[hecilopter]

I keep reading these threads about how Lycoming says to "not lean above 75%" power etc, etc. I simply don't understand this and need someone to clarify what this means.

My example will be taking off, at sea level, full throttle, full rich, full rpm, 100% power. I know nothing about what the air : fuel ratio would be but for argument's sake let's say it is 1 : 1. Upon takeoff, I make note of my EGT of 1250. As I climb, the mixture gets richer and richer because the air is thinner and thinner. I watch the EGT drop thru 1200, 1150, 1100. I have not touched anything the entire climb to 8500 feet. I am now so rich that my mixture is say .5 : 1 vs the 1 : 1 it was at sea level. I am also probably making nowhere near 75% power because I am so rich.

However, it is now "safe" to start leaning? Lean to what? Back to the air : fuel ratio of 1 : 1 which was OK on the ground? I now pull the mixture 2/3 of the way back, the engine smooths out, the EGT climbs to 1300, everything is OK now?

Why could I not lean slowly as I climbed, maintaining the 1250 EGT and air : fuel ratio the whole way?

Why does the POH for many planes say that at high density altitude it is OK to lean to smoothness/RPM for takeoff? After takeoff it says nothing about returning the mixture to full rich until below 75% power, then lean again?

Very confused....

 

[penguin]

Hey Rusty,

John Deakin on AvWeb recommends that you start leaning as early as you like to maintain the take-off EGT. This seems really sensible to me. You are clearly a long way from peak EGT, you will not degrade that detonation margin by maintaining that EGT. As you pointed out, the mixture is richening up as soon as you start climbing. I can't see why gentle leaning is not a good idea.

I think Lycoming's advice is an attempt to stop people leaning heavily when at a power setting that could possibly induce detonation. If you lean gently then I don't see there is any danger.

Peter & Casper

 

[gmcjetpilot]

Apples & Oranges, Lycomings & Continentals

Why could I not lean slowly as I climbed, maintaining the 1250 EGT and air : fuel ratio the whole way? Very confused....

As penguin wrote: "Hey Rusty, John Deakin on AvWeb recommends that you start leaning as early as you like to maintain the take-off EGT."

First, Lyc FI & Carbs are somewhat self leaning in a climb. More about that below but first the problem with Continentals and mixture during climb.

Deakin mostly writes about 350HP 6-cyl Continentals, most of which have mechanical FI, which uses RPM & throttle position only to control mixture (and of course mixture control). Most Continentals have used this simple RPM controlled FI system till recently. To compensate for this drawback, some Continentals added an altitude compensating fuel pump. As you climb to lower air density, the fuel pump lowered fuel pressure to "trim" the fuel with out pilot input. Bottom line old Continentals YOU almost NEED the pilot to lean in climb. Also those big twins and singles climb slowly compared to a RV which take little time to get to altitude. With the fuel flow difference between one/two BIG 6 bangers and one little 4-banger, fuel saving by leaning during climb is not great for the Lyc. Also the Lyc has the big advantage of a MASS AIRFLOW controlled fuel delivery system.

Your assumption that you are getting richer and richer is true, but you may be over stating how rich. Lycs have MASS AIR FLOW FI. As you climb it senses less air and therefore supplies less fuel. Even the Carbs are mass airflow devices and self leaning as you climb, by design. The less air through the carb (due to air density) the less fuel, automatically. Further some Carb models have an economizer circuit which kicks in when not at WOT.

Typical SOP for a Lyc/CS prop: takeoff full power, goto 25 square about 1000' agl (Carb into econ mode), MAP drops an inch per thousand feet, increase throttle to maintain 25" square as you climb, level off and set cruise power & mixture. DONE and DONE. Leave the mixture alone until you are above 8,000'. A little extra gas in climb helps temps. The extra gas we are talking about is so small for a 8 minute climb to cruise.

Even Lyc allows you to lean for engine smoothness and for high altitude takeoffs, typically at airports of 5,000 feet density altitude or more. So Lyc does understand to get PEAK POWER you may have to lean a little above 75% power.

I would not personally lean at 2,700 RPM and 27" at say 2,000 feet, just after take off. That is pushing it in my opinion. The "target EGT" deal I recall does not hunt for peak EGT, which would be a bad thing to do. It's just leaning with reference to the peak EGT at takeoff. However do you WANT peak takeoff EGT in climb. Don't know, you decide. I like the ideas of letting the EGT's cool a little. However again with 8-channes of EGT/CHT we can watch our engines like we never have in the past. When I learned to fly one of the rental planes had a single cylinder EGT and a LORAN! WOW!

My self going through 5,000 or 6,000 feet I'll take the mixture off the full rich stop to note a slight EGT change. However I don't try to optimize it or watch and trim the mixture all the way from takeoff to level-off. One thing is I'm too lazy, and second I've got better things to do, like watch for traffic.

As long as you are still closer to 100% than 75%, I would NOT touch the mixture myself. Don't look for peak EGT while at high power, even just momentarilly to find peak. If you do the "target EGT thing read those Avweb articles. They are pretty good. Just keep in mind they are mostly referenced for Bonanzas, Barons and C421's, not little Lycs in RV's.

You can cause damage by leaning at higher power than 75%. Pays your monies, takes yo chances. The extra work load and saving is small.

Lyc makes their limits for worse case scenario and just a TACH. We have engine monitors and know our engines and watch them more than the average pilot. Still be aware anytime you lean above 75% you are running at lower detonation margins. You are using some of the margin built into the limit. Detonation can cause very real, expensive, subtle or sometimes catastrophic damage. It's your engine.


PS: ECI's sells a mechanical RPM based FI for the Lycs. It's a throw back (actually a copy) of the old style RPM controlled FI on Continentals. The idea of using RPM only does simplify the FI. It works the same as other FI when you lean, but it's DUMB and does need more pilot input, such as on a long climb to higher altitudes. The Precision, Bendix and AFP FI for Lycs are better in that they are all mass airflow controlled.

 

[hecilopter]

Further explanation on engine damage

George,

I hear what you are saying, but I typically leave the power to the firewall and pull the RPM back to about 25" for climb. With 28" of MAP, this results in about 155 HP or 86% power. Not far above 75%. If you pull back to 25/2500 at takeoff, you are getting about 132 HP or about 73% HP. You are already below 75%. Reference the Superior power chart below for confirmation of these numbers. To get 100% power you need 29" of MAP and 2700 RPM. I'm still not seeing how it will hurt the engine if the CHT's are OK. It seems like it would save fuel and keep the plugs cleaner and provide more power at the same throttle setting by leaning just a bit (around takeoff EGT), but not near peak.

http://www.superiorairparts.com/PDF/FlliersDocuments/XP-360Engine/XP360_SpecSht.pdf

 

[L.Adamson]

FWIW, at our airport altitude of 4600' msl before density altitude kicks in; we always lean immediately after engine start, and never go full rich again...

Unless --------we descend to lesser altitudes.

Of course, flat lander ocean dwelling dudes, never seem to take these principals to heart, and we always have to argue with them.

If we don't lean, we loose significant takeoff or go-around power. That's all there is to it!

L.Adamson