Chieftain power setting

[Lumps]

Can someone explain this to me:

In the Piper Chieftain POH, for 65% power (230BHP) it lists 'best economy' fuel flow as 30.8 GPH (both engines). Seeing as the POH specifically prescribes LOP setting, one has to assume that 'best economy' in this case is LOP.

Problem is, these are 7.3:1 CR engines, so using 230 bhp ÷ 13.5 you get 17 GPH, or 34 GPH both engines LOP. How does Piper do it!?

* My guess is the 65% power in the title of the chart refers to the Best Power column.. seeing as TAS is lower for best economy. But it's a guess

[Jabawocky]

I just happen to be in a three letter town…..and the one place on earth where the answers to such mysteries exist.

I will dig out a POH and also compare that to Dyno data before I double check with George, Tim or Walter.

This would not be the first example of a POH containing fanciful approved data if your very acute senses are correct. And I know yours are pretty sharp :ok:

Talk soon.

[Jabawocky]

I just had a look at dyno data, 31" / 2400 and 16.5GPH was about 220-224 BHP.

I think you did another POH MYTH BUSTERS program :ok:

Seems to me at least the POH is predicated on the wrong compression ratio….or as we have just decided to call it the expansion ration more correctly ;)

They have used a calculation based on the typical 8.5-8.7:1 engines. Oops! :ooh:

Will have a chat with the guys after breakfast.

[oggers]

While you're at it how about digging out the "lycoming data" you claim exists there which you think proves that EGT and exhaust valve temperature "are in no way related".

[The Green Goblin]

From memory (it's been over ten years since I flew a PA31), I ran a Navajo at 30"/2200 rpm and about 17.5 gph. A chieftain about 31" 2200 rpm and 18 gph.

[sillograph]

remember the analogue gauge is not normally calibrated every year, in fact I don't think it is mandated on the annual, hence its never checked... How old are these aircraft.
The chieftains we operated in the past had digital fuel flows installed and k factor adjusted. When you check fuel flows on digital against analogue the digital reading is about 20.5 and analogue 18 gal go figure.

[Lumps]

Will have a chat with the guys after breakfast.

Good stuff! Wish I could too. While you're there if you could ask GB why the single drive, dual mags (J2BD) have different characteristics as far as running LOP etc is concerned, I'd love to know. (How does a spark know whether it came from a dual mag? Or is their dynamic timing inherently worse than their single brothers...)

[Jabawocky]

George is of the view that piper used a guess from who knows where. There is correlation of the 14.9 factor, but back then he doubts they had used that much effort to get it wrong.

So best you take that up with the local CASA folk and see how they answer it ;):ok: Ask them what else is wrong in there :ooh:

[Tinstaafl]

Thread drift:

"...the single drive, dual mags (J2BD)...". What a bastard idea that is. That's one of the reasons I like Colemill's (now Mike Jones) Panther upgrade of Navajos. The engine has separately driven mags.

[Lumps]

My theory is a little different: The 65% (230 bhp) column lists two airspeeds, best power, best economy. Obviously 230bhp is 230bhp no matter where the mixture is, the airspeed should be the same for a constant power output.

So only one of these columns is correct.

My guess is the best power column is and refers to 230bhp, and the much lower fuel flow (30.8gph) and slower airspeed is an engine actually producing about 210 BHP, or 60% power, LOP. Would explain the speed drop, and they'd have got these numbers from flight testing? It was the 1970s, almost the apogee of piston engine management ignorance. So maybe whoever was doing the flight testing had forgivably poor grasp of LOP procedure and neglected to add the 2" or so manifold pressure (in fear of frying some valves, Oggers), like what the FE's of yesteryear did with their BMEP gauges and whatnot.

"...the single drive, dual mags (J2BD)...". What a bastard idea that is.

Apart from the obvious single point of failure bit, why else are these bastards? I've heard of a failure in a Mooney or something where the whole unit moved a bit and timing got so far out the aircraft forced landed. Anything else?

[Jabawocky]

Lumps,

Best thing to do is go fly and record the result. Then YOU know what is correct.

As for Piper and their XX% performance for two mixture settings, this is not the only one. Makes you wonder about the accuracy or relevance of much more important data :uhoh:


"...the single drive, dual mags (J2BD)...". What a bastard idea that is. That's one of the reasons I like Colemill's (now Mike Jones) Panther upgrade of Navajos. The engine has separately driven mags.

Yeah……but the down side is the separate mags Vs the dual mags is that Lycoming and Piper never knew the undesirable result of going to the separate mags.

I bet everyone thinks the lower TIT was a good outcome := Probs not ;)

Do you know what the static timing is on the Panther setup? Is it the same as the standard?

[Bevan666]

Apart from the obvious single point of failure bit, why else are these bastards? I've heard of a failure in a Mooney or something where the whole unit moved a bit and timing got so far out the aircraft forced landed. Anything else?


The O-360 in my Socata Tobago had these too. A single fault (internally, both mags grounded at the same time due to both capacitors touching) turned it into a glider one day. Ended up on its nose in a paddock near Tamworth.

I know of another TB-10 in the US where the same thing happened. In their case, when they landed (on a runway) the engine then started just fine (the jolt of the landing caused the capacitors to move apart, 'fixing' the problem. Took the ginger beers quite a while to find that one!

Dual mags exist for a reason - redundancy.... :confused::confused:

[Jabawocky]

I bet that Diamond thing you have been flying is 2 x magneto Lycoming powered…..for other good reasons ;)

[Walter Atkinson]

Concerning the dual-drive mags:
They are properly timed at 20dBTDC. BUT, the sparks actually occur at approximately 16dATDC. This is quite probably not known by Lycoming. (Actually, I'm very, very confident that Lycoming does not know this.) The good news is that this rather unusual, serendipitous happening keeps these engines from detonating rather severely on TakeOff. The single drive mags do not have the same delay and those engines suffer more detonation than the ones with duel-drive mags. SOOOOOO, thinking the single-drive mags are better--on this engine--is not always the better thought.

It's that pesky data, again.......

[pcx]

Walter, is there any chance you can explain how or why the delay between mag timing and actual spark happens please.

[Lead Balloon]

They are properly timed at 20dBTDC. BUT, the sparks actually occur at approximately 16dATDC.Whattha?

I think you meant 16 degrees before TDC?

[Lumps]

Whattha?

I think you meant 16 degrees before TDC?

Plus one. Or I gotta go back to school.

Jaba I take it the lower TIT for the separate mags means they are more advanced (dynamically anyway) as what Walter was eluding to? So Colemill = not so good. Unless you could get your LAME to set them at 18btdc or something.

[Stationair8]

In the early model PA-31's, the EGT gauge was an optional extra and according to my source a fair few Navajo's in Australia did not have EGT gauges fitted.

The PA-31 350 had them as standard equipment.

[Jabawocky]

Lumpy….pretty much :ok:

They probably need about 4 degrees of retard. Now ask the LAME how the CASa dude will cope with that concept :}

[Walter Atkinson]

Sorry for the brain fart. The timing is set at 20dBTDC and actually occurs at about 16dBTDC, obviously not at 16dATDC! I guess I got my As and Bs mixed up!!!

This probably happens as a result of the timing gear lash and delay, but there could be other reasons in addition. This does not happen with the two single mags. We have been watching this on the test stand for a long, long time.

[Lead Balloon]

You were just testing to see if we were paying attention! :ok:

(If I had a buck for every mistake I make, I'd be the richest man on earth!)

[Lumps]

Lumpy….pretty much :ok:

They probably need about 4 degrees of retard. Now ask the LAME how the CASa dude will cope with that concept :}

Heretic!

So with the (thankfully) retarded timing does the J2BD actually make 350bhp on the dyno - Or did the J series start life with separate mags and then no one checked when the single drives got introduced?

[Walter Atkinson]

The J2BD does make rated power with the timing the way it is. Actually, the retarded effective timing helps it produce more power at TO power. Most engines can make more power with slightly retarded timing at TO. The rub is that with fixed timing this results in decreased power at cruise power settings. Hence, the timing "compromise."

[Lumps]

bring on PRISM

makes you wonder how the engineers designing these engines arrived at the actual degrees BTDC:

A) Trying various timing positions and seeing what resulted in the optimum compromise, then certifying the engine with said timing position... in which case the difference between fixed and effective timing is of little consequence, as it was the effective timing that was being observed the whole time.

or

B) From theoretical principals i.e., 20°BTDC arrived at from piston velocity, CR.. god knows. seems hard) Without the knowledge that there was a significant delay in effective timing, in which case all these engines must be a bit hit and miss in their timing.

I hope A is closer to reality. Seems more practical and logical.

[Jabawocky]

Interesting point to add to Walters comments, depending on load and RPM I have seen the actual spark with a 6 degree lag, but at typical cruise it is in the 4-5 range, so it should be noted that the numbers are always dynamic and one should not get hung up on a "fixed number" state of mind.

I suspect you are right Lumpy, option A is likely, old guys like Walter might know better. And when the independent mag came along nobody would have had reason to give it a second look would they?

[Walter Atkinson]

I'm pretty sure they went with option "B", then refined with option "A" if things turned out differently than expected. Many times, the testing to find out if "A" was what they wanted was not attempted. They did "B", ran the tests and if everything was OK, moved on. That's the only thing that seems reasonable for the J2BD.

[Lumps]

well that does surprise me. Thanks for your input fellas, will go out and fly the test card shortly!

[James McIlvenny]

Can someone explain this to me:

In the Piper Chieftain POH, for 65% power (230BHP) it lists 'best economy' fuel flow as 30.8 GPH (both engines). Seeing as the POH specifically prescribes LOP setting, one has to assume that 'best economy' in this case is LOP.

Problem is, these are 7.3:1 CR engines, so using 230 bhp ÷ 13.5 you get 17 GPH, or 34 GPH both engines LOP. How does Piper do it!?

* My guess is the 65% power in the title of the chart refers to the Best Power column.. seeing as TAS is lower for best economy. But it's a guess

I'm about four years late, but I can give it a crack.
In my PA31-350 manual (early), it describes in the footnote for the cruise performance chart that at best economy power, power is reduced by 4.5%.
so 60.5% of 350 is 211.75 hp, using 13.5 to find hp, so that's<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> 15.68 gallons or 31.37 total gph.
However, I have a feeling the best economy the manual refers to is NOT lean of peak, given this plane is from 1973 and the reference to LOP is an update to the flight manual. I bet if you sit on 1650 egt (on the original factory gauge, which really is TIT, based on where the jpi probe sits) you could probably get to 30.8 gph... as your JPI is screaming at you with egt's over 1680 and hot CHTs. I've never tried it, but somebody let me know how you go :)

[Xeptu]

Used to fly these PA-31-350 for a few years about a hundred years ago, I'm surprised there are any still flying lol.
Basic facts from memory, where, 31 inches, 2200 rpm, 50 degrees "rich" of peak, I think 1650 (Fahrenheit) sounds about right quoted previously.
fuel burn 150 litres per hour cruise, 180 litres per hour block, over two thousand hours never cracked a cylinder.
From memory, lean of peak came around the time of the mojave when inter coolers were introduced, didn't fly one.
If it's not a turbine why would you even bother :)

[machtuk]

Used to fly these PA-31-350 for a few years about a hundred years ago, I'm surprised there are any still flying lol.
Basic facts from memory, where, 31 inches, 2200 rpm, 50 degrees "rich" of peak, I think 1650 (Fahrenheit) sounds about right quoted previously.
fuel burn 150 litres per hour cruise, 180 litres per hour block, over two thousand hours never cracked a cylinder.
From memory, lean of peak came around the time of the mojave when inter coolers were introduced, didn't fly one.
If it's not a turbine why would you even bother :)

yeah about the same for me when driving the old girls a hundred years ago. I remember one owner whom I did the odd charter for said ROP at all times, like you +50 or two divisions, no probs doing that they donks run like Swiss watches👍LOP was a no no back in my day!

[Blueskymine]

The complexities of a light piston twin. To complex for an airline pilot.

[James McIlvenny]

yeah about the same for me when driving the old girls a hundred years ago. I remember one owner whom I did the odd charter for said ROP at all times, like you +50 or two divisions, no probs doing that they donks run like Swiss watches👍LOP was a no no back in my day!

For about the last 1200 hours I ran the chieftain LOP, burning 120 lph instead of 160 lph as I did previously. This was at about the same speed, or faster. When it came time for overhaul, they told me the engines could have run another 500 hours easily. At local fuel prices 40 litres is about $88 Aussie dollars an hour ($65 usd today), about $105,000 AUD in fuel for no apparent damage.
Not to mention there were a huge number of jobs I did that I COULDN’T have completed otherwise, the extended range or greater useful load was a huge plus.
(sorry for the delay, wanted to see how the engines looked after the overhaul before making any statements!)

[Lead Balloon]

Who knew that the practical reality would be consistent with science and the data!

[holdingagain]

JM, was that achieved with just you flying or multiple pilots

[Lead Balloon]

JM, was that achieved with just you flying or multiple pilots
Let me guess: You're worried that JM flew only a few of those 1200 hours and the rest were flown by pilots who 'saved' the engines by running them ROP?

[Checkboard]

I would imagine that it’s more: If it was achieved by multiple pilots, then the procedure is not only cheaper, but also robust in terms of engine damage. If was achieved by one, then some specific training may be required to make it that safe, and if you are renting a plane to multiple pilots then it might be best to require ROP ops to avoid damage. $100k over 1200 hours is great, but easily soaked up by a ham fisted pilot detonating an engine.

[43Inches]

I would imagine that it’s more: If it was achieved by multiple pilots, then the procedure is not only cheaper, but also robust in terms of engine damage. If was achieved by one, then some specific training may be required to make it that safe, and if you are renting a plane to multiple pilots then it might be best to require ROP ops to avoid damage. $100k over 1200 hours is great, but easily soaked up by a ham fisted pilot detonating an engine.

If they can't read basic instructions on how to run LOP, they probably will fail at setting it properly ROP. Incorrect leaning either side of peak will cause issues on these types of engines, and it's more a lack of basic engine knowledge that is the issue here rather than leaning procedure. If you can't trust them to lean it properly, why are they flying it, you will either have an expensive donk replacement or a hull loss when they run out of fuel mismanaging it.

[Lead Balloon]

I would imagine that it’s more: If it was achieved by multiple pilots, then the procedure is not only cheaper, but also robust in terms of engine damage. If was achieved by one, then some specific training may be required to make it that safe, and if you are renting a plane to multiple pilots then it might be best to require ROP ops to avoid damage. $100k over 1200 hours is great, but easily soaked up by a ham fisted pilot detonating an engine.
Most 'big' piston engines will run happily and healthily in a state of mild 'detonation' at most high power settings.

'ROP ops' might 'avoid damage', provided the engine is far enough ROP. But if it's not far enough ROP, that's where the biggest risk of damage arises.

[holdingagain]

No Leady you guessed wrong. I was just interested.

[Lead Balloon]

Roger. We'll both be looking forward to JM's response, then.

[Torukmacto]

Very interesting , 6 years flying chieftains and Navajos and company transitioned from ROP exactly as stated above to LOP towards the end of my time there . We had engines running well over their normal OH times by going on condition inspections . LOP was not popular at the beginning by us but after reading posts here it became a acceptable operating technique. I finished up with the aircraft just prior to the vortex generators arriving , just a happy coincidence in my mind . Good memories of helping in hanger and trying to get those 2 red lights to buzz and light up at same time .

[spektrum]

I distinctly remember how there was a direct correlation between a persons thirstiness for getting out of GA and their resistance to entertaining the thought of operating LOP.

[Torukmacto]

I distinctly remember how there was a direct correlation between a persons thirstiness for getting out of GA and their resistance to entertaining the thought of operating LOP.

Im laughing , it was one reason on a very long list of reasons by then but looking back it was some of the best times of my life , like most I’d say . Navajo still a favourite.

[megan]

Leaning in the old days, Constellation, LOP.


https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1103x1569/c_fc8c920eedf60f5dc7d5d94510c5f66f2dbac5d8.png

[SWMBO]

Leaning in the old days, Constellation, LOP.


https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1103x1569/c_fc8c920eedf60f5dc7d5d94510c5f66f2dbac5d8.png


megan,
Thank you so much for posting that.
I have done the APS class in Brisbane run by Andrew Denyer and David Brown. Using dyno data and literature from the days of Super Connie and so on, it became clear that all the methods that we were taught were in fact based on old wives tales. They even explained how that came about. It made some of my CPL theory rather confronting in its lack of truth in engine operations.

I sure hope they run a class again next year once all the covid restrictions ease. I was looking forward to booking in for the one this year but it was cancelled.

Torukmacto,
Your story sounds like yourself or a colleague was on the class a couple of years ago with me. An operator in Melbourne had made huge gains from following the APS instructors advice, especially magneto timing and spark plug maintenance. Too much of a coincidence for sure.

[43Inches]

I vaguely remember hearing that the Japanese may have used LOP technique to get the extreme range they achieved with zeros in the war. I've read a few books where they talk about some specific leaning procedure but don't expand on it much past that. Definitely have read the notes on big radials in DC-6/7 and Connies dramatically increasing reliability and life using LOP technique. They were getting some impressive TBOs on those big radials by the time the jets took over.

[Lumps]

I'm about four years late, but I can give it a crack.
In my PA31-350 manual (early), it describes in the footnote for the cruise performance chart that at best economy power, power is reduced by 4.5%.
so 60.5% of 350 is 211.75 hp, using 13.5 to find hp

Interesting, have not found any such reference in the newer manual where LOP is prescribed but maybe they were too busy at Piper back then for these little details. Like deleting the climb power leaning chart and replacing it with CLIMB FULL RICH!

We have many more hours on our PA31 nearly all LOP now. Zero issue except if anyone can share their CHT data would much appreciate it. Not sure if it's just ours but both sides CHTs are spread ridiculously - looks like most off the effective cooling air goes past No.6 and leaves Nos.3&4 warmer than they should be. I suspect the cowls were designed for looks back then, scant attention paid to where the air actually goes.

[43Inches]

If I owned a PA-31 these days I'd be at least strapping inter-coolers on them, if you are seriously wanting performance and economy. Especially for Australian conditions.