AOPA Pilot Magazine - October 2020 - 98

P&E 

SAVVY MAINTENANCE
is turbocharged, the wastegate is probably
hydraulically actuated by oil as well.
An enormously important function of
engine oil is protecting expensive steel components like crankshafts, camshafts, lifters,
and cylinder barrels from rusting during
periods when the aircraft is not being flown.
Because we tend to fly our airplanes far less
often and far more irregularly than we drive
our cars, the preservative requirements of
aircraft engines are vastly more demanding
than for automotive engines.
POLYMERS

FRICTION AND WEAR are caused by fracturing of "micro-welds" between moving surfaces.

hydrodynamic lubrication. It occurs when
a fluid-most commonly a liquid like oil-
is interposed between the moving parts.
The relative motion of the parts creates
sufficient pressure in the lubricant to keep
the parts from touching. Think of a water
skier being supported by the skier's relative motion to the lake and the resulting
pressure of the interposing water that prevents the skier from sinking. Or, consider a
car hydroplaning on a rain-slicked road, its
locked tires being separated from the pavement by water pressure.
Hydrodynamic lubrication works well if
the relative speed of the parts is high enough
to overcome the load pushing them together.
If the relative speed is not high enough, then
there will not be enough lubricant pressure to keep the parts separated. (Think of
the tow boat slowing down until the skier
sinks.) If hydrodynamic action cannot keep
the parts separated, we must rely on another
form of lubrication known as boundary
lubrication. Boundary lubrication relies
on a thin, soft, solid film deposited on the
moving parts-typically by chemicals called
"extreme pressure additives"-that reduces
friction and wear by chemically interfering
with micro-weld formation.
Rub your hands together vigorously and
you'll feel the friction between your palms
in the form of heat and resistance to movement. Believe it or not, you just created and
then fractured a few zillion micro-welds on
the surface of your palms. You can reduce

98 | AOPA PILOT October 2020

the friction in several ways: for instance,
by coating your palms with Vaseline, or by
dusting them with talcum powder. Think of
the Vaseline as hydrodynamic lubrication
and the talc as boundary lubrication.
WHAT ELSE DOES OIL DO?

Engine oil has to keep the engine clean.
Compared to car engines, piston aircraft
engines are positively filthy creatures. They
burn leaded fuel and allow large quantities of
lead salts, carbon, sulfur, water, raw fuel, and
other nasty combustion byproducts to blow
by the rings and pollute the bottom end of the
engine. The oil has to be able to keep these
contaminants dispersed and hold them in
suspension so that they can be drained away
at the next oil change and don't accumulate
on internal engine parts in the form of sludge.
Oil cools engine components like pistons that can't be air-cooled. The only thing
that keeps aluminum pistons from going
into meltdown is oil that is splashed and
squirted onto their bottoms to carry heat
away. Engine oil typically gains about 40
degrees Fahrenheit as it circulates through
the engine, after which it passes through
an oil cooler (radiator) that dissipates that
added heat via airflow through the cooler.
Oil also acts as a sealant to prevent the
leakage of gases and liquids past piston
rings, O-rings, gaskets, and other kinds of
seals. If your airplane has a constant-speed
propeller, oil serves as the hydraulic fluid
that is used to adjust the blade pitch. If it

Oil is made up of giant molecules called
polymers. Some are natural like mineral
oil from dead dinosaurs, others are manmade synthetics like polyalphaolefin (PAO)
and polyalkylene glycol (PAG). These various polymers have differing shapes. The
molecules of mineral oil have a lot of side
branches, while the molecules of synthetic
oil are smoother and less "branchy."
Mineral oil degrades the longer it
remains in service. The little branches
gradually shear off-a phenomenon known
as polymer shearing-and that causes viscosity to decrease. Because synthetic oil is
less branchy, it suffers far less from polymer
shearing and retains its viscosity better, so it
can go longer between oil changes (at least
in automotive applications).
Mineral oil's branchy molecules do a
much better job of holding particulate contaminants in suspension so that they can be
drained out at the next oil change instead of
settling out as sludge. In fact, the full-synthetic Mobil AV 1 was withdrawn from the
market in the late 1990s (in a hail of litigation from aircraft owners) because so many
engines were ruined by lead sludge deposits.
Synthetic oil simply doesn't deal with filth
as well as mineral oil.
MONOGRADE VERSUS MULTIGRADE

Monograde oil is simply mineral oil plus an
additive package. It has viscosity (thickness)
that varies considerably with temperature. At
operating temperature-around 200 degrees
Fahrenheit-it's quite thin and flows freely.
At room temperature, it's thick and gooey. If
it's cold enough, it won't pour at all.
Multigrade oils are much less thick and
gooey at cool temperatures. They still get
AOPA Pilot Magazine - October 2020

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