The Mysterious Mixture Control

[RoboRay 126]

If you ask five different pilots how to properly lean-out an aircraft’s mixture, expect to get five different answers. All pilots should have a general understanding of why you need to adjust the fuel/air mixture in the engine to maintain the optimal ratio (air pressure decreases as altitudes increases, requiring less fuel flowing to the engine), but methods to actually go about doing so seem to range from theoretical science to voodoo.

 

An engine performs most efficiently when provided just enough air to burn every drop of fuel you feed it. The problem is that is also where the engine runs at its hottest, which over time can reduce the operating time between overhauls or other major maintenance. Conventional logic says it's better to run your engine with the fuel-flow slightly above optimal, allowing the excess fuel flowing through the engine and not being burned due to lack of sufficient oxygen to act as a coolant. This works well, which is a big part of why running slightly Rich of Peak, or RoP, is the way most pilots learn to do things. But RoP wastes fuel.

 

The engine will also run cooler if you don't give it enough fuel for the air volume, meaning the engine would simply produce less heat to begin with and the excess airflow through the engine will also help cool it. It’s certainly cheaper to save fuel than to save air, and you can fly a little further before you need to land and refuel. The problem with running Lean of Peak, or LoP, is that carbureted engines rarely have precisely the same amount of fuel flowing to every cylinder. This isn't a problem when running rich as all of the fuel isn't being used anyway; every cylinder is already burning its maximum amount of fuel. Attempting LoP operations on the same engine may result in a situation where the engine runs roughly because one or more cylinders is producing significantly less power than the other cylinders due to fuel-flow variations.

 

The simplest way to set your mixture, and usually the first method a student pilot learns, is once you've reached your cruising altitude and throttled back to your cruising speed, is to begin pulling back the mixture control, reducing the amount of fuel entering the engine. When the engine begins to sound rough, enrich the mixture again until it's smooth then add just a little more. In theory, this should place you slightly on the RoP side of the temperature curve. In reality, it's not a very precise method. Unfortunately, this doesn't work in Flight as the engine never seems to run rough until you pull the mixture so far back that the engine simply stops running from fuel starvation.

 

Another method is to watch for the RPM to fluctuate when leaning. Pushing the mixture back in slightly until the RPM goes over the “bump” again should put you at about the same point as the method above, but again does not work well in Flight as the only aircraft without a constant-speed propeller (the RPM does not directly correspond to engine power output with a constant-speed prop) is also equipped with an engine that automatically adjusts the mixture for you, and it’s not clear that the RPM actually does change at the peak in Flight, anyway.

 

The most precise way to set your mixture is by measuring the engine’s Exhaust Gas Temperature. Yet again, though, this is not an option in Flight. Only one plane, the Maule M-7, is equipped with an EGT monitor and it doesn’t work. The RV-6 carries an engine analyzer that can also be used to monitor EGT (among many other things) but in Flight it appears to be limited to show only a single cylinder head temp (presumably the hottest one). An EGT gauge would make finding peak temperature trivial as it shows it to you clearly. In general, you would adjust the mixture until the EGT indicates around 50°F below peak, but this varies by aircraft.

 

I'm aware of only one way that may actually find the peak in Flight… by careful monitoring of the fuel-flow. Fuel-flow should drop fairly slowly above the peak when leaning the mixture, but should drop quickly below the peak. Observing how quickly the rate changes while moving the mixture control may be enough to pin down approximately where the peak occurs. The problem here is that only the Maule and Stearman are equipped with fuel-flow gauges.

 

In the real world, as fuel-injected engines became more common, Lean of Peak operation became a more viable proposition. Fuel-injection can reduce or eliminate the problem of fuel-flow variations between cylinders, as can more modern designs for carbureted engines, meaning you can often expect to obtain smooth operation below the peak. This saves fuel, as you aren’t wasting any of it to cool the engine.

 

For standard LoP operations, if your engine tolerates running leaner than peak, set your cruise speed as normal, find the peak with your preferred method, but then adjust the mixture to place you slightly on the lean side of the peak rather than the rich side. Enjoy the greater fuel efficiency and range of a LoP configuration.

 

A somewhat controversial leaning method, but which appears to be gaining converts, is called Wide-Open Throttle/Lean of Peak (WOT/LoP). In this method, you never pull the throttle back from the fully-forward takeoff position until you are descending to land.

 

The throttle is commonly referred to as the “power” control, but that is a bit of misnomer. The throttle, prop control and mixture are effectively all “power” controls as they all influence how much power the engine is actually providing to the aircraft. Closing the throttle limits total airflow through the engine. If your intention is to fly well Lean of Peak, reducing the available air will just force you to reduce fuel-flow (and effective power) even further.

 

Upon reaching your cruising altitude, set your desired RPM with the prop control (if so equipped) and pull the mixture back, way back, to reduce fuel flow and set your cruise speed. This should place you well below the peak, but a well-designed, properly adjusted motor should be able to still operate smoothly. If it doesn’t, push the mixture back in and revert to a more traditional approach to leaning.

 

Running with the throttle wide open guarantees that you are always running as lean as you possibly can for any given speed/altitude combination, as the mixture has become the only variable in the power equation.

 

I’ve experimented with WOT/LoP in both the M-7 and the RV-6A and did not note any problems. If Flight does get a fixed-pitch prop aircraft with a manual mixture control, this method will still work. The RPM will drop as you pull the mixture back, even though the throttle remains wide open. With aircraft that run smoothly LoP, and the lack of any reliable way for finding the peak, Wide-Open Throttle/Lean of Peak appears to be the way to operate in Flight.

 

 

Further reading:

http://flighttrainin...al/mixture.html

http://flighttrainin...lls/engine.html

http://www.avweb.com...n/182179-1.html

http://www.avweb.com...n/182176-1.html

http://www.avweb.com...n/182583-1.html

http://www.avweb.com...n/183094-1.html

http://www.flyplatinum.com/blog/?p=577

http://lahso.meggins.../12/23/leaning/

 

(Just remember, the more you try to learn about leaning, the more different opinions you will find.)

 

I have left out some things for simplification, mainly the effects on your motor of both RoP and LoP that can impact it's service life between overhauls (higher cylinder head temps when RoP than LoP, fouling of the internals by unburned fuel when RoP, slower combustion when LoP, etc.) as none of those really matter in Flight. When MS implements a damage-model for the aircraft systems, I'll reconsider. :Waiting:

[jcomm 6,050]

Great reading Ray(?). Thx a LOT for it + links !!!

 

It would be great if future panels included a functional EGT gauge. Otherwise, MS FLIGHT users had better use the Auto-Mixture Option under Gameplay...

[Mork 34]

Thanks for posting this. I like these topics with some theory being taught. You describe it very well, very easy to understand.

 

I use auto-mixture now, since i am really just a beginner. First i learned VOR, then ILS (and reading a lot about other Nav aids in the mean time as well). I recently experimented with the RPM control because of a topic here, and really gotten an advantage out of that, because i can fly faster and climb easier now. Before that particular topic i always thought the more RPM, the more power.

 

After reading this i'll look in to the fuel/air-mixture as well, eventhough it's isn't (fully) implemented yet. I'm getting there step by step, thanks to topics like these.

[Bobsk8 12,940]

Well I posted a couple of hours ago the results of some playing around with the mixture on the Maule, using the fuel flow gauge which looks like it might be working. Here are the results, the Maule was using about 16 gph at cruise, 60% prop. I started leaning the mixture control watching the fuel flow, 90%, 80%, 70%, ....... 30% 20% 10% 5% 4% 3% 2%...... Fuel flow reading at 2% (ready for this) 16gph........ :Phbbbt: OK, now to 1%, the fuel flow started heading towards zero and the engine sound like it was shutting down, back to 2%, 16gph and smooth as silk. The fuel flow gauge is bogus or the mixture control needle valve is broken. Anyway I posted this on Steam.

 

PS... I just read and forgot about auto mixture. maybe this is why the mixture control doesn't reduce the fuel flow. Going to take off again in a few minutes and check that out with auto mixture turned off.

[zinfinion 0]

Just a note that WOT/LoP only applies to cruising above 8000' where you can never get more than 75% power anyways. At lower cruise altitudes the WOT no longer applies and you need to throttle back to 75% or less power (which you need the engine power curve charts to fully determine). The quick and dirty method is to add manifold pressure + (RPM/100) and then run that against known values (again derived from those pesky charts). The Maule's Lycoming IO-540-V hits 75% around a sum 48. If you're cruising down low (clandestine!) set RPM to 2200 and throttle back to 26". Then lean from there. I tend to aim for 10 lbs/hr flow rate LoP, since the book suggests 12, which is RoP.

 

With regards to the Cub, it is only allowed to use the full 180HP for 5 minutes after takeoff due to FAA regs for the class. After that it cant be operated above 80 HP, hence the altitude/RPM placard on the dash. As long as you stick to the printed values, you'll never exceed 80HP. With regards to mix, you should see the RPM's spike a bit while leaning if MS gets it right since it is a fixed pitch prop.

 

And for some truly quick and dirty numbers, 100% throttle, 60-70% prop, and 35-40% mix is a good starting point for 8000'+ cruising. Maule or RV-6.

 

And one other thing: Takeoff climb should always be balls to the wall throttle/prop/mix to keep the engine cool while running full out. Unless above 3000', then lean to max RPM before taking off. And yes, I have been waiting for an excuse to say balls to the wall.

 

I'll leave the rest of the explaining to the articles linked.

[Oracle427 45]

The Stearman, Maule and RV all have fuel flew gauges.

[zinfinion 0]

The Stearman, Maule and RV all have fuel flew gauges.

Stearman and Maule, yes. RV, no. Unless I am oblivious to something here.

[RoboRay 126]

The Stearman, Maule and RV all have fuel flew gauges.

 

Flight's RV-6 doesn't, to the best of my knowledge. If it does, please let me know where so I can update the cockpit panel tutorial.

 

The Stearman may, though. I honestly never look inside except to check my speed on final. Guess I should look at the panel more!

[zinfinion 0]

The Stearman may, though. I honestly never look inside except to check my speed on final. Guess I should look at the panel more!

 

It's in amongst the oil gauges. I had no idea until just now.

[RoboRay 126]

Oh, and the "cruising WOT at low altitudes is bad for the motor" thing can certainly be true. I glossed over that for Flight. There are some pretty good explanations and charts exploring that in the "avweb" links.

 

From the engine's point of view, flying higher and throttling back are the same thing. So, if you stay low, you likely will need to throttle back to reach a safe power level for cruising.

 

The actual lower altitude-limit for safe WOT/LoP operation depends on the engine, but it's not always as high as 8000'.

[Oracle427 45]

It will have to wait until I am home. I am fairly certain that it is a digital readout just above the oil temp or below the fuel gauge... I know one is the manifold pressure.

 

Watch the numbers go up and down as you play with the mixture. The MP shouldn't change unless you shut the engine down.

[RoboRay 126]

It's in amongst the oil gauges. I had no idea until just now.

 

I've been meaning to do a full panel explanation for the Stearman (and the Icon), so I'll be sure to include that.

[zinfinion 0]

I actually ran across all of John Deakin's articles on this about a week ago. Really fascinating stuff. I actually even mentioned it with regards to one of my screenshots since my prop and mix were not where the POH says to put them. :lol:

 

I am fairly certain that it is a digital readout just above the oil temp or below the fuel gauge.

Watch the numbers go up and down as you play with the mixture.

 

That's the temp probe. Which does seem stuck on CHT.

 

[RoboRay 126]

It will have to wait until I am home. I am fairly certain that it is a digital readout just above the oil temp or below the fuel gauge... I know one is the manifold pressure.

 

Watch the numbers go up and down as you play with the mixture. The MP shouldn't change unless you shut the engine down.

 

If it's the Engine Analyzer display you are thinking about, it appears to be set to show Cylinder Head Temp. At least, I assume it is because of the way it slowly drops off after the engine is shut down. Note that it's still showing "128" in my picture for the panel tutorial, with the aircraft fully shut down.

 

The readout on the Fuel Gauge itself is a fuel quantity totalizer.

 

The readout on the oil monitor toggles between oil pressure and oil temp.

[Bobsk8 12,940]

Well I posted a couple of hours ago the results of some playing around with the mixture on the Maule, using the fuel flow gauge which looks like it might be working. Here are the results, the Maule was using about 16 gph at cruise, 60% prop. I started leaning the mixture control watching the fuel flow, 90%, 80%, 70%, ....... 30% 20% 10% 5% 4% 3% 2%...... Fuel flow reading at 2% (ready for this) 16gph........ :Phbbbt: OK, now to 1%, the fuel flow started heading towards zero and the engine sound like it was shutting down, back to 2%, 16gph and smooth as silk. The fuel flow gauge is bogus or the mixture control needle valve is broken. Anyway I posted this on Steam.

 

PS... I just read and forgot about auto mixture. maybe this is why the mixture control doesn't reduce the fuel flow. Going to take off again in a few minutes and check that out with auto mixture turned off.

 

 

Well, Pilot error!!!!!!! I turned off the auto mixture control, and now I can lean it out with the fuel flow gauge and everything works perfectly.

 

Reading the other comment about reducing power in the Maule below 8,000 feet, I wasn't doing that either... I guess my TBO will be alot quicker than 2,000 hours now..... if the engine even makes it that far......