The blue knob

[coloradobluesky]

Here a simplified version of how I do mine in a 0-360 with a Hartzell Constant Speed.
Take off with prop knob all the way in. Climb at high rpm to cruise altitude.
Pull the throttle back and look at the manifold pressure gauge.
Economy cruise calls for pulling throttle back to 18".
Twist the prop knob from 2700rpm to 2000rpm (and the manifold gauge goes from 18" to 20").
Now in economy cruise of 20"/2000rpm (20/20)
Its not the only way, but its a pretty common way and works well.
Of course you can substitute your own numbers. Fastest speed is both prop and throttle full forward.

 

[SkyDog58]

This is true, but shy of something like a fully automatic prop on something like a PC12, there is a reason that knob turns.

Oh, agreed! I was just being picky.

 

[LDJones]

I can't think of a reason, can't find a reason, not to leave it full forward.

A well-respected Mooney instructor had the philosophy that an engine had a finite number of revolutions in it, so advocated using the lowest RPM appropriate for the phase of flight and mission at hand. If 22"/2600 RPM gave the same power as 24"/2400 RPM why burn through 200 more revs per minute than you need to?

 

[James331]

You also won't find any working piston pounders that leave the prop at max forward

 

[Checkout_my_Six]

depends.....

 

[MAKG1]

A CS prop and a fixed setting might as well be a fixed pitch.

In climb having a high rpm and taking a smaller bite of air makes sense, but in cruise when the engine doesn't need to put as much work into climbing, let the prop take a bigger bite of air and spend the climb energy going forward. If that makes sense.

My R1 can do nearly 100mph in first gear, but it dos well over that in 6th and its a lot happier doing it.

Not the same.

If I leave a 182 full forward and dive up to Vno, it will stay at 2400 RPM, even at full throttle. If I dive a 172 at full throttle, it will overspeed.

Fixed RPM requires a variable pitch and a governor.

 

[MAKG1]

A well-respected Mooney instructor had the philosophy that an engine had a finite number of revolutions in it, so advocated using the lowest RPM appropriate for the phase of flight and mission at hand. If 22"/2600 RPM gave the same power as 24"/2400 RPM why burn through 200 more revs per minute than you need to?

To keep the cylinder pressures and temperatures down, to maximize lubrication and engine cooling (much of it is done by the oil), and perhaps to reduce configuration complexity with a lot of altitude changes. I'll leave the blue knob high for CAP searches and photos over nontrivial terrain, for the last reason, where I may have to adjust altitude by 500 feet every few minutes.

Lower speed WILL decrease the tach times, which can be relevant for maintenance costs. But detonation can dramatically reduce TBO.

 

[evapilotaz]

It's also a volume control for the noise from the prop tips. Also comes in handy for lowering CHTs sometimes.

Take off with all knobs forward (if using sea level mixture), pull back the blue knob a little so as not to bother the neighbors in the climb, pull it back a bit more if you want a comfy cruise, leave it back until base or final to make the neighbors happy again, then full forward again prior to landing.

Also, some emergency checklists say to haul back on the blue knob to get best glide in some planes.

I like your simplied answer. Thanks

 

[James331]

Not the same.

If I leave a 182 full forward and dive up to Vno, it will stay at 2400 RPM, even at full throttle. If I dive a 172 at full throttle, it will overspeed.

Fixed RPM requires a variable pitch and a governor.

This is true, but not really what I was getting at.

 

[FastEddieB]

Horsepower=RPM x Torque x constant. Pull back on the throttle and you reduce HP. Pull back on the prop and you also reduce HP.

That's not necessarily true, especially LOP if you keep a constant fuel flow.

Increased TAS at lower rpm on the same fuel flow is what pilots are seeing on the Cirrus forum.

Here's what I found, autopilot hugging altitude in calm air @ 11,500' in my SR22:

I gained just a tad by coming back about 50 rpm. Other pilots are finding other "sweet spots".

 

[FastEddieB]

A well-respected Mooney instructor had the philosophy that an engine had a finite number of revolutions in it...

Well, I respect him but not his philosophy.

Sounds logical, but there does not appear to be much if any data to support it. When Continental gives a recommend TBO of 2,000 hours for an IO-550N @ 2,700 rpm, nowhere do they state nor imply that if you turn the motor slower it will last longer. I've never seen a chart correlating reduced rpm to longer life, have you?

In a properly designed engine, there should be effective no metal-to-metal contact regardless of rpm - if and when there is the engine is minutes or even seconds away from failure. It is also conceivable that running an engine under more load at a reduced rpm may decrease longevity by increasing internal combustion chamber pressures and CHT's.

That's my non-engineer take on the matter, anyway!

 

[flyingron]

A well-respected Mooney instructor had the philosophy that an engine had a finite number of revolutions in it,

This premise is indeed bunk for the reasons you state. Higher MP/lower RPM does tend to be more efficient in most prop configurations. However, it's not an engine life thing. In fact, Continental on some engines advises keeping the RPMs at 2400 during cruise because it enhances lubrication.

What kills engines is usually HEAT: either insufficient lubrication or ineffective cooling. It's right up there with the shock cooling myth. They pretty much beat the cylinder failure to death to find it is almost always excessive heating that causes the failure, not excessive cooling.

 

[Ted]

Amazing. Yes how about RTFM.

It amazes me how many people reject this concept. The OEMs did a remarkable amount of testing to approve particular RPM/MP combinations, and the ones they didn't approve have good reasons for it.

Keep in mind this can also change with time. For example, Continental has a service bulletin out that prohibits cruise RPM below 2300 for certain engines. This is a good one to follow since they discovered there are harmonics that will literally tear the engines apart internally, making for a very bad day. I'm not aware of any similar SB for Lycomings, but Continental owners should look into this SB. I only run the 414 at 2300+ RPM because of it.

 

[James331]

It amazes me how many people reject this concept. The OEMs did a remarkable amount of testing to approve particular RPM/MP combinations, and the ones they didn't approve have good reasons for it.

Keep in mind this can also change with time. For example, Continental has a service bulletin out that prohibits cruise RPM below 2300 for certain engines. This is a good one to follow since they discovered there are harmonics that will literally tear the engines apart internally, making for a very bad day. I'm not aware of any similar SB for Lycomings, but Continental owners should look into this SB. I only run the 414 at 2300+ RPM because of it.

Agreed, always best to go off the POH, I selected the altitudes and setting which work for my mission and put it on the back of my checklist.

 

[Timbeck2]

It's an airplane transmission.

You take off in low gear and can back it off in cruise.

CS props are not limited to HP airplanes. They were even in some 172s.

Forrest Gump voice on:

"MAKG1 always had a way of explainin' thangs so's I can unnerstand theum."

Forrest Gump voice off

 

[eman1200]

huh uh, huh huh huh uh....you said 'knob'

 

[Fearless Tower]

It amazes me how many people reject this concept. The OEMs did a remarkable amount of testing to approve particular RPM/MP combinations, and the ones they didn't approve have good reasons for it.

Keep in mind this can also change with time. For example, Continental has a service bulletin out that prohibits cruise RPM below 2300 for certain engines. This is a good one to follow since they discovered there are harmonics that will literally tear the engines apart internally, making for a very bad day. I'm not aware of any similar SB for Lycomings, but Continental owners should look into this SB. I only run the 414 at 2300+ RPM because of it.

Not disagreeing with you, but isn't it a bit ironic that we follow the OEM guidance for the blue and black knobs but reject what they say about the red knob?

 

[JimNtexas]

My first car was a 1962 Ford Falcon, and my next car was an early 60s VW bug. It's always been obvious to me that the prop is the airplane's transmission.

I'd suggest every flight school get an old manual transmission beater car with a small engine, have the students drive up and down hills. The Falcon would be best because the windshield wipers are vacuum powered. So it teaches the relationship between manifold pressure and rpm (at least if you drive in the rain).

Edited to add:

An airplane with a fixed pitch prop is like a VW bug that is stuck in a gear. If the prop is a climb prop our hypothetical VW is stuck in second, if the prop is a cruise prop our VW is stuck in third gear.

If we replace our fixed pitch prop with a constant speed prop then our airplane now has essentially four gears. Low gear for takeoff, second gear and third gear for climbout, and high gear for cruising flight.

 

[denverpilot]

Not disagreeing with you, but isn't it a bit ironic that we follow the OEM guidance for the blue and black knobs but reject what they say about the red knob?

Some of the aircraft manufacturer's guidance in the POH is flat useless - the tach green arc on an O-470 for one example. Continental has no upper limit for continuous ops on the engine but Cessna painted on on the tach for cruise. Many people faced with a high DA climb in the mountains will refuse to push the blue knob forward not realizing that green arc is meaningless to the folks at Continental.

Not true for all engines or aircraft. But on the 182 if you need to climb at reduced power capability at DAs way up above 10,000 on a hot day, just firewall the thing.

It's worth digging to see why stuff is painted on tachs and what not. The paint on the ASI has a legal meaning. On the tach, not so much.

 

[mondtster]

Some of the aircraft manufacturer's guidance in the POH is flat useless - the tach green arc on an O-470 for one example. Continental has no upper limit for continuous ops on the engine but Cessna painted on on the tach for cruise. Many people faced with a high DA climb in the mountains will refuse to push the blue knob forward not realizing that green arc is meaningless to the folks at Continental.

Not true for all engines or aircraft. But on the 182 if you need to climb at reduced power capability at DAs way up above 10,000 on a hot day, just firewall the thing.

It's worth digging to see why stuff is painted on tachs and what not. The paint on the ASI has a legal meaning. On the tach, not so much.

Beware of advocating that sort of stuff. Some airframe manufacturers impose engine limitations that are more restrictive than what the engine manufacturers do.

A great example of this is the early Bonanza. No engine limitations per Continental but Beech put max power and max continuous RPM limitations on it. There are some engine and prop combinations on Mooneys that are restricted as well, if I remember correctly.

 

[N747JB]

Beware of advocating that sort of stuff. Some airframe manufacturers impose engine limitations that are more restrictive than what the engine manufacturers do.

A great example of this is the early Bonanza. No engine limitations per Continental but Beech put max power and max continuous RPM limitations on it. There are some engine and prop combinations on Mooneys that are restricted as well, if I remember correctly.

The geared engines on 421's have a fairly wide band of do not use RPM settings in between take off and cruise, it's 300 or so RPMs IIRC.

 

[denverpilot]

Beware of advocating that sort of stuff. Some airframe manufacturers impose engine limitations that are more restrictive than what the engine manufacturers do.

A great example of this is the early Bonanza. No engine limitations per Continental but Beech put max power and max continuous RPM limitations on it. There are some engine and prop combinations on Mooneys that are restricted as well, if I remember correctly.

That's why I said it's worth digging. If you do it without doing your homework, it's on you.

 

[Ted]

Not disagreeing with you, but isn't it a bit ironic that we follow the OEM guidance for the blue and black knobs but reject what they say about the red knob?

Point taken. Let's consider why.

The MP/RPM combinations they came up with had to do specifically with harmonics that can exist in the engine which will physically cause damage and can destroy the engines. This is also one of the reasons why you have so many variants of 360s, 540s, etc. - a number of them have different counterweights to allow for the harmonic properties of particular engine/propeller combinations. You also have no indication of what the harmonics are in your engine at any point in time. No gauge, and your butt-o-meter won't tell you, either.

When you look at the rationale behind the red knob (specifically prohibition of LOP ops), if the manual allows you to run at 25-50 ROP (or even peak EGT), there is nothing bad that can happen from running LOP. Additionally, if you have an engine monitor, you are able to monitor CHT and EGT. While those aren't direct indicators of detonation/pre-ignition, if you are having detonation or pre-ignition, they will manifest themselves in the form of a rapidly increased CHT.

I do often see people who run LOP operating outside of the factory MP/RPM combinations, claiming that's how they "gain back the power" and that it's therefore irrelevant. There may be something to that, but personally I won't do it.

 

[MAKG1]

Some of the aircraft manufacturer's guidance in the POH is flat useless - the tach green arc on an O-470 for one example. Continental has no upper limit for continuous ops on the engine but Cessna painted on on the tach for cruise. Many people faced with a high DA climb in the mountains will refuse to push the blue knob forward not realizing that green arc is meaningless to the folks at Continental.

Not true for all engines or aircraft. But on the 182 if you need to climb at reduced power capability at DAs way up above 10,000 on a hot day, just firewall the thing.

It's worth digging to see why stuff is painted on tachs and what not. The paint on the ASI has a legal meaning. On the tach, not so much.

On a 182 tach, the green arc ends at the redline, 2400 RPM.

Do you mean to describe the MP gauge? The green arc tops out at 23 inches, but takeoff power can be well above that, as can a "maximum performance" climb. But you'll never get over 23 inches at 10000 DA without a turbocharger.

 

[denverpilot]

On a 182 tach, the green arc ends at the redline, 2400 RPM.

Not on mine. It's lower. Could just be that some ninny installed the wrong tach, but AFAICT its original. I looked for a panel photo that sees it well, but don't have one. There's definitely a gap between end of green and redline in ours, however.

 

[denverpilot]

On a 182 tach, the green arc ends at the redline, 2400 RPM.

Ahh, that's it. Remembered what it is. Only the -U model has a 2400 redline.

Previous models prior to the higher compression (and longer TBO) were 2600. The green to redline gap is that. Kinda. See POH below.

Ours is an O-470-S. The last of the low compression models put in the 182. Wikipedia talks about a -T model but I've never seen one on a 182. Went from -S to -U. The -U can't get a mogas STC.

(I suppose it's PoA so I have to mention I normally have no reason to flog the poor thing and pulling the prop back after takeoff is the norm around here. It's just commentary that there's no limitation from Cessna nor Continental on continuous ops at 2600 on this particular engine so the green arc is misleading.)

The POH has "normal climb" and "maximum performance climb" sections and calls out using the 2600 RPM setting for the max one.

 

[MAKG1]

There is no 182U (yet). Cessna is still selling 182Ts. At least Q, R, and T models have a 2400 RPM red line and a green arc ending at the same place. And Qs and Rs had the O-470.

But sure, there were a lot of different models made.

Most of the newer CAP planes are 182Ts, and I'm sure if you ask real nice, the local squadron will let you take a look. Though maybe COWG goes for T206Hs due to the altitude. 182Ts are fat pigs, especially with all the extra CAP crap.

 

[denverpilot]

There is no 182U (yet). Cessna is still selling 182Ts. At least Q, R, and T models have a 2400 RPM red line and a green arc ending at the same place. And Qs and Rs had the O-470.

But sure, there were a lot of different models made.

Most of the newer CAP planes are 182Ts, and I'm sure if you ask real nice, the local squadron will let you take a look. Though maybe COWG goes for T206Hs due to the altitude. 182Ts are fat pigs, especially with all the extra CAP crap.

All of my references were to the engine sub-model, not the Cessna model. O-470-S, O-470-U... Read again.

 

[asicer]

MP is how hard you push on the piston. RPM is how often you push on the piston.

Before anyone takes this too seriously, I would like to point out that this is a gross oversimplification. As an example, before you start the engine have you ever noticed that the MP gauge is close to 30"?

 

[asicer]

That's not necessarily true, especially LOP if you keep a constant fuel flow.

No, it's still true that HP=Torque*RPM*Constant. That's a fundamental law of physics. It's also still true that the throttle controls torque. However, LOP and "constant fuel flow" introduce a new variable that affects the throttle-to-torque relationship and MP-to-torque relationship. In other words, 19.5" on the MP gauge may mean a certain amount of torque with the red knob full forward and a different amount of torque with the red knob pulled back a lot.

 

[asicer]

Ok. But isn't thrust (force pulling airplane forward) a function of RPM only? You can keep available horsepower constant, but drop into a lower gear, your top speed is reduced? (To make a probably incorrect analogy.)

Not exactly. As a thought experiment, let's say the prop is capable of zero pitch and you set it for zero pitch. You can spin the prop as fast as you want and still get zero thrust.

 

[FastEddieB]

No, it's still true that HP=Torque*RPM*Constant. That's a fundamental law of physics.

Beyond my pay grade, so I'll assume that's correct.

But that has to fit in with what Cirrus owners are observing - that a slight reduction in rpm gives a small boost in TAS if fuel flow and MAP are kept constant. Working theory is that props have a speed at which they are most efficient, and dropping the rpm a bit while keeping MP and fuel flow constant does achieve slight gains in efficiency, and hence TAS.

 

[FastEddieB]

Not exactly. As a thought experiment, let's say the prop is capable of zero pitch and you set it for zero pitch. You can spin the prop as fast as you want and still get zero thrust.

One can work that thought experiment the other way as well. "Coarsen" the pitch until the blade is perpendicular to the crankshaft and likewise there will be about zero thrust.

From those two "absurdum" extremes, I imagine one could plot a graph of thrust that would follow a curve - probably much like Laffer proposed about tax rates!

 

[FormerHangie]

Well, I respect him but not his philosophy.

Sounds logical, but there does not appear to be much if any data to support it. When Continental gives a recommend TBO of 2,000 hours for an IO-550N @ 2,700 rpm, nowhere do they state nor imply that if you turn the motor slower it will last longer. I've never seen a chart correlating reduced rpm to longer life, have you?

In a properly designed engine, there should be effective no metal-to-metal contact regardless of rpm - if and when there is the engine is minutes or even seconds away from failure. It is also conceivable that running an engine under more load at a reduced rpm may decrease longevity by increasing internal combustion chamber pressures and CHT's.

That's my non-engineer take on the matter, anyway!

I'm with Fast Eddie on this one. There's no evidence that after a certain amount of revolutions an engine will need an overhaul. I suspect that for any given power output, you would see more wear from a high manifold pressure/low RPM setting than from a low manifold pressure/ high RPM one. In either case, there's are a lot of factors affecting engine longevity and this is probably one of the less important ones.

 

[MAKG1]

I'm with Fast Eddie on this one. There's no evidence that after a certain amount of revolutions an engine will need an overhaul. I suspect that for any given power output, you would see more wear from a high manifold pressure/low RPM setting than from a low manifold pressure/ high RPM one. In either case, there's are a lot of factors affecting engine longevity and this is probably one of the less important ones.

It's not hard to see where such a misconception might come from. The tach time counts prop revolutions, so the engine will meet its TBO earlier with higher RPM. That's a very different statement from engine wear, and I'd suggest a faster spinning engine would make it to higher time before needing overhaul. Just because of the way "time" is measured.

 

[Sac Arrow]

Wait a minute!

My plane has a blue lever, no blue knob. Is there something wrong with it? (Am I supposed to be moving it or something?) Just like that red lever thing, seems best to leave it be.

 

[Fearless Tower]

Wait a minute!

My plane has a blue lever, no blue knob. Is there something wrong with it? (Am I supposed to be moving it or something?) Just like that red lever thing, seems best to leave it be.

The WHOLE lever is blue? Or just the KNOB that screws into the top of the lever?

 

[Sac Arrow]

The WHOLE lever is blue? Or just the KNOB that screws into the top of the lever?

Hmmm... I think it's a snap on attachment. But I could be wrong. Regardless, I don't think the KNOB moves independent of the lever. Or at least I don't think it's supposed to.

 

[Fearless Tower]

Hmmm... I think it's a snap on attachment. But I could be wrong. Regardless, I don't think the KNOB moves independent of the lever. Or at least I don't think it's supposed to.

It's all good. On my Baron, all 6 knobs were black.

 

[Sac Arrow]

This discussion should be the subject of an @SixPapaCharlie parody. "As the knob turns."