EAA Sport Aviation - December 2010

propeller hub and blade retention components during flight. That’s like hanging a full dump truck off one blade. Add to that uneven power pulses of pistons firing, along with aerodynamic loads, and you have components under immense strain the entire time they’re in use. Propellers are engineered to endure these stresses, but improper maintenance, neglect, and the passage of time add up and may eventually lead to failures.

The rust on this hardened-steel bearing shows that dry air offers no assurance that rust won’t develop.

WATER INFILTRATION

Many assume dry air helps preserve metal. Moisture is a necessary component of rust and corrosion processes, so the less present the better. Russ dispelled any myths about dry air by displaying a variety of propeller components in different stages of decay.

Variable-pitch and constant-speed propellers ride on hardened steel ball bearings. These bearings allow the blade’s pitch to change in flight. One such set displayed by Russ had rusted so badly that the propeller couldn’t even change pitch. This particular airplane had lived its entire life in the dry air of the Rocky Mountains, and the propeller was only 6 years old. How is that possible?

Water infiltration occurs from four sources: pressure washers, rain, condensation, and grease. Pressure washers used near a propeller hub exert enough force to blast water past gaskets, seals, and grease, damaging sensitive metal components within the hub. Russ’ advice: Never use a pressure washer on an airplane.

A propeller stored with its blades in a
vertical position will channel rain into the
hub. The standing water eventually makes
its way past gaskets and seals.

Condensation will occur any time the temperature of the propeller assembly drops below the dew point. Finally, grease acts much like a sponge, according to Russ. Over time, grease absorbs moisture and breaks down, making its regular replacement a good preventive maintenance procedure.

ALUMINUM COMPONENTS

And what of the hub’s aluminum components? Corrosion and stress fractures are common occurrences. One such hub had a stress fracture that went from inside, through the hub, and out to the mounting flange. Had it propagated any further, it would’ve led to a catastrophic failure of the entire assembly, similar to the MU- 2 incident. That same hub assembly had a blade-retaining ring with corroded threads. Russ noted that the ring, once mounted in the propeller assembly, has a torque specification in the hundreds of foot-pounds. Corrosion concentrates stresses and may lead to a catastrophic failure of the component.

The tuning fork effect of the propeller concentrates stress in blade anomalies. One blade from a constant-speed propeller was missing 4 inches of metal from the tip. What had started out as a nick (not a gouge) on the leading edge progressed into a crack, ultimately leading to separation. The leading edge of the blade had many nicks into which a dull pencil could fit; the paint along the edge was worn away, and the bottom blade surface had many small nicks from gravel strikes. What was most interesting about the blade was that it didn’t look much different from blades on

The gouge inside this propeller governor is advanced corrosion; it would’ve gone undetected during even the most thorough annual inspection. A hole in the governor can lead to engine seizure due to oil loss.