Cruise RPM Setting
- Thread starter sr30sb
- Start date
Good you started this thread. this topic has been discussed elsewhere in the forums, but mostly as thread drift, so very difficult to recall and/or find.
Preface: I do not fly xc for economy, I fly for speed. My Tiger could be leaned and tweaked to about 9gph, at slightly under 130kts, it would go 137 at 10+gph. The extra fuel was worth it to me.
In my mouse I typically pushed the throttle in and left it there for the day, 2600 rpm above 5000 DA, about 9gph. Down low, looking at the lake on a joy ride, 2350. In all cases I lean agressively, the only time the red knob is all the way in is at startup and climbout. Once above 5000DA, I lean for best power and CHT cooling. After installing the PF exhaust, I did have to reduce throttle a bit to stay under 2700rpm, but typically cruised at 2550 to 2600 at around 8k ft, it seemed to like that altitude and throttle combo. 115kts.
Others will vary. However on the lycoming O-320 and O-360 especially with chrome cylinders you really need to keep the power above 2500rpm or you will start using oil. Marty has a good story about low power settings on his Sierra.
Preface: I do not fly xc for economy, I fly for speed. My Tiger could be leaned and tweaked to about 9gph, at slightly under 130kts, it would go 137 at 10+gph. The extra fuel was worth it to me.
In my mouse I typically pushed the throttle in and left it there for the day, 2600 rpm above 5000 DA, about 9gph. Down low, looking at the lake on a joy ride, 2350. In all cases I lean agressively, the only time the red knob is all the way in is at startup and climbout. Once above 5000DA, I lean for best power and CHT cooling. After installing the PF exhaust, I did have to reduce throttle a bit to stay under 2700rpm, but typically cruised at 2550 to 2600 at around 8k ft, it seemed to like that altitude and throttle combo. 115kts.
Others will vary. However on the lycoming O-320 and O-360 especially with chrome cylinders you really need to keep the power above 2500rpm or you will start using oil. Marty has a good story about low power settings on his Sierra.
spannaviation
Orbiting Earth
In my Sierra, I fly Throttle full open, 2500, and 10.3
Rick
Rick
Bill this is RapTheory, pls wait for MartyFact, MarkFact, etc.
At higher power settings, which translates to higher RPM the explosion in the cylinder is much more violent thereby pushing harder on the rings and making them expand in the cylinders. That seals the rings against the cylinder walls better and IMHO makes for less chance for oil to creep into the combustion chamber and a few milliseconds later your exhaust. I think my theory is supported by Lycoming when they insist upon high rpm/pressure to "seat the rings" at break in. Seat the rings is a misnomer. You are seating the cylinder walls if you have steel, and only seating the rings if you have chrome cyls. Semantics. I still call them Kleenex even if I use Kroger brand.
Rap
markweiss
Headed to Andromeda
“Explosions and violenceâ€
Rap, have you been talking to Arnie again?
Higher power settings result in greater ICP (internal cylinder pressure) which forces the rings against the bottom edge of the ring land (piston groove), allowing that force to be exerted on the inner face of the top compression ring, forming a better seal.
This, in turn, reduces blow-by (of the cylinder/ring face) and crancase pressure/venting.
At higher power settings, (higher ICP’s) do you burn less oil, or do you chuff less oil?. Answer.....Both.
Rap, have you been talking to Arnie again?
Higher power settings result in greater ICP (internal cylinder pressure) which forces the rings against the bottom edge of the ring land (piston groove), allowing that force to be exerted on the inner face of the top compression ring, forming a better seal.
This, in turn, reduces blow-by (of the cylinder/ring face) and crancase pressure/venting.
At higher power settings, (higher ICP’s) do you burn less oil, or do you chuff less oil?. Answer.....Both.
mvanover
Headed to Andromeda
I've been under the weather lately, sorry for the hiatus.
Mark has it right. However, RPM is not the only factor. These big bore cylinders (both the O-320s and IO-360s have 5-1/8" dia cylinder bores) need the "internal cylinder pressure" (aka "Brake Mean Effective Pressure") to keep the rings expanded. Therefore, manifold pressure is an important factor. Of course, with the fixed pitch props you only have RPM to control. There is no real limitation on RPM, except redline. These engines are certified for continuous operations at max power. Of course, you have to be at sea level to do that. The pilot selects the RPM for the type of flight he/she is engaged in. I tend to set 65-75% power by the POH on all airplanes. You can go lower, but your oil consumption will increase as does the risk of plug fouling.
Forgetting this, I flew along side a Sundowner for a day and found I had burned about 3 of my 7 quarts of oil on arrival at the intermediate stop. My RPM was 2500 (as my IO-360 seems to be smoothest there) but the manifold pressure was just 17-18 inches. Backing the RPM down to 2360 and bringing the manifold pressure up to 21-22" produced the same airspeed but reduced the oil consumption to almost normal levels. On most cross countries I leave the throttle forward and would reduce RPM to 2600 for second segment climb. At 5,000 ft I lean to best power mixture. At cruise I reduce the RPM to 2500 and lean until the 3rd cylinder peaks (average of ~peak EGT). This setting costs only 4% of cruise power for ~9 gallon per hour fuel flow and would lose about 4 kts over my best power mixture fuel burn of 12 gph. When Mark W flies my airplane we loose 6 kts with just under 8 gph. I guess at his fuel prices you squeeze as much out of a gallon as you can.
Mark has it right. However, RPM is not the only factor. These big bore cylinders (both the O-320s and IO-360s have 5-1/8" dia cylinder bores) need the "internal cylinder pressure" (aka "Brake Mean Effective Pressure") to keep the rings expanded. Therefore, manifold pressure is an important factor. Of course, with the fixed pitch props you only have RPM to control. There is no real limitation on RPM, except redline. These engines are certified for continuous operations at max power. Of course, you have to be at sea level to do that. The pilot selects the RPM for the type of flight he/she is engaged in. I tend to set 65-75% power by the POH on all airplanes. You can go lower, but your oil consumption will increase as does the risk of plug fouling.
Forgetting this, I flew along side a Sundowner for a day and found I had burned about 3 of my 7 quarts of oil on arrival at the intermediate stop. My RPM was 2500 (as my IO-360 seems to be smoothest there) but the manifold pressure was just 17-18 inches. Backing the RPM down to 2360 and bringing the manifold pressure up to 21-22" produced the same airspeed but reduced the oil consumption to almost normal levels. On most cross countries I leave the throttle forward and would reduce RPM to 2600 for second segment climb. At 5,000 ft I lean to best power mixture. At cruise I reduce the RPM to 2500 and lean until the 3rd cylinder peaks (average of ~peak EGT). This setting costs only 4% of cruise power for ~9 gallon per hour fuel flow and would lose about 4 kts over my best power mixture fuel burn of 12 gph. When Mark W flies my airplane we loose 6 kts with just under 8 gph. I guess at his fuel prices you squeeze as much out of a gallon as you can.
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markweiss
Headed to Andromeda
On the “oil consumption†topic, the throttle position has a major influence.
The induction side of the engine includes more than the intake manifold, and, in a simple aircraft engine, is always at a lower pressure than anything else (crankcase or atmosphere). The inlet valve rattles around in the valve guide and the valve stem is lubricated by oil that is moved by this pressure differential. Ultimately, that oil passes the valve guide and mixes with the intake charge, where it is (mostly) burnt.
WOT results in the lowest possible pressure differential between the intake system and the atmosphere, so less oil passes the inlet valve.
Note; this refers to normally aspirated, basic aircraft engines. It is not intended to be a universal comment.
The induction side of the engine includes more than the intake manifold, and, in a simple aircraft engine, is always at a lower pressure than anything else (crankcase or atmosphere). The inlet valve rattles around in the valve guide and the valve stem is lubricated by oil that is moved by this pressure differential. Ultimately, that oil passes the valve guide and mixes with the intake charge, where it is (mostly) burnt.
WOT results in the lowest possible pressure differential between the intake system and the atmosphere, so less oil passes the inlet valve.
Note; this refers to normally aspirated, basic aircraft engines. It is not intended to be a universal comment.
I have found that in the Bonanza my manifold pressure drops off as I climb (as all engines do) which allows me to leave the throttle in at most cruise altitudes. I lean above 8’ k to best power but I watch my CHTs to see if the engine is happy. I’ve found at 8.5 I can only lean to 11.5 GPH. At 9.5 I can go to 10 GPH and the settings continue down as I go up. I figure at 18k the plane would owe me fuel.
mvanover
Headed to Andromeda
I've been to 17,900 feet in my Sierra. All indications are really low. Especially, OAT, fuel flow and airspeed. I was leaning to best power to top a thunderhead with WOT, 2700 RPM getting almost 100 fpm ROC. I topped the clouds level at 17.9K, OAT -15C, FF 4.8 gph, KIAS 83 (111 KTAS). I didn't think to go lean of peak to see what minimum fuel flow would keep me at cruise. I'll bet it wouldn't have been much. I made it to lower altitudes shortly afterwards without incident, but I doubt I'd ever do that again.
Col Mike
Climbing to Cruise
Wow are any you gentlemen have any upcoming classes?[emoji851]
My SD 2161W is in the mid teens if I don’t try to lean but only at 2500-3000MSL usually. I just got a PowerFlo on her and hope that helps as I learn to lean but your saying not too much tho.
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My SD 2161W is in the mid teens if I don’t try to lean but only at 2500-3000MSL usually. I just got a PowerFlo on her and hope that helps as I learn to lean but your saying not too much tho.
Sent from my iPhone using Tapatalk
I typically fly between 8500 and 10500 in my Sundowner in the Western US. And WOT most flights, where I usually see 2570 to 2600 RPM in level flight. Last week, I flew a x-c flight from Mesquite 67L to Fallon KFLX @10500 and burned 9 gph, leaned aggressively. And I have a Powerflo exhaust.
Col Mike
Climbing to Cruise
Good to hear, thanks.
mvanover
Headed to Andromeda
You might want to attend BACFest this year.
