTs are LOWER at the peak EGT as compared to being slightly rich of peak EGT! I think I've been running rich of LOP for too long. This ins interesting and counter-intuitive. I normally equate higher EGT with higher CHT but this turns out not to be true at reduced power settings and peak EGT. I tried this on a flight last night - monitoring the EGT carefully on my GEM Engine Monitor - and it does work. At 65% and Peak EGT the CHTs were a comfortable 360 on all cylinders. FYI - I am not going to bother LOP operations till our engine is entirely broken in.
View attachment Lycoming_O-540_Operators Manual.pdf
View attachment PA-32-300 CHEROKEE SIX Power Settings Table (POH page 37) with disclaimer.pdf
