Please explain propeller rpm vs. altitude vs. ground speed?

[1RTK1]

Didn’t cut off the last sentence on purpose, thought I copied it all.

 

[coloradobluesky]

Control systems always have a delay. Nothing happens instantaneously. It takes time for the governor to adjust the blade angle to keep the rpms steady when there is a change in power, whether the change in power is from the mixture or the throttle. By making the valve a smaller opening the designer can increase the delay. The increase in delay makes the rpm drop noticable, which you want so you can lean the engine by watching for an rpm drop, then richen it slightly.

 

[flyingcheesehead]

Yes there is:
You know you are in governing range if you turn the prop knob out and you get an rpm drop.
And if you get an rpm drop, when you pulled the prop knob out, you know you are now NOT at the stops.

That is true, but ONLY so long as you do not change anything else that affects the prop - Namely, throttle, mixture, and airspeed. As soon as you begin leaning the mixture, the prop is going to change pitch, but you won't know it until you're no longer in the governing range.

Easier thought experiment, since you didn't seem to get the first one: If you pull the prop out to get an RPM drop, and then pull the throttle out until you get another RPM drop, are you or are you not at the flat pitch stop with the prop knob not full?

It's the same with the mixture, which also affects power. No matter where the prop knob is, if you get a noticeable drop without moving it, you're either dropping the power low enough that you're not in the governing range, or there is something wrong with your airplane.

Stop thinking you know the answer to this puzzle (that it is on the stops and there can't be any lag in governing rpm), open your mind, sharpen your senses, and just go out and do a runup with the prop knob pulled a bit so you KNOW you arent on the stops,

Again - If you think having the prop knob pulled out means that you aren't on the stops, then you don't know how the system works. Period.

What people have a misunderstanding of if they think at runup, 1800-1900 rpm or so, is they think if the prop knob is all the way in that the prop is at the stops, fully flat.

On a normal runup (ie mag check power/RPM, full rich), the prop IS fully flat. If it weren't, you wouldn't get a drop in RPM when you went to one mag.

You can proove you are NOT on the stops by backing the prop knob off a bit, seeing an rpm drop, so you know you are in governing range, and doing the runup from that configuration. Because the prop knob has reduced rpm, you KNOW that the prop is not full flat from there.

Again, that is only true if you change nothing else. As soon as you start pulling the mixture or the throttle, you can put it back on the stops and drop it out of the governing range again.

And if you get an rpm drop from the lag at runup, you get it in the air at cruise power, though it is less noticable at higher power. This technique gives pilots a way to lean without having full instrumentation and balanced fuel injectors. It also gives the pilot a way to lean at runup when you need it at high altitudes.

Nope, nope, and nope. You can lean to roughness if you'd like, but if you get an RPM drop before that, you're either dropping it out of the governing range or flying a broken airplane. (speaking of broken things, I feel like a broken record...)

I know there is nothing wrong with my prop or governor. Its a Lycoming 0-360 with a Marvel Schebler carburetor and a Hartzel constant speed prop. And other aircraft exhibit the same behavior.

Other aircraft, except every aircraft that every other poster here has flown. Seems awfully coincidental, don't you think?

Control systems always have a delay. Nothing happens instantaneously. It takes time for the governor to adjust the blade angle to keep the rpms steady when there is a change in power, whether the change in power is from the mixture or the throttle.

True, but this lag is so short that it's not perceptible by the pilot... UNLESS the airplane is broken. Like I said, the only time I've seen this happen was when we landed, pulled the oil filter and found it full of metal.

By making the valve a smaller opening the designer can increase the delay. The increase in delay makes the rpm drop noticable, which you want so you can lean the engine by watching for an rpm drop, then richen it slightly.

OK, let's take a different tactic here. If the reason for the drop is a very restrictive pilot valve, then why is it that you don't first see a rise in RPM as you approach peak power?

 

[EdFred]

Kent,

You are wasting electrons. If somebody doesn't understand the system you can't explain to them why it doesn't work. They need remedial education before you can try to explain it to them.

 

[flyingcheesehead]

Kent,

You are wasting electrons. If somebody doesn't understand the system you can't explain to them why it doesn't work. They need remedial education before you can try to explain it to them.

Well, I was trying to provide some remedial education for free... And make it clear to anyone who happens upon this thread in the future that coloradobluesky has no clue what he's talking about. But yeah, I'm getting to that point where I feel like I'm talking to a rock. Maybe I am.

 

[coloradobluesky]

Boy, now I know what Galileo must have felt like.
Now go out and drop two balls of the same size, one made of lead and the other made of copper...

This is easy people, just to out and conduct two experiments.
1. Do a runup, but pull the prop back a bit so you know its not on the stops
2. Observe what happens to the rpms when you pull the mixture back.

Then in cruise at less that full throttle, say 20"/2000 or some other economy cruise setting
1. Pull the mixture knob back
2. Immediately observe what happens to the rpms when you pull the mixture back

Now, at economy cruise setting
1. Pull the THROTTLE back, not all the way but more than a little
2. Immediately observe what happens to the rpms when you pull the throttle back

Write your results down on paper. You know, its an experiment.

 

[flyingcheesehead]

Boy, now I know what Galileo must have felt like.
Now go out and drop two balls of the same size, one made of lead and the other made of copper...

This is easy people, just to out and conduct two experiments.

At least Galileo could count to three. But I'm a sucker for an argument so I'll indulge this for a bit.

1. Do a runup, but pull the prop back a bit so you know its not on the stops
2. Observe what happens to the rpms when you pull the mixture back.

How far do I need to pull the prop back to know it's not on the stops? Is it enough to just pull the knob out a little, or do I need to pull it out until the RPM goes down?

How far do I need to pull the mixture back, and how do I know the prop is still off the stops?

(One.)

Then in cruise at less that full throttle, say 20"/2000 or some other economy cruise setting
1. Pull the mixture knob back
2. Immediately observe what happens to the rpms when you pull the mixture back

Why is it important for this to be at such a low power setting? There's actually an SB out for my engine that it shouldn't be run at less than 2300 RPM in cruise. Will your experiment work at 23"/2300? Why or why not?

(Two.)

Now, at economy cruise setting
1. Pull the THROTTLE back, not all the way but more than a little
2. Immediately observe what happens to the rpms when you pull the throttle back

OK, if I do this experiment at, say, 7000 feet so that my 23" is at full throttle - Is it enough for me to pull back to 15" of manifold pressure? Or should I do this down lower, where 23" is not full throttle? I ask because I know I'll be dropping out of the governing range not far below 15" and I don't want to invalidate the experiment in your eyes.

(Three!)

Write your results down on paper. You know, its an experiment.

If I were to do this, would you accept my results "on paper" if they disagree with your theory, or would I have to take video to prove this to you? (I'm not sure exactly how I'd mount my GoPro where it can see my tiny engine gauges and my throttle quadrant... Open to suggestions that don't cost money.)

 

[EdFred]

I had to go out to the airport at lunch today to replace a bad EGT probe. While out there I decided to do Galileo's experiment and these are my results from a static run up on the ground. I didn't have a chance to go up in the air.

This is a voice to text so you might have to interpret it as I'm not going to reinterpret what I said. I started up the engine and ran the RPMs up to 2000 RPM act the propeller knob back so I got a 100 RPM drop down to 1900 RPM. I then dial the mixture out and kept dialing it out and dialing it out and dialing it out until I got an RPM drop down to 1800. I then push the prop control full-forward and I did not see an increase in crop speed. So at some point during the Leaning of the mixture my prop was back on the stops. This is why Galileo's method doesn't work the way he thinks it is supposed to. It basically becomes a fixed pitch prop at that point.

 

[Sinistar]

@coloradobluesky - Even though this thread has sorta diverged I have been following all the way as I learning! And I'm amazed that you guys have kept it civil

But I've wanted to ask this for clarification: What type of plane? Carb or injected? Engine(s) type? Prop(s) manufacturer? Blades per prop? Factory or non-factory governor? Is there a chance there is something different about your setup than the others? Are you using a engine monitor for prop RPM or the actual factory RPM gauge? Please don't shoot the messenger, just though it was important to ask.

 

[Checkout_my_Six]

....and what if there are four or five blades.....would this happen twice as fast?

 

[Clark1961]

....and what if there are four or five blades.....would this happen twice as fast?

Half as fast, it takes longer to change rpm with the larger mass.

 

[coloradobluesky]

Husky with a carburetor Lycoming 0360, Hartzel prop, marvel schebler carb. Same as Mooney and others
Think about it. If you lean it enough, the rpms gotta go down. In the extreme case where do rpms go when you pull the mixture to cutoff?
Of course you don't pull it nearly that far. Do it at economy cruise and it stays well within the governing range. Just enough rpm drop to notice it.

How do you think they leaned before they had all those why gauges?

Now I wish I could get Avare maps to work well.

 

[Sinistar]

Husky with a carburetor Lycoming 0360, Hartzel prop, marvel schebler carb. Same as Mooney Nov and others
Think about it. If you lean it enough, the rpms gotta go down. Th extreme case where do rpms go when you pull the mixture to cutoff?

Thanks!

 

[EdFred]

But the RPMs only go down after the power loss from leaning puts the CS back on the stops. That's what you are missing.

 

[Checkout_my_Six]

Half as fast, it takes longer to change rpm with the larger mass.

mine doesn't change all that slow....compared with what I use to have. Now with cold oil?....that's a pot of a different kettle.

 

[Clark1961]

mine doesn't change all that slow....compared with what I use to have. Now with cold oil?....that's a pot of a different kettle.

That’s what you get for using that cold oil stuff. I use hot oil in mine. Most FBOs don’t stock it so ya gotta order it from the distributor.

 

[denverpilot]

Husky with a carburetor Lycoming 0360, Hartzel prop, marvel schebler carb. Same as Mooney and others
Think about it. If you lean it enough, the rpms gotta go down. In the extreme case where do rpms go when you pull the mixture to cutoff?
Of course you don't pull it nearly that far. Do it at economy cruise and it stays well within the governing range. Just enough rpm drop to notice it.

So how does that apply to what you said about peaking? Where is @EdFred ‘s peak in the experiment you asked us all to run?

Hint: It’s not at the RPM decease point. That’s too lean for peak.

 

[Checkout_my_Six]

when I lean I see a power drop....and a decrease in airspeed by 2-3 kts....but, that's all.

 

[flyingcheesehead]

That’s what you get for using that cold oil stuff. I use hot oil in mine. Most FBOs don’t stock it so ya gotta order it from the distributor.

Amazon sells it. LOL https://www.amazon.com/Dynasty-Hot-...=UTF8&qid=1525719906&sr=8-13&keywords=hot+oil

(Now I'm going to be getting ads for that stuff for the next year.)

 

[dmspilot]

This is easy people, just to out and conduct two experiments.
1. Do a runup, but pull the prop back a bit so you know its not on the stops
2. Observe what happens to the rpms when you pull the mixture back.

Or you can just use the throttle.

When power is reduced regardless of why, the governor will attempt increase the RPM, and the prop will again hit the flat pitch stop; making your runup experiment invalid.

 

[Sinistar]

Amazon sells it. LOL https://www.amazon.com/Dynasty-Hot-...=UTF8&qid=1525719906&sr=8-13&keywords=hot+oil

(Now I'm going to be getting ads for that stuff for the next year.)

There's no way I'm clicking that link.

 

[Checkout_my_Six]

awe ...it ain't all that bad.

 

[EdFred]

There's no way I'm clicking that link.

It was for a hot chili sauce.

 

[flyingcheesehead]

There's no way I'm clicking that link.

It's for Chinese hot oil. Meant for eating, as a condiment. Probably NOT a good idea to use it for whatever you're thinking of.

 

[coloradobluesky]

Take a look at what LYCOMING says, its essentially saying the same thing I am saying which is lean to an rpm drop (some call it a "stumble") then richen to peak RPM! RPM! Right from Lycoming.

LYCOMING SAYETH:
"For direct-drive and for normally aspirated engines with a prop governor, but without fuel flow or EGT, set throttle at full power and lean mixture at maximum RPM"

You find too lean, then richen to peak.
Just go out and try it. It works!

 

[EdFred]

How do I get full power if I have pulled the prop off the stops? And since when do we do full power run-ups?

 

[denverpilot]

How do I get full power if I have pulled the prop off the stops? And since when do we do full power run-ups?

Many here at altitude recommend them, but most of us find them fairly useless below about 10,000 DA since normal leaning at RPM settings that don’t suck crap into the prop are plenty close enough on carb’d engines where the carb is going to enrichen at full throttle anyway. Way easier to see where the peak really is.

(I think where this is falling apart is @coloradoblueskies assumes peak is at the point where power falls off, but it’s slightly richer than that. If you want to SEE it on the RPM gauge and get it dead nuts on peak, you can’t do it at full RPM. It’s simply not possible. The prop will start to adjust as it goes up in tiny increments and then RPM will fall when you “go over the cliff” onto the overly lean side.)

You do need to be above the altitude where the engine CAN’T produce more than 65% power before there’s any point to a full power run-up if you’re planning on departing at peak lean. Otherwise you’re flirting with pre-ignition.

All you’ll manage to squeak out of it is what you’re going for, and if you’re planning a departure so tight that you’re going to smack the trees with a few percent of power loss either way (lean or rich) you planned badly anyway.

Offload some crap or wait for it to get cooler. It’s a fools errand to lean for absolute peak if it means you won’t clear an obstacle or need that extra 20 RPM unless you’re already so hot and high that peak operation won’t matter at all, and you’ve already lost 35%+ of engine power.

And really you can get it close enough prior to taking the runway that you can just advance the throttle and hold brakes and then lean to the drop and back in for smooth ops and that’s all you’re really going to get out of that engine today anyway, in practical operating terms.

(Doesn’t matter if it’s a constant speed or not, smooth operation just above where it’s dropping off in power is close enough to peak, and you’re not going to harm anything at 65% power or less anyway.)

Pick your abort point and stick to it, after that.

We rarely see problems with visitors who did SOME sort of leaning process, as long as they stay on the slightly rich side of things if it’s cold out, we see problems with people trying to take off full rich and losing tens of percent more engine performance than they expected and NOT having done takeoff calculations nor picked an abort point. They just wait and wait and wait and maybe abort late or mush it into the air barely flying and then maybe remember to crank that mixture out and get a nice surge of power when they finally do.

Dancing on the head of a pin for 20 RPM just doesn’t matter in any practical sense.

But I know YOU know that. Was just starting with the concept that some DO full power run-ups up here. I’ll do it like I said as a final check that I’m “close enough” on the runway at KLXV or on a 100F day down here in the Front Range, but at KLXV we’re talking reasonable temperatures.

Down here if I’m doing it, CHTs are going to severely limit the climb out anyway at 100F on the surface. Other things become the limits besides leaning. 100F is just hard on things here.

 

[Clark1961]

Many here at altitude recommend them, but most of us find them fairly useless below about 10,000 DA since normal leaning at RPM settings that don’t suck crap into the prop are plenty close enough on carb’d engines where the carb is going to enrichen at full throttle anyway. Way easier to see where the peak really is.

(I think where this is falling apart is @coloradoblueskies assumes peak is at the point where power falls off, but it’s slightly richer than that. If you want to SEE it on the RPM gauge and get it dead nuts on peak, you can’t do it at full RPM. It’s simply not possible. The prop will start to adjust as it goes up in tiny increments and then RPM will fall when you “go over the cliff” onto the overly lean side.)

You do need to be above the altitude where the engine CAN’T produce more than 65% power before there’s any point to a full power run-up if you’re planning on departing at peak lean. Otherwise you’re flirting with pre-ignition.

All you’ll manage to squeak out of it is what you’re going for, and if you’re planning a departure so tight that you’re going to smack the trees with a few percent of power loss either way (lean or rich) you planned badly anyway.

Offload some crap or wait for it to get cooler. It’s a fools errand to lean for absolute peak if it means you won’t clear an obstacle or need that extra 20 RPM unless you’re already so hot and high that peak operation won’t matter at all, and you’ve already lost 35%+ of engine power.

And really you can get it close enough prior to taking the runway that you can just advance the throttle and hold brakes and then lean to the drop and back in for smooth ops and that’s all you’re really going to get out of that engine today anyway, in practical operating terms.

(Doesn’t matter if it’s a constant speed or not, smooth operation just above where it’s dropping off in power is close enough to peak, and you’re not going to harm anything at 65% power or less anyway.)

Pick your abort point and stick to it, after that.

We rarely see problems with visitors who did SOME sort of leaning process, as long as they stay on the slightly rich side of things if it’s cold out, we see problems with people trying to take off full rich and losing tens of percent more engine performance than they expected and NOT having done takeoff calculations nor picked an abort point. They just wait and wait and wait and maybe abort late or mush it into the air barely flying and then maybe remember to crank that mixture out and get a nice surge of power when they finally do.

Dancing on the head of a pin for 20 RPM just doesn’t matter in any practical sense.

But I know YOU know that. Was just starting with the concept that some DO full power run-ups up here. I’ll do it like I said as a final check that I’m “close enough” on the runway at KLXV or on a 100F day down here in the Front Range, but at KLXV we’re talking reasonable temperatures.

Down here if I’m doing it, CHTs are going to severely limit the climb out anyway at 100F on the surface. Other things become the limits besides leaning. 100F is just hard on things here.

TLR yeah, the schools do full power run ups. No one else does.

 

[denverpilot]

TLR yeah, the schools do full power run ups. No one else does.

My school didn’t, but then again the airplane owner is an A&P/AI and doesn’t like beating hell out of equipment for no practical reason.

I think the era of that is waning amongst CFIs I’ve talked to at the local rent-a-wreck flight schools too. It comes and goes.

Some idiot starts it during the hot of the summer and then it sticks around until they leave for an airline or something. LOL.

 

[Clark1961]

My school didn’t, but then again the airplane owner is an A&P/AI and doesn’t like beating hell out of equipment for no practical reason.

I think the era of that is waning amongst CFIs I’ve talked to at the local rent-a-wreck flight schools too. It comes and goes.

Some idiot starts it during the hot of the summer and then it sticks around until they leave for an airline or something. LOL.

I took a flight recently with an instructor in a 182. I found peak rpm then announced the five half turn standard for best power. Instructor quickly said two half turns. On take off the instructor added three half turns because the temps were high. I just smdh. It isn’t rocket surgery...

 

[Tarheelpilot]

Take a look at what LYCOMING says, its essentially saying the same thing I am saying which is lean to an rpm drop (some call it a "stumble") then richen to peak RPM! RPM! Right from Lycoming.

LYCOMING SAYETH:
"For direct-drive and for normally aspirated engines with a prop governor, but without fuel flow or EGT, set throttle at full power and lean mixture at maximum RPM"

You find too lean, then richen to peak.
Just go out and try it. It works!

Manual reference?

 

[denverpilot]

I took a flight recently with an instructor in a 182. I found peak rpm then announced the five half turn standard for best power. Instructor quickly said two half turns. On take off the instructor added three half turns because the temps were high. I just smdh. It isn’t rocket surgery...

LOL! Funny thing is that could’ve been me as the CFI in my own airplane. BTDT. Did it to myself as temps got hotter in the late spring. Hahahaha.

Two works good — in winter — in my airplane though. Summer, it needs mo’ go juice and a shallower climb to keep CHT numbers happy.

But I know it wasn’t me with you, because we flew in that fancy airplane of yours where full rich always works.

That turbo twin I flew was also a lot happier on hot days with keeping those red knobs all the way forward until we got up high. And if we were going to shut one down, we’d do it before we leaned... just kept the numbers in a much happier place overall.

Like @Ted DuPuis says engine limits aren’t goals.

 

[dmspilot]

Take a look at what LYCOMING says, its essentially saying the same thing I am saying which is lean to an rpm drop (some call it a "stumble") then richen to peak RPM! RPM! Right from Lycoming.

LYCOMING SAYETH:
"For direct-drive and for normally aspirated engines with a prop governor, but without fuel flow or EGT, set throttle at full power and lean mixture at maximum RPM"

You find too lean, then richen to peak.
Just go out and try it. It works!

First, you truncated the quote to make it look like it's saying something it's not. Intellectual dishonesty at its finest.

The rest of the sentence that you left off is "lean mixture at maximum RPM with smooth operation of the engine as a deciding factor."

------------

Second, you need to learn the difference between the words "at" and "to".

Note the difference in technique between fixed-pitch props and governed props:

"For fixed-pitch propellers, lean TO maximum RPM at full throttle prior to takeoff where airports are at 5,000-feet density altitude or higher."

"For direct-drive and for normally aspirated engines with a prop governor, but without fuel flow or EGT, set throttle at full power and lean mixture AT maximum RPM with smooth operation of the engine as a deciding factor."

One you lean "TO" maximum RPM, the other you lean "AT" maximum RPM. Two completely different things! Leaning "TO" maximum RPM means that the leaning procedure involves finding the mixture that produces the maximum RPM. Leaning "AT" maximum RPM means that you sent the RPM to the maximum, and then perform the leaning procedure as described, which does not involve any further reference to RPM.

Notice it says "smooth operation of the engine as a deciding factor." If peak RPM was the deciding factor, then it would say "peak RPM as the deciding factor"!

------------

Third, when the quotes are put back into context, we realize they are talking about leaning for TAKEOFF at high density altitude! No wonder they tell us to lean AT maximum RPM. Because that's what you use for takeoff.

The cruise recommendations are entirely different:

"For engines equipped with fixed-pitch propellers, gradually lean the mixture until either the tachometer or the airspeed indicator reading peaks. For engines equipped with controllable pitch propellers, lean until a slight increase of airspeed is noted."

So LYCOMING SAYETH, with governed props, TO LEAN UNTIL THE AIRSPEED INCREASES. YOU DON'T PAY ANY ATTENTION TO RPM.

------------

Read your own sources before you try to use them to back up a flawed argument, and don't lift quotes from them and take them out of context to make them look like they're saying the opposite of what they actually say.