M20J Cruise Power Settings

[Slick Nick]

It's not "a few minutes" though. It's 8-10 minutes a flight, times that by tens of flights per year, and you've got hours and hours of overly rich engine operation. Who fed you the BS regarding detonation if you don't run full rich below 75%?

You're doing more damage to your engine now than you'll ever do by properly leaning it. It's sloppy airmanship not to look after your engine, especially in a single it's basically your lifeline. 

Edited Friday at 02:56 AM by Slick Nick

[Bolter]

5 hours ago, Shiroyuki said:

Maybe I should start leaning it then. 

But here's my question, when should I be worried about detonation? I have this thought which come from primary training that leaning above 75% may cause detonation and damage the engine, that's why I keep it rich. Above 7000ft the engine won't make 75% power anymore so I start leaning. That was my rational and I never put too much thought in it. I also see little benefit in leaning early, maybe 200fpm faster climb, saving a gallong of fuel... And I doubt three minutes of running rich will kill an engine...

Detonation risk is from running within the "red box".  If go overly lean at high power.  You can run 150F ROP at any power setting.  You can only run 0-50F ROP when power has dropped.  The red box is smaller as your power drops.  At 10k, I would often run right at peak EGT since power was down so low.  

[0TreeLemur]

7 hours ago, Shiroyuki said:

Maybe I should start leaning it then. 

But here's my question, when should I be worried about detonation? I have this thought which come from primary training that leaning above 75% may cause detonation and damage the engine, that's why I keep it rich. Above 7000ft the engine won't make 75% power anymore so I start leaning. That was my rational and I never put too much thought in it. I also see little benefit in leaning early, maybe 200fpm faster climb, saving a gallong of fuel... And I doubt three minutes of running rich will kill an engine...

As stated before, there are many reasons to lean in the climb.  Provided that you have an engine monitor that displays EGT and CHT for each cylinder, there is very little risk.  On the other hand, not leaning and running way rich invites creation of carbon deposits that can glow and act like little spark plugs and cause pre-ignition. 

Notice your EGTs the next time you take off.  That's the full power EGT target.   Mine is 1270F.  As you climb,  slowly lean once every couple of minutes to re-establish that EGT target.  Watch your CHT's.   If your engine has good cooling air distribution they will stay below 400F.   In my J they only climb to about 390F in the summer and 350F in the winter while leaning in the climb this way.  If your CHTs start to reach or exceed 400F, then richen it up a bit.  By maintaining the EGT target, you are still running quite far ROP.  In my J, 1270F is about 200F ROP because my cruise max. power EGT's up high tend to be about 1470F.

[0TreeLemur]

On 12/18/2024 at 7:50 PM, PT20J said:

Mooney had different versions of the power charts in different POH versions. Who knows why? Here’s one that shows 2500 rpm. I use 2500 most of the time. Bob Kromer (former factory test pilot) has written that the prop on the J was optimized for 2500 rpm.

I find it surprising here that this data sheet says that best economy is 25F ROP.   I'd wager that at MP>23" for a NA IO-360 that's in the red box.  I recall reading somewhere that to stay out of the red box for ROP ops, you need to be at least 80F ROP at 75% power?

[Bolter]

28 minutes ago, 0TreeLemur said:

I find it surprising here that this data sheet says that best economy is 25F ROP.   I'd wager that at MP>23" for a NA IO-360 that's in the red box.  I recall reading somewhere that to stay out of the red box for ROP ops, you need to be at least 80F ROP at 75% power?

If not in the detonation regime, it is at least an operating point of high internal pressures, making excess heat and stress.  

[ArtVandelay]

If not in the detonation regime, it is at least an operating point of high internal pressures, making excess heat and stress.  

Higher pressures = higher horsepower.

As long as the engine is operating correctly, it’s no problem. That’s reason they went to 20° timing BTW, was to improve the detonation margin.

[Bolter]

8 hours ago, ArtVandelay said:

Higher pressures = higher horsepower.

As long as the engine is operating correctly, it’s no problem. That’s reason they went to 20° timing BTW, was to improve the detonation margin.

Per the Lycoming chart, peak power is not at the peak CHT (which i think is peak internal pressure).  

[PT20J]

2 hours ago, Bolter said:

Per the Lycoming chart, peak power is not at the peak CHT (which i think is peak internal pressure).  

I believe that George Braly has shown that peak pressure occurs at about peak CHT. 

The shape of the power curve as a function of mixture strength is not obvious. LOP it is driven by thermodynamics. ROP it is driven by combustion chemistry. What George has done is explained it in terms of peak pressure as a function of crankshaft angle which is more easily visualized. 

Peak EGT occurs at the stoichiometric mixture where all fuel and all oxygen molecules should combine. This never happens in a practical engine because the mixture isn't perfectly homogeneous and there isn't enough time for the reaction between the hydrocarbons and the oxygen (which actually involves a number of intermediate reactions) to complete.  What does occur at this mixture is high temperature. The high temperature causes chemical dissociation of the combustion products that frees up additional oxygen. This is why a slightly richer mixture (which provides additional fuel to combine with the freed up oxygen) produces higher power and higher CHT at the max CHT mixture. Adding yet more fuel actually cools the mixture but adds more substance to the cylinder which expands during the power stroke providing more power at the max power mixture. 

[0TreeLemur]

14 hours ago, Bolter said:

If not in the detonation regime, it is at least an operating point of high internal pressures, making excess heat and stress.  

Agree.  Lycoming documentation is inconsistent over LOP ops.   The power table that @PT20J posted for the M20J says that best economy occurs 25F ROP, while the O-360 chart he provided in his last post shows the best economy range LOP.  Aside from fuel injection, they are in many ways the same engine, right?   I'd wager that nobody would recommend running an engine 25F ROP except at MPs that result in less than about 70% power.

[PT20J]

2 hours ago, 0TreeLemur said:

Agree.  Lycoming documentation is inconsistent over LOP ops.   The power table that @PT20J posted for the M20J says that best economy occurs 25F ROP, while the O-360 chart he provided in his last post shows the best economy range LOP.  Aside from fuel injection, they are in many ways the same engine, right?   I'd wager that nobody would recommend running an engine 25F ROP except at MPs that result in less than about 70% power.

Best economy is always LOP. The Mooney power chart showing numbers for 25 ROP, indicates that’s an economy cruise setting. Mooney never said it was best economy.

EDIT: Looks like I was mistaken. Although the power table doesn’t say best economy, the text and range charts do.

[A64Pilot]

People want to poo poo Lycoming without trying to understand what motivates them, they have to write procedures that an idiot can do and not hurt the engine because they will be paying if they do, secondly they don’t want to publish procedures that degrade the engine performance.

Anyway their recommendations are worth reading. Lycoming is well aware that LOP is where best BSFC is and if you read this they do tell you to lean as much as it will continue to run smooth but contradict themselves at times saying best economy is peak, but also remember they are having to keep it simple and cover every model of engine they make. They know LOP is better BSFC, but they also know the majority of their engines won’t run LOP, and power drops off rapidly when LOP, and peak is pretty close to -25 LOP once you factor in speed difference.

What is interesting that over several different pubs they state for max engine longevity cruise at 65% or less.

https://www.lycoming.com/content/leaning-lycoming-engines

 

Having done quite a bit of instrumented flight testing myself I can tell you that 99% of expert recommendations can’t be backed up by testing, and statements like props are optimized for xx RPM etc just isn’t possible, due to huge variations of density altitude, power etc. You can optimize a prop for a narrow density altitude and speed for a set power. For example Hartzell designed a prop for me for a 800 SHP engine that turned 2080 RPM on takeoff at sea level and the goal was short takeoff roll, it performed much better than their existing prop, their motivation was me informing them that Avia had a better performing prop and I was considering going with the Avia, unless they could meet or come close to it.

Pumping losses etc are real, but so small it’s nearly impossible to back them up in testing, perhaps it’s because the charts just aren’t that accurate because they are most certainly interprelated not every point is a test point.

High RPM is an increase in friction of both the engine and prop, but if everything else is equal higher RPM = higher power. As a general statement at least at normal altitudes the lower the RPM the more efficient the prop, efficient as in most thrust per torque applied, this isn’t usually what is meant by “optimized” though, especially from the manufacturer that means best speed without too much noise and vibration, aircraft are rarely sold for MPG, they are almost sold based on speed.

What usually happens is for a NA engine you can run LOP below say 9000 ft or so at a speed that you can live with, above that you have to start increasing RPM and or fuel flow until you get to 100 ROP, then no matter what else you do you can’t get any more power.

Now 9000 ft is just a statement, some can be happy higher, others say I didn’t buy a Mooney to go slow and can’t, personal preference. At least we have a choice, if we were FADEC we wouldn’t, you lose speed increasing efficiency and lose efficiency to increase speed TANSTAAFL.

Now when I was flying the C-210 as the Company paid the bills I fire walled it at takeoff, and reduced power to land, I flew it at best power for max speed and 2500 RPM for noise and it was smooth there.

Now that I’m retired and only fly for fun and pay all the bills, I don’t fly my Mooney that way, but understand the I want speed, nothing wrong with that but it does cost more.

Most of the CFI’s I’ve had experience with have essentially no maintenance experience, and often have very little if any aircraft ownership experience so other than the POH they don’t often have any real experience, and every POH to include the few I wrote emphasize performance, because performance sells. Very few people that can buy a new airplane are more concerned with fuel burn than speed.

The reason most engines can be run past TBO if well treated is because TBO is set for the guy that flies like I did the C-210, always as fast as it could go. If you do as Lycoming recommends and forget that 500F head temp redline and your redline is 400F and oil temp between 165 and 200 and fly at or below 65% power the engine will most likely go way past TBO safely, probably, unless the cam goes or you let it sit too much or etc.

 

 

 

[PT20J]

I believe that propeller efficiency is related to both rpm and TAS so that an increase in one requires an increase in the other. Therefore, Bob Kromer’s comment that the M20J prop was optimized for 2500 rpm likely means that rpm is best at cruise speed. Since Mooney was all about speed back then, that would likely be best power at 75% speed. If you slow down, perhaps a lower rpm would be more efficient. 

[jetdriven]

As @A64Pilotsays, as you go higher, you have to start raising the RPM and enriching the mixture to keep the true air speed where you want it. For our plane in around 12,000 feet that’s going to be 2700 RPM and 80 rich of peak. Any other power setting is slower. Interestingly, that’s about 9.9 or 10 gallons per hour whether it’s 7000 feet or 12.

I recently flew 1000 mile leg last summer, and the only altitude that really worked was 9500 and just slightly lean of peak. There was kind of basically only about nine knots average tail wind, and it’s hard to get that kind of range out of a 64 gallon airplane.    

[A64Pilot]

8 hours ago, PT20J said:

I believe that propeller efficiency is related to both rpm and TAS so that an increase in one requires an increase in the other. Therefore, Bob Kromer’s comment that the M20J prop was optimized for 2500 rpm likely means that rpm is best at cruise speed. Since Mooney was all about speed back then, that would likely be best power at 75% speed. If you slow down, perhaps a lower rpm would be more efficient. 

Respectfully efficiency isn’t really what is most often meant and Bob probably was using it to mean best speed as you state, but that’s not efficiency. I ASSUME he said 2500 instead of 2700 for the reduced NVH at 2500, I believe he was selling airplanes who’s niche was speed after all, but the increase in speed at 2700 probably wasn’t worth the extra noise and vibrations, a 4 cyl engine has many advantages, but smooth isn’t one of them.

Think of it this way, best speed for example is 100 ROP, I think that’s generally accepted, but best efficiency is low engine RPM and about -25 LOP, it’s most efficient, but quite a bit slower than best power mixture and 2700 RPM. That gets you best engine and prop efficiency, but there is more to be had, the airframe becomes more efficient as it slows down with its peak at L/D airspeed. But who could stand that? I couldn’t and I don’t mind slowing down quite a bit, But if max time aloft was for some reason what you needed that’s how to get there.

But other than hangar talk it’s often just not relevant, if we accept that at 2500 the prop is most efficient it’s really sort of irrelevant in the real world, if you want to go faster turn up the RPM and lowering it decreases speed, any increase in efficiency just really isn’t measurable, trust me I’ve tried. It’s dead easy to try just roll back RPM and increase throttle until indicated A/S is identical, check fuel flow. Most often you will find that rolling back RPM you don’t have enough throttle to fully recover the A/S though so you have to try this experiment at lower than most cruise A/S.

Many years ago I was certain that there was a sweet spot, that is an RPM/ mixture/ throttle setting that gave the most speed for the least fuel. I spent significant time trying to find it by graphing different combinations and while admittedly I was in an inefficient airframe I just couldn’t find one. Want to save fuel? Slow down, sure yiu can get small gains from RPM and mixture, but the big savings come from reduced A/S.

  Years before that I tried the same thing in a twin engine Sportfishing boat that had flo scan fuel flow meters, I was certain that once you broke onto plane that the boat would accelerate at the same fuel flow and there was a sweet spot, so I went out and graphed speed and fuel flow, and while the graphs curve did reduce the steepness there was never a point where you could go faster with less fuel burn.

Even cars many people swear their car, usually higher HP ones have a sweet spot, a point that the engine is most efficient at and this efficiency more than makes up for the higher drag at speed, many think for example their Corvette or whatever gets better fuel milage at 80 than 70 or 60, but it’s unfortunately incorrect.

To continue the rambling I can remember even as a boy flying with my Father that many, particularly Mooney drivers swore that you could get one on step, that was a point where the airplane accelerated and would maintain a higher airspeed than one that wasn’t on the step, usually they said to get on the step climb a couple of hundred ft above desired cruise then slowly descend picking up speed, they were certain that the final stabilized airspeed was faster than if they just leveled off and let speed stabilize, they were even convinced you could tell if the airplane was on the step because the deck angle was slightly lower.  

I’ve even spent some time trying in vain the find this step, why not? sometimes tribal knowledge is correct, not often but sometimes. The Gurney flap comes to mind.

 

Edited Saturday at 02:04 PM by A64Pilot

[A64Pilot]

Now you can get a sort of sweet spot with altitude, for my Maule that was 10k to 12k, flaps at -7 at 10K and 0 at 12K at 12K and above reflexed flaps slowed you down. I’ve not done much long distance flying in my Mooney and to get the gain from altitude the distance flown has to exceed the fuel and time you burn climbing to get there, so I don’t know what that spot is for my Mooney, I suspect it might be lower than the Maule though, in this instance I’m calling the sweet spot to be the fastest speed obtainable, at a power setting that’s kind on the engine.

As you climb airframe drag at first falls off faster than engine power decreases, but at some altitude that reverses and you lose HP faster than airframe drag.

That’s where forced induction comes in of course, you get to continue to go faster as airframe drag decreases but don’t lose as much power.

[PT20J]

The standard equation for drag is D = C_DqS where S is wing area and q is dynamic pressure which is proportional to air density. Thus, drag decreases linearly with air density (altitude)

Anderson Jr., John D, Aircraft Performance and Design, shows that engine power decreases approximately linearly with air density. Thus, engine power also decreases linearly with altitude.

So, the effect of increasing altitude is approximately the same (percentage wise) on engine power and aircraft drag.

Propeller parameters are usually described in terms of advance ratio, J, using the formula J = V/nD where V is the TAS, n is the rpm, and D is the diameter. David Rogers (NAR-Associates) has shown that for a Bonanza, the maximum efficiency occurs at about J=1. He determined this from a propeller map from McCauley. If we assume this is also true for the Mooney prop, Roger's formula for a rule of thumb to obtain the airspeed that maximizes propeller efficiency is:

J = 1.0 = (60*1.69*12*KTAS)/(rpm*74) where 60 converts rpm to rev/sec, 1.69 converts knots to ft/sec, 12 converts inches to feet, and 74 is the Mooney prop diameter in inches.

This yields rpm = TAS*16.4.

So, at 155 KTAS (where I often cruise at altitude), the optimum rpm comes out to 2542. 

Note: Rogers' work is well known in the Bonanza world and he has a wealth of detailed technical analysis of many areas related to airplane performance on his website https://www.nar-associates.com/. Unfortunately, this website appears to no longer be maintained and it's security certificate has expired, so some browsers may block access.