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had not been warmed in the fi rst 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 fl uid 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 due
to the added heat, and the cylinder is cooling with the cold
radiator water, resulting in piston seizure. To prevent this,
slowly add power well before getting close to the ground
where power is needed. This gives the system a chance to open
the thermostat gradually and warm up the radiator water.
Just as it takes time for the engine crankcase and bearings
to warm up, it also takes those steel parts a long time to cool
down. If a pilot lands, refuels, and wants to take off again
quickly, there is no need to warm up again for 5 minutes.
The lower end of the engine stays warmed up after being
shut down for short periods.
4-8
rpm
x1000
0
1
2
3 4 5
6
7
8
ENGINE HOURS
Shown when engine is shut off
ENGINE RPM
6500MAX CON 6800MAX
Figure 4-9. Engine rpm is indicated on the analog gauge (top) and
the digital gauge (bottom).
Any engine restart is an example in which it would be
appropriate to warm the engine up until the gauges reach
operating temperatures. The lower end of the engine is warm
and now a pilot needs to be concerned only with preventing
the pistons from seizing.
Four-Stroke Engine Warming
A four-stroke engine must also be warmed up. The fourstroke
engine has a pressurized oil system that provides
more uniform engine temperatures to all of its components.
Takeoff power can be applied 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 most WSC reciprocating engines to
take the rotational output of an internal combustion engine
which is turning at a 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.
Some examples are a two-stroke rpm reduction from 6,500
engine rpm with a 3.47 to 1 reduction, resulting in 1,873
propeller rpm. A four-stroke rpm reduction could be 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 fi lings into
the oil. If the oil is not changed, the abrasive fi lings cause
even more wear.
Some gearboxes have a built-in electric starter motor. When
activated, the motor turns the gearing which cranks the
engine.
Four-stroke propeller reduction gearboxes use oil from the
engine oil system for lubrication.
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 without the load of the
propeller. Otherwise, two-stroke engines can generate a great
deal of vibration at low rpm when loaded. As the engine
speeds up, the centrifugal clutch engages and smoothly starts
the propeller spinning. When the engine is brought back
to idle, the clutch disengages and allows the engine to idle
smoothly again; the propeller stops when on the ground and
windmills when fl ying.
Propeller
The propeller provides the necessary thrust to push the WSC
aircraft through the air. The engine power is used to rotate the
propeller, which generates thrust very similar to the manner
in which a wing produces lift. The amount of thrust produced
depends on the airfoil shape, the propeller blade angle of
attack (AOA), and the engine rpm. [Figure 4-9] Light-sport
aircraft (LSA) are equipped with either a fi xed-pitch or a
ground adjustable-pitch propeller.
Fixed-Pitch Propeller
The pitch of the fixed-pitch propeller is set by the
manufacturer and cannot be changed. Refer to the Pilot’s
　
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