‘Waggling my little red knob...’

[NT5224]

That’s what the bloke said. ‘You should waggle your little red knob better’.

 

He was in his eighties with 60 years of flying everything from fast jets to commercial multi engine in his logs. When he spoke I listened.

 

The question was around achieving peak fuel efficiency in cruise, and by implication he was suggesting I might not be optimising my mixture settings.

 

So I’m interested to know how much a well leaned mixture should save on fuel flow?

 

So ladies and gents. Let’s assume a cruise at about 30 litres per hour, 5000 feet. If mixture is leaned correctly, what reduction in fuel flow might be achieved? Just rough approximation will suffice. 1 litre? 3 litres? Or even more?

 

One more thing. I was always taught that leaning was only necessary over about 5000 ft where the air density becomes significantly lower. This made sense to me because when you lean you are maintaining the optimal fuel/ air ratio, and that ratio begins to change the higher you get. But my experienced friend said that you could adjust mixture at any altitude.

 

Thoughts?

 

Alan

[pmccarthy]

Adjust at any altitude. My experience with the O360 in my Archer is that it gets 37-38 l/h if leaned badly and 32-33 l/h if leaned correctly. All for the same airspeed. At 60% power you can lean until rough and then just ease it forward until smooth. At 70% power you have to look at the chart and calculate, or you might run it too lean. For a long time I was running too rich, it used to soot the plugs. You should lean on the ground too.

[facthunter]

Be under about 75% power is the general rule, before you lean. Some engines do run excessively rich on the ground. IF you lean when taxiing, makes sure you put it back to full rich before taking off, or you risk engine damage or failure. TO power does enrich engines and you are not encouraged to take of at reduced power to save the engine as you do with a jet.. for that reason. One way of ensuring your engine is not left lean is to lean it so much it falters if you open the taps IF you forget .There's a considerable risk in changing your usual technique and doing such things. if you rely on not forgetting , one day you will. Nev

[Thruster88]

That’s what the bloke said. ‘You should waggle your little red knob better’.

 

He was in his eighties with 60 years of flying everything from fast jets to commercial multi engine in his logs. When he spoke I listened.

 

The question was around achieving peak fuel efficiency in cruise, and by implication he was suggesting I might not be optimising my mixture settings.

 

So I’m interested to know how much a well leaned mixture should save on fuel flow?

 

So ladies and gents. Let’s assume a cruise at about 30 litres per hour, 5000 feet. If mixture is leaned correctly, what reduction in fuel flow might be achieved? Just rough approximation will suffice. 1 litre? 3 litres? Or even more?

 

One more thing. I was always taught that leaning was only necessary over about 5000 ft where the air density becomes significantly lower. This made sense to me because when you lean you are maintaining the optimal fuel/ air ratio, and that ratio begins to change the higher you get. But my experienced friend said that you could adjust mixture at any altitude.

 

Thoughts?

 

Alan

Alan, we will guess that you have a 160hp 0-320 lyc. 30lph agrees with the Musketeer's performance chart for 65% power. It is 160hp and achieves 2350 static rpms with a 74x60 prop. This chart will give you a rough guide to power settings, you may be able to get a more suitable chart from another aircraft(propeller and cruise speed). As others have said, lean any time with 75% power or less. At 65% power you can not hurt this engine with a lean mixture. Lycoming have good operator manuals describing leaning, carb heat etc.

[kgwilson]

In the C172 & PA28-181 I used to run full rich till at cruising altitude (depending on the day & hemispherical, anywhere from 4000 to 10,000) & then lean till the engine began to lose power. I'd then wind it in till it ran smooth & regained rpm again & leave it there till I was descending to land. I never did any checks but always used less than my planned burn which was based on the manual.

[turboplanner]

In the C172 & PA28-181 I used to run full rich till at cruising altitude (depending on the day & hemispherical, anywhere from 4000 to 10,000) & then lean till the engine began to lose power. I'd then wind it in till it ran smooth & regained rpm again & leave it there till I was descending to land. I never did any checks but always used less than my planned burn which was based on the manual.

This is around the bottom threshold for GA and the first experience of leaning the mixture for pilots; While there's nothing to prevent mixture control being added to a Recreational Aircraft, it's really getting into GA territory, and what kgw has posted is good for understanding the principle of optimising mixture for altitude by leaning the mixture.

 

With it comes responsibilities. You need checks to remember to go to full rich before you descend from altitude and cause problems to the engine at lower altitudes. The two aircraft mentions are good practice for getting used to high altitude engine management.

 

The next step up usually comes with a Constant Speed Prop, and then you need to pay very close attention to the settings required by the manufacturer because they are not all the same.

[facthunter]

Set revs and let speed stabilise .Go to just becoming rough then get it smooth and then a" tiny "bit more rich. Your flight times V/s fuel used will confirm your compliance with the figures in your POH. The idea of running rough as I've seen suggested sometimes, is crazy. A rough running motor is not running as it was designed to do. Nev

[Steve L]

In the Bulldog, IO360 and CSU I kept full rich until passing 5000 which didn’t take long, then leaned it out till temps climbed in the upper green arc. Leaning any further till lean of peak only saved me about 3 litres out of 40 so I stayed rich. I figured it’s not worth risking an engine to save 3 litres. The Bulldog only had a temp gauge on one cylinder and you would expect an injected engine would have fairly constant temps, had gauges been fitted on each cylinder I would have more game

[Yenn]

To do the job properly you need an EGT gauge. Watch your EGT at take off, full rich and full throttle. What you see at about 50' is what you want to see all the way. You can keep adjusting your mixture to achieve that target, but don't lean it further without closing the throttle, or you risk detonation.

Of course if you are taking off from 5000' you are going to be too rich right from the start and it would be possible to run at max throttle for a short time ad lean till you get max rpm.

I definitely suggest leaning when taxying, but lean it back until the motor won't respond to throttle, just to prevent you taking off too lean. I found that ssaved me from getting fouled plugs in my O-320.