Best power vs economy mixture setting

[PT20J]

The tables from the 1977 POH are labelled ECONOMY CRUISE and BEST POWER. Section IV of that POH describes economy cruise as 25 F ROP and best power as 100 F ROP. Note that Mooney's economy cruise mixture is NOT a "best economy" (i.e., lowest BSFC) mixture. 

With the rpm held constant, apparently very little change in manifold pressure is required to generate the same power at 25 F ROP as at 100 F ROP. Some entries in the tables show the same manifold pressure for each mixture at a given rpm and power, and some entries show a slightly higher (0.1 - 0.3 ") manifold pressure for economy cruise. 

Had Mooney chosen to publish a best economy table, the manifold pressures would be notably higher to generate the same power at the same rpm as the best power mixture.

Skip

[CoffeeCan]

I realize it’s a lot of fun to debate how many EGT’s can dance on the head of a cylinder head pin (apologies to any medieval angel scholars present), but going back to the OP, it seems to me this debate isn’t helping our new Mooniac, who is only trying to figure out how to make sense of his engine settings in light of the POH tables, graphs, and figures. 

The plain truth is the POH’s  for our M20J and K airplanes were written in the 1960’s and 70’s at best, and are (as others have pointed out) quite outdated if not actually useless.

When I bought my first Mooney in 2018 and I was seeking advice on engine management here on Mooneyspace, I was in a similar place to the OP. I did my best to reconcile all the bits of advice gleaned from these pages, and to interpret the POH graphs and tables in light of this advice, but in the end was no wiser than I was when I started. 

My confusion was resolved when I became a SavvyMX subscriber and bought Mike Busch’s book on airplane engines and learned how to properly run my engine. I’ve attached a short article here by Mike that gives the Cliff’s Notes version. Basically, throw out the POH tables and run your engine at either 25-50 degrees LOP or best power (~100 ROP) while maintaining safe CHT’s (380-400F).  It’s that simple. 

I always cruise LOP, which is cleaner and greener and considerably more economical. My CHT’s, TIT, and EGT’s remain safely in the green, and I burn 9-10 gph. 

Sure, I could run at best power at 100 ROP, and gain 10-15 KTAS, and burn 12-13 gph.  But on a 6-hour XC flight that extra speed will get me there only 20-30 minutes sooner, and I’ll burn 20-24 more gallons of fuel, which means $300+ more of my hard-earned dollars being turned to water vapor and CO2. And unless I’m carefully monitoring my CHT’s and keeping them well down, I’ll be significantly shortening the life of my engine. 

But I ain’t an engineer, so don’t take my advice as if I was one. 
 

https://www.avweb.com/ownership/the-savvy-aviator-59-egt-cht-and-leaning/

Edited September 18, 2022 by CoffeeCan

[AH-1 Cobra Pilot]

On 8/31/2022 at 9:55 PM, PT20J said:

Here you go...

AIAA.1980.1847.B.H.Carson.pdf 547.23 kB · 8 downloads

I went through the paper a couple of times, and I conclude the airspeed to be arbitrary.  He sets the slope of the previously mentioned line at 45^o, (why not 40^o, 50^o, or 30^o?), then does a lot of hand waving to prove that is what it should be. 

Everyone uses his/her own "best speed" as determined by his/her own estimate as to the factors involved, but to set something for everyone would have to be based on either a derived term or a fundamental constant of nature, which this paper does not.

That slope should be much more dependent on the price of fuel, which he neglects entirely.  i.e. If AVGAS were $.01/gallon, the appropriate slope of the line would be almost infinity; if the price were $1000/gallon, the slope would approach zero.

[AH-1 Cobra Pilot]

18 hours ago, CoffeeCan said:

Basically, throw out the POH tables and run your engine at either 25-50 degrees LOP or best power (~100 ROP) while maintaining safe CHT’s (380-400F).  It’s that simple. 

The discussion the rest of us are engaging concerns power and the resulting airspeeds.  Power is determined by RPM, and since you never mention RPM in your post, you are neglecting a most important variable and cannot "throw out the POH tables."

[Shadrach]

9 hours ago, AH-1 Cobra Pilot said:

The discussion the rest of us are engaging concerns power and the resulting airspeeds.  Power is determined by RPM, and since you never mention RPM in your post, you are neglecting a most important variable and cannot "throw out the POH tables."

That is only true if the load is constant.  Power cannot determined by RPM with a constant speed prop

 

9 hours ago, AH-1 Cobra Pilot said:

I went through the paper a couple of times, and I conclude the airspeed to be arbitrary.  He sets the slope of the previously mentioned line at 45^o, (why not 40^o, 50^o, or 30^o?), then does a lot of hand waving to prove that is what it should be. 

Everyone uses his/her own "best speed" as determined by his/her own estimate as to the factors involved, but to set something for everyone would have to be based on either a derived term or a fundamental constant of nature, which this paper does not.

That slope should be much more dependent on the price of fuel, which he neglects entirely.  i.e. If AVGAS were $.01/gallon, the appropriate slope of the line would be almost infinity; if the price were $1000/gallon, the slope would approach zero.

You read it 3 times and you're referring to the "Gabrielli Von Karman limit" referenced in the paper as Dr. Carson's work?  How does the  price of Avgas have anything to do with this paper. A unit of fuel is a unit of fuel is a unit of fuel...what it costs is not a factor.

The goal was to derive the speed that coincides with the least amount of time spent in cruise for the least amount of fuel burned over a distance. Stated simply it is a fuel efficiency calculation that factors in speed/mpg rather simple fuel consumed over distance.

All vehicles have a speed that yields the most speed for a unit of energy.  That speed is somewhat higher than the speed the yields the most distance for a unit of energy. 

[Shadrach]

45 minutes ago, AH-1 Cobra Pilot said:

The discussion the rest of us are engaging concerns power and the resulting airspeeds.  Power is determined by RPM, and since you never mention RPM in your post, you are neglecting a most important variable and cannot "throw out the POH tables."

Power is not determined by RPM nor can it be determined by RPM unless the engine is under a known load. 

[AH-1 Cobra Pilot]

10 minutes ago, Shadrach said:

Power is not determined by RPM nor can it be determined by RPM unless the engine is under a known load. 

In Steady State, a particular airspeed is a known load.  P = Tϖ

[Shadrach]

21 minutes ago, AH-1 Cobra Pilot said:

In Steady State, a particular airspeed is a known load.  P = Tϖ

Agreed, so why did you say P = RPM? 

RPM is not a known load because neither prop efficiency nor pitch is constant. 

This is getting pedantic but surely you understand that an engine with a CS prop can make the same power at different RPMs? 

[AH-1 Cobra Pilot]

22 minutes ago, Shadrach said:

Agreed, so why did you say P = RPM? 

RPM is not a known load because neither prop efficiency nor pitch is constant. 

This is getting pedantic but surely you understand that an engine with a CS prop can make the same power at different RPMs? 

I never said that.  Reread my post.

[Shadrach]

7 minutes ago, AH-1 Cobra Pilot said:

I never said that.  Reread my post.

Perhaps you should reread your post...

1 hour ago, AH-1 Cobra Pilot said:

The discussion the rest of us are engaging concerns power and the resulting airspeeds.  Power is determined by RPM, and since you never mention RPM in your post, you are neglecting a most important variable and cannot "throw out the POH tables."

 

[Shadrach]

On 9/1/2022 at 7:45 PM, MikeOH said:

@redbaron1982

I've slogged through this entertaining (kinda), tangential (gee, what a surprise), and informative (sometimes) thread you kindly (I think) started and I'm going to be the heretic and render my $0.02:

Both tables in your POH are just fine.  It is not suffering from '60s era mistakes (any more than today's era mistakes)

There is nothing incongruous about 65% power yielding the same speed at two different fuel flows; 65% ROP is the same power as 65% LOP. Thus, speeds in both table are the same.  Thing is, ROP you are THROWING away excess fuel, but NOT getting any more POWER.

MAP and RPM are red herrings.  I can cut mixture completely (zero fuel flow) and the MAP and RPM will NOT change if I push the nose over to maintain the same speed.

So, it should not be any surprise that 65% can have the same MAP, RPM, and speed but different fuel flows.

Flame suit on

"There is nothing incongruous about 65% power yielding the same speed at two different fuel flows; 65% ROP is the same power as 65% LOP"

That's a bit of an intellectual slight of hand.  Your statement is true. You can make 65% ROP or you can make 65% power LOP. However, it will take more MP to make 65% LOP. That's really not applicable to the OPs quandary as to how the POH shows identical MP, RPM and % power at best economy and best power mixture.  To be clear the actual power delta between 100ROP and 25ROP is not huge, but there is a measurable difference. Power changes over the mixture spectrum. Those changes are almost imperceptible between some settings (say 100ROP and 125ROP) and obvious at others (25LOP and 50LOP).  MAP and RPM are most definitely not "Red herrings".  They are very usable parameters when setting power though they are not needed to set cruise for NA engines, it's certainly usable information.

[Shadrach]

On 9/2/2022 at 1:47 PM, MikeOH said:

Your third paragraph is what requires investigation.  It appears your position is that you cannot achieve the same power output with the same MAP and RPM both ROP and LOP.  My position is that you can, and thus the POH that the OP referenced is, in fact, correct.  When ROP you are just wasting fuel, when LOP you are burning just what is required to achieve, for example, 65%.

This is not a true statement. You're confused.

[jaylw314]

12 hours ago, AH-1 Cobra Pilot said:

I went through the paper a couple of times, and I conclude the airspeed to be arbitrary.  He sets the slope of the previously mentioned line at 45^o, (why not 40^o, 50^o, or 30^o?), then does a lot of hand waving to prove that is what it should be. 

Everyone uses his/her own "best speed" as determined by his/her own estimate as to the factors involved, but to set something for everyone would have to be based on either a derived term or a fundamental constant of nature, which this paper does not.

That slope should be much more dependent on the price of fuel, which he neglects entirely.  i.e. If AVGAS were $.01/gallon, the appropriate slope of the line would be almost infinity; if the price were $1000/gallon, the slope would approach zero.

I think the point of his mentioning the GvK line is that their work suggested a line of some slope in that graph of P/WV vs V (not 45 deg, where did you get that from?) that may be a theoretical limit.   His calculation of a Carson's speed approximates what the slope of that line should be.

IOTW, I think you have it backwards.  The slope of the line does not suggest Carson's speed, it's Carson's speed that suggests the slope of the line.

FWIW, the cost of fuel affects this calculation not at all. 

Edited September 20, 2022 by jaylw314

[MikeOH]

3 hours ago, Shadrach said:

This is not a true statement. You're confused.

You need to be a bit more convincing as to why I'm confused.  I'm not going to claim their isn't a slight difference in efficiencies between LOP and ROP at the same MP/RPM, but I posit that they are small and, thus, the OP's POH is, in fact, correct.

[PT20J]

1 hour ago, MikeOH said:

You need to be a bit more convincing as to why I'm confused.  I'm not going to claim their isn't a slight difference in efficiencies between LOP and ROP at the same MP/RPM, but I posit that they are small and, thus, the OP's POH is, in fact, correct.

This statement makes sense if you substitute "... Economy Cruise and Best Power at the same power ..."

It doesn't work as written because you cannot get the same power using the same MP/RPM LOP and ROP and so it becomes an apples to oranges comparison.

As pointed out earlier in the thread, the reason the tables in the POH are correct is that Mooney's Economy Cruise is not LOP -- it is 25 deg F ROP and so the difference in manifold pressure between Economy Cruise and Best Power (100 deg F ROP) at the same power is half an inch or less. 

Skip

[MikeOH]

36 minutes ago, PT20J said:

It doesn't work as written because you cannot get the same power using the same MP/RPM LOP and ROP and so it becomes an apples to oranges comparison.

Skip

Hmm, I guess I'm just not getting this. Why can't I get the same power ROP and LOP with the same MP/RPM?  (And, yes, I do recognize you are not going to achieve ALL power levels both ROP and LOP as you may well be in the red box when LOP at higher power levels)

[PT20J]

10 minutes ago, MikeOH said:

Hmm, I guess I'm just not getting this. Why can't I get the same power ROP and LOP with the same MP/RPM?  (And, yes, I do recognize you are not going to achieve ALL power levels both ROP and LOP as you may well be in the red box when LOP at higher power levels)

Good question 

Manifold pressure is really just a measure of how much air is flowing through the engine. At a constant altitude, it is a function of throttle position and rpm.

Power comes from burning the fuel. There is more fuel available ROP than LOP, so ROP generates more power for the same airflow.

Skip

[MikeOH]

4 minutes ago, PT20J said:

Good question 

Manifold pressure is really just a measure of how much air is flowing through the engine. At a constant altitude, it is a function of throttle position and rpm.

Power comes from burning the fuel. There is more fuel available ROP than LOP, so ROP generates more power for the same airflow.

Skip

Well, I would argue that LOP all the fuel is being consumed, while ROP there is EXCESS fuel that is NOT being burned.  Thus, for a given MP/RPM I contend that you can find TWO mixture settings that achieve the same power.

Where am I going wrong with that logic?

[PT20J]

Just now, MikeOH said:

Well, I would argue that LOP all the fuel is being consumed, while ROP there is EXCESS fuel that is NOT being burned.  Thus, for a given MP/RPM I contend that you can find TWO mixture settings that achieve the same power.

Where am I going wrong with that logic?

As you richen from peak EGT, essentially all the additional fuel is being burned up until best power mixture, which is why the power increases. Richer than best power, there is excess fuel that is not burned and which acts to slow the combustion process which is why the power falls off slightly at full rich compared to best power mixture. But the CHT falls off faster than the power which makes rich mixtures useful for cooling with minimal power loss.

For mixtures leaner than peak EGT, all the fuel is being burned, but there is less of it and so the power decreases.

[jaylw314]

36 minutes ago, MikeOH said:

Well, I would argue that LOP all the fuel is being consumed, while ROP there is EXCESS fuel that is NOT being burned.  Thus, for a given MP/RPM I contend that you can find TWO mixture settings that achieve the same power.

Where am I going wrong with that logic?

That's sort of true, but impractical.  There's a point richer than best power that will produce the same power as a mixture LEANER than best power.  In practice, there's not an easy way to figure where those points are.

I don't think any of us actually run anything richer than best power other than full rich) during normal operations.

[MikeOH]

27 minutes ago, PT20J said:

As you richen from peak EGT, essentially all the additional fuel is being burned up until best power mixture, which is why the power increases. Richer than best power, there is excess fuel that is not burned and which acts to slow the combustion process which is why the power falls off slightly at full rich compared to best power mixture. But the CHT falls off faster than the power which makes rich mixtures useful for cooling with minimal power loss.

For mixtures leaner than peak EGT, all the fuel is being burned, but there is less of it and so the power decreases.

I kind of see what you are getting at.  I think I'm just going to go flying and try it at some low power setting, say 21"/2400, ROP and note airspeed, then go LOP and see if I can achieve the same airspeed at the same 21"/2400.

[MikeOH]

5 minutes ago, jaylw314 said:

That's sort of true, but impractical.  There's a point richer than best power that will produce the same power as a mixture LEANER than best power.  In practice, there's not an easy way to figure where those points are.

I don't think any of us actually run anything richer than best power other than full rich) during normal operations.

If you are speaking of best power to mean MAXIMUM, as in 200 HP for an IO-360, then I agree completely.  But, if you are at say, 65% ROP, at a given MP/RPM, I contend that you can go LOP at that same MP/RPM and still achieve 65%.  I also agree that there's no easy way to figure the two points out. As I said, I need to go try it!

[Shadrach]

19 minutes ago, MikeOH said:

If you are speaking of best power to mean MAXIMUM, as in 200 HP for an IO-360, then I agree completely.  But, if you are at say, 65% ROP, at a given MP/RPM, I contend that you can go LOP at that same MP/RPM and still achieve 65%.  I also agree that there's no easy way to figure the two points out. As I said, I need to go try it!

I’ve marked lycomings graph to depict what Jay is saying. 
The only way to burn enough fuel LOP to make the same power as a ROP setting is to add more air.

 

[Shadrach]

10 hours ago, MikeOH said:

I kind of see what you are getting at.  I think I'm just going to go flying and try it at some low power setting, say 21"/2400, ROP and note airspeed, then go LOP and see if I can achieve the same airspeed at the same 21"/2400.

To make it even more complicated, the power generated by the fuel consumed is not linear as you know. On the rich side, the engine makes more heat and less power per unit for fuel which is why you can induce a much larger increase in CHT than airspeed by moving the mixture from say peak (which is the true economy setting) to 80 ROP. More fuel burned, more pressure generated, more heat to off load but less power per unit of fuel being transferred to the crank. This is clearly illustrated by the BSFC curve. When thinking about power produced by a fixed timing engine, how the fuel burns is as important as how much. There is a window between peak and about 30 LOP that produces a flame front that propagates in the most optimal way. It acts upon the piston at the most optimal time to produce the greatest mean pressure per unit of fuel while the piston is at the most optimal crank angles to harness that energy, again depicted by BSFC curve.

To your ROP vs LOP confusion, think of it this way. You are right that the engine is using all fuel to produce power when LOP. So what’s the ROP analogue?…the engine is using all of the available O2 to make power on the rich side.

So, at a fixed MP, ROP is making all the power possible with the available O2.  LOP, by definition has excess O2 after combustion (or it would not be a LOP setting). Even though the engine is able to extract more power for a unit of fuel LOP, that additional efficiency is suffcient to make as much power as the less efficient ROP setting. There is no way to burn as enough gas using less O2 to equal the ROP setting. This is why Peak EGT is a desirable engine setting if CHTs allow. It’s the theoretical intersection of what’s best about ROP and LOP. No wasted O2 or Fuel, efficient flame front characteristics with reasonable CHTs. Peak EGT is almost always my goal power setting, I only run leaner to control CHTs. That’s typically only needed on hot days or at power settings above 70%. 

The above is often misunderstood or not even a consideration when people ignorantly say things like "LOP fuel savings and/or reduced CHTs are mostly from reduced power". I was reading an "Ask a CFI" website (not a blog) the other day and read several paragraphs of drivel written by a highly credential Aviation pro.  I wanted to send him an email suggesting that he'd be better off to admit ignorance and not pretend to be an expert on something rather than to prove the latter with poorly informed writing.  I have seen this sort of thing everywhere.

[MikeOH]

10 hours ago, Shadrach said:

I’ve marked lycomings graph to depict what Jay is saying. 
The only way to burn enough fuel LOP to make the same power as a ROP setting is to add more air.

 

The red line is exactly what I was visualizing. But, I can see the curve cuts off, so to speak, at full rich. So, I understand now Jay’s point about it being impractical. Thanks for the graphic!