HANDS ON
FIREWALL FORWARD
This propeller lost four inches from the tip when a stress
fracture grew from a nick in the leading edge.
The uneven line along the right side of the propeller shank
is a crack. The only way to find it is to disassemble the
propeller hub.
Corrosion caused the small pits along the threads of this
propeller-retaining ring. This highly stressed part is subject
to failure when corrosion develops.
many airplanes out on ramps today. The tuning fork effect will also
concentrate stress in areas of corrosion anywhere on the blade sur-
face, with the same possibility of stress fractures developing.
Because propellers are mounted to a crankshaft or gearbox,
they’re actually part of a much larger system that includes the engine
and engine mounts. Aerodynamic loads and engine pulses induce
vibrations into a propeller that are accounted for during the design of
the propeller—but only on a relatively fresh engine, with engine
dampers in good condition. Worn or damaged engine dampers produce additional harmonics, which contribute to metal fatigue in the
propeller, not just the crankshaft and other engine components.
Did you know a tachometer could cause a propeller to fail? Russ
pointed out that many tachometers might deviate as much as 100 to
200 rpm from the actual engine rpm. That means you could be over-revving your propeller, or operating within a restricted rpm range,
without being aware of it. Any problems already present can be
exacerbated. Depending on the amount of discrepancy, a propeller
may need a full teardown and inspection. Have your mechanic check
the accuracy of your tachometer at every annual.
Corrosion concentrates stresses and may lead to a
catastrophic failure of the component.
Variable-pitch and constant-speed propellers present their own
set of challenges. Additional moving parts offer more opportunities
for failure. Additionally, the mechanisms used to change propeller
pitch are tied to the engine’s oil system, so additional precautions
must be taken.
Constant-speed and feathering propellers use a governor to regulate the amount of engine oil sent to a piston in the prop hub. With
feathering propellers, oil pressure pushes the piston, reducing the
pitch of the blades, causing the engine to turn at a higher rpm.
Lower pressure causes the blades to increase pitch, decreasing rpm.
With constant-speed propellers, the operation is the opposite.
The propeller governor plays an important role in maintaining
proper oil pressure; if its internal pressure regulator fails, control of
the propeller can be lost, or in the extreme, the cylinder assembly of
the propeller may rupture. That is essentially like opening the oil
drain plug and pumping engine oil overboard at 290 psi (the rated
output pressure of most governors). A rupture of the propeller cylinder results in an almost immediate need to shut down the engine,
not to mention a couple gallons of oil covering the windscreen.
CORROSION
Corrosion can also be an enemy of the governor. Russ had a governor
on hand with internal corrosion so advanced it had almost pene-
trated through the assembly, creating another potential oil drain.
If you have a fixed-pitch propeller, don’t think it’s immune from
corrosion or fatigue. When was the last time your mechanic
removed the safety wires from the propeller bolts, removed the nuts,
and looked at the back side of the propeller? Have the bolts recently
been retorqued to the proper specifications? Was the spinner assembly checked for proper torque and cracks?
