This past week I had the opportunity to attend a presentation on constant speed propeller refurbishing and repair. Our local Experimental Aircraft Association hosted Mr. Fix at their June meeting in Olathe, Kansas. Fix-It Prop Shop, Inc was established in 1973 at Lincoln Air Park in Lincoln, Nebraska. Today they are a certified repair station specializing repair of 2024 grade Aluminum props.
Mr. Fix's slide presentation took us thought the processes he uses to measure and correct the pitch, track and bow in a propeller. He explained that it is rare to find a perfectly matched prop from side to side, even brand new ones with factory certifications are typically less than perfect from inconsistencies in the manufacturing process. Each prop tells a story, if the back of a prop is splattered with chips, the nose gear strut is most likely lo, or was low at one time, allowing the debris thrown up from the front wheel to reach the prop tip. We were presented us with a collection if prop tips that had been gathered, showing us what types of damage looked like, we saw gravel pits, cracks from strikes, what a water strike can do, even one that was damaged with buckshot.
Many things can effect the performance and lifespan of a propeller. Some of the most common issues are damage from gravel, wear of the leading edge and bowing. Chips in the blade, even bugs plastered to it can cause it to be out if balance and cause vibrations. Propellers need to be matched to the particular engine they are to be run on. A unit too large or small can have significant consequences.
Bending and straightening a prop is as much of an art as it is a science. The tools he uses are homemade. The primary bench is a large piece of one inch steel, about 12 feet x 2 feet, precision etched with stations at one inch intervals similar it those used to measure weight and balance in an aircraft. With the prop placed on a steel shaft in the middle of the table he can measure the pitch, bow and track at each station interval along the prop for accuracy and consistency from side to side. Pitch can only be measured with scribes placed on the flat surface from station to station.
Adjustments are done cold, the prop is not heated prior to being adjusted. Adjustments are made by hand using home made tools and long steel bars. Bending and twisting of a blade is done by feel because the temper on blades differ. The equipment will allow a pitch to be adjusted at any point along a blade that is not exact. The procedure is to repair and tweak the worst blade first to perfect, then change the opposite blade to match. He can re-pitch a prop, a three tenths of a degree change in pitch equates to 50 rpm. Each engine manufacturer has a table of recommended prop configurations to go with it.
Mr. Fix talked about periodic maintenance. All props have a lifespan established by the manufacturer plus inspection and usage limits. That doesn't mean one can put on a prop and forget about it. Over time a unit will develop a dead spot in the middle of the blade that effects the harmonics of the vibrations created as it travels through the air. Harmonics normally will travel on the surface to the ends of the blades, over time the dead spot will force the vibrations deep into the prop and potentially cause fractures inside the aluminum piece. For this reason, the dead spots need to be ground out periodically. It should not be universally concluded that any prop will just last it's lifespan. Periodic inspections of a prop by a knowledgeable A&P can help reveal when a refurbishment should be considered. A simple metal straight edge run up and down a blade can reveal bowing and twisting that can be seen with the naked eye.
Leading edge maintenance is a very important part of maintaining the integrity if the aluminum blades. A blade is in effect a twisted wing that creates lift, the force of the lift creates the airflow that is pushed rearward over the aircraft and its wing allowing powered flight. This "wing's" shape is just as critical as the shape of the wings attached to the fuselage. If the pitch or cord line of the prop gets out of shape, lift is lost and efficiency deteriorates. An aircraft mechanic can make surface only corrections to a damaged prop edge but they must stay within limits set by the directives. If large chips or chunks are missing from a blade, the blade must be replace or repaired by a certified propeller repair station technician.
Each propeller is balanced horizontally and vertically before it is ready for surface prep. Once a propeller is repaired and refurbished, sanding of the surface is completed, the entire prop is dipped in alodine, then anodized. The units gets dipped again to neutralize the anodizing process before it goes into the plating tank. This last process will expand the raw metal revealing cracks.
The final surface preparation of a finished prop consists of alodining and etching the edge, the application of epoxy to protect the surface and finally a lacquer finish that may contain color. Mr. Fix warns against ever using a powered buffing wheel on a propeller, it creates heat, weakening the unit. A propeller should never be oiled he insists, instead the recommended coating is a good quality automotive paste wax.
When asked about service costs, he explained it varies on the work needed. An average refurbishment with blade alignments takes 20 man hours plus one day for the finishes to dry and runs around $600. He does not repair wood or steel props. He encourages owners to visit his shop and deliver their props in person so he can explain in depth the process. He can be contacted at 402-470-2346.
