Chieftain power setting
You were just testing to see if we were paying attention! 
(If I had a buck for every mistake I make, I'd be the richest man on earth!)
(If I had a buck for every mistake I make, I'd be the richest man on earth!)
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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?
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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."
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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.
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.
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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?
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?
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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.
Last edited by Walter Atkinson; 11th Jun 2016 at 21:58.
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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
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
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
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
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
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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
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
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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!)
Who knew that the practical reality would be consistent with science and the data!
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.
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.
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.
'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.
Roger. We'll both be looking forward to JM's response, then.