Best power vs economy mixture setting

[carusoam]

Better timing = better efficiency + better power

At the cost of higher CHTs…

Very low cost, for some nice improvements…. 
 

Makes you want to review the 10:1 compression pistons again….  
 

Best regards,

-a-

[Pinecone]

15 hours ago, PT20J said:

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.

 

That is because there is not perfect mixing nor perfect combustion.  So you need a slight increase in fuel to make sure you consume all the oxygen.

ROP, power is based on air intake (MP and RPM).

LOP, power is based on fuel delivered.

[Igor_U]

21 hours ago, jaylw314 said:

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!)

??? Where am I? Cessna space? 

 

Joke aside, you should NOT see any speed reduction going from 65% Best power (100 def ROP) to 65% best economy (20 ROP) as the same power results in the same speed.

Speed reduction means you are applying less power regardless of the fuel burn (efficiency). I think Red Baron is asking a good question: how can 65% 100ROP have the same MF and RPM settings as 65% 20ROP (wherever best economy is as per Chapter IV)? In my book, it can't! if you start rich and go to peak or LOP, you will lose power and will have to compensate with throttle (or RPM increase) to achieve same speed. As mentioned, curve between 100 and 20 RPM is sufficiently flat and flight testing in 1970's imprecise to showing old gages but speed will be the same as both settings are 65%.

 

 

 

[PT20J]

5 hours ago, Pinecone said:

 

That is because there is not perfect mixing nor perfect combustion.  So you need a slight increase in fuel to make sure you consume all the oxygen.

ROP, power is based on air intake (MP and RPM).

LOP, power is based on fuel delivered.

Even with perfect mixing, dissociation would cause the effect I noted. There is a good discussion of this in Winterbone and Yuran, Advanced Thermodynamics for Engineers, Chapter 8: Effect of Dissociation on Combustion Parameters. It is also discussed in Heywood, Internal Combustion Engine Fundamentals.

At fixed airflow, power is always a function of fuel flow, regardless of the mixture. However, the BSFC curve is relatively flat for lean mixtures which gives rise to the observation that horsepower can be computed by applying an appropriate multiplier to fuel flow when sufficiently LOP. When ROP, the horsepower/fuel flow relation is not linear, so it is not possible to predict horsepower with a simple multiplier applied to fuel flow.

Skip

[jaylw314]

1 hour ago, Igor_U said:

Joke aside, you should NOT see any speed reduction going from 65% Best power (100 def ROP) to 65% best economy (20 ROP) as the same power results in the same speed.

Speed reduction means you are applying less power regardless of the fuel burn (efficiency). I think Red Baron is asking a good question: how can 65% 100ROP have the same MF and RPM settings as 65% 20ROP (wherever best economy is as per Chapter IV)? In my book, it can't! if you start rich and go to peak or LOP, you will lose power and will have to compensate with throttle (or RPM increase) to achieve same speed. As mentioned, curve between 100 and 20 RPM is sufficiently flat and flight testing in 1970's imprecise to showing old gages but speed will be the same as both settings are 65%.

There's a difference between "negligible" and "unmeasurable."  The speed decrease would be negligible, but measurable  

[A64Pilot]

Guys 65% power is 130 HP of a 200 HP engine, if power and RPM are the same speed will be identical, only thing RPM plays into this is a slower turning prop is more efficient, not a whole lot, but it’s there.

The charts if they show identical power output at both best economy and best power with the same MP and RPM are obviously incorrect. TANSTASFL.

Yes you can make 130 HP at best economy, but it’s going to take more MP or RPM or both than best power, you’ll run out of MP and RPM at a much lower altitude at best economy than best power, keep climbing and you will be pushed toward best power trying to maintain power. 

Its pretty simple 

Best Power = Most power possible, fuel flow be damned, but heat is also a limit

Best Economy means least fuel burned per HP, power made be damned, heats still a limit but as max power is much lower so is heat, so it’s not as much a limit.

So this works out to the best MPG will be flown at best economy and slower airspeed, within reason the slower you fly the better the MPG, from my informal non scientific OFO flying it seems max MPG occurs in my J at about 120 kts true and I think that’s about 50% power, just as a reference.

[PT20J]

5 minutes ago, A64Pilot said:

it seems max MPG occurs in my J at about 120 kts true and I think that’s about 50% power, just as a reference.

Probably about right. The APS folks like to point out that the airlines ran the big radials deeply LOP, but they leave out the fact that they ran at about 55% rated power during cruise for fuel efficiency. Even when avgas was cheap, it was a significant operating cost to the airlines.

[A64Pilot]

10 hours ago, PT20J said:

Probably about right. The APS folks like to point out that the airlines ran the big radials deeply LOP, but they leave out the fact that they ran at about 55% rated power during cruise for fuel efficiency. Even when avgas was cheap, it was a significant operating cost to the airlines.

Range was a big factor too, it’s amazing what slowing down and leaning it out can do for range. Then there is engine life, an engine operated at lower power / RPM has been proven to last for many more hours than if it’s run hard.

Lots of differences between auto engines and aircraft but one big difference is auto engines loaf around almost their entire lives, rarely making high power, our Prius when we got rid of it had over 8,000 hours on it, and burned no oil. Not saying you could get an aircraft engine there you can’t, but if the Prius had been run like an aircraft it wouldn’t have either.

[Pinecone]

10 hours ago, A64Pilot said:

So this works out to the best MPG will be flown at best economy and slower airspeed, within reason the slower you fly the better the MPG, from my informal non scientific OFO flying it seems max MPG occurs in my J at about 120 kts true and I think that’s about 50% power, just as a reference.

BEST MPG happens at the Carson Speed.  Which is about 1.3x best glide speed.

Best efficiency, for NA, is to fly at an altitude where WOT, lowest RPM gives you an indicated air speed that is your Carson Speed.

 

[A64Pilot]

My best glide based on weight is about 90 kts, I think, 1.3 times 90 is 117, very close to 120. Plus or minus 5 kts or so the curve is pretty flat. I can speed up to 130 and not burn much more if MPG is the metric, but above that MPG begins to decrease significantly.

High altitude only becomes a benefit if it’s a long distance flight, most mine aren’t and the few that I do, I usually get impatient and go on, but your right altitude is your friend if the distance is great enough to overcome the fuel burned to get up there. Plus of course 120 KIAS is actually a pretty good cruise speed up high.

I fly at 2200 when I’m not in a hurry, because in my airplane it’s smooth, lower RPM in theory has less frictional losses, full throttle has lower pumping losses and a more efficient prop (friction again), back to that Prius, it’s computer would drive the engine RPM to the min RPM that the car could maintain speed at full throttle, put a scan gauge on a Prius and you will see 850 or 900 RPM at times going 60 mph or so.

Having said that I have flown some test points at low RPM in an instrumented airplane with low MP high RPM vs high MP / low RPM at identical speed. and just didn’t see any significant fuel flow difference, there is a difference but it’s very small, hard to quantify. So I pick RPM’s that give smooth operation myself

So in other words it’s my opinion that 120 KIAS and pick a smooth low RPM, run LOP gives you about the best your going to get, any increases over that will be small

 

[PT20J]

3 hours ago, Pinecone said:

BEST MPG happens at the Carson Speed.  Which is about 1.3x best glide speed.

Best efficiency, for NA, is to fly at an altitude where WOT, lowest RPM gives you an indicated air speed that is your Carson Speed.

 

Best MPG occurs at L/D max speed, which is the max range and best glide speed, and the most aerodynamically efficient speed. Recognizing that this speed was too slow to be practical, Carson set out to define a speed that would better balance speed and efficiency, or as Carson put it, “the least wasteful way of wasting.” His analysis showed that an optimum balance is obtained by multiplying L/D max speed by the fourth root of three (1.32).

[AH-1 Cobra Pilot]

1 hour ago, PT20J said:

Best MPG occurs at L/D max speed, which is the max range and best glide speed, and the most aerodynamically efficient speed. Recognizing that this speed was too slow to be practical, Carson set out to define a speed that would better balance speed and efficiency, or as Carson put it, “the least wasteful way of wasting.” His analysis showed that an optimum balance is obtained by multiplying L/D max speed by the fourth root of three (1.32).

I first heard of this from one of my squadron's 'Academy Boys' who had classes with Carson about the time, (apparently), he published it.  It still seems arbitrary and depends on what you think is the cost/benefit relationship, i.e. the slope of the speed vs. fuel burn line.

The real purpose of "design" is not just to find the best solution, but to interpret the solution that provides the slightly less best solution that provides more of other benefits and becomes the ultimate solution.  When you have 10 variables and 10 equations determining gross weight, highest speed, fuel consumption, lowest landing speed, payload, stability, etc. the best solution is the point at which you still have water when the rest has evaporated from your 10-dimensional lake.  The ultimate solution requires you to interpret how far you want to go up the slope from that bottom, (best solution), of any and/or all of the variables to give you better results, (for instance, lots more speed for only a little more fuel burn or reduced payload), again, in 10-dimensions.  There is nothing magical about how high up the slope you quit and take that for your ultimate solution.

I have not studied Carson's idea since then, so if there are newer data to say why that number is not arbitrary, I welcome enlightenment.

[201er]

22 minutes ago, AH-1 Cobra Pilot said:

I first heard of this from one of my squadron's 'Academy Boys' who had classes with Carson about the time, (apparently), he published it.  It still seems arbitrary and depends on what you think is the cost/benefit relationship, i.e. the slope of the speed vs. fuel burn line.

It's not arbitrary. It's the switch point where the fuel burn increases more quickly than speed due to parasitic drag.

[AH-1 Cobra Pilot]

1 hour ago, 201er said:

It's not arbitrary. It's the switch point where the fuel burn increases more quickly than speed due to parasitic drag.

I still do not see it.  What is the slope of the red line and why?

[201er]

https://cafe.foundation/v2/pdf_tech/MPG.engines/AIAA.1980.1847.B.H.Carson.pdf

[AH-1 Cobra Pilot]

7 hours ago, 201er said:

https://cafe.foundation/v2/pdf_tech/MPG.engines/AIAA.1980.1847.B.H.Carson.pdf

I cannot get to that site.  Can you post the file?

[carusoam]

Who is our Carson guy….?

Let’s invite Norman…. @testwest

He likes this topic….  

Best regards,

-a-

[PT20J]

24 minutes ago, AH-1 Cobra Pilot said:

I cannot get to that site.  Can you post the file?

Here you go...

AIAA.1980.1847.B.H.Carson.pdf

[Mooney in Oz]

On 8/30/2022 at 6:37 AM, PT20J said:

…usually peak for long distance flights at 8-12,000 MSL.

Skip

First or last to peak?

[carusoam]

26 minutes ago, Mooney in Oz said:

First or last to peak?

V,

When going LOP… or peak…

My desire is to not let any fuel intentionally go un-burned…

So I am waiting for the last to peak…

The larger the Gami spread the bigger the difference…

But, at peak… it’s not a lot either way…

 

Kind of more of a theoretical argument… 

Best regards,

-a-

[PT20J]

48 minutes ago, Mooney in Oz said:

First or last to peak?

I usually stop when I hit the first peak. Mine are pretty close together. For some reason, after a few minutes is seems to go a bit leaner and I end up somewhere in the middle of the pack.

 

 

[MikeOH]

@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

[A64Pilot]

22 hours ago, Mooney in Oz said:

First or last to peak?

If you have a decently balanced engine it’s splitting hairs, remember honestly that close is good enough.

These things we think of as absolutes really aren’t like best efficiency is 25 LOP, well in truth it’s correct for one DA and one set of MP/RPM, but in reality it’s close enough, driving down to the absolute may be entertaining and good for table talk, but in the airplane it’s close enough. by close enough if we get to within .2 GPH and the mission is 5 hours, well that’s 1 gl difference, just isn’t relevant, then of course my engine will be a little different than yours so how to you compute for that?

[A64Pilot]

11 hours ago, MikeOH said:

 

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.

The first is where your incorrect, but seems to be what’s taught by the LOP folks. If your LOP flying along in level flight and touch nothing but the mixture and begin slowly to enrichen it, power will increase and continue to increase until you hit best power, often about 100 ROP. If you continue to enrichen, power will begin to decrease and that’s the point where your throwing fuel away, Cyl head temps decrease substantially also, and that’s why we takeoff and climb full rich and not best power because of temps, so we throw away fuel to preserve engine longevity.

Yes you can often achieve 65% LOP, but not at the same MP and RPM as best power, that’s the point that gets glossed over sometimes, that there is always less power LOP.

Secondly MP and RPM as well as fuel flow are what generate power, sort of like the fire triangle, remove any one and no fire, so yes when driven as an air pump an engine can maintain MP and RPM, but without fuel there will be no power. So in normal flight if you reduce any of the three you will reduce power, only exception is if your rich of best power a reduction will initially result in a power increase until best power, but then any further reduction will result in a decrease in power.

So why don’t we fly at best power in normal cruise, if it gives the fastest cruise? Reason is efficiency, there is an increase in efficiency lean of best power until about -25 lean of peak, so if we are willing to give up a little speed we can save some fuel LOP, there will be a speed reduction, at no point will you not be slower LOP than at the same MP / RPM at best power. But if you bought your airplane to go fast and want to go fast, you fly at best power and keep an eye on cyl head temps, because best power is, well best power, it’s in the name.

[MikeOH]

10 hours ago, A64Pilot said:

The first is where your incorrect, but seems to be what’s taught by the LOP folks. If your LOP flying along in level flight and touch nothing but the mixture and begin slowly to enrichen it, power will increase and continue to increase until you hit best power, often about 100 ROP. If you continue to enrichen, power will begin to decrease and that’s the point where your throwing fuel away, Cyl head temps decrease substantially also, and that’s why we takeoff and climb full rich and not best power because of temps, so we throw away fuel to preserve engine longevity.

Yes you can often achieve 65% LOP, but not at the same MP and RPM as best power, that’s the point that gets glossed over sometimes, that there is always less power LOP.

Secondly MP and RPM as well as fuel flow are what generate power, sort of like the fire triangle, remove any one and no fire, so yes when driven as an air pump an engine can maintain MP and RPM, but without fuel there will be no power. So in normal flight if you reduce any of the three you will reduce power, only exception is if your rich of best power a reduction will initially result in a power increase until best power, but then any further reduction will result in a decrease in power.

So why don’t we fly at best power in normal cruise, if it gives the fastest cruise? Reason is efficiency, there is an increase in efficiency lean of best power until about -25 lean of peak, so if we are willing to give up a little speed we can save some fuel LOP, there will be a speed reduction, at no point will you not be slower LOP than at the same MP / RPM at best power. But if you bought your airplane to go fast and want to go fast, you fly at best power and keep an eye on cyl head temps, because best power is, well best power, it’s in the name.

My first statement is NOT incorrect!  65% power is ALWAYS 65% power.  I don't know how much clearer I could have made my first statement!  Are you claiming 65% power is somehow not the same power when LOP vs. ROP???  The rest of your first paragraph is obvious; start enriching from LOP and power will increase to peak and then decrease as you get richer.  I didn't think anyone was debating that.  Certainly not me.

Your second paragraph is completely off point.  I NEVER claimed you could get 'best power' while LOP!  Sure, there is LESS power available LOP, but heck there is LESS power ROP, too!  The OP was not asking about what was best power.  We are, specifically, looking at power UNDER max/best power.

Your last paragraph is a nice discussion but not what the OP was asking about.

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%.

When the power levels start to go over 75% I'm not going to argue that you may not be able to run the same power level both ROP and LOP due to other considerations (temps).

Frankly, I think this whole thread is because of the poor choice by Mooney to label the COMPLETE tables "Best Power" and "Economy Cruise".  My POH only has one table labelled "Best Power" it is left as an exercise to the student to figure out LOP!  The tables should have been titled ROP and LOP  As you point out, there is only ONE best power setting if you want to fly fast!