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Chieftain power setting

Old 5th Jun 2016, 00:42
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Chieftain power setting

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
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Old 5th Jun 2016, 10:54
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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

Talk soon.
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Old 5th Jun 2016, 11:30
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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

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!

Will have a chat with the guys after breakfast.
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Old 5th Jun 2016, 12:14
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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".
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Old 6th Jun 2016, 00:37
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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.
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Old 6th Jun 2016, 01:04
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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.
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Old 6th Jun 2016, 11:56
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Originally Posted by Jabawocky View Post

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...)
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Old 6th Jun 2016, 13:47
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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 Ask them what else is wrong in there
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Old 6th Jun 2016, 21:56
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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.
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Old 7th Jun 2016, 12:01
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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?

Last edited by Lumps; 7th Jun 2016 at 12:18. Reason: better
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Old 7th Jun 2016, 12:39
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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

"...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?
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Old 7th Jun 2016, 16:53
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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....
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Old 7th Jun 2016, 22:35
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I bet that Diamond thing you have been flying is 2 x magneto Lycoming powered…..for other good reasons
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Old 7th Jun 2016, 22:37
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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.......
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Old 8th Jun 2016, 01:28
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Walter, is there any chance you can explain how or why the delay between mag timing and actual spark happens please.
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Old 8th Jun 2016, 08:02
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They are properly timed at 20dBTDC. BUT, the sparks actually occur at approximately 16dATDC.
Whattha?

I think you meant 16 degrees before TDC?
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Old 8th Jun 2016, 12:12
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Originally Posted by Lead Balloon View Post
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.
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Old 8th Jun 2016, 15:40
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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.
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Old 9th Jun 2016, 02:41
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Lumpy….pretty much

They probably need about 4 degrees of retard. Now ask the LAME how the CASa dude will cope with that concept
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Old 9th Jun 2016, 03:45
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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.
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