There was a time when propeller blades were made of wood — until metal blades were invented. Now, to round out the selection, propellers are also being made of composited layers of carbon fiber or Kevlar (or both) surrounding a shock-absorbing foam core. The reason: composite blades are lighter than metal and stronger than wood. In fact, a four-blade composite weighs the same as a three-blade metal, providing a quieter, smoother flight without extra fuel consumption.
Hartzell Propellers of Piqua, OH, knows all about composite props. The company that made its first wooden propellers for the legendary JN4 Jenny in 1917 ventured into advanced composite propellers more than 30 years ago. Today, Hartzell composite props can be found on the Cirrus SR22, many aerobatic aircraft, Beech 1900C and D, and the Shorts SD3-60 (a.k.a the 360), among others.
So how does one maintain and repair a composite propeller blade? To find out, we spoke to Mark Spoltman, Hartzell’s engineering manager of composites.
Composite blades face the same threats as their wooden and metal cousins. The most common are nicks caused by stones or other objects getting swept up in the propeller wash. Other risks include damage caused by impact with external objects such as fences, vehicles, other aircraft on the ground, or the ground itself.
With metal propellers, the remedy is to remove material by filing, sanding, and polishing. “Unfortunately, metal can only be removed and each repair distorts the aerodynamic shape and steals a little bit of life,” says Spoltman. “Once minimum dimensions have been hit, the blade must be replaced — a costly proposition.”
Composite blades also get damaged, but whereas metal can only be removed, composite can be restored. In fact, “Almost all Hartzell propeller blades are certified with an infinite fatigue life,” he says. “Many metal blades must be retired due to on-condition issues caused by the removal of material during repair. Therefore, when the metal blade owner is looking at costly blade replacement, the composite blade owner is still going strong with all of the original performance still in place.”
The repair of composite blades starts out essentially the same as metal with removal of the damage, but there’s an extra stage required.
“The aircraft technician does sand down nicks on a composite blade,” says Spoltman. “To do this properly, you begin by starting at the deepest part of the nick, then working your way back to the undamaged area in a 360-degree pattern. This smooths out the divot while minimizing the amount of material being removed.”
Once this stage has been completed, the sanded area is built up using layers of fiberglass and resin. When this has cured, the fiberglass is then sanded down such that the patched blade shape matches the original in size, shape, and thickness. At this point you paint it, and you are ready to go.
Composite blades are coated with a durable paint that can be time-consuming to replace. To speed the process, smaller repairs are permitted to use an aerosol process.
“The limits on the faster repair are not arbitrary and must be adhered to,” he warns. As well, “Many composite blades have a conductive paint layer for the proper dissipation of precipitation static. Therefore, larger repair areas require the re-application of the conductive coating. You do not want the propeller to be the source of a p-static problem.”
Sometimes the damage is to a blade’s leading edge. On a metal propeller, the answer is once again to remove the material. On a composite propeller, the leading edge is protected with a secondary structure manufactured from nickel (an erosion shield) that can be replaced.
The damaged erosion shield is removed by first debonding it from the blade using a propane torch, while being careful not to damage the composite material. Once removed, the leading edge area is cleaned of residual adhesive and inspected for damage. A new erosion shield, which has been prepared for bonding by the factory with a metal preparation process followed by an adhesive primer, is glued in place using a special adhesive paste purchased from the manufacturer.
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