Power setting. Why so difficult?

airdale

airdale

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airdale
As I was putting together yet another power settings kneeboard chart for yet another airplane, I came to the following tentative conclusion: All these charts with seas of numbers for different altitudes, MP, and RPM are way more information than I need.

For normally-aspirated flying in roughly the 4K-10K feet, I need only the following:


  • The lowest available RPM and associated manifold pressure for the two or three power settings that I might use. Say 55%, 65%, and 75%. So, six numbers total.

Rationale:


  • One Engine RPM: The lowest RPM setting permitted by the POH minimizes noise and friction losses. Why would I use anything else in cruise?
  • One Manifold Pressure: The manifold pressures for a given RPM at different altitudes differ by only small amounts. And why do I care whether I am exactly hitting 75.00% anyway? 76% or 74% would be fine. I can't read 0.1" increments on the MP gauge plus is it probably not anywhere near 1% accuracy anyway, so that's another reason to skip the fine details.

This is really not a new idea, the Mooney Safety Foundation training manual talks about a "key number" where, for example, any combination of MP + RPM/100 that adds up to 50 will give you 75% power in an M20J. Key number for 65% is 47%, etc.

Comments? Maybe I'm just the slow student in the class?
 
It's difficult to generalize like that and engine/airplane combinations are different. But the lowest RPM for a given power setting (i.e. 65% cruise) may not necessarily be the most efficient. Compare fuel consumption between different RPM/MP combinations.

Also, running flat out at 75% in cruise tends to shorten engine life. Shouldn't normally be an issue above 5,000 feet though.
 
No, you've pretty much got it right.
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Thank you.
It's difficult to generalize like that and engine/airplane combinations are different.
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Oh, I'm not saying that it's six numbers for everything, just six numbers for a given airplane. The PA28R-201 Arrow and the Mooney M20J numbers are significantly different even though they use the same engine. The Arrow POH numbers match Lycoming's but the Mooney's do not. I'm still scratching my head on that one.

Compare fuel consumption between different RPM/MP combinations.
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For the 540 and 360 Lycomings anyway, the factory operating manuals show a few percent less fuel needed at each RPM reduction. For example, from 2300 to 2200. For the IO-360 A,C, 65% power fuel consumption is roughly 9% less at 2200 vs 2700. Slower is always better.

running flat out at 75% in cruise tends to shorten engine life.
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Different issue. I wasn't recommending or not recommending, just using it as an example.
 
anything above about 3500 is full throttle - you did not buy an airplane to get passed by 1.3 liter Civics and Chevy's did you?

You can then figure out what 2500/2400 and 2300 rpm get you, but whats the point?

What you need to really do is discover your airplane's sweet spot . . . mine is about 70% power - there I can put along at 155ktas at 2400rpm and see about 13.8gph .. . .

can't beat that with a stick. If I am under 7000 I can go LOP to about 11.5 - and get 15.5mpg - or just about what my ML350 gets . . .
 
comanchepilot said:
anything above about 3500 is full throttle
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Depends on the plane. Do that in my fixed pitch prop Tiger, and you'll be about 100-150 above redline RPM, and that's not a real good idea. In your Comanche, it will also depend on prop setting, as your engine will react badly to too much MP for the commanded RPM, although I'm estimating that with your IO-540, the min RPM for full throttle at 4000 MSL (about 26 inches) would be around 2100, and you're probably not pulling the prop back that far. And I assume you have good CHT instrumentation so you know what you're doing to your cylinders, because at that altitude, full throttle and a more typical 2400 RPM is probably close to 80% power.
 
airdale said:
For the 540 and 360 Lycomings anyway, the factory operating manuals show a few percent less fuel needed at each RPM reduction. For example, from 2300 to 2200. For the IO-360 A,C, 65% power fuel consumption is roughly 9% less at 2200 vs 2700. Slower is always better.
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To a point - it depends on where the BSFC curve starts to drop off rapidly, which is about 2200 rpm for an Lycoming IO 360. If you were to extrapolate and take it down to 2100 or 2000 rpm you would find a rise in fuel consumption. Some AC might have tabulated operation down to 2100 but have a better fuel burn at 2200 or 2300.
 
comanchepilot said:
anything above about 3500 is full throttle - you did not buy an airplane to get passed by 1.3 liter Civics and Chevy's did you?

You can then figure out what 2500/2400 and 2300 rpm get you, but whats the point?

What you need to really do is discover your airplane's sweet spot . . . mine is about 70% power - there I can put along at 155ktas at 2400rpm and see about 13.8gph .. . .

can't beat that with a stick. If I am under 7000 I can go LOP to about 11.5 - and get 15.5mpg - or just about what my ML350 gets . . .
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It would be awesome if everyone could do this but without some form of JPI's and Gami's it's not even recommended to go LOP even though I consider it the best way to fly if you ask me as well.

Like most people who fly, he's probably stuck flying ROP there for those MP/RPM settings are hit or miss until he finds out which setting works best for his engine.

Even for a given percentage to achieve LOP it's not always going to be 1475/380. You just gotta find which setting work best like you did with the 70% power of course and see what EGT/CHT numbers you get.

Per airdale's comment I agree, all those numbers are just mumbo jumbo in the grand scheme of things. Most pilots will tell you not to even bother with MP/RPM POH settings.
 
Thanks for the comments, guys.

My situation is probably a little different than most. In a year I will have flown at least a half-dozen airplanes between club airplanes, CAP airplanes, and other situations. Everything from 160hp fixed prop to 300hp IO-540. I am not smart enough to remember all the numbers and issues, so my flight gear includes both cheat sheets and customized checklists. Hence my poring over this latest POH.

Re "sweet spot" I am also not much of a touchy-feelie guy. To me there are various settings that produce various results and different flights have different objectives. I may lollygag on a $100 hamburger flight and press harder on a long cross country. Also, if I am flying a SAR grid or some of the other missions I have flown, I will be at 90-100kt regardless of what the airplane can do. Hence, again, my poring over the POH and wanting to have some reference numbers handy on my kneeboard.

Even if I believed in "sweet spots" I wouldn't have time to find them all.

I have also found that leaning with a JPI or a G1000 sounds wonderful but is a lot more Ouija-board and Art than science. Lean too slowly or too fast and you get false peaks. Mixture controls with lots of backlash make the experience worse. So I am also looking at the OEM specs for Best Economy and will be experimenting with whether just setting the OEM fuel flow number will work as well as fussing with Lean Find. But that is another thread.

Some AC might have tabulated operation down to 2100 but have a better fuel burn at 2200 or 2300.
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Could be. The Lycoming 540 and 360 books that I have been looking at, which are the two engines I fly most often, do not show that even down to 1800 RPM. The lines do get closer together, though, so the improvement with RPM reduction is less as you point out. The last time I actually saw a specific consumption graph was in the service manual for my 1962 Austin Healy 3000. (Incidentally, IIRC the Healy's best number was around 2300 rpm. Not that I cared.)
 
airdale said:
I have also found that leaning with a JPI or a G1000 sounds wonderful but is a lot more Ouija-board and Art than science. Lean too slowly or too fast and you get false peaks.
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That's because when you are operating LOP you want to monitor EGT not CHT. EGT is an almost instantaneous result, CHT are delayed and require time to stable. After a while you get to where you know where your EGT numbers should be and thus you lean for a particular EGT readout and monitor CHT and adjust from there.

There's a lot more involved in running LOP though, you have to really know what's going on inside that engine understand what is happening to the engine as you increase or decrease the mixture LOP, what's also going on with brake horsepower as well as brake specific fuel consumption for a giving setting. The normal combustion cycle as well as the peak pressure pulse for each stoke at the correct top dead center operation. When you get on the LOP side there's just as many variables to know and understand.
 
That's because when you are operating LOP you want to monitor EGT not CHT.
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Oh, both the JPI and the G1000 Lean Find functions monitor EGT. It's just that they often falsely declare a peak. I often find it easier to just monitor EGT directly and not try to use the Lean Find. Actually, I often just set the fuel flow about where it needs to be and don't try to optimize. Sloppy, I know, but particularly where there is a lot of backlash in the mixture adjustment it is a PITA to do better.

The issue is finding the peak accurately, not whether you go ROP or LOP after you find it.

... you have to really know ... what's also going on with brake horsepower as well as brake specific fuel consumption for a giving setting. The normal combustion cycle as well as the peak pressure pulse for each stoke at the correct top dead center operation.
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Really? You know all that about your engine? I'm impressed. I raced sports cars for many years using engines from and talking frequently to one of the top builders in the country and I never heard of anyone who had instrumented an engine to the point of knowing what the pressure pulses looked like. Probably the F1 teams have done it. What does "the correct top dead center operation" mean?
 
airdale said:
I never heard of anyone who had instrumented an engine to the point of knowing what the pressure pulses looked like. Probably the F1 teams have done it.
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Pretty typical to have that kind of instrumentation on an engine on a dyno at one of the major auto companies. Peak pressure at about 10-15 degrees ATDC (IIRC) is about optimum.

I've seen piles of pressure / volume traces.
 
This problem is solved in FADEC engines, and to a lesser degree, in Cirri.
 
Most of my flying is above the "full throttle" altitude. What I mean is even with full throttle I'm getting 22MP or less. Climb at 25 Square, until full throttle will not give me 25MP, fuel to 11GPH for climb. I reduce RPM to about 2300, fuel flow to 1380F EGT, yields between 9.5 to 11.5 GPH dependent on altitude and 125KTAS.

This is for a Lyc IO360, 210 HP, 2800 RPM at T/O. Cessna T-41B.
 
Ron Levy said:
Depends on the plane. Do that in my fixed pitch prop Tiger, and you'll be about 100-150 above redline RPM, and that's not a real good idea. In your Comanche, it will also depend on prop setting, as your engine will react badly to too much MP for the commanded RPM, although I'm estimating that with your IO-540, the min RPM for full throttle at 4000 MSL (about 26 inches) would be around 2100, and you're probably not pulling the prop back that far. And I assume you have good CHT instrumentation so you know what you're doing to your cylinders, because at that altitude, full throttle and a more typical 2400 RPM is probably close to 80% power.
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I'm in SoCal - 3500 puts me about 2000AGL or less in most places - I"m NEVER that low - and yeah - 2400 and full throttle is prob 80% - and I do have a GEM 6 cylinder. Remember that the Lyc engine manual says full rich at WOT under 5000 . . . . I think I can lean that a little and still keep from turning it into a slag pile.

I would never consider 27" of MP and 2100 RPM. 2200 maybe. .. when I had the Turbo Viking I generally ran it 26" and 2200rpm with the 3 blade - was real smooth there - leaned to a TIT of 1550 or so. Engine liked that - had scattershot compressions in the low 70's and by the last year I owned it they all were 76/77 -

2600 or 2700 rpm is just added noise for not along of extra power. I rarely run 2500 rpm but the other I ran it at 2500 rpm 75% at 7500' power at for a while to see if I could make book - and did.
 
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