AOPA Pilot Magazine - December 2020 - 88

P&E

SAVVY MAINTENANCE

THE 210 OWNER'S IDEAS for modifying his cooling baffles would not have been a good idea.

FLEXIBLE SEALS prevent air from escaping

through the gaps between the enginemounted baffles and the cowling, and must
curve toward the high-pressure chamber.
EARLY AIRCRAFT ENGINES were velocity

MODERN 'PRESSURE-COOLED' ENGINES rely

cooled, but this isn't practical for today's
high-performance engines and low-drag
airframes.

on rigid baffles and flexible baffle seals that
divide the engine compartment into highand low-pressure chambers. Cooling airflow
is usually top to bottom, not front to back.

That better system was known as "pressure cooling" and is the method used in all
modern piston aircraft. Pressure cooling is
accomplished by placing a cowling around
the engine and using a system of rigid baffles and flexible baffle seals to produce
the volume and pattern of cooling airflow
necessary to achieve even cooling with
minimum drag.
The volume of cooling airflow that
passes across the cylinders is a function of the pressure differential between
the upper (high-pressure) chamber and
the lower (low-pressure) chamber of the
engine compartment. This pressure differential is known as "delta P." Cowl flaps
are often used to modulate the cooling airflow. Opening the cowl flaps reduces the
air pressure in the lower chamber, thereby
increasing delta P and consequently the
volume of cooling air that passes vertically
across the cylinder fins.
It's important to understand that the
pressure differential between the upper
and lower chambers is remarkably small:
A typical high-performance piston aircraft
generally relies on a delta P of just 6 or 7
inches of water-about 1/4 PSI! Aircraft

designers try to keep this delta P to an
absolute minimum, because higher delta
P means higher cooling drag.
Because delta P is so tiny, even small
leaks in the system of baffles and seals can
have a serious adverse impact on cylinder
cooling. Any missing, broken, or improperly positioned baffles or seals will degrade
engine cooling by providing an alternative
path for air to pass from the upper chamber to the lower chamber without flowing
across the cylinder cooling fins. (That's
what would have happened if the T210
owner had cut holes in his baffles.)
One of the most problematic parts of the
cooling system is the system of flexible baffle
seals. These strips-usually made of hightemp silicone rubber-are used to seal the
gaps between the sheet metal baffles and the
cowling. These gaps are necessary because
the baffles move around inside the cowling as
the engine rocks on its shock mounts.
To do their job, the seals must curve up
and forward into the high-pressure chamber, so that the delta P presses the seals
tightly against the cowling. If the seals
are permitted to curve away from the
high-pressure area-not hard to do when

88 | AOPA PILOT December 2020

reinstalling the cowling if you're not careful-they will blow away from the cowling
in flight and permit large amounts of precious cooling air to escape.
I recall some years ago inspecting a
Cessna TR182 whose pilots had complained
of high CHTs. Upon removing the top engine
cowling, I immediately spotted the problem:
One of the ignition leads was misrouted and
became trapped between the baffle seal and
the cowling, preventing the baffle seal from
sealing against the cowling. The ignition
lead had become severely chafed where it
rubbed against the cowling, and an A&P had
wrapped the chafed area with electrical tape
but failed to reroute the tape-wrapped lead
to keep it away from the baffle seal. Clearly
that A&P didn't understand the importance
of an airtight seal between the baffle seals
and the cowling. Repositioning the ignition
lead solved both the cooling problem and the
chafing problem.
Another common problem is that seals
may develop wrinkles or creases when the
cowling is installed, preventing them from
sealing airtight against the cowling and
allowing air to escape. It's important to look
carefully for such problems each time the
cowling is removed and replaced, and especially important when new seals have been
installed (as was the case with the T210).
Yet another common cooling system problem involves the intercylinder



AOPA Pilot Magazine - December 2020

Table of Contents for the Digital Edition of AOPA Pilot Magazine - December 2020

Contents
AOPA Pilot Magazine - December 2020 - Intro
AOPA Pilot Magazine - December 2020 - Cover1
AOPA Pilot Magazine - December 2020 - Cover2
AOPA Pilot Magazine - December 2020 - Contents
AOPA Pilot Magazine - December 2020 - 2
AOPA Pilot Magazine - December 2020 - 3
AOPA Pilot Magazine - December 2020 - 4
AOPA Pilot Magazine - December 2020 - 5
AOPA Pilot Magazine - December 2020 - 6
AOPA Pilot Magazine - December 2020 - 7
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AOPA Pilot Magazine - December 2020 - Cover4
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