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| VOLUME 3 |
FEBRUARY, 1950 |
NUMBER 5 |
MODEL 18 SERVICE NOTES AND CHANGES |
Hydromatic Propeller Care
Single-acting, hydromatic, Hamilton Standard propellers are offered as optional
equipment for the Model D18S airplanes in lieu of the constant speed Hamilton
Standard Propellers provided as standard equipment. These hydromatic propellers
are standard equipment on the Model D18C and Model D18C-T airplanes. These
propellers are controlled by individual, single-acting, Hamilton Standard
propeller governors, and feathering or unfeathering action is accomplished by
individual, electric motor-driven pump feathering systems. The feathering pumps
are located in the engine nacelles and the propeller feathering oil supply is
contained in the engine oil tank. A standpipe extending into the oil tank at the
engine oil outlet fitting prevents the escape of oil below the level of the
standpipe opening through the engine oil system, thereby insuring a sufficient
supply of oil for propeller feathering in an emergency.
Three fundamental forces are utilized to control the blade angle variation
required for constant speed propeller operation.
1. A centrifugal twisting moment which tends at all time to move the blade into
low pitch.
2. Oil at engine oil pressure on the inboard piston side *the outboard piston
side on early model propellers) which is introduced to supplement the blade
centrifugal twisting moment toward low pitch.
3. Oil, taken from the engine supply and boosted in pressure by the
engine-driven propeller governor, is supplied to the outboard piston side (the
inboard piston side on early models) to balance Forces 1 and 2, and used to move
the blades toward high pitch.
These hydraulic and centrifugal forces are transformed into mechanical force by
the piston and cylinder, and the cams transmit this force to the blades through
the bevel gears. The required balance between the three control forces is
maintained by the propeller governor which, in addition to boosting the engine
oil pressure, meters to or drains from the piston the exact quantity of oil
necessary to maintain the proper blade angle setting for constant speed
operation. See figure 4 |
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Circulation of oil through the propeller dome is very slow, since it is limited
to the amount of oil that can flow through the piston bleed holds. The propeller
dome, due to its revolving motion, acts as a centrifuge and tends to throw any
foreign particles which are in suspension in the oil toward the circumference of
the dome. This centrifugal action causes sludge deposits to collect on the
piston and inside surfaces of the dome, and the slow circulation of oil is not
sufficient to wash them loose and carry them away. Excessive accumulation of
these deposits will cause the propeller to respond sluggishly to governor
control. To insure proper operation, the propeller dome should be removed at
each 100-hour inspection and cleaned in unleaded gasoline. It is not necessary
to disassemble the piston and cams, but they should be removed from the dome to
facilitate cleaning.
The propeller should be operated through its complete pitch range a number of
times during preflight inspection after the engine oil temperature has reached
normal operating range. This operational check serves to expel air which might
have been trapped in the propeller system, and at the same time it will reveal
improper operation of the propeller, governor, or engine. With the engine
operating at approximately 1500 RPM and 22 inches HG manifold pressure, depress
the feathering switch. When the propeller has reached the full-feathered
position, the switch will automatically open and the engine will have slowed to
approximately 500 RPM. Immediately depress the feathering switch again and hold
it down while the propeller is being unfeathered, until the engine speed has
increased to approximately 1000 RPM. At this engine speed, the feathering switch
may be released and the propeller will return to governor control. This
feathering and unfeathering check during preflight inspection is especially
important during cold weather operation, since it will tend to replace the cold,
viscous oil in the propeller dome with warm engine oil and cause the propeller
to respond more quickly to governor control. It will also reduce the time
required to feather the propeller if an emergency should arise on take-off.
Model D18 airplanes that are equipped with hydromatic propeller installations
are provided with oil tanks having an 8-gallon capacity. Approximately 6 1/2
gallons of oil are available for engine lubrication. A standpipe in the tank
reserves the remaining 1 1/2 gallons of oil for feathering the propeller. This 1
1/2 gallons of oil should be replaced with fresh oil at each oil change;
however, it can not be drained from the oil tank through the oil drain "Y". Too
drain the oil tank completely, the flexible propeller feathering line should be
disconnected forward of the firewall at the hydromatic adapter fitting. The
feathering motor may then be used to pump the oil from the tank. After the oil
tank has been refilled, the engine should be started and the propeller should be
feathered and unfeathered to eliminate any air which is trapped in the
feathering line. |
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