Proper Mixture Control

[pmccarthy]

After trouble with plug fouling and higher than expected fuel consumption in my Lycoming O360 engine, I asked around about proper use of the mixture control. The advice was to watch a YouTube video on Leaning Basics by Mike Busch.

 

It turns out the way I was taught was wrong, and everyone was taught wrong until Lycoming reported a study based on experience at the Embry-Riddle flying school after 1999. My settings were too rich most of the time. In summary, Mike Busch says that for a fixed-pitch propeller there are very simple guidelines:

 

·         Full rich only for start and take off

 

·         Ground operations – lean for maximum RPM

 

·         For take-off below 3000ft – full rich

 

·         For take-off above 3000ft – lean for maximum RPM

 

·         Climb above 3000ft – lean for maximum RPM

 

·         Cruise – lean to onset of roughness

 

At Kyneton in summer, we often have take-off DAs above 3000ft. If this is interesting to you, or you disagree with these guidelines, I suggest you spend 91 minutes watching the video.

 

 

[old man emu]

It seems odd that during ground theory courses on engine performance we learn a lot about the "chemically correct" fuel/air mixture, then when we start doing our practical flying and mixture control becomes part of a checklist litany where mixture is set to "rich". From then on, we are made to believe that the red knob is untouchable.

 

It's also forgotten in flight planning. How many times does the average pilot divert from the "2500 RPM will give you 100 knots"? The chart shows how the range of a C-172 at various altitudes with the mixture leaned. These are speeds in still air.

 

 

If you take the figures for Standard Temperature at 4000'

 

2400 RPM > 100 kts @ 27 litres per hour

 

2200 RPM >   99 kts @ 22.5 litres per hour

 

So the fuel saving is 4.5 litres per hour. If avgas is $2.20/ litre this is a saving of $9.90 per hour

 

100 NM @ 99 kts takes 60.6 minutes

 

100 NM @ 100 kts takes 54.6 minutes

 

You would waste nearly six minutes at the end of a trip just in stretching your legs.

 

 

[turboplanner]

then when we start doing our practical flying and mixture control becomes part of a checklist litany where mixture is set to "rich". From then on, we are made to believe that the red knob is untouchable.

 

Not in PPL training (or shouldn't be), and mixture control for cruise is taught in the Navex phase.

 

It's also forgotten in flight planning. How many times does the average pilot divert from the "2500 RPM will give you 100 knots"?

 

In flight planning you'll be calculating the fuel burn based on the way you are going to manage the engine

 

 

[skippydiesel]

It was a while ago now but from faulty memory Pmccarthy's "YouTube video on Leaning Basics by Mike Busch." was what I was taught - didn't know there was another way.

 

 

[pmccarthy]

Before 1999 most were taught to stay full rich on the ground and below 5000. And for cruise to lean to rough, then push it back a half inch or so, well away from rough.

 

 

[turboplanner]

I don’t think so.

 

 

[facthunter]

How do any of you KNOW what others have been taught? at best its an informed guess. .Without tables and  a fuel flow meter it's a lot of guesswork. The drag characteristics of the whole plane including it's weight, and altitude flown at, decide what TAS is suitable for econ flying. As always  when a head wind exists you fly a bit faster and can use a tailwind to advantage by flying a bit slower so Ground distance for fuel used is maximised. That's the aim if you care to do it . If you are flying a long distance for the planes capability it means the difference between getting there and not quite getting there.  Nev

 

 

[Yenn]

What I was taught was incorrect, but I found that out much later.

 

For best leaning methods you need EGT gauges and fuel flow for even better usage.

 

Normally full rich would be used for takeoff, then lean as necessary to keep the EGTs at what you saw at about 1000’ on climb. At cruise you pull the mixture well back until the engine runs rough and then richen the mixture to smooth. The EGT reading then is just about your target.

 

Lycoming preach that you should run on the rich side of peak EGT, but that in my opinion is wrong, it is where you are likely to burn valves.

 

Lean on the ground when taxing, but lean aggressively, so that the engine will not run with the throttle being advanced, that way you will not oil your plugs and also will not be able to take off without advancing the fuel flow.

 

If you have fuel flow metering you will be able to use the mixture control more precisely with experience of your engine.

 

Th one thing to be aware of is do not lean out at anything more than 75% power, to avoid detonation.

 

 

[facthunter]

  A rich mixture won't burn Valves. It's a safe (if sooty) method to get extra power out of an engine  especially useful for short periods

 

      .Avgas has 2 octane figures one for rich (the higher one) and one for lean. You get soot because there's not enough oxygen to burn ALL of the fuel so the hydrogen burns first and some of the carbon doesn't get burned at all and exits as soft soot which is visible.. If your mixture is rich (er) than you need the oil will get dark sooner as it does in diesels.  IF you have excess oxygen, THAT will remain at the completion of normal combustion to oxidise other things, like red hot exhaust valves and microns thick oil film on the cylinder walls.  Lean mixtures also have a habit of not responding when the throttle is opened wider and sometimes backfiring into the inlet manifold which can do a LOT of damage to a motor .Nev

 

 

[turboplanner]

  A rich mixture won't burn Valves.

 

So.......a lean mixture might, could possibly, maybe burn valves?

 

 

[skippydiesel]

So.......a lean mixture might, could possibly, maybe burn valves?

 

For sure!! Speculating -however we are talking overly lean for an extended period (period will depend on how lean & to some extent load)

 

 

[Yenn]

Yes an over lean mixture could burn valves. What I should have said was that running on the rich side of peak EGT means you will run hot, resulting in higher CHT.

 

 

[skippydiesel]

Yes an over lean mixture could burn valves. What I should have said was that running on the rich side of peak EGT means you will run hot, resulting in higher CHT.

 

Its such a long time since I managed an in flight mixture control, so I probably have this wrong  - should not the rich side of peak EGT run cooler than the lean side ? I was trained to find max EGT, then turn the control back to rich ,from memory 1/4 turn but it might have been more.

 

 

[Thruster88]

This is a very good vid of the practical and theory of running lean of peak. It is a Beech Bonanza with a big Continental but the laws of thermodynamics apply to all engines.

 

 

 

 

[old man emu]

Is it correct that a richer than a "chemically correct" mixture can be used to reduce CHT if they have been raised as a result of over-leaning? 

 

I think that an engine's exhaust valves can handle the temperatures associated with a chemically correct fuel/air mixture, ie properly leaned, but an overly leaned mixture will result firstly in one cause of detonation and subsequently in "burning" of the exhaust valves.

 

If there was one person whose spirit is probably still cursing correct leaning techniques, it would be the Japanese Admiral Yamamoto. His aircraft was attacked and downed by  the 339th Fighter Squadron, 347th Fighter Group, flying P-38G Lightning aircraft, equipped with drop tanks. To avoid detection by radar and Japanese personnel stationed in the Solomon Islands along a straight-line distance of about 400 miles (640 km) between U.S. forces and Bougainville, the mission entailed an over-water flight south and west of the Solomons. This roundabout approach was plotted and measured to be about 600 miles (970 km). The fighters would, therefore, travel 600 miles out to the target and 400 miles back.

 

Such a long range flight required careful fuel management, which was only possible through accurate mixture control.

 

 

[Yenn]

Rich of peak is producing more heat than lean of peak. That increases CHT.

 

How to alter mixture depends upon the aircraft, in mine it is by a push - pull control.

 

Normally you would take off full rich, except at high altitude airstrips. You would not touch the mixture until after you reduced throttle, then you could lean, while watching the EGT and look for the peak temp. The lean until it dropped about 75deg.

 

I find it easier to pull the mixture back quickly until I hear the engine run rough and then increase flow to make it smooth. The EGT when it is smooth is then what I aim for at any other rpm.

 

 

[old man emu]

Don't Jabirus have an altitude compensating carburettor? I know that VW Beetle carburetors  had this feature, but that was probably only fitted to European and USA engines.

 

 

[facthunter]

 A rich mixture has no oxygen left to burn anything. If your oxytorch has too much oxygen it becomes a metal cutter.  The Octane rating is reduced with a LEAN mixture as well. Nev

 

 

[Thruster88]

Cirrus, the most successful GA manufacturer in recent history also recommend LOP, it is in the POH.

 

 

 

 

[skippydiesel]

18 hours ago, Thruster88 said:

 

This is a very good vid of the practical and theory of running lean of peak. It is a Beech Bonanza with a big Continental but the laws of thermodynamics apply to all engines.

 

When I did my training way back - I started adjusting mixture from about 3,000 ft. Running full riich up above this is going to have a negative impact on engine performance - she would be spitting out unburnt fuel (joking only a little) on your climb to 9500.

 

 

[facthunter]

 I'm not against leaning and you certainly need to above about 5,000. but there's a good way of doing it. Running too lean will damage your engine. Most of the Jabiru issues started when they leaned them out for endurance on drones. Cruising with it leaned till it's running rough is Not a good idea . Save some fuel? Maybe you do, but there's more to it than that. Also if your engine doesn't have even fuel distribution, you are looking for trouble pushing the lower flows. One backfire in a common manifold can destroy an engine, by ramming mixture into another cylinder. It only has to do it once and you can have a shortened or bent conrod. or stretched studs  etc Nev

 

 

[Thruster88]

Not arguing with you Facthunter but the two vids I linked show that running lean at cruise power will not damage an engine, leaning somewhat to best power mixture creates to most stress on the engine, as the Cirrus guy said if the CHT is high lean more.

 

I would be very interested to know if a jab ever runs lean of peak at cruise .  So jab operators does the egt rise and then fall back to a number similar to full throttle climb power when slowly decreasing throttle at top of climb say 6500 feet.

 

One question for the CD, Bing carbureter experts, will the vacuum slide throttle be in the same position at 65% power at sea level as wide open throttle at altitude (about 9500) when the engine is also producing 65% power.

 

 

[facthunter]

It's a "heat" engine and if you put less fuel through it, it may well run cooler but you risk making it more prone to detonate and backfire as well as dry out "Burn" the oil film off the bores. Jabiru frequently have scratched bores that can only be due to breakdown of the oil film. Rings also need some oil to help seal them and pass heat away.. Any roughness in the engine is a sign you are making the burning unstable. it's harder to ignite a lean mixture.. Residual oxygen WILL  attack hot metal surfaces. If it's been consumed by the adequate fuel there, it can't do any damage. We also seem to continually ignore the Octane rating being LOWER when leaned. Non of these LOP people address that. Nev

 

 

[Yenn]

How is the octane rating changed by leaning?

 

 

[facthunter]

 Avgas has 2 tests done for any particular fuel, one rich and one lean and the octane varies That's shown as  two numbers after the basic description Like Avgas 130 -145. (Now unavailable) I have no reason to NOT believe that All "petrols" behave the same. Also where a fuel is poorly atomised the "mixture " behaves as if it's leaner for the ease of ignition. (range over which it will ignite) requiring more fuel for starting and perhaps other situations than might otherwise be the case.. Nev