Operating below the Green - slow cruise

skidoo

skidoo

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skidoo
So, my POH (Turbo 182T )charts operating conditions from 18" mp to 28" mp and 2000 to 2400 rpms. This is shown as the Green area on the instruments. At 18" and 2000 rpm, which yields 108 kts at 10,000 ft. Now, if I just want to putt around at 75 kts instead, I would have to reduce power to say 14" mp. This is 4" below the green area and the POH doesn't chart that power level which could be near 35% power.

Does any one see anything wrong with or bad for the engine by operating below the green like this for hours at a time? Any ideas why the mfg would stop charting at 18" mp?
 
I think the bottom of the green on the prop is a limitation on the prop more than the engine, isn't it?

Kinda like driving the stick shift around in 5th gear at 10 MPH for the engine.

But isn't the limitation on how long and how much oil pressure it takes to hold the prop at that steep a pitch for hours on end? Something about pushing on the spring or the distance the piston is forward being a problem?

You've got me curious now too.

I do know at higher altitudes if I pull back the prop to the bottom of the green at WOT, I'm not going anywhere fast. Going further would be so slow I might as well grab a Skyhawk for the trip. The Skyhawk might even beat me there.
 
Ooh I just noticed you're turbocharged too. That'd really be "lugging" the engine. Mine won't produce enough power up here to worry about being crazy "over-square" anyway.

At sea-level you'd have to grab a handful of throttle out of your cruise flight config with that prop that far back.
 
How is it lugging the engine? The a/c has a constant speed prop that adjusts to keep RPM set where it needs to be.

If low torque/power is input to the crankshaft, the pitch will flatten out to maintain rpm. Your fuel flow will be lower, matching the lower power generation.

Lugging would be a fixed pitch prop in level flight below a max endurance airspeed. If then.
 
Its possible that with the MP that low, your engine would not produce enough heat to keep operating temps within a normal range if you operate for an extended time. Possibly it would get cold and cause spark plug fouling etc...

Other than that, I can't see any reason why not. Lugging is a high MP and low RPM condition... not a low MP and normal RPM setting (i'm assuming the prop - and RPM will be set to keep it in the "green" range)
 
dell30rb said:
Its possible that with the MP that low, your engine would not produce enough heat to keep operating temps within a normal range if you operate for an extended time. Possibly it would get cold and cause spark plug fouling etc...

Other than that, I can't see any reason why not. Lugging is a high MP and low RPM condition... not a low MP and normal RPM setting (i'm assuming the prop - and RPM will be set to keep it in the "green" range)
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Yes, I'm talking about rpms about 2000 to 2200 with MP below 18" such as 12 to 16" MP, where 18" mp is the Green/White transition.

Also, my CHT's could be say from 280 to 320 deg F.
 
Someone may well provide proof that such operation will allow the engine to survive for 10,000 hours...but you know what - it just feels bad to do it!
 
Let'sgoflying! said:
Someone may well provide proof that such operation will allow the engine to survive for 10,000 hours...but you know what - it just feels bad to do it!
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Well, we are all taught to be able to operate slow flight. But, in order to do so, and maintain altitude, it would need to operate below the MP green. So, I am wondering what is wrong (if anything) with operating this way for a long time.
 
Let'sgoflying! said:
Someone may well provide proof that such operation will allow the engine to survive for 10,000 hours...but you know what - it just feels bad to do it!
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A good portion of engine wear occurs at engine start up as the oil pressure hasn't reached all the moving parts. Operating at different power settings will have little to do with wear so long as temperatures are kept in the propper ranges
 
Continental suggests not operating the engine with low MPs on long descents. Ring flutter or something along those lines.
 
skidoo said:
Well, we are all taught to be able to operate slow flight. But, in order to do so, and maintain altitude, it would need to operate below the MP green. So, I am wondering what is wrong (if anything) with operating this way for a long time.
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just make sure your temps aren't dropping too low. if you find your temps sinking and you still want to maintain 75 knots, why not throw in some flaps to generate some drag and require a higher power setting to maintain your airspeed
 
And I realize that I *completely* misread the original question and had it backward. I thought he wanted to operate below the bottom of the green arc on the RPM.

Disregard all after Good Morning, Over. :)
 
dell30rb said:
just make sure your temps aren't dropping too low. if you find your temps sinking and you still want to maintain 75 knots, why not throw in some flaps to generate some drag and require a higher power setting to maintain your airspeed
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Changes how the plane flys
 
flyingron said:
Continental suggests not operating the engine with low MPs on long descents. Ring flutter or something along those lines.
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Well in a decent you could end up with the prop driving the engine, that might make the rings flutter a bit, but in cruise that won't happen
 
And now that I read through the thread... :rofl: ...

Yes, extended operation at low MP can cause the problem where the ignition stroke may not provide enough pressure to properly push the rings against the cylinder wall.

Off-shore fishing patrols fly like this all the time "loitering", and I've seen some articles on the differences they see in engine care in the past, but having a heck of a time finding one of them right now.
 
flyingron said:
Continental suggests not operating the engine with low MPs on long descents. Ring flutter or something along those lines.
Click to expand...

I suppose I can see that.. the higher pressures will keep the rings pressed tight against the cyl wall.

Possibly you could set your prop at the bottom of the green arc which may balance things out a bit. I can see a high rpm, low MP condition as one which may cause "ring flutter". Piston / rings moving up and down really fast with not enough manifold pressure to keep them seated at such a high rpm. By lowering the rpm you could better match RPM and MP so that the engine is not lugging nor is it revved up way high.
 
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Skylane81E said:
Changes how the plane flys
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True I can see how it might be a little awkward at that speed. I wouldn't go full flaps obviously. But it will improve his over the nose visibility.. i'm guessing he is doing some inspecting or just sightseeing?
 
denverpilot said:
I think the bottom of the green on the prop is a limitation on the prop more than the engine, isn't it?

Kinda like driving the stick shift around in 5th gear at 10 MPH for the engine.

But isn't the limitation on how long and how much oil pressure it takes to hold the prop at that steep a pitch for hours on end? Something about pushing on the spring or the distance the piston is forward being a problem?

You've got me curious now too.

I do know at higher altitudes if I pull back the prop to the bottom of the green at WOT, I'm not going anywhere fast. Going further would be so slow I might as well grab a Skyhawk for the trip. The Skyhawk might even beat me there.
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Kinda, but if you want to do 10mph in 5th gear, and the car will do it smoothly, it will be the most efficient gear to use.

Maximum no wind efficiency is going to come when you pull the prop as far back as you can and set the throttle to whatever power required to fly at L/D Max.
 
dell30rb said:
I suppose I can see that.. the higher pressures will keep the rings pressed tight against the cyl wall.

Possibly you could set your prop at the bottom of the green arc which may balance things out a bit. I can see a high rpm, low MP condition as one which may cause "ring flutter". Piston / rings moving up and down really fast with not enough manifold pressure to keep them seated at such a high rpm. By lowering the rpm you could better match RPM and MP so that the engine is not lugging nor is it revved up way high.
Click to expand...

+1

Low cylinder temperatures mean a higher likely hood of fuel dilution in the oil. Carbon build up on the pistons and possible ring jacking. Spark plug fouling from low combustion temps. Carbon build up on the valve seats, etc.
 
dell30rb said:
Its possible that with the MP that low, your engine would not produce enough heat to keep operating temps within a normal range if you operate for an extended time. Possibly it would get cold and cause spark plug fouling etc...
Click to expand...
That's the biggie. If the oil temp stays above 160-170F to burn out contaminants, the oil pressure stays up in the green arc, and the CHT stays above 150F (Lycoming says "for maximum service life of the engine maintain CHT between 150F and 400F during continuous operation"), you should be fine as long as you lean enough to keep the EGT up somewhere near peak to activate the lead scavenging agents in the fuel. And on that last point, you may be amazed how close to cutoff that is.

As for operating at reduced RPM and higher MP as discussed above, consult your Engine Operator's Manual for the maximum MP at any given RPM. If you don't have the manual for your Lycoming engine, see Lycoming SL 114 for how to get one -- and it's really worth the price.
 
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Ring flutter occurs when the propeller is windmilling at a higher rpm than the engine power setting would produce.
 
I don't see a problem with it per se, I also don't see why you'd do it. You might burn a bit less fuel, but be going that much slower. The engines aren't as efficient in that range. You've got a turbo, use it.
 
There are so many misconceptions in this thread I don't know where to start.

But you will not harm the engine at low power settings. But you will not get efficient operation either.

the green arc on the gauge tells you that is where the cam grind is at its best operating range, that's all.

If you could harm an engine at low power settings how could you idle it?

For the OP, get the publication from TCM called "Tips On Engine Care" and read it, and read it, over again..

You can not harm any engine by removing fuel or manifold pressure. all it will do is quit, when too much is removed.

My Edit, the 182T has the Lycoming IO-540 Lyke
 
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Steve said:
Ring flutter occurs when the propeller is windmilling at a higher rpm than the engine power setting would produce.
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When you are maintaining a level attitude you have enough power applied to prevent that from happening.
 
flyingron said:
Continental suggests not operating the engine with low MPs on long descents. Ring flutter or something along those lines.
Click to expand...

When the prop is driving the engine that can happen, but in a level attitude, no. you will have enough power applied to prevent this.
 
skidoo said:
So, my POH (Turbo 182T )charts operating conditions from 18" mp to 28" mp and 2000 to 2400 rpms. This is shown as the Green area on the instruments. At 18" and 2000 rpm, which yields 108 kts at 10,000 ft. Now, if I just want to putt around at 75 kts instead, I would have to reduce power to say 14" mp. This is 4" below the green area and the POH doesn't chart that power level which could be near 35% power.

Does any one see anything wrong with or bad for the engine by operating below the green like this for hours at a time? Any ideas why the mfg would stop charting at 18" mp?
Click to expand...

In reality if you are flying at 75Kn, in a 182 you will have plenty of power applied to not worry about harming the engine. or you will be sinking like a rock.
 
Tom-D said:
In reality if you are flying at 75Kn, in a 182 you will have plenty of power applied to not worry about harming the engine. or you will be sinking like a rock.
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So true. 90 knots with 10 flaps is a nice stable comfortable "Let's not get anywhere in a hurry" speed in 182s.

75 is a good number for base leg if decelerating... 85 downwind, 75 base, 65 initial on final... then keep slowing to Vref for your load... ;)

90 with 10 degrees of flap hanging out is also really stable for aerial photography, and isn't so wicked fast you'll go blowing by whatever you're trying to take a photo of.
 
TripleZ said:
+1

Low cylinder temperatures mean a higher likely hood of fuel dilution in the oil. Carbon build up on the pistons and possible ring jacking. Spark plug fouling from low combustion temps. Carbon build up on the valve seats, etc.
Click to expand...


Not if you are properly leaned.
 
Henning said:
Kinda, but if you want to do 10mph in 5th gear, and the car will do it smoothly, it will be the most efficient gear to use.

Maximum no wind efficiency is going to come when you pull the prop as far back as you can and set the throttle to whatever power required to fly at L/D Max.
Click to expand...

Kinda

You want to use as low of an RPM as possible - but there is a point where you go so far behind the engine's power curve that you actually shoot yourself in the foot.

The motor, due to cam setup (and a host of other factors such as compression and ignition timing) has a sweet spot for efficient operation. For my v8 jeep this is around 1700-2500 rpm. I can go smoothly at 30 MPH in 5th gear at 1200 rpm, but its way behind the power curve and takes about half throttle to stay there. If I downshift one gear and run at 1700 rpm, the motor will run more efficiently because the cam / ignition / compression ratio is designed to be most efficient in that rpm zone.

What is best glide speed for a 182? I'd imagine the speed for maintaining altitude with the least amount of power is just above that. If 90 knots and 10 degrees of flaps is comfortable then wouldn't 75 knots and 20 deg. be pretty comfy?

If you're properly leaned but running cold you can have issues. They will be much slower to materialize than if you are rich and cold however.. to the point where its probably negligible, as long as its not really cold.
 
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dell30rb said:
Kinda

You want to use as low of an RPM as possible - but there is a point where you go so far behind the engine's power curve that you actually shoot yourself in the foot.

The motor, due to cam setup (and a host of other factors such as compression and ignition timing) has a sweet spot for efficient operation. For my v8 jeep this is around 1700-2500 rpm. I can go smoothly at 30 MPH in 5th gear at 1200 rpm, but its way behind the power curve and takes about half throttle to stay there. If I downshift one gear and run at 1700 rpm, the motor will run more efficiently because the cam / ignition / compression ratio is designed to be most efficient in that rpm zone.

What is best glide speed for a 182? I'd imagine the speed for maintaining altitude with the least amount of power is just above that. If 90 knots and 10 degrees of flaps is comfortable then wouldn't 75 knots and 20 deg. be pretty comfy?
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Right, but that all comes when the plane is correctly designed, propped and powered. If the aircraft is properly engined and propped It will be "Coming on the pipe" and operating within a comfortable torque band when at L/D max even at "full coarse". That is a big part of what the bottom end of the governor and max course pitch are designed to keep you from getting into trouble with.
 
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Yes, I like going slow for photography. I take lots of pictures. But, I like the option to get somewhere faster when needed and climb over the mountains as needed or high elevation fields, hence desire for the turbo.

So, I think I will go out tomorrow and make some notes on a few slow settings. Thanks for the input!
 
Somthing I have noticed in my NA 182 is that I almost always get the same MPG. This makes sence if you think of LD max being below where the engine has it's best BSFC (brake specific fuel consumption, or hp/fuel burn) so as the airframe becomes less efficent the engine becomes more so. Now this stops somewhere over 75% power when the power enrichment is dumping in more fuel but for the most part KIAS/10=gal/hr, or close enough for government work anyway:wink2:
 
I can not feel my fingers. Wooooooo and since when did...: it was only a BJ become slang for I don't want to see u again. Whore
 
bob_albertson said:
I can not feel my fingers. Wooooooo and since when did...: it was only a BJ become slang for I don't want to see u again. Whore
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:rofl::rofl::rofl::rofl::rofl: That has got to be in contention for "greatest degree of thread drift in one post" ever....
 
Bob's either drunk posting or someone else has gotten ahold of his phone and he's going to beat their ass later...
 
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