a different approach to comparing actual performance to POH...

[dominikos]

just to be clear... I'm just trying to figure out how much off POH my plane is. Old engine (1500 hours), old paint... etc.  

went to do a few landings in prep for OSH - first time flying in - any takers to join as a spotter on the fligth from texas to OSH?

anyhow, I thought to myself... it is hot, POH wants me to do adjust for temperature... what if instead of playing with MP/RPM I will aim for %BPH. I have Dynon as EIS, it actually has a profile for my Lycoming... what if I try for %BPH. I saw that in POH, if I get %BPH, the speed does not change between the settings. A page for my POH ('78 M20J below). 

I know, I know... only 2000 ft but that's what a flight was today...

and here is my dynon screenshot... actually two. first, shows me not adjusting for temperature... the speed numbers are below the book and then, using % of power, the numbers are almost spot on (picture 2). Should I always aim for the %BPH instead of MP/RPM?

[A64Pilot]

%Power should be a constant only IF you fly density altitude and not pressure, and of course if the Dynon has it right, I’ve not seen an engine monitor get it right, but then I’ve not used a Dynon either

Edited July 9, 2023 by A64Pilot

[ArtVandelay]

Use fuel flow.
And don’t forget humidity will hurt performance of the engine.

[dominikos]

14 minutes ago, ArtVandelay said:

Use fuel flow.
And don’t forget humidity will hurt performance of the engine.

sigh.... and that's Texas for you... and Florida....

[Danb]

Interesting the LOP speed is considerably higher than ROP 

[ArtVandelay]

Interesting the LOP speed is considerably higher than ROP 

It is NOT LOP, the difference between economy cruise and best power, best power is 100° ROP, economy cruise is 25° ROP.

From the POH:
Lean the mixture until temperature peaks on the EGT indicator..
ECONOMY CRUISE - Enrich mixture (push mixture lever forward) until the EGT indicator drops 25°F or more below peak.
BEST POWER MIXTURE - Enrich mixture until EGT indicator drops 100°F below peak.

ROP only gives more power because of increased fuel consumption:
Compared to Economy cruise, best power mixture will result in a speed increase, an increase in fuel flow and a reduction in range.

[PT20J]

Best power mixture is pretty clearly the mixture that produces the most power. But there is no standard definition for economy mixture. A lot of manufacturers use peak because that may be the leanest mixture where the engine runs smoothly. Some may cite leaner, some richer. When these Mooney manuals were written, Mooney's selling point was speed. Gas was cheaper and economy was not as much of a concern, though leaner mixtures allowed advertising an increased range. Mooney probably chose 25 ROP as a compromise between economy and speed.

Some get worried about the dreaded red box at this mixture, but it's not a problem at cruise powers as long as the CHTs are in check.

[dominikos]

I have to admit that I equated LOP and economy cruise. Now you guys sent me to a drawing board. I need to do redo some of my tests. I guess the observation that matching POH %BPH give me good performance on LOP still stands but now I have to redo my tests to match POH economy and best performance cruise numbers. Will do at 6,000 sometime this week and report back 

[ArtVandelay]

I have to admit that I equated LOP and economy cruise. Now you guys sent me to a drawing board. I need to do redo some of my tests. I guess the observation that matching POH %BPH give me good performance on LOP still stands but now I have to redo my tests to match POH economy and best performance cruise numbers. Will do at 6,000 sometime this week and report back 

The way you calculate HP % is completely different when LOP vs ROP. The engine monitor assumes ROP I think. The ROP involves MAP, RPM, OAT, and maybe some other parameters, LOP is just the engine constant x fuel flow, I think the IO360 engine constant is 14.9.
I’m vague on the details, hopefully someone will answer the specifics. @PT20J ?

In other words, don’t use HP % if comparing LOP vs ROP.

[dominikos]

With engine monitors it looks like LOP vs ROP and %BPH is a bit a fluid concept and red box.

if I keep CHT under 400 and happy with fuel flow and KTAS I should disregard the rest?

I fly with Dyson reporting LOP and making sure that CHTs are stable but O don’t get the speeds I want. Hence attempt to figure out better settings to get the higher speeds 

[EricJ]

1 hour ago, dominikos said:

I have to admit that I equated LOP and economy cruise. Now you guys sent me to a drawing board. I need to do redo some of my tests. I guess the observation that matching POH %BPH give me good performance on LOP still stands but now I have to redo my tests to match POH economy and best performance cruise numbers. Will do at 6,000 sometime this week and report back 

On my airplane 5000-6000 feet or so is the sweet spot for speed, so that's probably a good altitude to experiment at, especially if it is easily available for cruising in your area (it isn't here, hafta go higher to stay out of the dirt).

And +1 that any engine monitor's indication of % power should be considered a rough estimate.

[PT20J]

Power is always a function of fuel flow; after all, no fuel flow, no power. Brake Specific Fuel Consumption is gallons per hour per horsepower. So, if you have a BSFC formula for your engine, you just divide the fuel flow by the BSFC and you have your horsepower. The problem is that the BSFC curve is not linear and there is no simple relation between it and the MAP, rpm and fuel flow over the entire range of mixtures that the engine will operate. 

ROP, it's simplest to use the power charts in the POH. Many have simplified this to "key numbers" noting that, to good approximation, MAP + rpm/100 is nearly constant for any particular power (say 65%) in the chart. It's not exact, but it's simple enough and who really cares if you are set at 64% or 66% anyway?

LOP, the BSFC reaches a minimum and is then relatively constant over a range of mixture until the mixture gets so weak that the engine begins to misbehave. This allows a simple estimate of power. I'll attach an article that goes through the details, but for an IO-360 you can just take the fuel flow and multiply by 15 (technically it might be 14.9, but it's an approximation anyway, and 15 is easier to do in my head )

Skip

 

Determining Engine Power.pdf

[A64Pilot]

To compare one day to the next as in temp differences, pressure etc without using the book charts you need to fly density altitude, charts of course correct for density altitude. Flying 6000 PA today and trying to compare it to 6000 PA tomorrow may give you different results because the DA is significantly different and DA affects drag quite a bit.

Humidity isn’t usually taken into account for density altitude and it’s not a large difference, it does effect engine performance more though, but I don’t know how to correct engine power for humidity, I don’t have much experience with determining engine performance in piston engines. None officially actually.

Turbo props % power is easy as they have torque meters simply do whatever it takes throttle movement wise until you get the torque that equates to the %power you want, some torque meters even read in %, with a piston engine good luck, because as was stated mixture has a large effect on power, other than possibly best power I don’t see how an engine monitor could compute % power, at best it’s a SWAG, without a dyno or torque meter how do you even know what your engines power output really is?

If you’re out just looking into things, try this. Compare 50 ROP at a set RPM and set speed, but a lower MP, one that you can go say 25 LOP keep the RPM the same and adjust MP until airspeed is identical, compare fuel flow. I’ve done it on two different airframes but not a Mooney, I’d be interested in your results.

Best done one test right after the other and let it sit for several minutes to make sure A/S is dead stable, you guys with autopilots have a definite advantage here, best to creep to the A/S from a lower one for each test. I know it’s not supposed to make a difference but I swear if your going faster and reduce power and let speed stabilize sometimes you end up faster than if you set a power and let it accelerate until speed stabilizes.

I’m not saying the “step” really does exist, just take out all the variables you can when comparing things.

Also read up on what a Phugoid is, because ALL aircraft exhibit it, and it does effect A/S if altitude is held exact, not much just take a few readings and average them out.

Normally the pilot dampens the Phugoid without even realizing it

https://en.wikipedia.org/wiki/Phugoid

https://www.pilotscafe.com/glossary/phugoid-oscillations/

You can notice it in dead still air, the airplane will require very minor trim adjustments to maintain level hands off flight, it’s minor but real. If you have an autopilot put it in heading mode but not altitude you’ll notice very slight slow climbs and descents, or with altitude hold on, very slight changes in A/S. You just have to average the reading out.

 

Edited July 9, 2023 by A64Pilot

[Pinecone]

One nice thing about LOP operation is that HP is directly related to fuel flow.

For my TSIO-360, that is 13.7 HP per GPH.   So 10 GPH is 137 HP, or 62% on my -SB

[kortopates]

I wouldn't trust ROP %power at all till you've calibrated or at least checked it for its accuracy against the POH using a known POH cruise power setting.

Edited July 11, 2023 by kortopates

[PT20J]

George Braly once told me that all the engine monitors he looked at were inaccurate in %power calculations. He did a proprietary algorithm for Cirrus for their Perspective system and it leaked out to other Garmin products. My G3X seems pretty accurate LOP and ROP. I don't know about the current versions of other monitors -- perhaps they have improved.