Lean for rpm increase while taking off

[Salty]

No, but we know more about it.

Well, I do and maybe you do, but I bet the guys that wrote that don’t.

 

[farmrjohn]

It's a lot more complex than that. I have run up engines before inspections first thing in the morning, then found water inside the rocker covers when they were removed for various reasons. In our colder climate we normally find the oil on the dipstick looking like foamy coffee with cream after a shorter flight, and that's the oil/water emulsion that results from crankcase moisture accumulating and mixing with the oil on short flights.

The rest of it is chemistry. Water provides oxygen and hydrogen. Oil supplies sulfur, chlorine and nitrogen, as do the other blowby gases. The metals in the engine--aluminum, iron, copper, zinc and tin--act as catalysts. Leave them sitting together and over time we get sulfuric, hydrochloric and nitric acids, and none of those are friendly to engines. Hydrobromic acid can be formed when engines are run on leaded fuels. These reactions also form sludges. These are some of the major reasons we change oil periodically. Just coating the engine parts with oil will do little good if there's water in there, too. Water mixes with the oil during operation, and when the engine is shut down it's left in places like the lifter bodies, oil galleries in the case and crank, in the accessory case and between the pistons and cylinder walls. I have pulled cylinders off after a short runup and found water droplets in there.

You have experience with one or two airplanes. Licensed mechanics get many years of experience with hundreds of airplanes representing a wide range of aircraft and engines, owners and their various habits and levels of care, the conditions they're operated and stored in and other factors, and we learn what to look for. There are preservative storage oils like Aeroshell Fluid 2F that are put into engines and run for a while to get them all through the engine, and then the oil is also sprayed into the cylinders to coat the upper end as well. Preservative oils neutralize the acids.

Arizona is not immune to corrosion. Most of the water comes from the combustion process, not the environment.

Will the accumulated water show up in an oil analysis after an oil change?

 

[Dan Thomas]

Will the accumulated water show up in an oil analysis after an oil change?

I never saw water mentioned on any oil analysis. They burn the oil sample and take a spectroscopic picture of the flame. The various contaminants in the oil like aluminum, zinc, iron, copper, silicon and so on, burn with a different color, and the picture can give an idea as to how much of each is in it. Water doesn't burn, so...

If the drained oil has a milky appearance, there's plenty of water in it.

 

[flyingron]

Even if they tested for it, you wouldn't necessarily see it. Water doesn't dissolve in oil (or vice versa). As Dan says, the milkiness is the stuff being shooken up into suspension. Given a bit of rest, most of the water will settle out. If you've done your own oil change you've probably noticed some water coming out in advance of the oil. Best suggestion is to just run the engine regularly.

 

[timwinters]

I never saw water mentioned on any oil analysis.

Blackstone does, water is listed at a percentage of the total volume. But, since the oil sample is supposed to be taken from the middle third of what's drained...maybe the water has already drained out by the time the sample is taken.

OTOH...I always run the engine right before draining so it should still be amply mixed? Maybe??

 

[David Megginson]

No, but we know more about it.

The sad thing is, no. Engineers knew all about LOP as far back as the Second World War, and the early version of the power/EGT/CHT chart that LOP advocates use dates back to engine manuals from the 1940s. They just figured private pilots were too stupid to understand that stuff, especially since they were trying to market airplanes as a "family car in the sky" during the post-war years. Plus, many carbureted engines had fuel/air distribution that was too uneven to run LOP smoothly, and fuel injectors weren't as common (or even well-calibrated where they did exist).

You don't even need fancy engine monitors to fly LOP safely, though it's a bonus. It's especially easy with a fixed-pitch prop:

1. Note your desired RPM for your power setting and density altitude (just like you would ROP).
2. Leave the throttle wide open.
3. Pull back the mixture until you have your target RPM.

That's all there is to it. You're now running at your engine's leanest-possible mixture for your current power setting, and you don't need anything to confirm it, as long as your tach is properly calibrated. Some carbureted engines (especially 6-cylinder ones) will run rough LOP---nothing to do there but fly ROP---and there are extra steps for a constant-speed prop, which is trickier (and the engine monitor might pay off there).

 

[David Megginson]

The MA-3SPA in that installation doesn't have any enrichment circuit. It's a pretty crude affair that doesn't carburate as well as my old (1946) Stromberg carb did.

But then, funny things can happen. We had a 172 that I'd installed a new engine in, complete with new carb (MA-4), and that engine ran a little rough on the climbout, and leaning didn't help. Pulling the throttle back a hair got a few extra RPM and the engine smoothed out. I couldn't find anything wrong with the carb or linkages, so I figured that the fuel nozzle in that unit was less than perfect and didn't atomize the fuel properly. Closing the throttle plate a little made the fuel stream strike it and break it up better. Precision had a series of ADs in the '90s regarding the two-piece venturi. The single-piece that replaced it caused all sorts of troubles so they issued another AD requiring a cross-drilled nozzle which caused a different set of troubles in a few cases, so they issued yet another AD allowing the reinstallation of the old parts as long as the venturi was inspected for looseness every 100 hours.

Some people find that cocking the throttle very slightly, to create more-turbulent airflow, helps with roughness in a carbureted engine. But obviously, it's better to hunt down the underlying issue, as you narrated.

 

[Dan Thomas]

Even if they tested for it, you wouldn't necessarily see it. Water doesn't dissolve in oil (or vice versa). As Dan says, the milkiness is the stuff being shooken up into suspension. Given a bit of rest, most of the water will settle out. If you've done your own oil change you've probably noticed some water coming out in advance of the oil. Best suggestion is to just run the engine regularly.

The oil and water form an emulsion that can stay mixed for a long time. The worst of it is that the emulsion has been getting pumped through the engine and after shutdown the galleries in the case and crank and inside the valve lifters and prop and governor and oil pump and on the accessory gears and between the bearings and crank and the pistons and cylinders. Anywhere oil goes, so does the emulsion. The water therefore is spread everywhere that it can possibly cause corrosion and sludging as the engine sits. It doesn't just fall out and isolate itself in the oil pan, though some of it will do that. The thin oil sumps on the big Continentals sometimes rot out from it.

There is no substitute for running that engine hard and long enough to get it hot enough to clear that stuff out. On landing the engine will be hot enough that any blowby during taxi is minimal so that there will be little moisture left anywhere in the engine. In cold weather one should use whatever winter stuff is available to get the engine temps up to normal ranges. Air-cooled engines are simpler and there's less to go wrong, but they definitely have their drawbacks.

 

[Dan Thomas]

OTOH...I always run the engine right before draining so it should still be amply mixed? Maybe??

Yup. And so will the other contaminants like carbon. You want them in suspension so they'll get drained.

 

[MooneyDriver78]

When I land after even long flights, removing the dipstick will allow plenty of water vapor to escape...I don’t think all water is being boiled out as suggested. There is plenty left, the best strategy is to frequently change the oil so there is less bad stuff in the oil.


Tom