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Chapter 4 — Powerplants
Four-Stroke Engine Warming
A four-stroke engine must also be warmed up. The
four-stroke engine has a pressurized oil system that
provides more uniform engine temperatures to all its
components. You can apply takeoff power as soon
as the water, cylinder head temperature (CHT), oil
temperatures and oil pressure are within the manufacturer’s
recommended tolerances for takeoff power
applications.
Gearboxes
Gearboxes are used on all powered parachute reciprocating
engines to take the rotational output of an
internal combustion engine which is turning at a very
high RPM and convert it to a slower (and more useful)
RPM to turn the propeller. Gearboxes come in different
gear ratios depending on the output speed of the
engine and the needed propeller turning speeds.
A typical two-stroke RPM reduction is from 6,500
engine RPM with a 3.47 to 1 reduction, resulting in
1,873 propeller RPM. A typical four-stroke RPM reduction
is from 5,500 engine RPM with a 2.43 to 1
reduction, resulting in 2,263 propeller RPM.
A gearbox is a simple device that bolts directly to the
engine and in turn has the propeller bolted directly
to it.
A two-cycle engine gearbox is kept lubricated with
its own built-in reservoir of heavy gearbox oil. The
reservoir is actually part of the gearbox case itself.
The gearbox oil has to be changed periodically since
the meshing of the gears will cause them to wear and
will deposit steel filings into the oil. If the oil is not changed, the filings themselves are abrasive and will
cause even more wear.
Some gearboxes have the electric starter motor built
into it. When activated, the motor turns the gearing,
which in turn cranks the engine itself.
Four-stroke propeller reduction gearboxes use oil
from the engine oil system for lubrication.
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