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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
The cylinders have steel cylinder walls that expand
slowly compared to aluminum pistons that expand
quickly. If an engine is revved too quickly during
takeoff before warming up, a lot of heat is generated
on top of the piston. That quickly expands the piston,
which can then seize in the cylinder. A piston seizure
will stop the engine abruptly.
The second area of concern is lower in the engine
around the engine crankshaft. This is an area where
things may get too loose with heat, rather than seizing
up. Additionally, the crankcase has steel bearings set
into the aluminum which need to expand together or
the bearings could slip.
Many two-stroke engines have steel bearings that normally
hug the walls of the aluminum engine case. The
crank spins within the donuts of those steel bearings.
If you heat up the engine two quickly, the aluminum
case will out-expand those steel bearings and the
crank will cause the bearings to start spinning along
with it. If those steel bearings start spinning, they can
ruin the soft aluminum walls of the case, which is
very expensive.
If heat is slowly added to an engine, all the parts will
expand more evenly. This is done through a proper
warm-up procedure. Many two-stroke engines are
best warmed up by running the engine at a set RPM
for a set amount of time. Follow the instructions in
your POH; however, a good rule of thumb is to initially
start the engine at idle RPM, get it operating
smoothly, and then warm the engine at 3,000 RPM
for 5 minutes.
Once the engine is warmed up and the powered parachute
is flying, it is still possible to cool down the
engine too much. This will happen when the engine is
idled back for an extended period of time. Even though
the engine is running, it is not generating as much heat
as the cooling system is efficiently dumping into the
atmosphere. An immediate power application with a
cooled engine can seize the engine just as if the engine
had not been warmed in the first place.
In water-cooled engines, on a long descent at idle, the
coolant cools until the thermostat closes and the engine
is not circulating the radiator fluid through the
engine. The engine temperature remains at this thermostat
closed temperature while the radiator coolant
continues to cool further. If full throttle is applied, the
thermostat can open allowing a blast of coolant into
the warm engine. The piston is expanding because of
the added heat and the cylinder is cooling with the
cold radiator water resulting in a piston seizure. To
4-6
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.
Centrifugal Clutch
Some gearboxes come with a built-in centrifugal
clutch, and others have allowances for installation. A
centrifugal clutch is very useful in a two-stroke engine
because it allows the engine to idle at a lower speed
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Powered Parachute Flying Handbook动力伞飞行手册(30)
