and forth while the propeller is turning. Flutter-
ing causes a distinctive noise, which is nearly
drowned out by the exhaust noises of the engine.
The greatest force acting upon the propeller
Fluttering will weaken the propeller and may
blade is centrifugal force. This force tends to pull
result in structural failure unless detected early and
the blade of a spinning propeller out of its hub.
To prevent the blades from breaking into
fragments or flying off into space, the blade is
PROPELLER SYSTEM ASSEMBLIES
thicker near the hub. The hub is made from a
strong steel forging.
The propeller system has one primary
function--increasing or decreasing pitch as
Thrust Bending Force
required by power lever movement. Safety
features incorporated in the 54H60-77 propeller
Thrust bending force causes a rotating pro-
system include the automatic mechanical pitch-
peller to try to pull away from the aircraft.
lock, the automatic negative torque control, the
Because it is held back by the hub and the load
mechanical low pitch stop with a secondary
of the aircraft it is pulling, the blade tips, which
hydraulic low pitch stop (referred to as the beta
are thinner and lighter than the blade shank, bend
follow-up), and an emergency feathering system.
forward. The sum of these bending forces on the
The complete 54H60-77 model propeller,
blades is carried at or near the hub. Hence,
shown in figure 8-13, consists of the front anti-
the section of the blade at the hub must be
icing propeller spinner, the hub mounting
proportionately thicker. Centrifugal force
bulkhead assembly, the variable pitch aircraft pro-
counteracts thrust bending force by its tendency
peller (propeller assembly), the rear deicing
to pull the blades in a straight line.
propeller spinner, the air baffle assembly, the
propeller control (integral oil control) assembly,
Torque Bending Force
and the propeller afterbody assembly.
Torque bending force is the tendency for a
Spinners and Afterbody Assemblies
blade to bend backwards, throughout its length,
in a direction opposite rotation. This bending
The main purpose of the front and rear
force is created by the density of the air.
spinners is to streamline the airflow around the
outside of the propeller assembly for cooling. The
Aerodynamic Twisting Force
front and rear spinner assemblies improve the
aerodynamic characteristics of the whole propeller
Aerodynamic twisting force tries to rotate the
assembly. They enclose the dome, barrel, and oil
blades in the hub to an increased blade angle. The
control assemblies. The front spinner has an air
point at which this force is exerted most strongly
inlet in the middle of it. Cooling ram air enters
on the chord of the airfoil is known as the center
to cool the dome, barrel, and oil control
of pressure. During normal cruise conditions, this
center of pressure is nearer the leading edge of
The propeller afterbody assembly streamlines
the propeller, so the force tends to rotate the
airflow into the engine air inlet. The afterbody
blades to a higher pitch.
assembly has a top and bottom half. The two
halves of the afterbody have electrical deicing
Centrifugal Twisting Force
wires to prevent ice buildup on the back side of
the propeller assembly.
The centrifugal twisting force on the blades
tends to twist them to a lower pitch angle. This
Hub Mounting Bulkhead Assembly
occurs because all parts of the propeller try to
and Propeller Assembly
remain in a plane parallel to the plane of rotation.
The hub mounting bulkhead is the mounting
surface for the front and rear spinner assemblies.
The variable pitch aircraft propeller (propeller
Sometimes, in the face of these forces, a pro-
assembly) has four major subassemblies. They are
peller loses some of its rigidity. The result is a
the barrel assembly, the blade assembly, the dome
flutter, which is a type of vibration in which the
assembly, and the pitchlock regulator assembly.
tips of the blades attempt to twist rapidly back