LOP RPM

[201er]

I'm a little confused about how RPM affects fuel saving and efficiency when LOP.

While ROP, using lower RPM with higher MP (if available) yields a reduction of fuel flow (albeit at the cost of speed but fuel flow seems to win over amount of speed loss). For example from the POH for 2000ft all of the following yield 75% power:

2700RPM, 23.3", 10.8GPH

2600RPM, 24.4", 10.5GPH

2400RPM, 26.8", 10.3GPH

According to the POH speed is identical across all settings which is a strong justification for reducing RPM when MP is available. In practice it seems that speed suffers a tiny bit but not substantially (getting into the lower 2000s).

Now my question is how to pick RPM for LOP operations? If we're leaning lean of peak and controlling power with mixture, then what kind of gas saving role does lower RPM provide? Being at a higher RPM, you'll still be leaning to a target fuel flow (say 10GPH for 75% in 201) so it doesn't seem to affect gas savings what RPM you're at like it would when ROP (although it may change how far LOP you can get and affect cooling). So what is the method for selecting an optimal RPM setting to use for LOP operations and how does this vary by altitude?

[jetdriven]

Mike, at low altitude you limit % of power with fuel flow.  It can make more than 75% power.  RPM for noise and comfort. 

  At cruise altitude, say 5K on up, you are limited by air density.  So, what i have been doing is set RPM based on how fast I want to go. Basically, set mixture  at 10-15 LOP  (max BSFC efficiency) and use the RPM you need to get the job done.   2500 RPM is faster, and uses more fuel. 2400 RPM is quieter (cabin din), and NMPG goes up a little.   2100 RPM is most efifcient from a NMPG standpoint, but you lose even more speed.  FWIW, I did a test last summer, it was approximately 1.3 KTAS of speed loss per 100 RPM below 2500.    NMPG goes up by ~1/2 per 100 RPM as well. Difference from 2500 RM to 2100 RPM was ~ 7 KTAS.  This was before the 25 degree timing, and drag mods so I would say that 2100 RPM would fare better for us now.  

 

Fuel flow decreases with RPM because the mass of air processed is lower.  The sense tubes in the fuel injector servo sense this and lower FF automatically. IF you set LOP, and your FF is 10.0 GPH at 2500 RPM, simply reducing RPM to 2400  the fuel flow goes down to 8.7.   Voila, 75% power to 65% power, with one adjustment.

 

I know Ross explains Theta PPP better than I do, but here's my attempt.  A LOP mixture burns slower, so a lower RPM extracts that peak power pulse more efficiently and turns it into torque at the prop because there is more dwell time with the connecting rod 90 degrees to the crankshaft pin at lower RPM.  So, a low RPM while LOP will be more efficient because of that as well.

[PTK]

Mike, I had this same question about this. I found this article to be an excellent resource. As are most other articles by John Deakin, but this I think is the best:

http://www.avweb.com/news/pelican/182583-1.html

I have read all his discussions on the subject and I think this one sums it all up very well from a procedural perspective.

[jetdriven]

PK, thats it exactly.  From the article:

"If I'm at a high cruise setting ("Go Fast mode"), and I don't like the range (or reserve fuel) numbers I see, I reduce the airspeed a bit by leaning more for optimum brake specific fuel consumption (BSFC), then pulling the RPM back as needed. I'll let that stabilize for a minute or so, then I'll take another peek at the numbers. It doesn't take a lot of speed reduction to have a major impact on "Predicted Fuel Remaining." (Under NO circumstances will I reduce throttle: That remains full open on all normally aspirated engines, and on turbonormalized engines. We can talk later about turbocharged engines that pull 35" MP and more.)

Why the sequence of leaning first, then RPM? For simplicity. The idea is to attain optimum BSFC first, then set the RPM as needed for the mission.

As a general rule in normally aspirated engines, you won't go too far wrong by leaning until you have a MIXTURE setting of about 15 to 25F LOP (at lower manifold pressure settings) or 25 to 50F LOP at higher manifold pressure settings in order to obtain the most favorable (minimum) BSFC. This is not really critical, anywhere around 50 LOP is fine, if the engine runs smoothly. Thereafter, reduce RPM as needed to get the reduced AIRSPEED you want or need to get the range you require to complete the trip with adequate reserves. On most airplanes, mixture and RPM give ample control to do anything needed."

[1964-M20E]

To elaborate on what Byron said

 

“Fuel flow decreases with RPM because the mass of air processed is lower”

 

On a 4 stroke 4 cylinder engine at 2500RPM you have 2500 power strokes per minute burning fuel and at 2400 RPM you have 2400 power stokes burning fuel.  Less power stokes equals less fuel burned and less HP.  Also when you reduce RPM MP will go up.

[Shadrach]

We are getting pretty good! Only 3 posts after the OP and we've really gotten into the meat of the matter. The only thing that I would add is that RPM does have an effect mixture.

As was mentioned in the Deakin article, once your leaned out you can adjust RPM accordingly and still be in the ball park because the servo will compensate for the reduced airflow through the engine at the lower RPM setting. It's still a good idea to check the numbers to make sure (that means referenced to peak, not raw EGT number. The reason for checking is that peak EGT will vary a bit with RPM. The other thing to consider is that how far LOP an engine will run smoothly is also affected by RPM and MAP. I can run smoothly to well over 100LOP at 2700RPM, at 2400 RPM it becomes noticeably rough around 75-80 depending on alt (MAP), 2500 will let me go further LOP than 2400...and so on.

All of this is kind of academic in a NA aircraft because no matter the RPM, we can typically get lean enough to control temps, but with a turbocharged/normalized engine these things need to be considered.

 

[Shadrach]

dupe...

 

 

[Parker_Woodruff]

My M20K does best LOP between 2500 and 2575 RPM it seems.  Below that, I can't get it as far LOP without a bit more engine stumble...

[Shadrach]

Quote: 1964-M20E

To elaborate on what Byron said

 

“Fuel flow decreases with RPM because the mass of air processed is lower”

 

On a 4 stroke 4 cylinder engine at 2500RPM you have 2500 power strokes per minute burning fuel and at 2400 RPM you have 2400 power stokes burning fuel.  Less power stokes equals less fuel burned and less HP.  Also when you reduce RPM MP will go up.

[1964-M20E]

Ross

“Are you sure about that?  If that's the case, how many power strokes does my Triumph inline twin have in a minute at 2500RPM? ”

 

Assuming your Triumph is a 4 stroke engine then you will have 1250 power strokes per minute at 2500rpm if it is a 2 stoke engine then you will have 2500 power strokes per minute.  If it is a one stroke engine then it just spits and sputters and does not run and then my Honda Shadow will out run it.

:-)))

[Piloto]

After over 2000hrs on my 1982 M20J I found that the overall best cruise setting for above 7000ft to be:

MP: Max

RPM: 2500

Mixture: 10deg LOP

I get:

TAS 160kts

FF: 9 GPH

Oil Temp: 150F

Avg CHT: 360F

Avg EGT: 1390F

On mine at ROP the CHT temps go above 380F, oil temp 200F and FF to 10gph.

 

José

[Shadrach]

Quote: 1964-M20E

Ross

“Are you sure about that?  If that's the case, how many power strokes does my Triumph inline twin have in a minute at 2500RPM? ”

 

Assuming your Triumph is a 4 stroke engine then you will have 1250 power strokes per minute at 2500rpm if it is a 2 stoke engine then you will have 2500 power strokes per minute.  If it is a one stroke engine then it just spits and sputters and does not run and then my Honda Shadow will out run it.

:-)))

[Shadrach]

Quote: Piloto

After over 2000hrs on my 1982 M20J I found that the overall best cruise setting for above 7000ft to be:

MP: Max

RPM: 2500

Mixture: 10deg LOP

I get:

TAS 160kts

FF: 9 GPH

Oil Temp: 150F

Avg CHT: 360F

Avg EGT: 1390F

On mine at ROP the CHT temps go above 380F, oil temp 200F and FF to 10gph.

 

José

[jetdriven]

Ross, you mean 80 F ROP? 

[Shadrach]

Quote: jetdriven

Ross, you mean 80 F ROP? 

[smccray]

John Deakin posted this slide from his APS online course over at Beech Talk a while back.  It shows how the relationship between ROP and LOP affects the timing and amount of peak pressure on an engine producing the same 244 HP.

[1964-M20E]

Quote: Shadrach

 In a 4 stroke engine, each "stroke" is 180 degrees of crank motion.

In a single cyl 4 stroke, the piston fires every 720 degrees of crank rotation.

In a twin (like my Triumph), a piston fires every 360 degrees of rotation.

The engine in my Mooney (and I suspect yours), a piston fires every 180 degrees of crank rotation.

Are you not getting a compression on every blade when you crank the starter (as in 2 per 360 degree revolution)?

At 2500RPM,

A single = (2500*360) = 900000/720 = 1250 power strokes per minute.

A twin = (2500*360) = 900000/360 = 2500 power strokes per minute.

A 4 cyl = (2500*360) = 900000/180 = 5000 power strokes per minute.

One of us has not had enough coffee this morning...and it might be me, please explain what I'm missing.

P.S. Your shadow would likely out run my 40 year old 500cc Trumpet, it's only a 40hp machine...

[danb35]

Quote: Shadrach

The engine in my Mooney (and I suspect yours), a piston fires every 180 degrees of crank rotation.

[KSMooniac]

One other variable not mentioned is the change in propeller efficiency with RPM and air density.  Some props might be optimized for cruise at a specific RPM, and anything more or less than that RPM will result in a slower TAS for the same power setting.  Practically, this is difficult to measure in our planes as RPM reduction will result in power reduction (and vice versa) unless you adjust the mixture to compensate.  If LOP, you can simply keep a constant fuel flow (if equipped) but it is difficult while ROP.  

My MT prop is optimized for a much lower RPM than my OEM McCauley, and it was pretty obvious when I did some testing.  I believe this is because Europe is much more noise-sensitive than we are, and that is a design choice they made. 

[jetdriven]

Scott, I have never seen a propeller which will return a faster TAS (or even the same)  with less RPM, everything esle constant.   Maybe more efficient, but never faster.

[PTK]

Quote: jetdriven

PK, thats it exactly.  From the article:

"If I'm at a high cruise setting ("Go Fast mode"), and I don't like the range (or reserve fuel) numbers I see, I reduce the airspeed a bit by leaning more for optimum brake specific fuel consumption (BSFC), then pulling the RPM back as needed. I'll let that stabilize for a minute or so, then I'll take another peek at the numbers. It doesn't take a lot of speed reduction to have a major impact on "Predicted Fuel Remaining." (Under NO circumstances will I reduce throttle: That remains full open on all normally aspirated engines, and on turbonormalized engines. We can talk later about turbocharged engines that pull 35" MP and more.)

Why the sequence of leaning first, then RPM? For simplicity. The idea is to attain optimum BSFC first, then set the RPM as needed for the mission.

As a general rule in normally aspirated engines, you won't go too far wrong by leaning until you have a MIXTURE setting of about 15 to 25F LOP (at lower manifold pressure settings) or 25 to 50F LOP at higher manifold pressure settings in order to obtain the most favorable (minimum) BSFC. This is not really critical, anywhere around 50 LOP is fine, if the engine runs smoothly. Thereafter, reduce RPM as needed to get the reduced AIRSPEED you want or need to get the range you require to complete the trip with adequate reserves. On most airplanes, mixture and RPM give ample control to do anything needed."

[KSMooniac]

Quote: jetdriven

Scott, I have never seen a propeller which will return a faster TAS (or even the same)  with less RPM, everything esle constant.   Maybe more efficient, but never faster.

[jetdriven]

Scott, if you keep the FF the same while LOP and lowering RPM, it requires richening.  It may mess up your test.   The engine will make more power with more RPM, and hence more thrust, and speed.  Try this,  Go WOT at 1000 AGL up there, and leave all 3 knobs to the firewall.  You should see 160-169 KIAS after about 90 seconds.  Now, pull the prop back, try 2450 RPM.  wait a minute, and then record that value.     You can do it at 5K feet too, but down low, you can make more horsepower.  It wil be interesting, but I bet you lose 3-4 KIAS bringing down RPM.   You couls also do a time to climb to 5K feet.  2700 vs 2500 RPM.

[jetdriven]

Quote: allsmiles

Yes it is Byron! I have read probably all John Deakin's articles on the subject. But that particular article is probably the best one. I must have read it four or five times and each time I learned something new! Obviously I'm not as versed on LOP ops as you and others are so I'm being overly cautious. I'm learning as much as I can before I jump in with all fours so to speak. The danger I want to avoid is doing something incorrect that can damage the engine, and not realizing it until it's too late! 

[201er]

At my last oil change I had my mechanic check the plugs and he gave me the usual line of "looks like you're running a bit too rich, try leaning it some more." How is that possible when I've been flying almost exclusively lean of peak (except climbs) since my last plug cleaning? Or is that just a standard line they read to everyone? Is it possible to get the apperance of too rich by running too lean? Or is being too rich in the climbs enough to give this apperance?