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Leaning normal aspirated engines

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Leaning normal aspirated engines

Old 28th Feb 2003, 14:35
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Leaning normal aspirated engines

Hi everybody,

I've been reading about leaning and engine management and found myself quite confused about what to do and what not to do.

For example, the normal leaning method, in the absence of good and accurate engine data (EGT ,CHT for every cylinder) is to lean until EGT reaches a peak and a little further. But I also read that operating near the max EGT region is very damaging for the engine. And then there's roughness and RPM clues about what's happening.

So, in the normal C1X2/PA28 environment, normally aspirated, fixed pitch propeller and with little or no engine information what's the best procedure to lean the engine without blowing it up (figuratively speaking... I hope)? and also at what density altitude can I expect to get enough range improvement to justify the increase in work load?


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Old 28th Feb 2003, 15:37
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Hi Mak
First Both Lycoming and Continental state you can lean their engines until they stop AS LONG AS THEY ARE ON 70% POWER OR LESS.
Given enough time I could find the service letter numbers,back in the late 70,s or early 80's.
The airframe manufacturers try to make it idiot proof by stating that on normally asperated engines you should not lean below 5000 feet - at this altitude the engine is only producing about 70% power !!!
Second:- as you pull out the mixture control the engine eventually runs rough, this is because the engine has a carb. the fuel distribution between cylinders is not even ,so one cylinder is generally running slightly leaner than the rest. So as you lean ,this cylinder stops working first - thus rough!!
The best you can do is then ease the mixture back in until the roughness has abated.
If you try leaning up fuel injected engines there is no roughness,
they just stop producing power because the fuel distribution is usually very good.
As an addon , the most economic is at full thottle altitude say 6000 feet (60% output) and lean .

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Old 28th Feb 2003, 16:35
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you could always go to www.avweb.com and read John Deakin's column Pelican's Perch. He'll give you more information than you could believe.

Remember to come back to prune though
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Old 28th Feb 2003, 21:58
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After leaning to rough and enriching back a bit until smooth, keep an eye on oil temperature.

While many SE a/c don't have a CHT, they have to have oil temp.

If too lean the engine will run hot; in fact, at full power excess fuel is used for cooling.
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Old 28th Feb 2003, 23:37
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Right, pulling this out of the archives(from before I converted to the Arrow and spent money even quicker!!).......

Lean the engine by going to max egt then back by one division or use the above rough running procedure to acheive what you perceive to be the correct mixture setting. Now apply Carb Heat. This shold ensure that you have not gone too lean (which leads eventually to PA28 Gliders) because:-

Too lean a mixture will give NO RPM drop

The correct mixture or richer WILL give a RPM drop

So this can be used even without a CHT gauge. The trick is to set the RPM accurately so that the beginning of the reduction of the RPM drop with Carb Heat is noticable. It is VERY important to remember to go FULLY rich before any change in power setting or climb/decent is made and that FREDA chacks are made properly with special attention given to oil pressure as well as temperature.

However I do not remember the reason why this carb test works(first channel crossing to Alderney with the Club CFI right seat). If anyone thinks they know I would love to hear.
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Old 1st Mar 2003, 15:24
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The carby heat test is based on the principle that max. RPM is produced at best power mixture. If you've leaned to achieve that then any change to the mixture (richer or leaner) will reduce the RPM from this 'best' setting.

Carby heat increases air temp --> less dense air but still getting the same amount of fuel added --> richer mixture.

If you have over-leaned the mixture ie running 'lean of peak' then applying carby heat will most likely cause an increase in RPM. Presuming no carby ice, of course...

There are a several acceptable ways of leaning the mixture, some are the airframe manufacturer's stated method, other's the engine manufacturer, and other's based on very accurate metering & measuring devices.

Generally the equipment in the a/c as supplied by the manufacturer is not adequate to use the accurate metering/measuring device method to operate lean of peak.

That leave engine/airframe recommendations which are broadly similar: The conventional wisdom is to lean until max EGT or rough running then enrichen by a certain amount and enough to restore smooth running.

In all these manufacturer's cases there will be a specified percentage of max power below which leaning is allowed. Above that power setting full rich must be used to provide adequate cooling using excess fuel. At high density altitudes the mixture must be adjusted for take-off to correct any massively over-rich rough running & restore a little bit of the lost horsepower.

Generally cruise power may be leaned at altitude.

Without an EGT:

Lean until roughness is detected. Enrichen until smooth. Sometimes this also includes advice to enrichen further by some amount. Check your a/c's manual.

With an EGT:

Lean until Peak EGT is found. Enrichen until EGT reduces by a certain amount. This varies by a/c type but is typically 25 deg F to 100 deg F.

Make sure you check your a/c's manual for the correct temp. drop.

Some regulators have required amendments to this for various reasons eg Oz & PA31s.

Using appropriate very accurate metering & measurement equipment it becomes justifiable to operate lean of peak for many power settings. Chimbu Chuckles has a thread running somewhere on PPRuNe where he's now able to do this after having the appropriate injectors & monitoring equipment installed in his A36.

Last edited by Tinstaafl; 2nd Mar 2003 at 15:22.
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Old 2nd Mar 2003, 02:46
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Leaning on Takeoff

Doing a checkout in a Cheetah on a nice Summer day at Springbank airport West of Calgary, approx 3,500' I was instructed to run the engine up to full power and then lean to max revs.

The touch and go at full rich had the gophers diving back into their holes
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Old 2nd Mar 2003, 02:59
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Operating out of Kariba (Zimbabwe) in summer required a super lean mixture - calculate the DA for 2000ft AMSL and 45*C OAT! Another little gotcha was that the fuel was a sea level blend i.e. high vapour pressure.

The BAe 146 wasn't too happy either.
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Old 4th Mar 2003, 10:03
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Cool thanks

Thanks all for the input.

I did come across John Deakin's column in avweb and that's what got me confused because he focusses on operation lean of peak EGT and that didn't match what I found elsewhere. Frankly it made me afraid of leaning anything in case I found myself operating within the "red box" around peak EGT.

But that's all clear now (i.e. a case of RTFM or more to the point, RTFPOH)

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Old 5th Mar 2003, 04:10
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flyer key re-prints are a very good source of info, try this link

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Old 5th Mar 2003, 04:25
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I say RTFPOH and Engine Manufacturer's Recommendations as well.

But a rule of thumb is: Best Range - 50 degrees rich of peak
Best Endurance -25 lean of peak
I know it sounds a bit dodgy and backyardish but never had any complaints from the engineers using this for a coupla years flying Islanders and Shrikes for Coastwatch.

Safe flying, hoss
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