Best power vs economy mixture setting

[redbaron1982]

Since I did my PPL training I have a dumb doubt I can not find an answer to. 

Why we can best power mixture "best power"?

Looking at the M20J power schedule at 65% (is the same for all, just to put an example), there is no difference between best power and economy: same rpm, same mp, same power, same TAS. 

The only change is fuel flow, from 9.2gph to 10.7gph. 

I get that the engine runs cooler with richer mixture, but why call it best power of it gives you the same power? Am I missing anything here?

[Raptor05121]

Check the Lycoming manual. The difference is the red knob. Best power is usually in the most strenuous regime of mixture settings which will achieve best power and ultimately, highest CHTs and ICP (Internal Cylinder Pressure). Conversely, the best economy is much leaner and kinder to the engine. Where are you seeing same power between two mixture settings?

[redbaron1982]

15 minutes ago, Raptor05121 said:

Check the Lycoming manual. The difference is the red knob. Best power is usually in the most strenuous regime of mixture settings which will achieve best power and ultimately, highest CHTs and ICP (Internal Cylinder Pressure). Conversely, the best economy is much leaner and kinder to the engine. Where are you seeing same power between two mixture settings?

 

These are pictures from the M20J POH. 

 

[carusoam]

Alex, great job supporting the answers!   (Pro pilot on the move!)

 

RB,

One thing you are going to find out…

Especially if your POH is really old…

 

The phraseology being used in power charts probably comes from the 1930s…

The charts, often, are a cut and paste from the engine manufacturer’s documents…

 

The mistakes you might find are often results of transcribing data by hand…. The word processor didn’t become useful until the mid 1980s…

Still… for quite some time the tech people making the data… handed it off to a non tech person writing and typing… 

 

The important part…

Why nobody uses Best Power…. Who is it best for?

Read Alex’s comments above… for what happens when best power is used…

 

Some of this gets covered in Aviation 101… to non-owners… expect that memory may have faded….

It gets covered during Transition Training… if you bought the full package… 

 

Essentially we are using technical documents that were designed in the 1930s… the words have specific meanings that are not exactly how we interpret them in modern times… sort of like legal documents… it sounds like regular English… but each sentence is crafted to mean something specific…

 

Some of the engine data is terribly inconsistent…

As if the data was collected over several different days with different weather conditions… on old analog instruments with user error thrown on top…

Data collection back in the day wasn’t that good…

Data printing back in the day wasn’t that good…

Data usage back in the day wasn’t that good…

 

Today… we can use the data because our planes have digital instruments that are better than the engines that collected the data…

 

The odd thing… the data came from the engine suppliers… and mostly got re-printed by the aircraft manufacturer…

For fun… grab the manuals from the engine manufacturer… and see what has been left out… 

Guess who wrote a long chapter on all of the various starting and shut-down techniques… and who cut it down to size to fit it into the aircraft’s POH…?

Every now and then… an engineer at Mooney grabs the bull by the horns and makes a huge difference… 

Find a POH from the 90s to see the big changes…

Best regards,

-a-

[carusoam]

RB,

Part 2…

Looks like you are comparing lines on the chart for 65%bhp….

We can approach 65%bhp in a few several different ways…

1) A cool expensive way… ROP

2) A not as cool less expensive way… ROP that is tough on hardware…

3) A peak hot way… saving on the FF… at Peak

4) A cool way… saving on FF, but less power (yet, still can be over 65%bhp) … LOP

 

Your charts show the first two options…

MSers use all four depending on the day….

Ovation power charts include 3 out of four…

 

Congrats on entering the ROP vs LOP discussion world….  
 

The red knob can be adjusted infinitely…

Your POH only included two hard lines… (two out of infinity…)

And the archaic 1930s language doesn’t help…

 

How was that?

Best regards,

-a-

[toto]

Just another chart for comparison.. My J flight manual has multiple mixture settings in one chart.

[redbaron1982]

Thanks @carusoam for the detailed explanation. I'll get the engine manual to see if I can have a more complete picture.

 

I think my error is to consider best power as if it was actually more power, if you do 65% of nominal you don't have best or worst power, you just have the same with different operating conditions of the engine (hotter or cooler). 

[carusoam]

Reviewing the terms….

• Best power
• Best economy

There is probably some extra words that go with them… that get left out for space.

Something more like…

• Best Power, considering all other variables…
• Best Economy, considering all other variables…

 

Around here we tend to consider all the other variables as well…. 

Hence the red box, peak, and LOP… discussions…

 

Best regards,

-a-

[PT20J]

Think of it this way: If you start at full rich and keep the MAP and RPM constant as you lean the mixture, the power will increase peaking at the best power mixture and then decrease as you continue to lean until eventually it quits running. 

[carusoam]

5 minutes ago, PT20J said:

Think of it this way: If you start at full rich and keep the MAP and RPM constant as you lean the mixture, the power will increase peaking at the best power mixture and then decrease as you continue to lean until eventually it quits running. 

Skip,   (Skip has tons of knowledge regarding operating old machines)

Do you have any insight on where the phraseology comes from?

Best Power and Best Economy….

The exact terms show up everywhere as if they have legal meaning…

Where there is nothing Best about either of them…. 

Engine longevity gets left out of the discussion….

50°F ROP is pretty good, compared to Peak…

100°F is more engine friendly than 50°F…

 

Just wondering if you have this insight….. in your tool box. 

Best regards,

-a-

[PT20J]

Just now, carusoam said:

Skip,   (Skip has tons of knowledge regarding operating old machines)

Do you have any insight on where the phraseology comes from?

Best Power and Best Economy….

The exact terms show up everywhere as if they have legal meaning…

Where there is nothing Best about either of them…. 

Engine longevity gets left out of the discussion….

50°F ROP is pretty good, compared to Peak…

100°F is more engine friendly than 50°F…

 

Just wondering if you have this insight….. in your tool box. 

Best regards,

-a-

Best power mixture is the fuel/air ratio that produces the most power for a given MAP/RPM. All engines will run well at that mixture, but it burns more gas than necessary at normal cruise power settings.

Best economy mixture is the fuel/air ratio that produces the lowest fuel flow for a given power output.

Not all engines will run smoothly at the true best economy mixture due to uneven distribution of the mixture between cylinders. Also, there is cycle-to-cycle power variation in each individual cylinder due to randomness in the mixture and combustion process, and this variation is increased at lean mixtures. So, best economy mixtures don't generally produce quite as smooth a running engine as richer mixtures. Also, while the engine runs with greater efficiency at best economy mixture, it is incapable of generating as much power as with richer mixtures, and for a normally aspirated engine this limits power available at normal cruise altitudes. For all these reasons, most manufacturers advertise an "economy" mixture somewhat richer than the best economy mixture. Piper favors peak EGT and Mooney went with 25F ROP as examples.

Skip

 

[carusoam]

Skip!

I voted for you… the Best Knowledge Collector award of the day!

And you share it all so well!

Thanks for going the extra nautical mile for everybody!

Best regards,

-a-

[A64Pilot]

There are many times I’ll use best power, just not often anymore cause I’m old and don’t travel like I used to, but if I’m climbing out of a valley to cross mountains and or at high altitudes where my non turbo is a pig, I’ll run best power just trying to get some power. My service ceiling LOP is significantly lower than ROP, same with airspeed, so if your in a hurry even at mid altitudes say 10,000 or so then best power may be called for. 

Everything back in the day was done as a close is good enough, and the deeper you dig into things and if you have a few years test flying, you learn close is good enough.

There is a point of diminishing returns with everything and then there are considerable variations from one engine / aircraft and another, so it was wisely chosen to give some charts to illustrate “best”. I say wisely because the average pilot has no need or use actually in determining actual “best” plus of course the aircraft as manufactured didn’t have the instrumentation to do so. To publish the method of determining best would have new pilots spending hours on something they don’t need to, time better spent on gaining a feel for the airplane etc.

Even doing airspeed cals is more complex than you may think, requires instrumentation not on a standard aircraft and in truth average pilot has no need for it.

As an example airspeed is all over the place flaps, CG, aircraft attitude all effect it, but most don’t get wrapped around the axle about it like we do the engine.

Bottom line you can spend thousands of dollars and many hours trying to run your engine at the perfect spot for improved performance and life span, but it doesn’t seem to really make as much difference as many want to believe. As a reference no flight school aircraft is ever operated that way, and yet their engines last as long or longer than average.

All of this is an opinion of course and flies in the face of an entire industry set to make money selling you things you must have, but really don’t have to have. The aircraft can be safely flown with the instrumentation and equipment it left the factory with engine wise. I’m not saying the ADF and Loran are all you need, speaking to engine instrumentation.

I believe if your a new pilot spend your $$$ on fuel and fly very often and learn to fly by the seat of the pants, then get your instrument and become proficient it instrument flying, then if you want to buy the gadgets etc and get deep into mixture control, by all means do so, but not until after you have become a very proficient pilot, both VFR and IFR.

 

The confusion that came from this post was the misunderstanding that best power and best economy give the same power at the same manifold and RPM, which they don’t of course. 

[redbaron1982]

2 hours ago, A64Pilot said:

There are many times I’ll use best power, just not often anymore cause I’m old and don’t travel like I used to, but if I’m climbing out of a valley to cross mountains and or at high altitudes where my non turbo is a pig, I’ll run best power just trying to get some power. My service ceiling LOP is significantly lower than ROP, same with airspeed, so if your in a hurry even at mid altitudes say 10,000 or so then best power may be called for. 

Everything back in the day was done as a close is good enough, and the deeper you dig into things and if you have a few years test flying, you learn close is good enough.

There is a point of diminishing returns with everything and then there are considerable variations from one engine / aircraft and another, so it was wisely chosen to give some charts to illustrate “best”. I say wisely because the average pilot has no need or use actually in determining actual “best” plus of course the aircraft as manufactured didn’t have the instrumentation to do so. To publish the method of determining best would have new pilots spending hours on something they don’t need to, time better spent on gaining a feel for the airplane etc.

Even doing airspeed cals is more complex than you may think, requires instrumentation not on a standard aircraft and in truth average pilot has no need for it.

As an example airspeed is all over the place flaps, CG, aircraft attitude all effect it, but most don’t get wrapped around the axle about it like we do the engine.

Bottom line you can spend thousands of dollars and many hours trying to run your engine at the perfect spot for improved performance and life span, but it doesn’t seem to really make as much difference as many want to believe. As a reference no flight school aircraft is ever operated that way, and yet their engines last as long or longer than average.

All of this is an opinion of course and flies in the face of an entire industry set to make money selling you things you must have, but really don’t have to have. The aircraft can be safely flown with the instrumentation and equipment it left the factory with engine wise. I’m not saying the ADF and Loran are all you need, speaking to engine instrumentation.

I believe if your a new pilot spend your $$$ on fuel and fly very often and learn to fly by the seat of the pants, then get your instrument and become proficient it instrument flying, then if you want to buy the gadgets etc and get deep into mixture control, by all means do so, but not until after you have become a very proficient pilot, both VFR and IFR.

 

The confusion that came from this post was the misunderstanding that best power and best economy give the same power at the same manifold and RPM, which they don’t of course. 

But then again, the M20J POH indicates that at 2400RPM and 22.8inHg both at best power and economy the engine produces the same power and the aircraft flies at the same TAS. The only difference is that with economy mixture it burns 12% less fuel. 

So I don't think I was confused, just didn't understand why it was that. 

[Will.iam]

5 hours ago, redbaron1982 said:

But then again, the M20J POH indicates that at 2400RPM and 22.8inHg both at best power and economy the engine produces the same power and the aircraft flies at the same TAS. The only difference is that with economy mixture it burns 12% less fuel. 

So I don't think I was confused, just didn't understand why it was that. 

Mistakes and typo’s

Look at my 2 charts on time to climb on max climb compared to cruise climb. Does it make sense that max climb time is - in the formula where as cruise climb would be a +? Of course not. It was a typo. Happen alot back then. 

[A64Pilot]

On 8/27/2022 at 1:51 PM, redbaron1982 said:

But then again, the M20J POH indicates that at 2400RPM and 22.8inHg both at best power and economy the engine produces the same power and the aircraft flies at the same TAS. The only difference is that with economy mixture it burns 12% less fuel. 

So I don't think I was confused, just didn't understand why it was that. 

It may say that, if it does, it’s wrong. I’ll admit to not being as up on the POH as I should be

As was already said best power is that magical mixture where the most power is made, any mixture other than it, decreases power.

For an angle valve 360 Lycoming I believe most cases 100 ROP is close to best power, could be wrong though.

Best economy is that magical mixture that produces the most power per fuel burned or best BSFC, for the same engine I believe that’s usually about 25 LOP, any mixture other than that increases fuel burn per HP made.

We often run much richer than best power of course and accept the lower power output and the excess fuel cools the engine, our engines don’t have the cooling capacity to run best power at full throttle.

One takeaway though is at lower power say 65% or less (Lycoming I believe says 75%) you can run any mixture you desire without harm.

[carusoam]

Will,

I’ll take your word for it!

And give you the Eagle Eye award today… 

Unfortunately, I can’t read the chart… my iPad is killing the resolution…

Best regards,

-a-

[jaylw314]

On 8/27/2022 at 10:51 AM, redbaron1982 said:

But then again, the M20J POH indicates that at 2400RPM and 22.8inHg both at best power and economy the engine produces the same power and the aircraft flies at the same TAS. The only difference is that with economy mixture it burns 12% less fuel. 

So I don't think I was confused, just didn't understand why it was that. 

I am going to take a guess that those airspeeds listed on your economy cruise power schedule are NOT flight tested speeds, but simply based off the chart of MP/RPM settings. My suspicion is that all the power-to-airspeed charts are done with specified MP/RPM settings at best power, even in the economy cruise power section.  Later M20J POH's removed the airspeed numbers, as above.

So yes, if you set the MP/RPM to 22.8 and 2400, at best power, I suspect a perfect plane would hit 154 knots KTAS, and if you leaned further it would be less than that by 4-5 knots

[PT20J]

I think we are overthinking this. Mooney defines best power as an EGT 100F ROP and economy cruise as 25F ROP. The drop off in power going from 100F to 25F is only 2-3% because the power vs fuel flow curve is nearly flat in that region. This small amount of change is probably lost in the accuracy of setting  rpm and manifold pressure adjusted for OAT. 

it seems reasonable to assume that Mooney chose 25 ROP as an economy cruise recommendation because the decrease in airspeed will be negligible, but the fuel flow is noticeably decreased compared to the best power mixture. 

Skip

[Pinecone]

And puts you right smack into the red fin/red box area. 

 

 

[jaylw314]

2 hours ago, PT20J said:

I think we are overthinking this. Mooney defines best power as an EGT 100F ROP and economy cruise as 25F ROP. The drop off in power going from 100F to 25F is only 2-3% because the power vs fuel flow curve is nearly flat in that region. This small amount of change is probably lost in the accuracy of setting  rpm and manifold pressure adjusted for OAT. 

it seems reasonable to assume that Mooney chose 25 ROP as an economy cruise recommendation because the decrease in airspeed will be negligible, but the fuel flow is noticeably decreased compared to the best power mixture. 

Skip

In practice, I see about that 4-5 knots of drop off, which is about 2-3% which I'd agree in percentages is pretty negligible (for 10% less fuel!)

[redbaron1982]

2 hours ago, PT20J said:

I think we are overthinking this. Mooney defines best power as an EGT 100F ROP and economy cruise as 25F ROP. The drop off in power going from 100F to 25F is only 2-3% because the power vs fuel flow curve is nearly flat in that region. This small amount of change is probably lost in the accuracy of setting  rpm and manifold pressure adjusted for OAT. 

it seems reasonable to assume that Mooney chose 25 ROP as an economy cruise recommendation because the decrease in airspeed will be negligible, but the fuel flow is noticeably decreased compared to the best power mixture. 

Skip

I haven't paid attention to how small the power loss was. I completely agree that for that small amount and for all the factors involved and measurement errors, both power settings can be considered as producing the same power output. 

Great answer Skip!

[PT20J]

2 hours ago, Pinecone said:

And puts you right smack into the red fin/red box area. 

 

 

Lycoming approves leaning below 75% rated power, so there really is no red box/fin as long as you are below 75%. Or, you can be more conservative and not lean below 65%. While it is true that 25 - 50F ROP causes higher CHTs than other mixtures, it really doesn't hurt anything as long as you keep them reasonable (say below 400F). Personally, I run LOP about 65% for short, low altitude flights, and best power for high altitude takeoffs and climbs, and usually peak for long distance flights at 8-12,000 MSL.

Skip

BTW, it's interesting that mixtures slightly richer than peak EGT yield the highest CHTs. Peak EGT occurs at the stoichiometric mixture where all the fuel molecules should combine with all the oxygen molecules. Since all the oxygen should have been consumed, it seems counterintuitive that adding more fuel would have any heating effect. But combustion of mixtures at or near stoichiometric produce temperatures high enough to cause some combustion products to dissociate. This chemical dissociation frees up oxygen and allows excess fuel to combust thereby increasing the heat generated and rejected to the cylinder components.

[Will.iam]

16 hours ago, carusoam said:

Will,

I’ll take your word for it!

And give you the Eagle Eye award today… 

Unfortunately, I can’t read the chart… my iPad is killing the resolution…

Best regards,

-a-

Here yea go a zoomed in shot on the chart example. Notice one chart has - and the other +? And they even did the addition which doesn’t make sense. The example is showing how to figure out time to climb when you start off at a field that’s already 1500 agl  so you run the numbers for 0 agl to 1500 and subtract that from the number you get going agl to 22,000ft as you already were at 1500ft to begin with  but the second chart showing same example of starting off at 1500 field elevation added the time so that would be like when you took off below sealevel like death valley but deeper by 1500ft if you added the time so of course a + is the wrong symbol and adding the wrong math but the proofreaders missed the error.

[201er]

Yeah and they claim mag timing doesn’t affect airspeed but it certainly does!