Leaning mixture

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fxyz
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Leaning mixture

Post by fxyz »

I'm 3 hours into ppl so don't be too hard on me if I ask something stupid.. I flew with two instructors so far. One of them insisted that I should lean mixture all the time, otherwise the spark plugs would be fouled, even on a local flight. The other said it's not necessary unless on a long xcountry. I'm a little confused. Is it something essential for a flight? Thanks :prayer:
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clear_blue
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Re: Leaning mixture

Post by clear_blue »

You should probably keep a more steady instructor :P but as for your question, the "proper" thing to do is to lean the mixture when in cruise flight (not including the downwind portion of a circuit however... you'll learn more about that in the future). It's a good habit to get into, leaning the mixture and doing the cruise checklist while in the cruise attitude. As you become more advanced however, you'll find that most instructors probably don't put as much emphasis on leaning the mixture (myself included) when conducting an upper airwork based flight, but keep in mind that mixture leaning does fall under Airmanship, and that is a topic graded throughout the entire flight test. So I would say it is a great habit (so you don't forget on a cross-country, you conserve fuel, and it's one less thing to worry about on a flight test), but depending on how much fuel you departed with, and how far away you are from your home airport, and how long you're going to be, not a complete necessity. I would say, at your stage, practice it, get it engrained, leaning and flying is almost a skill unto itself.
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GyvAir
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Re: Leaning mixture

Post by GyvAir »

This guy: http://www.avcanada.ca/forums2/viewtopi ... ng#p861930, grumpy as he may be sometimes, is pretty savy on how to operate a piston engine aircraft.
Search through some of his posts on leaning for more details.
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DanWEC
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Re: Leaning mixture

Post by DanWEC »

No stupid questions, only stupid people who don't ask them! :)

Whew, a circular container of earth dwelling invertebrates may have been unlatched.

Regarding fouled plugs- that instructor is hopefully talking about ONLY taxiing. The engine is at a very low power setting and should always be leaned for taxi. The plugs can/will foul otherwise.
For all intents and purposes, in-flight, plug fouling won't ever happen.

Talking about in-flight, you should lean while in cruise to reduce fuel consumption. Fuel costs money. Money, however, is an aspect of pilotage that doesn't rear it's head till usually much later in training.
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turkeycannon
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Re: Leaning mixture

Post by turkeycannon »

For all intents and purposes, in-flight, plug fouling won't ever happen.
The plane I did my PPL on started to develop occasional mag drops about half way through my training. One way of knowing whether or not I'd been good enough at leaning on an XC or even a local flight was checking the mags after getting back. I have fond memories of learning the burn-off procedure at the hold-short line at Chilliwack, wondering if we were going to get home!

As a result I got pretty obsessive about leaning, and regularly flew around with the mixture 1-1.5" out at 2000'. When it got towards my flight test, we actually pulled the plugs to have them cleaned... enough lead came off of them to make a pencil!

During my test I warned the examiner in advance that I'd be doing the airwork section leaned out rather than full rich, which seems to be the normally encouraged thing. He was fine with it, even seemed kind of impressed.

Which is all a long way of saying, I agree with the Colonel(!). Leaning is important and a good habit to get into, as long as you understand the effects and why you're doing it.
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B-rad
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Re: Leaning mixture

Post by B-rad »

At the risk of throwing a baseball bat in the wheels of a forward moving bicycle that is this discussion, what's better, lean of peak or rich of peak leaning?
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7ECA
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Re: Leaning mixture

Post by 7ECA »

Rich of peak is generally consider better in an FTU environment, where the engines already deal with enough abuse, and don't need students overheating them in an attempt to get LOP. Rich of peak is a simple enough for most students to get, and serves the purpose.
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davecessna
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Re: Leaning mixture

Post by davecessna »

Remember too, during the winter your engine is already running leaner than it would during the summer. The Old Grumpy Man™ referenced above once explained what was happening when I would perform a carb heat check during a cold winter night flight and my RPM would scarily drop when going carb heat cold: the cylinders went too lean.
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New_PIC
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Re: Leaning mixture

Post by New_PIC »

My local airstrip is above 3000 feet and the summer DA can be much higher. There, I was initially taught to always lean a bit before taxi (on the checklist), then to lean just rich of rpm drop during the run up, and to leave it there for the rest of the taxi and take off. Different procedures for different conditions.
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iflyforpie
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Re: Leaning mixture

Post by iflyforpie »

I'm willing to bet that the instructor who told you leaning is only for cross countries probably has no experience outside of the FTU environment. You should lean as often as you can... particularly on the ground.

One reason why the mixture is often left rich is because of the type of flying that training aircraft do... circuits, upper air work, precautionaries and forceds where the mixture is required to be rich for their execution and there isn't enough time to lean before it needs to go rich again.

But this creates an issue where students think it is 'rich all of the time'. It doesn't have to be... lean it out on the way to and from the practice area--it only takes a few seconds and is good practise and good for the engine. Then use your HASEL checks or pre-landing checks to ensure it is rich at the proper time.

The ground is a big one because of the nuances of aircraft fuel metering... we only have good control over it at max power, cruise, and idle (right to the stop... about 600RPM for most planes). Most planes when at the recommended 1000RPM on the ground will be too rich... resulting in plug fouling fairly quickly. Lean it out on the ground as you would in the air... and use pre-takeoff checks to ensure it's in. Another thing I do that they probably won't teach you is to use a wide hand to advance the throttle.... this ensures things like mixture, carb hear, or prop when you have it is all where it should be if you forget the checklist.



For lean of peak... completely irrelevant for flight school aircraft... and practically irrelevant for most privately flown aircraft where the average annual usage of <50 hours will never make you realize the fuel savings required to offset the increased cost of purchasing, installing, and maintaining the required equipment..... not to mention the endless troubleshooting and nervous jug replacements and early overhauls because of an odd reading that would never been an issue with a single probe EGT and CHT.

There are those who need to run LOP to get IFR reserves with any sort of useful load.... but if you need to do that... you might as well get a bigger, better airplane.
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cgzro
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Re: Leaning mixture

Post by cgzro »

The flat palm is good advice for 99 percent of aircraft. For some British a/c the mixture works backwards and you need to conciously split the mixture and throttle with your palm lest you lean out on an overshoot or something.

I think you only really learn to lean properly when you own the thing and see the cost of doing it wrong.
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B-rad
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Re: Leaning mixture

Post by B-rad »

iflyforpie wrote:For lean of peak... completely irrelevant for flight school aircraft.
While it may not have use in flight school aircraft, I think it's relevance is the understanding. Once you understand whats happening in LOP, you will not have an issue leaning ROP. It's not just as simple as the term; it takes an understanding in the science of leaning to know LOP. Maybe it is beyond the scope of what's needed but there are no traffic jams on the extra mile ;)
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B-rad
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Re: Leaning mixture

Post by B-rad »

I was over on PPrune and came across the leaning mixture topic and want to share what one poster had to say..
The reason for your confusion is that few people know anything about this subject. No instructor I've ever met (or got anywhere near) does. Plenty of owners of planes with engines whose management is regarded as important do know it but they aren't participating here

A good starting point is a read of articles by John Deakin at avweb.com. They do move around there a little so if you can't find them, PM me and I will dig them out.

Back to your questions:

How then, when you run lean, can less fuel make more heat

It doesn't, of course.

The highest CHT occurs in the vicinity (not quite "at") peak EGT. Once you lean past the peak, i.e. in the lean-of-peak (LOP) region, the temps all fall. Since all the fuel is getting burnt, you are extracting all the chemical energy out of it that can be extracted.

So, why not run LOP all the time (like modern car engines tend to)?

The answer, I think, is simply inadequate cooling

There is always a design compromise between getting rid of heat, engine weight, and cost. Liquid cooling makes this a lot easier but we don't have that option in those engines.

A design compromise has been chosen such that to reach their handbook max rated power, a typical Lycoming or Continental engine cannot get rid of the heat (into the exhaust, the oil and the airflow) and needs to have the combustion cooled by something. One could squirt water into it (like has been done elsewhere) but they just squirt fuel in there - in America fuel costs about the same as water

The fuel injection unit is configured so that with the mixture lever fully forward you are about 150F ROP and you can leave it there all the way through climb, wasting unburnt fuel happily out of the exhaust. Until you get to an altitude where the engine won't run because it's too rich, say 5000ft+, so you HAVE to lean, but by then it should be safe because it isn't doing anywhere near 100% power, more like 60-70%.

When the power setting is well below 100% (75% is Lyco's official position on certain engines), and the airflow (airspeed) is sufficient, one can lean to LOP. The only limit is that the engine design does not properly distribute air (and therefore fuel) equally to all cylinders, so the engine start to run rough because some cylinders develop more power than others.

The reason this doesn't matter when ROP is that if you have surplus (unburnt) fuel, uneven fuel distribution doesn't result in correspondingly uneven power distribution. Whereas at LOP (where fuel=power) it does.

So to operate LOP one needs an engine whose power distribution is balanced. A firm called GAMI has been selling matched injectors for this purpose but some engine owners don't need them as their stock injectors are well enough matched to start with. On carb engines the situation is usually hopeless.

I don't know if one can design an air cooled engine like a big Lyco which can run LOP at 100% power under all operating modes (slow speed climb and the rest). Possibly, it can't be done. These engines do deliver quite a lot of HP for their weight, considering the low RPM which is dictated by lack of a gearbox and the prop tips having to be sub-mach1.

Lycoming's position is complicated because they've been selling engines essentially unchanged for decades, and if they change any operating recommendations they open themselves to legal liability. This is especially hard if you sell an engine which is relatively easy to mismanage - if you fly mine (IO540) at 100% power 1000ft above the sea, leaned to 50F ROP, for half an hour, doing slow climbs and such, it will be scrap, 25 grand just like that. And it happens. People crack cylinders, lose camshafts, you name it. And they often try to go after Lyco, especially during the warranty. So anything Lyco do has to be ultra defensive, but providing they don't change their manuals they are OK. This makes a mockery of anything whatsoever they say on the subject.
Also, I think it's important for clarity that "lean of peak" is not the same as "not rich enough" and I have a feeling this is where being "too lean" gives LOP a bad rep.
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trey kule
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Re: Leaning mixture

Post by trey kule »

To the op.

1. Dont believe anyone who tells you just to pull it out an inch or so (giggity). The proper amount to lean is based on density altitude and power needs. When it is very cold you should not be leaning at all, or very little..on the other hand on a plus 30 day from a high elevation airport you need to lean lots just to get full power, and there is a technique for that.

Years ago, when 80.87 was used in the C152 which wanted 100.130 or 100 ll, there were constant fouling problems because of the extra lead in the 80.87. It is no more. But There was more than one C150 that had valves burned out because students were taught the pull it out an inch on the 152. The 150 liked 80.87, and that technique caused problems

Which is my way of saying, it is not simple with carbed engines, and someone who says lean at 5000 feet is forgetting the effect of temperature, and, to a lesser extent, pressure variations,
the leaning required at 5000 feet in minus 20 and plus 20 are very different,

In the old planes .carberated and without any guages, we used to lean them until they ran rough, enrich until they ran smooth again, and then add a little bit in case we had a cylander running lean.
It was simple and seemed to work.

With fuel injected engines, it is much easier, as they typically have better instrumentation.

As to the LOP....forget about that for now...just idle thread drift and should have its own thread.

So, to argue my point...leaning is more than just taxiing. More than just in X-country flying.
Take some time with and learn about it.

Then at the end you can get into the LOP discussion.

Enjoy the flying.
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fxyz
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Re: Leaning mixture

Post by fxyz »

Thanks everyone! Never expected such great replies.
GyvAir wrote:This guy: grumpy as he may be sometimes, is pretty savy on how to operate a piston engine aircraft.
Search through some of his posts on leaning for more details.
I searched his posts and they are extremely helpful. Looks like he stopped coming here now. Wish I was here earlier. :(
clear_blue wrote:You should probably keep a more steady instructor
I was experimenting with different instructors before and finally made up my mind now :D
iflyforpie wrote:Most planes when at the recommended 1000RPM on the ground will be too rich
So why not recommending idle at 600rpm?Is it harmful to the engine?
iflyforpie wrote:Another thing I do that they probably won't teach you is to use a wide hand to advance the throttle
My instructor taught me that!
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B-rad
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Re: Leaning mixture

Post by B-rad »

trey kule wrote:There was more than one C150 that had valves burned out because students were taught the pull it out an inch on the 152.
http://www.avweb.com/news/pelican/18215 ... directed=1
A cylinder in your piston aircraft engine flunks its compression check, with lots of leakage past the exhaust valve. The mechanic says you probably fried the valve by leaning too aggressively. Wrong, says AVweb's John Deakin! Lean mixtures don't cause burned valves — lousy valve-to-seat geometry does. It's probably the fault of the factory or overhaul shop, not the pilot.
As for forgetting about LOP for now, that's like saying let's stop half way to learning about something. Why not complete the picture? How can you understand proper leaning if you never explore what happens on the other side of peak? And what better time then when a student is asking to understand mixture leaning. It's not a thread drift, it's directly related and builds on the obvious little knowledge that is out there for the vast amount of students.
Without the full picture all students will understand is mixture full, mixture cutoff, and that they should be somewhere in between.

It's a little bit frustrating that flight instructors get such a bad rep for being inexperienced time builders but when someone tries to offer some knowledge and build on a concept that most students struggle with, it gets brushed off as a drift off topic.
Perhaps you'd actually prefer instructors to stick with the method of teaching students to pull it out an inch.
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trey kule
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Re: Leaning mixture

Post by trey kule »

The original poster is a new student pilot. His question was a primary question.
K.I.S.S. We, and instructors do not have to demonstrate our knowlege. Just answer his question.

If you are interested in LOP operations, start another thread. Dont try to confuse the OP. He does not need to know that with three hours of flight time.

If I came across as rude, it is because when a new student asks a simple question that they are confused about, they do not need a graduate level discussion. The clearest and simplest answer to match their level of training. And I think a good instructor understands that.. Read the posts from the very experienced instructors on this thread.
Once they understand the basics, they can build on it.

I would prefer instructors review the principles of teaching, and then have their CFI confirm with them to make sure they understand how to put them into practice...

As to your link.. I have read it before. But I have also witnessed that of which I wrote about. I am going to stick with my experience and the wisdom of those at the time who replaced and looked at the causes. My point was that a formula lean procedure does not work, and in the time I referenced, it was due to using different fuels.

Do youbreally think a 3 hour student needs to know about LOP ops when two instructors have already confused him?

Kind of like the instructors who want to start a PPL out in 20G30 , 45 degree x winds, because if they can learn in that they can learn in anything...students can learn better, a step at a time, and building from the simple to the complex...does your FGI not say sometning similar?

And I understand the irony of responding to your post and complaining about thread drift...hope the op got what he needed from this beforehand.
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B-rad
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Re: Leaning mixture

Post by B-rad »

trey kule wrote:The original poster is a new student pilot. His question was a primary question.
K.I.S.S. We, and instructors do not have to demonstrate our knowlege. Just answer his question.
You're right, I was mistaken by his question and assumed he wanted to learn something but in reevaluating his question he is just looking to be told what to do. Let me try this again using your K.I.S.S. method.
I flew with two instructors so far. One of them insisted that I should lean mixture all the time...The other said it's not necessary unless on a long xcountry. I'm a little confused. Is it something essential for a flight?
Yes
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fxyz
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Re: Leaning mixture

Post by fxyz »

She does want to learn something :lol:
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B-rad
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Re: Leaning mixture

Post by B-rad »

Thon came to the wrong place.
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