WestJetForLife
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Mixture Controls On Propeller-driven Aircraft

Sun Feb 24, 2008 6:59 pm

Hi, everyone in Tech-Land. Nik here with yet another question to ask.

When I was in Air Cadets, I was taken up numerous times in a Cessna 172, and even on one occasion flew on a Beech B58 Baron as part of my aviation training through the RCAC (Royal Canadian Air Cadets).

This question has been bugging me for four years, and I haven't found any answers yet, but how do the fuel mixture controls on a propeller-driven airplane work, exactly? I understand lean and rich, but everything else is Greek to me.

To refine the question: when we were up at high altitude (9,000 to 11,000 feet), the Pilot-in-Command pulled the mixture lever(s) back about halfway between 1/4 and 1/2 between lean and rich settings. Why do propeller pilots do this? Does it have something to do with engine control? Fuel mixture in the cylinders? Fuel economy? An explanation would be greatly appreciated.

Thank you,
Nik
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2H4
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RE: Mixture Controls On Propeller-driven Aircraft

Sun Feb 24, 2008 7:16 pm



Quoting WestJetForLife (Thread starter):
when we were up at high altitude (9,000 to 11,000 feet), the Pilot-in-Command pulled the mixture lever(s) back about halfway between 1/4 and 1/2 between lean and rich settings. Why do propeller pilots do this?

It's called "leaning" the fuel mixture, and it's done to maintain an acceptable fuel/air ratio for the engine.

At sea level, a fully-rich mixture setting will probably produce a perfectly acceptable ratio of fuel to air. That mixture will contain just the right amount of fuel and just the right amount of air to burn efficiently.

Now, as we climb higher, the air becomes thinner. If the mixture setting remains constant, the engine will receive the same amount of fuel, but much less air. Because there's so much more fuel compared to air, this mixture will not burn efficiently.

So, as we climb, and as the air becomes thinner, we "lean" the mixture (or reduce the amount of fuel being introduced to the engine) so that the ratio of fuel decreases accordingly. This maintains the optimum fuel to air ratio, and the engine keeps running smoothly.

2H4
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Mender
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RE: Mixture Controls On Propeller-driven Aircraft

Sun Feb 24, 2008 7:18 pm

It is to allow the pilot to compensate the engine fuel mixture for the aircraft altitude.

As the aircraft climbs and the air becomes thinner the air/fuel ratio or mixture in the engine will become rich as there is less air available to burn the fuel with. By using this control the pilot can manually correct the fuel/air ratio. More often than not the aircraft will have an instrument that displays the EGT or exhaust gas temperature. By watching the EGT gauge when the pilot is adjusting the mixture he can tell when it is "about right".
 
WestJetForLife
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RE: Mixture Controls On Propeller-driven Aircraft

Sun Feb 24, 2008 7:29 pm



Quoting Mender (Reply 2):
More often than not the aircraft will have an instrument that displays the EGT or exhaust gas temperature. By watching the EGT gauge when the pilot is adjusting the mixture he can tell when it is "about right".

What exactly would be the "about right" EGT for flying a prop-driven plane at, say, 10,000 feet MSL?

Another question: do turboprop aircraft (Ex. Dash 8, Dornier DO-328, ATR-42/72) use fuel mixture to enrich/lean their engines, or do these aircraft have computerized mixture controls?

Thanks,
Nik
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N353SK
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RE: Mixture Controls On Propeller-driven Aircraft

Sun Feb 24, 2008 8:50 pm



Quoting WestJetForLife (Reply 3):
What exactly would be the "about right" EGT for flying a prop-driven plane at, say, 10,000 feet MSL?

I'm not exactly sure, but one common technique is to set your cruise power to slightly below what the POH calls for with mixture full rich and then lean it out until the RPMs peak.
 
vc10
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RE: Mixture Controls On Propeller-driven Aircraft

Sun Feb 24, 2008 8:55 pm

Now I am no expert on light aeroplane piston engines, but I have had some experience on the larger 18 cylinder variety

Now as I understand it he mixture control on a piston engine sets the mixture ratio desired by the pilot for the engine power he is about to use. Also piston engines have a bad habit of detonating if they get too hot and detonation is definately a condition to avoid. When engines are operated at high powers take-off and climb they tend to get hot and so are more likely to detonate.

There is a specific air /fuel ratio which will give you the engine's best power, however the engine gets very hot [detonation] so the mixture is run at rich so there is excess fuel which can be used to cool the cylinders

As you climb the barometric control will keep the air/fuel mixture at this rich setting or indeed any other setting selected on the mixture lever

Once settled in cruise[ and using lower power] because the power curve drops off either side of best power mixture you can now safely lean the mixture to the lean side of best power with no loss of power compared to that on the rich side of best power mixture.

There are all sorts of arguments about how far you should lean or not lean , but that is another question.

I really just wanted to say that the mixture lever is not there to correct for the engine moving to a rich mixture as it climbs as this should be done by the barometric fuel control capsule

The mixture lever is there to allow the pilot to adjust the mixture ratio to suit the phase of flight and power being used

As I said I am no expert on small engines s if they do not have automatic altitude correction capsules I am quite wiling to be corrected

littlevc10
 
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tb727
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RE: Mixture Controls On Propeller-driven Aircraft

Sun Feb 24, 2008 10:14 pm



Quoting WestJetForLife (Reply 3):

Another question: do turboprop aircraft (Ex. Dash 8, Dornier DO-328, ATR-42/72) use fuel mixture to enrich/lean their engines, or do these aircraft have computerized mixture controls?

Nope. The only aircraft that need mixture control are normal aspirated aircraft. On advanced airplanes like jets, you have a fuel control unit that automatically changes fuel density for changes in altitude. I have limited Turboprop time but they basically work the same. I don't have much prop time at all, jets are so much easier, in thrust we trust!
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vc10
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RE: Mixture Controls On Propeller-driven Aircraft

Sun Feb 24, 2008 10:20 pm

Quoting Tb727 (Reply 6):
On advanced airplanes like jets, you have a fuel control unit that automatically changes fuel density for changes in altitude.

You also have this on piston engines, well at least on the big radials , where the fuel flow is altered automatically with changes in altitude to maintain the selected fuel/ air ratio. The mixture lever selects what that fuel/air ratio will be

littlevc10

[Edited 2008-02-24 14:28:06]
 
Mender
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RE: Mixture Controls On Propeller-driven Aircraft

Sun Feb 24, 2008 10:37 pm



Quoting WestJetForLife (Reply 3):
What exactly would be the "about right" EGT for flying a prop-driven plane at, say, 10,000 feet MSL?

Hopefully a pilot will answer this properly, it's nearly 20 years since I worked on piston aircraft but if I recall correctly the pilot leans the mixture until the EGT peaks and then richens it up slightly.

I'm the first to admit that I don't remember too much about the carburetor used on aircraft like the 172 or the Cherokee but I'm sure that it doesn't have any barometric correction, just the mixture control. This is partly because the aircraft is cheap/basic and also partly because they are mainly used at low altitudes. Remember the aircraft are unpressurized.

It wouldn't surprise me that you would have barometric correction on an 18 cylinder engine because the airframe is also likely to be pressurized AND it would have a variable pitch prop.

Once you have a variable pitch prop piston engine control gets more complicated because you have to monitor the torque on the engine. This is usually done by monitoring intake manifold vacuum. Basically the pilot will have to juggle the throttle, propeller pitch and mixture and adjust these at each phase of the flight such as climb, cruise and descent. I think the pilot sets the speed with the prop pitch, then the vacuum with the throttle, then adjusts the mixture.

Quoting VC10 (Reply 5):
Now as I understand it he mixture control on a piston engine sets the mixture ratio desired by the pilot for the engine power he is about to use. Also piston engines have a bad habit of detonating if they get too hot and detonation is definately a condition to avoid. When engines are operated at high powers take-off and climb they tend to get hot and so are more likely to detonate.

The pilot always sets the mixture to fully rich for take off and landing so detonation shouldn't be an issue.
 
DKCFII
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RE: Mixture Controls On Propeller-driven Aircraft

Sun Feb 24, 2008 11:01 pm

On the C-172 the pilot leans the mixture out until the RPM reaches a peak. At this point you note the EGT and the richen the mixture until the EGT is 50deg rich of peak. This gives you the "Best Power" mixture. If you want "Best Economy" then you leave the mixture at the max EGT. Also some planes don't have an EGT and in that case you just note the max RPM and then turn the mixture in (Richen) about one full turn. Each plane has it's own best economy and best power EGT, so always consult the POH for your airplane.

Quoting Mender (Reply 8):
The pilot always sets the mixture to fully rich for take off and landing so detonation shouldn't be an issue.

The mixture also isn't always set at rich for takeoff.....Where I fly out near Denver, CO we have to lean the mixture before takeoff since the field elev. is 5670msl.

-Dan K
 
WestJetForLife
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 12:47 am

Once again, Tech/ops never ceases to amaze me.

Thanks, everyone, for their responses. You all have answered my question in full.

Nik

[Edited 2008-02-24 16:47:59]
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KELPkid
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 6:59 am



Quoting 2H4 (Reply 1):
At sea level, a fully-rich mixture setting will probably produce a perfectly acceptable ratio of fuel to air. That mixture will contain just the right amount of fuel and just the right amount of air to burn efficiently.

Not really, but the manufacturers would like us to leave the mixture knob at full rich, because climbing out with the mixture full rich provides some margin against detonation and helps cool the engine. If you do a run-up leaning (take the engine up to 1,800 RPM and lean until you find peak, and then push the mixture rich for 3 notches), you will find that the engine, even when close to sea level, is still running pretty rich. You can use the above 3000 foot takeoff procedure in the POH at lower-elevation airports, but if you do, be particularly mindful of your CHT reading on climbout.

However, the manufacturers like you to climb out this way, because many have found that, starting at fields below 3,000 feet of elevation, the engine makes so much power that the primary consideration on climbout is proper cylinder cooling. It is possible that if you are careles with your airspeed on climbout at a low elevation airfield, you could cause the CHT's (Cylinder Head Temperatures) to rise above acceptable limits. Too rich of a mixture definitely helps keep cylinder head temps under control. Read John Deakin's articles from avweb.com from around the 2005-2006 timeframe...  Wink
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Analog
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 8:01 am

Manual mixture controls are an artifact of overbearing regulation and a stagnant industry. The designs used in piston aircraft engines are just amazingly primitive.

Quoting Tb727 (Reply 6):
Nope. The only aircraft that need mixture control are normal aspirated aircraft

Eh? Superchargers don't obviate the need for mixture control.
 
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 10:32 am



Quoting Tb727 (Reply 6):
The only aircraft that need mixture control are normal aspirated aircraft.

Proof, once again, that you'd better damn well take what you read on these boards with a huge grain of salt. There is some simply ignorant advice but there is, also, some information that is so far wrong as to be lethal.

I have, from long ago, about 600 hours of T210 time; should I have put duct tape over the mixture control?

Beware, especially, the MSFS "pilots" who seem to know everything about FADEC but couldn't land their fork on their plate.
 
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 3:44 pm



Quoting VC10 (Reply 7):
You also have this on piston engines, well at least on the big radials , where the fuel flow is altered automatically with changes in altitude to maintain the selected fuel/ air ratio. The mixture lever selects what that fuel/air ratio will be

Never flew a R4360 or even an R2800 but did fly a P&W R985 for a few years and yes you did lean out the mixture as you climbed.
 
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 3:49 pm

Quoting ThirtyEcho (Reply 13):

Proof, once again, that you'd better damn well take what you read on these boards with a huge grain of salt. There is some simply ignorant advice but there is, also, some information that is so far wrong as to be lethal.

Really?

[Edited 2008-02-25 08:00:47]
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DKCFII
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 4:15 pm

Sometimes I wonder....  banghead 
 
dragon6172
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 5:24 pm



Quoting ThirtyEcho (Reply 13):
couldn't land their fork on their plate.

Now thats good humor!! Big grin
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 5:32 pm



Quoting CosmicCruiser (Reply 14):
lean out the mixture as you climbed.

Is that really leaning out the mixture, or does that have the effect of maintaining the A/F ratio by compensating for the lower pressure?
 
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SEPilot
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 7:10 pm

Proper leaning of an aircraft piston engine is a very complicated subject. First, on takeoff where power output is maximized and airflow is low no air cooled aircraft piston engine gets adequate cooling airflow to keep cylinder head temperatures at acceptable limits, and so the fuel system is calibrated to give an overrich mixture, effectively cooling the cylinder with raw gas. This is very inefficient, but it works and doesn't impose a weight penalty (except for the excess fuel used.) This is one of the primary reasons for having a manual mixture control; you obviously do not want to use this "overrich" mixture except where you absolutely have to, as in addition to the excessive fuel consumption it causes spark plug fouling (especially with leaded fuel), oil dilution and contamination, and other undesirable effects. Under 75% power output on Lycomings and 65% output on Continentals you are free (according to the manufacturers) to lean to your heart's content. Most lean to approximately best power, which is when you lean to peak EGT and then enrich 50 degrees (when using the guage) or approximate by leaning to the onset of roughness and then enrich slightly. Much more efficient (when your engine can do it) is to lean "lean of peak"; the big radials did this routinely but most stock flat opposed engines do not have uniform enough fuel distribution to do this effectively. But with "Gamijectors" and newer engines (Continentals anyway; I don't know if Lycoming is doing it as well) the flows are better balanced and it is becoming possible. Lean of peak operation sacrifices some power output for much greater efficiency and significantly lower cylinder head temps. This has been frowned on in the past because of the uneven fuel distribution problem; the most dangerous point for a piston engine is actually just slightly rich of peak EGT; that is where CHT maximizes, and if you do not have good distribution you could have one cylinder there when the others are considerably leaner. But with better fuel distribution and much better instrumentation available it is becoming more and more acceptable.
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speedracer1407
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 7:27 pm

Quoting ThirtyEcho (Reply 13):
Proof, once again, that you'd better damn well take what you read on these boards with a huge grain of salt. There is some simply ignorant advice but there is, also, some information that is so far wrong as to be lethal.

Somehow, I doubt any pilot is gonna print out an A.net thread and bring it along in the cockpit for procedure.

So rather than the usual breathless astonishment at everyone else's stupidity, how about offering up clarification to what is obviously a simple misunderstanding and/or misuse of terminology.

The OP asked if turboprops need manual mixture adjustments like recips do. I take it the answer is no.

However, a turbo/supercharged piston engine (which would NOT be naturally aspirated) needs manual mixture control the same as any other recip, yes?

[Edited 2008-02-25 11:28:49]
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SEPilot
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 7:33 pm



Quoting Speedracer1407 (Reply 20):
However, a turbo/supercharged piston engine (which would NOT be naturally aspirated) needs manual mixture control the same as any other recip, yes?

Correct.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
KELPkid
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 7:41 pm



Quoting SEPilot (Reply 19):

Aaah, a man who's read John Deakin  Smile

However, I must pick a bone:

Quoting SEPilot (Reply 19):
First, on takeoff where power output is maximized and airflow is low no air cooled aircraft piston engine gets adequate cooling airflow to keep cylinder head temperatures at acceptable limits, and so the fuel system is calibrated to give an overrich mixture, effectively cooling the cylinder with raw gas.

Cessna recommends leaning before takeoff in many of their non-high performance birds (like the 152 and 172) at a field elevation above 3,000 feet. In fact, until I moved to the Pacific Northwest (having had learned to fly in a part of the desert southwest of the USA where you will never find a field elevation under 3,500 feet), I thought that this was standard operating procedure, until I went up for a rental checkout in a 172 here in PDX land, and the instructor asked what in the hell I was doing when, after runup, I took the engine up to 1800 RPM and started leaning...we had a thorough walkthrough of the 172 POH after that upon returning to the FBO  Wink

I'm guessing that above 3,000 feet, Cessna at least feels that the engine is no longer making high enough power that you could cook your cylinder heads by taking off with the mixture set for max. power.
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SEPilot
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 7:56 pm



Quoting KELPkid (Reply 22):

Cessna recommends leaning before takeoff in many of their non-high performance birds (like the 152 and 172) at a field elevation above 3,000 feet.

You are absolutely correct. However, the reason is that the full rich calibration is for sea level, and as altitude rises it becomes more rich and consequently power suffers even more, as well as the engine (unless it is turbocharged) is not capable of producing as much power due to the lower weight of air ingested by the engine. I did not address this issue in my previous post; but you are quite right.

Quoting KELPkid (Reply 22):
Aaah, a man who's read John Deakin

Yes, I have, as well as a number of others. John Deakin is the main source for running LOP information, however; he is very knowledgeable on the subject as well as being an excellent writer.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
KELPkid
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 8:11 pm



Quoting SEPilot (Reply 23):

I'd add you to my RU list for that, but it seems to not be working at the moment...anyone from Demand Media had a look at this?
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 8:42 pm



Quoting KELPkid (Reply 24):
I'd add you to my RU list for that, but it seems to not be working at the moment...anyone from Demand Media had a look at this?

You have to go in and edit your profile; add any names you want to manually and they will appear on your and their profile. I had the same problem, but someone else posted the solution. I tried it and it worked.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
JoeCanuck
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 9:35 pm

Good discussion about something not easily understood by folks without experience with carbs. Basically, you lean or enrich the air/fuel mixture to achieve optimum combustion efficiency. This usually is guided by the density of the air witch changes with temperature and altitude.

There are Rich of Peak and Lean of Peak variations to the theme as well. It's all about best power/economy ratios...and keeping the engine running at all.

Turbo/super charging helps maintain a denser air charge as the plane climbs. Mixture control can be more critical because the higher pressures in the engine can also cause significant damage.

Turbine engines don't have a direct air intake control, like a gas/petrol engine does. The air intake is always wide open and speed/power is controlled with fuel only. It is almost always a leaner burn than a convention gas engine.

Automobile fuel injection does this automatically.

Hopefully, I've not unduly repeated any responses or presented erroneous information. That's the way it works on the planes I fly, anyway.
What the...?
 
KELPkid
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RE: Mixture Controls On Propeller-driven Aircraft

Mon Feb 25, 2008 11:16 pm



Quoting SEPilot (Reply 25):
You have to go in and edit your profile; add any names you want to manually and they will appear on your and their profile. I had the same problem, but someone else posted the solution. I tried it and it worked.

Done! Welcome to my RU list!
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Analog
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 12:12 am



Quoting Speedracer1407 (Reply 20):
The OP asked if turboprops need manual mixture adjustments like recips do. I take it the answer is no.

Could the variable geometry intakes seen in some turbojets/turbofans (eg. J58, Olympus) have this effect (akin to throttling)? These devices represent an independent control over the mass of air pushed into the core.

Does this make sense, or am I completely off base?

Quoting Speedracer1407 (Reply 20):
However, a turbo/supercharged piston engine (which would NOT be naturally aspirated) needs manual mixture control the same as any other recip, yes?

It needs some form of mixture control; manual mixture control is not required, except perhaps by regulations & certifications.

Of course with a turbo/supercharger you could use the wastegate and/or blow-off valve to keep the mass flow constant versus altitude (or to adjust it) by keeping the mass air flow constant (use a MAF meter just like in your car). The variable boost is then your mixture control. I have no idea if this is done or is practical (this is not what I do).

Quoting SEPilot (Reply 19):
) This is one of the primary reasons for having a manual mixture control; you obviously do not want to use this "overrich" mixture except where you absolutely have to,

This sounds like the perfect reason to go with automated mixture control. With sensors to measure cylinder head and other critical temperatures the computer can overrich the mixture for cooling as needed. No more, no less. This would reduce the chance of the engine being harmed (overtemp or plugs fouled) by pilot error.

Quoting KELPkid (Reply 22):
Cessna recommends leaning before takeoff in many of their non-high performance birds (like the 152 and 172) at a field elevation above 3,000 feet.



Quoting SEPilot (Reply 23):
You are absolutely correct. However, the reason is that the full rich calibration is for sea level, and as altitude rises it becomes more rich and consequently power suffers even more, as well as the engine (unless it is turbocharged) is not capable of producing as much power due to the lower weight of air ingested by the engine. I did not address this issue in my previous post; but you are quite right.

Just a nitpick: this isn't really "leaning" the mixture, since you're actually trying to adjust the air/fuel mass ratio to keep it the same as compared to sea level.
 
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 12:18 am



Quoting Mender (Reply 8):
The pilot always sets the mixture to fully rich for take off and landing so detonation shouldn't be an issue.

It's not uncommon of course, to forget to enrich the mixture again when descending... so I'm told  Wink

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SEPilot
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 12:31 am



Quoting Analog (Reply 28):

It needs some form of mixture control; manual mixture control is not required, except perhaps by regulations & certifications.

Current production turbocharged (there are no currently produced supercharged certified engines that I am aware of) use exactly the same mixture control as non-turbocharged ones. All current production engines are fuel injected; there are of course many carbureted engines still flying, and one model (the Lycoming O-540) was used in a turbocharged application (the Cessna 182T). All use exactly the same mixture control as the others; they don't have the altitude issues to the same degree as non-turbocharged ones (although they do have some altitude issues; most of the aircraft that use them are certified to higher altitudes than what the turbocharger will maintain full power to) but they still have the same cooling and efficiency issues and are flown pretty much the same way. They actually require much more careful mixture and throttle control than non-turbocharged engines, and a ham-fisted pilot can completely ruin a $30,000 or more engine in one flight just by careless management.
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 12:34 am



Quoting Analog (Reply 28):

Just a nitpick: this isn't really "leaning" the mixture, since you're actually trying to adjust the air/fuel mass ratio to keep it the same as compared to sea level.

Well, we can get into a semantic debate, but the aviation world speaks of it as leaning, which when you consider that it is adjusting the mixture to make it leaner than it was is technically accurate. You may call it what you will, but if you refer to it as "leaning" every English speaking pilot will understand what you mean.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
Analog
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 12:50 am



Quoting SEPilot (Reply 31):
Well, we can get into a semantic debate, but the aviation world speaks of it as leaning, which when you consider that it is adjusting the mixture to make it leaner than it was is technically accurate. You may call it what you will, but if you refer to it as "leaning" every English speaking pilot will understand what you mean.

Well, you are leaning it versus the previous setting (on the knob, lever, misc. adjustment control), so in that sense you are leaning it, but you are not leaning it verus the sea level setting.

Quoting SEPilot (Reply 30):
Current production turbocharged (there are no currently produced supercharged certified engines that I am aware of) use exactly the same mixture control as non-turbocharged ones

Until some aviation guy told me, I never realized that turbocharger was short for turb[ine]-supercharger, but, yeah, everyone knows the difference.

Quoting SEPilot (Reply 30):
They actually require much more careful mixture and throttle control than non-turbocharged engines, and a ham-fisted pilot can completely ruin a $30,000 or more engine in one flight just by careless management.

It must be great to have regulations protecting you from innovations like automatic air/fuel ratio adjustments. Big grin  Sad
 
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 1:16 am



Quoting Analog (Reply 32):
It must be great to have regulations protecting you from innovations like automatic air/fuel ratio adjustments.

Actually, it is not regulations so much as economics. Continental has a full FADEC control system that they have been developing for years; it has been certified for a number of applications but has not sold well at all. There are two problems; first, the cost, and second, the fact that a good knowledgeable pilot can operate the engine more economically. I know this should not be the case, but what I have read indicates that it is. Part of the problem is that Continental's system does not know how to run lean of peak very well, and that is why a human pilot can beat it. This should just be a programming problem, but the cost barrier is still formidable.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
KELPkid
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 1:24 am



Quoting SEPilot (Reply 33):
Continental has a full FADEC control system that they have been developing for years; it has been certified for a number of applications but has not sold well at all.

I thought the Cirrus SR-20 and SR-22 were both using this...
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Analog
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 1:42 am



Quoting SEPilot (Reply 33):
Part of the problem is that Continental's system does not know how to run lean of peak very well, and that is why a human pilot can beat it. This should just be a programming problem, but the cost barrier is still formidable.

Why can't they use automotive engine controls? Take some Bosch ECU (or from another manufacturer willing to license), fiddle with the constants, and slap it on one of their own engines.Obviously it's not that simple, but has this been tried? Automotive ECUs work up to at least 14k ft. Obviously there are issues, like lead fouling the air/fuel or O2 sensors, but they don't seem insurmountable.
 
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SEPilot
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 2:47 am



Quoting Analog (Reply 35):
Obviously there are issues, like lead fouling the air/fuel or O2 sensors, but they don't seem insurmountable.

If it were that simple I'm sure Continental would have done it instead of developing a whole new system. I do not understand enough about the details to be able to say why, but in all that I have read (which has been quite a bit, as it is an area that I have been quite interested in) nobody who knows what they are doing has even suggested doing this. I really don't know why, but I am assuming that there are good reasons.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
Analog
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 2:55 am



Quoting SEPilot (Reply 36):
If it were that simple I'm sure Continental would have done it instead of developing a whole new system. I do not understand enough about the details to be able to say why, but in all that I have read (which has been quite a bit, as it is an area that I have been quite interested in) nobody who knows what they are doing has even suggested doing this. I really don't know why, but I am assuming that there are good reasons.

I certainly don't understand the details, but I would be willing to assume that the main reason is either regulatory or one of liability (i.e. if we change nothing we're legally safer than if a change saves 10 lives but kills one person). There are probably technical obstacles, but unfortunately those are probably less significant than the legal ones.

Yes, the above is BS, but it's probably correct.  Smile
 
JoeCanuck
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 3:54 am

The main reason fadec has such a tough time making into planes is product liability. Until very recently, light airplane engines were gasoline powered, fuelled by a single barrel carb with manual mixture control and sparked by a pair of fixed advance magnetos.

The engines are most commonly 4 or 6 cylinder horizontally opposed, air cooled engine, very similar to the type abandoned decades ago by volkswagon...except not as sophisticated. What they do bring to the table is proven reliability. Certifying a new aircraft part or modification is very expensive.

No manufacturer wants to be the one to bring new technology to the table. If it fails, they could be bankrupted...no matter how big they are.

It's why 1500 pounds of aluminum and plastic, designed 50 years ago will cost over 250 thousand dollars today.
What the...?
 
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SEPilot
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 11:13 am



Quoting Analog (Reply 37):

I certainly don't understand the details, but I would be willing to assume that the main reason is either regulatory or one of liability (i.e. if we change nothing we're legally safer than if a change saves 10 lives but kills one person). There are probably technical obstacles, but unfortunately those are probably less significant than the legal ones.

This may be, but I still point to the fact that Aerosance Controls (owned by Continental) has been working for years developing a totally new FADEC system for piston engine aircraft, and they started from scratch. They could have just as easily started with an automotive system; even if no existing manufacturer would sell the guts to them they could have copied the architecture (I'm sure there are enough of them out there on which the patents have run out). I suggest that there are enough differences in the way aircraft engines are managed that it was better and easier to start from scratch; otherwise I'm sure that they would have tried to adapt an existing system. The real barrier to using existing technology is not so much regulatory as the predatory legal environment that exists in the US today, as JoeCanuck brings up.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
Mir
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 11:33 am



Quoting Analog (Reply 28):
Could the variable geometry intakes seen in some turbojets/turbofans (eg. J58, Olympus) have this effect (akin to throttling)? These devices represent an independent control over the mass of air pushed into the core.

Does this make sense, or am I completely off base?

I believe that the variable geometry intakes are designed to ensure that the engines get air to the compressor at the right pressure and speed (ideally as low a speed as possible, since that will result in the highest pressure). A jet engine cannot work if the airflow through the compressor is supersonic, so if you're going to fly faster than the speed of sound you need some way of slowing the air down to subsonic speeds so that the engine can use it, and that's what those fancy intakes do. It doesn't have anything to do with metering the fuel flow.

-Mir
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CosmicCruiser
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 1:25 pm



Quoting Analog (Reply 18):
Is that really leaning out the mixture, or does that have the effect of maintaining the A/F ratio by compensating for the lower pressure?



Quoting SEPilot (Reply 31):
Well, we can get into a semantic debate, but the aviation world speaks of it as leaning, which when you consider that it is adjusting the mixture to make it leaner than it was is technically accurate. You may call it what you will, but if you refer to it as "leaning" every English speaking pilot will understand what you mean.

Thanks for beating me to it SEPilot.....Heck I used to "LEAN" the old R-985 by looking out at the exhaust flame at night.
 
N231YE
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 2:59 pm



Quoting SEPilot (Reply 39):



Quoting JoeCanuck (Reply 38):

I used to wonder about FADEC in general aviation, so much so that I asked in this thread: Why Is Fadec Progressing So Slowly In GA? (by N231YE Oct 5 2007 in Tech Ops) . Like many of the responses in the thread stated, the technology is too new and expensive, as well as what you guys stated above about legal problems.

I typically lean using the 50ºF Rich of Peak EGT. On aircraft without EGT gauges, I lean and watch the RPM (can also listen to the engine as well) for peak power, then enrichen the mixture a little, unless otherwise stated in the POH.

In some Piper PA28s, there is a "divet" in the mixture control, and I will usually pull the mixture lever back until it reaches this "divet."

Interestingly enough, there is a theory about Otto cycle engines that states that most of the engine's power is used to fight the throttle. Some some automobiles computer's take this into account, I have sometimes thought about opening the throttle full and controlling the engine via the mixture control. However, in the Cessna POH in big, bold lettering, states, Continuous operation at mixture settings lean of peak EGT is prohibited. My guess is this may damage spark plugs, etc, thus why I don't do it.
 
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SEPilot
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 3:53 pm



Quoting N231YE (Reply 42):
I have sometimes thought about opening the throttle full and controlling the engine via the mixture control.

Operating lean of peak you actually do this. The problem, as I stated earlier, is that most engines not specifically set up to allow it run poorly lean of peak because of uneven mixture distribution.

Quoting N231YE (Reply 42):
However, in the Cessna POH in big, bold lettering, states, Continuous operation at mixture settings lean of peak EGT is prohibited.

In the condition the planes came from the factory this is warranted. The O-200's, O-235's, O-320's, and O-470's which account for the vast majority of Cessna's are all totally unsuitable for lean of peak operation due to being carbureted and having poor flow balance (especially the O-470, which has the most spectacularly bad intake system of any known engine) and thus justify the warning. New 172's, 182's, plus all 206's and 210's can be modified to run lean of peak quite well, but as they came from the factory they probably can't.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
N231YE
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 6:48 pm



Quoting SEPilot (Reply 43):
New 172's, 182's, plus all 206's and 210's can be modified to run lean of peak quite well, but as they came from the factory they probably can't.

I based this off of a fairly new 172, with a fuel-injected IO-360.
 
JoeCanuck
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 7:12 pm

As I understand it, lean of peak works best on engines with constant speed props.

Some of the efficiency of a diesel comes from the fact that it is, essentially, wide open throttle all the time. A constant speed prop allows WOT operation and controlling the speed with prop rpm.

Legend has it that the procedure comes from Charles Lindbergh, who taught it to Doolittle in the Pacific. The extreme ranges of the pacific theatre required maximum fuel efficiency, which Lean of Peak operations allow...at least that's how I understand it.
What the...?
 
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SEPilot
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 8:01 pm



Quoting N231YE (Reply 44):
based this off of a fairly new 172, with a fuel-injected IO-360.

If you re-read my post I said that AS IT COMES FROM THE FACTORY it cannot run lean of peak effectively. Being fuel injected, though, I strongly suspect that GamiJectors can be had for it which would enable it to run lean of peak. But bear in mind that there are a large number of people out there that fear that lean of peak operations run the risk of burning valves (and some of them might be writing Cessna manuals); this is absolutely untrue. The problem is that variations in mixture once the leanest cylinder is about 50 deg. rich of peak don't affect the power output very much, while when you get around peak and lean of peak small variations in mixture affect power output quite significantly. This is why engines without calibrated injectors run very rough when attempting to run LOP. The other problem is that, as I mentioned before, the area where cylinder damage is most likely is right at peak to a few degrees rich of peak. If your injectors are not calibrated, you run the danger of having one cylinder in this range while the rest of them are lean of peak. So how did the big radials run lean of peak, since they were carbureted? It's just a matter of geometry; the carburetor was located so that the intake runners were equal length and so did a very good job of providing uniform mixtures to each cylinder.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
meister808
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 8:10 pm



Quoting SEPilot (Reply 43):
The O-200's, O-235's, O-320's, and O-470's which account for the vast majority of Cessna's are all totally unsuitable for lean of peak operation due to being carbureted and having poor flow balance

I always think this is funny, because the guys putting the same O-320s in airplanes down at Vero Beach (Piper) wholesomely support LOP operation in the Lycoming O-320 equipped Warrior. The procedure, as clearly stated in the POH:

"For Best Economy cruise, a simplified leaning procedure which consistiently allows accurate achievement of best engine efficiency has been developed. Best Economy Cruise performance is obtained with the throttle fully open. To obtain a desired cruise power setting, set the throttle and mixtrure control full forward, taking care not to exceed the engine speed limitation, then begin leaning the mixture. The RPM will begin to increase slightly but will then begin to decrease. Continue leaning until the desired cruise engine RPM is reached."

Same or very similar engine, different aircraft manufacturer, very different procedures. I didn't used to believe that a Lycoming O-320 would run LOP until I read that Piper POH, from which point I've used that procedure regularly. The engine runs great, and burns very little gas.

-Meister
Twin Cessna 812 Victor, Minneapolis Center, we observe your operation in the immediate vicinity of extreme precipitation
 
meister808
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 8:19 pm



Quoting KELPkid (Reply 34):
Quoting SEPilot (Reply 33):
Continental has a full FADEC control system that they have been developing for years; it has been certified for a number of applications but has not sold well at all.

I thought the Cirrus SR-20 and SR-22 were both using this...

Negative... the only control system Cirrus uses incorporates a mechanical linkage between the throttle and propeller controls, incorporating both into a 'Power Lever'. The idea is that the prop control and throttle are generally happiest and most efficient when moved in the same direction (full throttle, high RPM; decreased throttle, decreased RPM) so the Cirrus engineers came up with prop/power combinations that worked well together and tied them together via one handle.

I believe Mooney has been working with FADEC Continentals?

-Meister
Twin Cessna 812 Victor, Minneapolis Center, we observe your operation in the immediate vicinity of extreme precipitation
 
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SEPilot
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RE: Mixture Controls On Propeller-driven Aircraft

Tue Feb 26, 2008 8:42 pm



Quoting Meister808 (Reply 47):
"For Best Economy cruise, a simplified leaning procedure which consistiently allows accurate achievement of best engine efficiency has been developed. Best Economy Cruise performance is obtained with the throttle fully open. To obtain a desired cruise power setting, set the throttle and mixtrure control full forward, taking care not to exceed the engine speed limitation, then begin leaning the mixture. The RPM will begin to increase slightly but will then begin to decrease. Continue leaning until the desired cruise engine RPM is reached."

Not having had access to a late model Piper I did not know this. Thanks for the info. In fact, I did get the opportunity to fly a 1972 Piper Arrow for quite a bit a number of years ago, and I discovered that it did run quite well LOP.

Quoting Meister808 (Reply 48):
I believe Mooney has been working with FADEC Continentals?

Not to my knowledge, and I have been following Mooney information closely, as I hope to get one.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler

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