Need help leaning by EGT

Discussion in 'Pilot Training' started by kicktireslightfires, Mar 22, 2021.

  1. David Megginson

    David Megginson Cleared for Takeoff

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    I'm suggesting just rich of peak RPM to allow for some mixture spread among the cylinders (especially in a carbureted engine), so that the leanest cylinder has a better chance of landing outside of the red zone.

    Before the OP (or anyone else) gets too stressed about this, however, I want to repeat that below 65% power it doesn't matter where you lean with a regularly-aspirated engine. This is only for people who want to get the full performance out of their planes by cruising at (e.g.) 75% power. If I'm willing to give up 15–20 knots in my Piper PA-28-161, I can cruise at 55–60% power all day chasing birds through the sky, and set the mixture wherever the spirit takes me.
     
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  2. farmrjohn

    farmrjohn Pre-takeoff checklist

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    Just under 20% savings (1/6=.17). That's pretty much what I get with my O200.
     
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  3. Salty

    Salty Final Approach

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    Not only that, but a small engine like a O-200 is just about impossible to get stopped. I was flying around (at altitude over an airport) with the mixture out and the stall horn blaring and never could get the prop to stop spinning. It was running so slow the display went dark from lack of power from the alternator, but the prop would not stop. Nose down a bit, push in the mixture, and roar!
     
  4. David Megginson

    David Megginson Cleared for Takeoff

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    There is a chance of a backfire, which isn't great for the engine if it happens too often.
     
  5. Salty

    Salty Final Approach

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    If the engine is spinning with no fuel and then fuel is added with the engine spinning faster than the starter would spin it? How do you figure that?
     
  6. David Megginson

    David Megginson Cleared for Takeoff

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    To be fair, I've seen that only after overleaning on the ground, not in the air (though the prop was still spinning faster than starter speed).
     
  7. William Pete Hodges

    William Pete Hodges Pre-takeoff checklist

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    Peak RPM can be used to determine best power mixture setting on a fixed pitch prop at any given throttle setting, and it will be slightly richer than peak EGT. Depending on how much power you are producing the engine may or may not be running outside of the Red Box. I ran across the Red Fin concept and liked it because it shows directly how the Red Box changes with load. I used a set of graphs to generate my IO-360-200HP RED FIN Fuel and air flow chart (1st page of the attachment). The EGT-CHT-Power-SFC chart (the 2nd page of the attachment) was used to compute the first chart. Since best fuel economy is 14.9 HP-Hours/Gal I corrected the SFC chart to read 100% at Best SFC. All other mixture settings burn more fuel than that. My Red Fin fuel flow chart looks similar to the SFC chart turned sideways.
     

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    Last edited: Mar 24, 2021
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  8. GeorgeC

    GeorgeC Administrator Management Council Member PoA Supporter

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    Nice work! FWIW, I came up with a slightly different number for the IO360, but our methods may have varied slightly:

    https://www.pilotsofamerica.com/com...n-deakin-©june-2013.60964/page-3#post-1998074
     
  9. David Megginson

    David Megginson Cleared for Takeoff

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    This thread has been a lot of fun. Here's some further reading:

    Mike Busch's book Engines (part of a four-book series on aircraft ownership; much of it is also available as articles online)

    John Deakin's "Pelican's Perch" series from 20 years ago (start with post 15 and move forward).
     
  10. William Pete Hodges

    William Pete Hodges Pre-takeoff checklist

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  11. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    With a carbureted engine, it is very difficult to run lean of peak. Given that smoothness on most carbureted engines will happen near peak, enriching slightly from there likely puts you in the red box. I would not recommend this technique.

    Altitude matters when it comes to anything leaning unless you have a turbo, because as you climb there is less oxygen and you need a leaner mixture setting to compensate for that. That's why we lean relative to peak EGT most of the time. More on that below.

    I would say yes to what you've said above, EXCEPT that you don't do it at full throttle, you set the throttle where you want it for cruise first. Changing throttle also affects mixture, so you set throttle first, then lean the mixture.

    On all of the airplanes I've flown that had the instrumentation to measure such things, the difference in fuel burn between takeoff at full rich mixture and cruise at or lean of peak was close to 50%. That is, you're burning TWICE the fuel at takeoff as you are leaned for best economy cruise.

    In addition, flight school planes spend most of their time down low. Lots of time in the pattern at no more than 1000 AGL, and lots more time practicing airwork at 1500-2500 AGL. In both cases, it's probably best to leave the mixture full rich. Otherwise, a student may get a nasty surprise when they lean early in a flight and then try to practice stall recoveries and their engine quits when they try to go to full power.

    For the point-to-point flying that most people do outside of the training environment, it's no longer worthwhile to leave the mixture full rich even if you're down low. See the "When to lean" section of this article: https://www.avweb.com/features/the-pilots-lounge-30myths-for-the-last-millennium/

    Be careful with terminology here. When we say "rich of peak" and "lean of peak" we're talking about peak *EGT*, not peak power. Nothing @David Megginson said above is incorrect (because he did say peak power), but when you are lean of peak *power* you may still be rich of peak *EGT* - And in between those peaks is not a good place to be for engine longevity.

    So, when you hit peak *power* you're roughly 100º *rich* of peak EGT, and by enrichening slightly from there, you're ensuring that your leanest cylinder is rich enough to be out of the red box.

    Heck, start with post 1. It's all good stuff. But yes, #15 "Manifold Pressure Sucks" is by far the one I refer people to the most often. The FAA's explanations of how engines with constant speed props work are terrible, and "Manifold Pressure Sucks" clears it right up and makes it understandable.
     
  12. David Megginson

    David Megginson Cleared for Takeoff

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    Lots of good points. Agreed that the PA-24-250 pilot didn't know what he was talking about, because perceived roughness can occur when some cylinders are still rich of peak or all are lean of peak, depending on the mixture spread among the cylinders — "lean to roughness" is useless as a general guideline.

    As for carbureted engines, the ability to operate lean of peak varies. Four-cylinder carbureted engines have some success LOP, because it's possible to get fairly even fuel/air distribution to all of the cylinders — the Lycoming O-320 is particularly good for this.

    Six-cylinder carbureted engines are more hit or miss, because the mixture has to travel further from the carburetor to some cylinders than to others, though I do have one friend who manages to do all his flying lean of peak behind a six-cylinder carbureted engine (confirmed by a fancy engine monitor).

    And finally, agreed that my talking about "peak RPM" is confusing because people might read too fast and see "peak EGT." I'll talk about "max RPM" from now on.
     
    Last edited: Mar 26, 2021
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  13. kicktireslightfires

    kicktireslightfires Pre-takeoff checklist

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    All of this is very helpful and lots to take in. Thank you all!

    Another question: How do you know what % power you are at? Do your airplanes display the % of power on the avionics? Mine does not. I know the POH tells me % BHP under Cruise Performance. But is % BHP the same as % power? And even if it is, it's still a much slower process to know what % of power I am at by having to look at my OAT, pressure altitude, and RPM to see what % of power I'm running -- definitely not a real-time viewable metric.
     
  14. William Pete Hodges

    William Pete Hodges Pre-takeoff checklist

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    The Actual % of power is a combination of total airflow and fuel flow. Total airflow is basically RPM (volume displaced) combined with manifold pressure and temperature (air density). Most POHs and engine manuals will specify HP produced at these given variables at Best Power Mixture Setting. There is about a 10% loss of power from BPMS to Best Economy Mixture Setting, at about 86% of BPMS fuel burn. On the lean side, the engine will produce about 14.9 HP per Gal per hour. So 8.0 GPH at about 75F LOP is about 120HP, so long as enough airflow is available to get to LOP operation at 8GPH. 120HP / your HP rating is % power LOP. Be aware that the mixture control can reduce power from Best Power available down to zero at any setting of RPM and MP. The best way is to pick a combination of RPM & MP at various altitudes and OATs that give an acceptable max available % of power and lean from there. On my bird at 10k and O°C I'm looking at 2500 RPM and WOT, then do the Big Mixture Pull back to LOP. It ends up being about 8GPH and 60% load.
     
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  15. David Megginson

    David Megginson Cleared for Takeoff

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    Yes, you use the power setting tables. It's not as hard as you think. The OAT and pressure altitude together are just the density altitude, and you'll have a fixed relationship between RPM and % power at any given density altitude. If you know your planned cruise altitude, the altimeter setting, and the temperature aloft, you can pre-calculate on the ground. It's important to figure it out for long trips, b/c if you don't know your power setting, you don't know your fuel burn.

    Here's a little cheat — the same % power will give you roughly the same indicated airspeed at all density altitudes ("roughly," because gross weight and vertical air movement both affect it). So if I want to fly at 75% power in my PA-28-161, I ballpark by setting up for 108–112 KIAS first, then fine-tune afterwards. It's important to verify with the tables (or an engine monitor) before you fly too long, in case something else is going on.
     
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  16. David Megginson

    David Megginson Cleared for Takeoff

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    Here's another approach: look at the table like the one below in your POH and check where the 55%, 65%, and 75% power lines cross the 0 altitude line (without temperature compensation). That's where true and calibrated airspeed intersect, so you have your expected calibrated airspeed at any altitude with those power settings and that gross weight. The difference between calibrated and indicated airspeed is usually pretty small in the cruise regime, but you'll still want to adjust just to be sure.

    In my case, the lines cross zero altitude at 95 KCAS for 55% power, 105 KCAS at 65% power, and 112 KCAS at 75% power. Applying the adjustments from the airspeed calibration table, that means that, in a mint-condition plane — at 2,300 lb (140 lb under max) and with my wheel fairings installed — I should expect to see 96 KIAS (55%), 103 KIAS (65%), or 110 KIAS (75%) no matter what altitude I'm flying at. If I remove my wheel fairings, I need to subtract 7 knots. If I'm at a different weight, I can ballpark it (for a single-engine piston) by adding or subtracting 1 knot for every 100 lb, so at 2,100 lb, I'l add 2 knots to all of those speeds.

    Of course, my plane might not be perfectly rigged, my paint isn't in great shape, I have a couple of extra antennas, etc. etc., so I needed to figure out what the actual values are for my plane (about 2–3 knots less, as it turns out), but Piper's POH numbers got me very, very close.

    All that one-time prep work makes flying by power setting very simple. I still verify with RPM, of course (and I have a portable optical tachometer to cross-check my dubious mechanical one), but in general, if I set power so that I'm flying at 110 KIAS when it's just me on board in air without strong updrafts or downdrafts, then I'm flying at 75% power, and my fuel burn (which I've logged for every long flight over the past 19 years) has confirmed that, repeatedly. So this is my normal approach (flying LOP-WOT, which your POH doesn't recommend):
    1. Leave the throttle wide open and pull back the mixture until I'm flying at about 110 KIAS (if I want 75% power and it's just me on board).
    2. Check that the RPM is about where I'd expect it to be at my current altitude/temperature.
    3. Land 4 hours later, fill up, and find that my fuel burn was accurate within a gallon or so of what I'd planned. :)


    upload_2021-3-28_16-57-32.png
     
  17. Jeff767

    Jeff767 Pre-takeoff checklist

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    Running the engine as you describe will keep it in a safe zone. It will also be very inefficient and leads to excessive lead buildups in the engine. There is nothing wrong with operating most lower powered engines at peak EGT. Even though 50 ROP to peak ROP is the highest ICP and CHT it’s simply not a issue if the engine is properly baffled and installed. These engines are just not that stressed. It’s a different story with big bore high compression engines.
    What is not mentioned in the replies is the effects of carb verses injection and how every engine is somewhat different. In general carbed engines don’t like to run LOP because their fuel distribution is not good enough to provide smooth operations. There are exceptions and tricks to get better distribution but I suspect the recommendation to not run LOP is based on the manufacturer knowing the engine won’t operate smoothly in that area. Even injected engines often need to make changes in injector sizing to get all cylinders hitting peak EGT at about the same point. A perfectly balanced engine will not run rough regardless of leaning. It will simply quit when leaned to much. They are rare.
     
    Last edited: Mar 29, 2021
  18. David Megginson

    David Megginson Cleared for Takeoff

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    My preference is to operate my O-320-D3G LOP-WOT, where it runs well. Peak EGT is OK if you have EGT probes in all cylinders, and can make sure that the richest one is at peak, but with a single-cylinder EGT probe, it's not unlikely that one of the other cylinders is still around 50°F–75°F ROP, in the red zone. That's why, with a fixed-pitch prop and no advanced digital engine monitor, if you can't operate smoothly LOP-WOT, I suggested slightly rich of max RPM, which ensures you're well outside the red zone. True that you'll burn 15% more fuel than LOP, and will have to deal with lead fouling on your plugs (and a significantly-elevated risk of CO poisoning if there's a breach in the exhaust system), but them's the breaks.
    Below 65% power, We're in full agreement. Up at 75% power, you're right that the simpler engines are more robust, but the main reason we have a CHT limit isn't because of heat damage to the engine (as much as that matters for cylinder life), but because peak CHT—which we can measure easily—is more-or-less proportional to the maximum internal cylinder pressure. If the internal pressure gets too high, your cylinder might get confused and think it's in a diesel engine, and then detonate the fuel/air mixture (through pressure alone) at the wrong part of the stroke, before the spark fires. But fully agree that it's a lower risk — but still a real risk — with low-compression cylinders.

    Also note that the old-timers claimed that it was normal to need a top overhaul halfway to TBO, even on lowered-powered planes. Those are the same old timers who thought it was fine to operate their engines in the red zone, and even wrote that into the POHs. Draw your own conclusions.
    We discussed that, but easy to miss in such a long discussion. With a carbureted engine, you don't need a POH or digital engine monitor to know if it runs smoothly LOP — you'll feel it easily enough. My O-320 gets slightly throatier in the LOP range, but it will usually lean right to idle cutoff without any noticeable roughness (e.g. a pencil on top of the dash won't start vibrating). As I mentioned in an earlier post, 4-cylinder carbureted engines have a much better shot at running smoothly LOP than 6-cylinder, because with 6 cylinders the fuel/air mixture has to travel further to some cylinders than others, though I do have one friend — with fancy engine monitor — who manages to operate his 6-cylinder engine in a PA-28-236 LOP. Agreed that with a 6-cylinder carbureted engine (and some 4-cylinder ones), it's hit or miss.
     
  19. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    Mike Busch or John Deakin - Y'know, one of those engine guys - once said something about pulling the throttle back just a hair - maybe 1" MP - seemed to help some carbureted engines be able to run lean of peak, something about the turbulent airflow behind the throttle helping out in some fashion. But IME, on an O-470 or O-520 with a full engine monitor, I wasn't able to get either to run smoothly LOP. The O-320 thing is interesting, I'll have to borrow a friend's 172 with engine monitor and mess with that. He's been wanting me to teach him about the engine monitor too, so win win.

    It's fairly rare for avionics to say what % power you're at in a piston. There's a lot of variables, and even the avionics that do guess % power probably aren't accounting for all of them. Especially in between when the first cylinder peaks and the last cylinder peaks, it's not necessarily an easy calculation - Really, your engine is kind of four or six separate "engines" that happen to share a crankshaft.

    When you're rich of peak, you're limited by air flow. When you're lean of peak, you're limited by fuel flow. Your air flow will be roughly (RPM/Max RPM)*(MP/29) but since you have a fixed pitch prop and likely no idea what your manifold pressure is, that doesn't really help. Lean of peak, in theory your hp is 14.9*gph = hp, divide that by your rated hp and you've got percent power. So, for example, if I'm at 12 gph in my Mooney (280hp max), 12 * 14.9 = 178.8 / 280 = 63.8%.

    But, with all of the variables involved, including spark timing and such, and the fact that you likely don't have enough information to calculate percent power on the fly in the 162, your best bet is the performance charts in the POH. Not nearly as cool as having an actively displayed % power, but until we have FADEC and even better instrumentation, you probably won't see that... And I doubt we'll ever see that sort of thing become common, unfortunately.
     
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