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Author Topic: leaning  (Read 4897 times)
N737R
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« on: March 14, 2013, 06:29:55 PM »

I've read and have been told to always run rich of peak and never go lean of peak are there dangers of going lean of peak?
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sabre76
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« Reply #1 on: April 15, 2013, 08:54:46 PM »

Danger?  Probably not until you lean it to the point it quits.  Lower life on engine components?  Possibly, but that's a raging debate.  I suggest you follow the guidance from the engine manufacturer.
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OscarTango
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« Reply #2 on: March 18, 2014, 08:14:33 PM »

you might overheat the cylinder heads when going lean of peak.... fuel contributes to he cooling of the engine. If you go lean of peak, and forget to adjust in the descend-phase, you'll also get a rough running engine sooner.
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RNC
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« Reply #3 on: April 27, 2014, 04:45:47 AM »

Sorry but neither of those are true...depending on the engine.

http://www.avweb.com/news/pelican/182146-1.html?redirected=1

See the series of articles about "where should I be running my engine?"

1) Leaner is cleaner, cooler, and better for engine longevity and particularly plug longevity.  Leaded gas is 'dirty', for lack of a better word, and burning it incompletely will leave deposits of that lead in places you don't want them.  Piston GA engines should be aggressively leaned whenever they're not at full throttle, particularly when taxiing.

2) There is no reason to enrich your mixture during a descent.  If you have a constant speed prop, it won't let you over-rev the engine to the point that the mixture is too lean.  If you have a fixed prop, you'll be pulling back the throttle to keep the RPM down.  In either case it's impossible to get "too lean" during a descent.  You can leave the mixture at cruise settings until you level off at low altitude after the descent.

The exception to these is with a turbo.  The catch with the turbo is, it has a turbo intake redline temperature that's independent of the cylinder redline temperature.  It's very possible that the turbo will redline before you get the exhaust temperature to the lean side of peak.  In this case you either need enough fuel flow/engine temperature instrumentation to know where the proper lean of peak setting is and jerk the mixture to it, or just not do it.  You can't 'sneak up' to lean of peak operation with a turbo and in the process redline the turbo itself.  The flip side of an engine with a turbo is it's not as susceptible to the problems of overly rich mixture settings as a non-turbo engine, because the turbo is making more consistent intake air density for you, so it has more leeway for a range of proper operation in that regard.

This is all dependent on balanced fuel flow.  Some engines do this well, some do not.  It depends on the individual engine.  I've flown new'ish fuel injected airplanes that just don't want to do it.  I've also owned a 1979 carbureted Lycoming that would fly all day 100 degrees lean of peak like a champ.

Either way, short of the turbo caveat above, you're not going to destroy a normally aspirated engine from trying.  Worst case scenario is it just won't do it before it starts running rough.  Best case scenario it will.  Try it and see.  If it does happily run lean of peak you should run it that way, the benefits are well documented.
« Last Edit: April 27, 2014, 05:01:56 AM by RNC » Logged
InterpreDemon
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« Reply #4 on: April 27, 2014, 05:24:20 PM »

"There is no reason to enrich your mixture during a descent.  If you have a constant speed prop, it won't let you over-rev the engine to the point that the mixture is too lean."

I think that needs to be clarified a bit because with normally aspirated CSP, injected engines it may depend upon whether you are sitting behind a Lycoming or Continental. The Lycoming (RSA system) uses a Bendix fuel servo that provides a feedback loop based upon actual MAP, whereas the Continental system basically makes assumptions about throttle position and fuel flow rate based upon a mechanical linkage between the throttle and the injection pump. With the latter, if you make a powered descent from 9k to 3k at a constant RPM starting at cruise settings, especially if lean of peak, and do not touch your throttle or mixture settings during the descent, you will find yourself running increasingly over-square and lean as you come down, however if you adjust the throttle to maintain cruise MP or lower during the descent the engine won't know or care at what altitude it is. The issue is not that the CSP prevents the engine from over-revving, but rather that it causes the engine to pump a constant volume of air for a given manifold pressure. At the same time the continuous flow injection pump will deliver a constant volume of fuel at a given RPM and mixture setting without regard for actual MAP, meaning that as you descend without throttle adjustment the MAP will increase, the engine will pump more air for the same amount of fuel and the mixture will lean at the same time cylinder pressure is increasing, which is not desirable.

Anyway, the "lean vs rich" debate has been around as long as the single vs twin debate. For some, the rule may be, "If you own the plane, run rich, if you're just paying for the gas run lean", however systems vary in ways that can be significant, which is why one should always consult and follow the recommendations and procedures in the POH for the aircraft they are flying no matter how much more one believes he knows about the plane than those who built it.
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RNC
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« Reply #5 on: April 27, 2014, 11:56:42 PM »


I think that needs to be clarified a bit because with normally aspirated CSP, injected engines it may depend upon whether you are sitting behind a Lycoming or Continental. The Lycoming (RSA system) uses a Bendix fuel servo that provides a feedback loop based upon actual MAP, whereas the Continental system basically makes assumptions about throttle position and fuel flow rate based upon a mechanical linkage between the throttle and the injection pump. With the latter, if you make a powered descent from 9k to 3k at a constant RPM starting at cruise settings, especially if lean of peak, and do not touch your throttle or mixture settings during the descent, you will find yourself running increasingly over-square and lean as you come down, however if you adjust the throttle to maintain cruise MP or lower during the descent the engine won't know or care at what altitude it is. The issue is not that the CSP prevents the engine from over-revving, but rather that it causes the engine to pump a constant volume of air for a given manifold pressure. At the same time the continuous flow injection pump will deliver a constant volume of fuel at a given RPM and mixture setting without regard for actual MAP, meaning that as you descend without throttle adjustment the MAP will increase, the engine will pump more air for the same amount of fuel and the mixture will lean at the same time cylinder pressure is increasing, which is not desirable.

Good point, a better way to word it would be "maintaining cruise settings during the descent, there's no reason to enrich".  And yes you're right, in the Lycoming design, which I've always owned (Pipers and Commanders), you do not in fact have to do anything, other than trim down and avoid VNE.
« Last Edit: April 28, 2014, 12:00:46 AM by RNC » Logged
InterpreDemon
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« Reply #6 on: April 28, 2014, 12:07:57 PM »

And that goes for carbureted engines as well since a carburetor delivers fuel according to air flow, but there are not many out there with a constant speed prop. Folks have to keep in mind that these engines are "loafers" with big displacement and only about 1/2hp per CI at rated power, so they can take quite a bit of abuse for short periods at lower altitudes and at higher altitudes (without a turbo) it's almost impossible to damage them via mixture settings since they will only produce half their rated power anyway even running at peak EGT. At cruise I usually just lean to "feel" peak power, note the EGT and enrich fifty or so, but that's just because I believe that gas is cheaper than metal.
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