Proper fuel mixture adjustments in long climbs/descents...?

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WastedFlyer
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Proper fuel mixture adjustments in long climbs/descents...?

#1 Post by WastedFlyer » Thu May 11, 2017 11:55 pm

So we all know that the fuel mixture should be adjusted "for best performance" for altitude, but how often should the mixture being adjusted during climb/descent? Say, every 2000 ft? Every 1000 ft? Every 500 ft? Adjusting continuously every second?

Is it true that the engine should be a little rich-of-peak ("to safeguard it against detonation, preignition and possible overheating of exhaust valves")? Or would that cause "fouled spark plugs and lead deposits"?

Would the mixture adjustments during descent be different than during climb? (as the engine would be cooler, and not a full-power)

I'm thinking particularly about bush flying operations... Is there any "standard" procedure to keep the engine in top shape? (taking into consideration performance, fuel efficiency and maintenance costs)

Just curious...
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Re: Proper fuel mixture adjustments in long climbs/descents...?

#2 Post by PilotDAR » Fri May 12, 2017 4:35 am

Aircraft engines have a wide tolerance of poor mixture setting. Yes, those pilots who optimize mixture settings get a bit more fuel economy, and perhaps increased engine life, but it must be remembered that an engine is a long term consumable, and fuel is a cheap coolant compared to the affects of letting it get too hot.

I'm content to adjust the mixture every thousand feet or so, and when leveling off. If in doubt, I'll enrichen a little. Depending upon how well the aircraft is instrumented, and how well you perceive the nuances of how the engine is running, you can refine your technique.

My experience with the two engines I own, has been that the difference between casual leaning, and very precise leaning is not worth the trouble. If I'm bored on a long flight, yeah, I'll play with the mixture to optimize things. This will be most easy with a temperature scanner, and will often require optimizing the throttle too for carburetted engines.

Considering and caring for the engine is great, but running it on the rich side of lean will be fine for any common float flying. If you're climbing a C P210 up to the high 'teens, then worry more about optimum leaning.....

But certainly a good question to ask.....
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Re: Proper fuel mixture adjustments in long climbs/descents...?

#3 Post by photofly » Fri May 12, 2017 5:55 am

Full rich for climb below 3000'. Above 3000', during a climb lean for best performance, as desired.
Level flight - lean as desired.
Descent, don't bother adjusting the mixture until you've levelling.
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Re: Proper fuel mixture adjustments in long climbs/descents...?

#4 Post by WastedFlyer » Sat May 13, 2017 10:47 am

PilotDAR wrote:the difference between casual leaning, and very precise leaning is not worth the trouble.
Interesting... Thank you both for the feedback! :)
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Re: Proper fuel mixture adjustments in long climbs/descents...?

#5 Post by anofly » Tue May 16, 2017 3:16 pm

Boy this one could start a lot of fights. DAR is right on. I would modify the part about a climb, by photofly, by all means lean a tad, but leave it well on the rich side, as you need the cooling extra fuel gives you during a high power lower speed climb.. But not so rich it is choking on fuel.
Low power descents you can lean brutally, but don't forget, if you go around , and or you might have to richen up to keep it purring nice.
I think if you had a big displacement , fuel injected engine that you move to 10, 000 feet and back every flight you have more mixture to fool with than a c 150 banging around below 5000 feet most of the time.

DAR is correct in that if you are really trying to entertain yourself on a long trip , with an egt, moving the throttle a bit and the mixture a bit you can get it sweet, but with a carb, you can only do so well, lean it in cruise till it complains a bit, and richen it up till it runs sweet, that's about it. Fuel injection can be a bit more exacting, with a 6 probe egt, but don't forget to look out the window once in a while......
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Re: Proper fuel mixture adjustments in long climbs/descents.

#6 Post by photofly » Tue May 16, 2017 8:08 pm

If you want to get a normally aspirated aircraft anywhere close to its service ceiling you're going to have to lean hard in the climb.

If you don't want your climb to result in any increase in altitude and you don't mind running out of fuel or dying of old age in the attempt, then sure, climb with the red knob all the way in, "for cooling".

Pay attention to the rubric:
IMG_2075.JPG
IMG_2075.JPG (80.12 KiB) Viewed 2398 times
When you're cruising around Ontario at 2000 feet density altitude at 75% power, you have the luxury of excess available air, and you can get that 75% power with a whole range of mixture settings, anywhere from way too rich to way too lean.

When you're at 8000 feet you can't ever have more than 75% power because there isn't enough air flowing through your cylinders to burn enough fuel to give it to you. What's more, you can only get that 75% power with the throttle wide open and the mixture at one exact setting which you'll find is with the red knob about one third to half way out. If you get the mixture wrong you're throwing performance away, as well as depositing shit on your plugs.

Which is a silly and unnecessary thing to do because the 75% power that you can get with a correctly leaned mixture at 8000 feet doesn't generate any more heat than the 75% power you were happy to cruise with at sea level, only there you had the throttle partly closed and the mixture leaned or not, as you please.

Lean in the climb.
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Re: Proper fuel mixture adjustments in long climbs/descents...?

#7 Post by WastedFlyer » Mon May 22, 2017 10:02 pm

Thank you anofly and photofly for the feedback... So then, putting both of your perspectives together, what I'd do is lean above 3,000 - 4,000 ft, but rich-of-peak (to prevent overheating) and when I start getting way up, say 6,000 - 8,000 ft, and see the engine struggling to get me a descent rate of climb at full throttle, then I'd lean to peak, as in those cooler levels, with the engine well below "full power", overheating would be less of a concern (right?)... For the descent, part of the checklist should be mixture full rich below 3000 ft... That should get me covered...
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Re: Proper fuel mixture adjustments in long climbs/descents...?

#8 Post by photofly » Tue May 23, 2017 4:07 am

Why would you want full rich descending below 3000 feet? You should only ever be full rich when climbing at low altitude with the throttle wide open.
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Re: Proper fuel mixture adjustments in long climbs/descents...?

#9 Post by SuperchargedRS » Tue May 23, 2017 7:33 am

What does your engine analyzer say?

I lean/cowl flap for target CHTs

It's not exactly paint by numbers.

Also toss out that rich below whatever AGL, it's all about DA, again engine analyzer, and don't be ham fisted with ANY control.
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Re: Proper fuel mixture adjustments in long climbs/descents...?

#10 Post by ScottS » Fri Jul 21, 2017 3:47 pm

After take off I note where the EGT's are during the first 1000 feet of climb. I just adjust the mixture whenever that EGT has dropped (getting richer) back to where it was on take-off. Still plenty rich, but I am maintaining approximately the same mixture as max power take-off.
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Re: Proper fuel mixture adjustments in long climbs/descents...?

#11 Post by WastedFlyer » Sat Jul 22, 2017 3:29 pm

ScottS wrote:After take off I note where the EGT's are during the first 1000 feet of climb. I just adjust the mixture whenever that EGT has dropped (getting richer) back to where it was on take-off. Still plenty rich, but I am maintaining approximately the same mixture as max power take-off.
That makes perfect sense, yes... Thanks for the feedback!
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