Setting Piston Engine Power By Fuel Flow?

[Sunfish]

I was always taught to set power with a Constant speed prop by reference to MAP. Is it a recognised method to set it by fuel flow instead?

[Mark__]

Yes, in aircraft like the Cirrus with modern very accurate fuel metering systems. It is standard when say setting up a 65%LOP cruise in a SR22 with a Continental IO-550 to first set the power to 75% and then swiftly move the mixture control until a FF of 13.5GPH is achieved (sometimes referred to as the "big mixture pull"). This then results in 65% power LOP, which can be verified using the Lean Assist function on the Garmin Perspective avionics.

[Walter Atkinson]

When ROP, power is a function of mass airflow only. That is determined by MP and RPM. Fuel flow is inconsequential as there is more fuel than can be burned already.

When LOP, only FF determines HP. If the mixture is LOP, then MP and RPM play no part, because there is already more air than can be used.

For example, when LOP all of the below settings produce the same HP:
25/2500/14 gph
25/2400/14 gph
23/2400/14 gph
25/2700/14 gph
28/2700/14 gph
as so on....

When ROP, all of the below settings are the same HP:
23/2300/18 gph
23/2300/17 gph
23/2300/16 gph
23/2300/15 gph
23/2300/14 gph
as so forth.

The above is a very important concept to appreciate.

[Walter Atkinson]

BTW, as an important aside, there is no reason one cannot run the engine at power settings above 65% when LOP. My SOP is 87-90% power LOP. The engine loves it and it goes very fast on 17.2 gph with all CHTs hovering around 360-370dF. I can't go the same speed ROP because the CHTs would be excessively high--above 420dF.

As a proof of concept flight I have run the TNIO-550 at max power on takeoff, LOP at 20.1 gph (300HP). It takes over 36 gph to get the 300 HP with controlled CHTs when ROP (full rich). Your engines will not do this because we had to make a few changes to make it work at 300 HP LOP--but it is quite possible to set the engines up to do that (as we proved).

[Lead Balloon]

Hi Sunfish

After having:

- studied everything John Deakin has written,

- attended APS, and

- set up my normally aspirated injected engine properly,

my standard operating procedure is to leave the throttle wide open from the start of the take off roll until joining the circuit before the next landing. Engine management is almost entirely by mixture (with a little bit of fiddling with RPM when settling into the cruise and the occasional glance at the engine monitor).

That's why the APS T shirt says "WOTLOPSOP": Wide open throttle and lean of peak (in the cruise) as a standard operating procedure.

If you're running a normally aspirated, injected engine like mine, the MAP adjusts itself when climbing and descending! :ok:

(Of course, if I'm doing circuits I don't leave WOT turning downwind, otherwise it gets a bit fast to lower the gear...)

[Squawk7700]

Personally, I would not rely on the Cirrus LOP computer based calculations. I know of a pilot who hired a Cirrus to fly to Birdsville from Melbourne and he cracked 5 cylinders by running "best economy" on the Avidyne.

[Lead Balloon]

The algorithms that calculate these magic settings are notoriously inaccurate and 'laggy' on most monitors. I gave up using the "Lean Find" function on my EDM years ago.

Much better to use the 'raw' EGT numbers, provided the monitor has been set to the highest sensitivity. Good EGT probes are usually quick to follow real EGT changes.

[kingRB]

That's why the APS T shirt says "WOTLOPSOP" :rolleyes:

Doesn't sound like Paleo / Crossfit evangelists at all

[currawong]

Cessna I used to fly had a placard showing altitude vs. fuel flow at given RPM for max performance.

Mixture adjusted during take off roll was called for in the manual.

[Lead Balloon]

Indeed, KingRB. It's all snakeoil.

Don't believe a word of it. :ok:

Of course, currawong. If you don't adjust the mixture properly for take off and climb, you aren't going to get the best compromise between power and CHT (or so the snakeoil salesmen would have you believe).

[The Green Goblin]

You guys are really taking it a step too far.

I operated big contys and lycomings for many years with museum spec instrumentation.

I flew them as per the POH and never had a problem. Except for the faulty batch of pots in the 550s at one point.

Provided you don't do anything too stupid, they are fairly robust.

[Stationair8]

Didn't Cessna for the 300 and 400 series aircraft have a power/ fuel flow whiz wheel computer?

[Aussie Bob]

Personally, I would not rely on the Cirrus LOP computer based calculations. I know of a pilot who hired a Cirrus to fly to Birdsville from Melbourne and he cracked 5 cylinders by running "best economy" on the Avidyne.

How do you know it was this pilot who cracked the jugs? How do you know if the cylinders were conforming prior to this flight and did not have casting flaws in them? With respect, I think the above is incorrect.

[Lead Balloon]

It's the unwritten law of GA, AB: No piston engine problem could possibly be caused by manufacturing or maintenance defects or deficiencies.

That's why the revival of LOP was a God-send to manufacturers and maintenance organisations. Every problem that arose while running LOP was, by definition, caused by running the engine LOP. Just ask "yr right" in his various guises.

No engine problem ever happened running ROP and, even if it did, the pilot was at fault. :ok:

It's ironic that APS are accused of being the snakeoil salesmen.

[Aussie Bob]

Thanks LB, that explains it well :ok:

Regarding this, given the speed of a Cirrus, but taking into account the pilot may have tracked to Birdsville via Whoop Whoop and Nowheresville, let's say for arguments sake he did 30 hours.

I would defy any pilot out there to crack a single quality Continental cylinder in 30 hours regardless of what they did to the engine. Perhaps if, by some combination of settings I know not of, the pilot managed to run CHT's in excess of 500 F for the full 30 hours, I would still be surprised if more than one cylinder failed. My guess is that an engine with quality cylinders would take this abuse for 100+ hours before any one cylinder failed. For 5 to fail simultaneously, I would be gobsmacked.

[Squawk7700]

All good points. I'll find out where in the 100 hourly cycle it was.

[Walter Atkinson]

***Personally, I would not rely on the Cirrus LOP computer based calculations. I know of a pilot who hired a Cirrus to fly to Birdsville from Melbourne and he cracked 5 cylinders by running "best economy" on the Avidyne.***

While it may be an accurate observation that there were 5 cracked cylinders, it is essentially impossible that a LOP, best economy mixture could have had anything to do with it... at any power setting.

[Walter Atkinson]

***
Cessna I used to fly had a placard showing altitude vs. fuel flow at given RPM for max performance.
***

That was to encourage a specific mixture setting, having nothing to do with HP produced.

***
Mixture adjusted during take off roll was called for in the manual.
***

ONLY for high DA departures, I hope. NEVER lean during max power unless that is the situation (NA). NEVER, EVER lean a TC engine for takeoff--at any altitude.

[currawong]

From the manual -

"For maximum engine power (300HP Engine), the mixture should be adjusted during the initial take-off roll in accordance with the fuel flow vs altitude placard. The power increase is significant above 3000 feet, and this procedure always should be employed for field elevations greater than 5000 feet above sea level"

Unquote.

Engine is a Continental IO-520D in this case.

[Lead Balloon]

And what did the fuel flow card say for a sea level standard atmosphere take off?

It may be that the practical outcome is the same as indicated by the data and advocated by APS, depending on how the fuel flow has been set up on that engine.

[currawong]

It says -

Sea Level...............2850 RPM/24 gal/hr ................. 2700 RPM/23 gal/hr


Interesting some say never.

Manufacturer states always...


Incidentally I was mostly operating that machine at/near sea level in the tropics.

Not entering the LOP/ROP debate. Response was to the original post.

[Lead Balloon]

No, the manufacturer's "always" you quoted only applies to field elevations greater than 5,000'.

What was the fuel flow at sea level with the mixture full rich?

[gassed budgie]

Personally, I would not rely on the Cirrus LOP computer based calculations. I know of a pilot who hired a Cirrus to fly to Birdsville from Melbourne and he cracked 5 cylinders by running "best economy" on the Avidyne

I flew an SR22 for 800 hours and of those 800 hours, I'd say 750 of them were spent running it at "best economy" on the Avidyne. It ran smooth, it ran clean, it ran cool (CHT's all around 300°F, EGT's all 50°F to 70°F LOP). Never ever came close to cracking a cylinder.

[currawong]

No, all instructions are prefixed by "should".

Therefore not mandatory at all.

If, however one is requiring max power, that is the procedure.

From memory fuel flow was 25 gal/hr before adjustment (red line)

Again for clarity, not entering the LOP/ROP debate.

[Lead Balloon]

So the "should" procedure at sea level is to reduce FF from 25 GPH to 24 GPH at 2,850 RPM.

This is not about the ROP/LOP debate. It's about whether the engine is sufficiently ROP when ROP at high power settings.

[Walter Atkinson]

Testing has shown repeatedly that 24gph at 300 HP is an inadequate FF. CMI has begun to agree with our recommendations and over the years their SID97-3x has changed to reflect their appreciation that their FF recommendations have been lower than optimal. At 300 HP, the FF should be a minimum of 29gph. We show hard data to prove our point. The same engine at 285 HP needs 27gph. The FF placard for altitude adjustments is an equally suboptimal, inadequate recommendation. The reason CMI has been slow to change, is the fact that having the FF where it should be costs their 300HP engine 3 to 5 HP--thus they are not delivering what they promise. Lycoming's FF settings are at the same "richness" as we recommend for CMI, but their 540 produces 290HP. TCM had to have a 300 HP engine to outsell Lycoming, so they reduced the FF to get it. Since the health of the engine is not their concern after the sale, delivering the engine with suboptimal FF has worked for a marketing standpoint. From an engine health/longevity standpoint, not so much.

The inadequate FF is a major reason why the CHTs tend to run high in those aircraft.

OTOH, it's your airplane and your engine. Operate it any way you choose.

[Lead Balloon]

Surely the procedures in the manufacturer's manuals and charts are driven by safety and efficiency considerations alone? They are, after all, the fonts of the collected technical wisdom.

Surely marketing imperatives wouldn't override longevity and efficiency considerations? Heck, if they did, they'd be recommending mixture settings that give the engine the hardest beating it can be given, just for bragging rights to a few HP and a couple knots over the competition. I can't believe manufacturers would do that. :rolleyes:

[Walter Atkinson]

***Surely the procedures in the manufacturer's manuals and charts are driven by safety and efficiency considerations alone? They are, after all, the fonts of the collected technical wisdom.

Surely marketing imperatives wouldn't override longevity and efficiency considerations? Heck, if they did, they'd be recommending mixture settings that give the engine the hardest beating it can be given, just for bragging rights to a few HP and a couple knots over the competition. I can't believe manufacturers would do that. ***


<smirk> Had that been posted on BeechTalk, it would have been in green!

[Jabawocky]

You two lads are too funny! ;)

Interesting note to support Walters comments regarding the Lycoming F/A ratios being aligned with what APS teaches as compared to the TCM/CMI data.

At the APS class in Ada OK in 2013 Bill Ross from CMI attended. The next two classes had around half a dozen CMI technical folk attending. A big step in the right direction.

I caught up with Bill at the CMI stand, he was very appreciative of the APS class, so much so singled me out in the crowd that was jammed into the tent classroom. I was impressed to see the company promote APS.

Next impressive thing was they had taken a bunch of data off their test stand and created a classroom emulator of an IO550. The photo below is as best I could get zoomed up from the back corner of this packed "tent theatre".

You may not be able to see clearly, but look at the fuel flow at 100% power as per the engineers at CMI, the data hovered around 29 GPH and at the time I took the photo it showed 28.9GPH.

When questioned about the higher value by someone in the audience, he carefully explained a bit more than the SID was better, and if you used the service centre (center) in Fairhope AL you would see more flow….. ;)

http://i849.photobucket.com/albums/ab58/jaba430/CMI%20IO550%20Simulator_zpslxzzore6.jpg

[currawong]

Not my argument, but here goes...

The manual/placard states for "max performance", not "max engine longevity" or suchlike.

The manufacturer acknowledges this is less than ideal by imposing a 5 minute limitation on that power setting.

This type of use is reflected in the TBO of only 1200 hours.

Seems like plain English to me.

"having nothing to do with HP produced"

"FF where it should be costs their 300 HP engine 3 - 5 HP"

With respect Walter, which is it? All about HP? Or nothing to do with HP?

FWIW I have not flown behind one of those infernal devices in over a decade:ok:

[Walter Atkinson]

***
With respect Walter, which is it? All about HP? Or nothing to do with HP?
***
Both will be correct, depending on the question being asked. My responses were to two different issues.