By Joe Escobar
Composite propellers are seeing more and more use these days. But there are inspection issues that are unique to these propellers. With proper maintenance and inspection, these sturdy propellers can provide a long service life. But neglect can lead to catastrophic failure. This article will discuss some inspection tips for composite propeller blades and take a behind the scenes look at a composite propeller blade overhaul.
Propellers, both metal and composite, are some of the most critical components of an aircraft. Failure of a propeller can lead to much more than loss of thrust. If a propeller blade is thrown, the result is catastrophic, much more so than an engine failure.
There are five operational forces that act on a propeller simultaneously. These are centrifugal force, thrust bending force, torque bending force, aerodynamic twisting moment, and centrifugal twisting moment. Of them all, centrifugal force causes the greatest stress on propellers. This is the force that tends to pull the blades from the propeller hub. It is related to RPM - the higher the RPM, the more centrifugal force the blades are subjected to.
John DeJoris of Wheeling, IL-based Aircraft Propeller Service, Inc. explains: "Propellers are subjected to great amounts of centrifugal force. There can typically be 25 tons of centrifugal force at the root end of metal propeller blades, minimally 20 tons. It is an exponential load. In other words, it's not a straight line relationship. As the RPM goes up, the amount of centrifugal force acting on the propeller blades goes up exponentially."
With so much centrifugal force acting on the propeller blades, it is easy to see how the loss of a blade in flight can be catastrophic. The danger is not necessarily all from the unrestrained propeller blade that can impact the aircraft and cause serious damage. Major danger lies in the transfer of energy. It goes back to Newton's Law of momentum conservation. The force that the propeller blade was subjected to is transferred to the system when it departs the aircraft. This can be an enormous amount of force that can rip the engine from its mounts and cause severe damage to the aircraft structure.
Benefits of composite blades
One of the most evident benefits of composite blades is their weight. They offer a substantial weight reduction compared to metal propellers, thereby offering more efficient operation (less horsepower is needed to produce the same thrust).
Another advantage to composite propellers is the fact that they don't shrink dimensionally after rework. As a typical metal blade experiences damage, the damage is blended out according to the repair manual. So, over time more and more material is taken away until eventually the blade reaches its minimum limits. That is not the case with composite blades.
Keith Wendell, a Quality Assurance inspector for Aircraft Propeller Service, explains: "The composite blades never wear out. They may eventually have a shank go bad, or a bearing go out, but the blades themselves never change. They are always reworked to the same size. As you repair them, they go back to the original dimensions, where with metal blades as you keep on taking metal away, sooner or later they are going to go undersize."
Not losing dimensional area is a definite advantage. DeJoris explained the danger of metal blades that go under dimension: "If a prop blade is undersize, it has a tendency to be susceptible to resonance. Every metal propeller blade is like a tuning fork. If it finds a sympathetic frequency that it can respond to, it's tip can deflect up to 6 inches. This can cause it to fail instantly. When the blade is within dimensional limits, it can't do that."
The health of your composite blade begins with inspection. Following the recommended inspection schedule is essential. The main inspection for composite prop blades is the tap test. A metal tool is tapped on the propeller blade surface to look for delaminated areas. Damaged areas sound like dull thuds compared to the light sound of the normal structure. The defect may not be visible, but the tap test will give a definite indication of a flaw.
In addition to the tap test, a good visual inspection of the propeller is in order. Check the condition and security of the leading edge and de-icer. A nickel covering protects the leading edge of the propeller from erosion and impact damage.
The de-ice boots should be checked for any evidence of overheating. Excessive heat can be very damaging to composite prop blades.
Wendell explains: "If a boot goes bad and burns, blade damage to the structure beneath is assured. Temperatures over 140 degrees can cause the Hartzell composite to 'pop' - basically delaminating on the inside. Hamilton blades will withstand approximately 100 degrees more. Most of the time the prop blades aren't destroyed by this heat damage. They can be fixed, but the cause of the overheating should be addressed."
The paint coating should be examined carefully for eroded areas. Areas where the coating is eroded could allow fluids to enter and saturate the composite. This can be especially damaging to fiberglass prop blades. When oil or grease are allowed to saturate the material, it causes the fiberglass to deteriorate quickly. The Kevlar® type prop blades aren't as susceptible, but they can still be heavily damaged by infiltration. Pay close attention to the blade coating and repair any eroded areas in accordance with the maintenance manual.
Composites are also susceptible to lightning strikes. Therefore, various methods are employed to protect composite propellers from the damage caused by lightning strikes. This can be in the form of metal spars, erosion sheaths, and/or special metal-based coatings placed over the composite that helps dissipate the electricity from a lightning strike. Whatever the type of protection on your prop blades, make sure that it is intact. With no path to airframe ground for the energy to dissipate, the propeller blade could literally blow apart if struck by lightning. Also, be aware of the signs of a lightning strike during your inspection. A tell-tale sign is a dark, discolored brown or black spot on the outer trailing edge of a blade that looks like an overheated area. If a lightning strike is suspected, the use of a gauss meter to check for residual magnetism in the ferrous attaching parts of the propeller is recommended. If a strike is verified, the propeller should be sent to an authorized repair facility for inspection and disposition.
The overhaul process for composite propellers begins with an incoming inspection. Any areas that may require follow-up are noted.
Next, the paint coating is removed as necessary. This process is usually done by hand to ensure that only the protective paint coating is removed without going into the composite layer.
The propeller blade is then inspected visually and re-tap tested. All damaged areas are repaired as necessary. The leading edge is inspected for security and integrity. The nickel leading edges are sacrificial, protecting the composite structure underneath. Eventually they have to be replaced. To remove edges from Hamilton propeller blades, they are ground down the center line of the blade's leading edge and then each remaining side is pried off the blade using a thin chisel. On the Hartzells, a propane torch is used to flash heat the edge in order to soften up the glue, paying close attention not to overheat the underlying blade. The edge is then removed once the glue has softened. Structure under the edge is inspected and repaired as necessary before installing a new edge.
When ready to install the new edge, a template is first placed over the repaired bare leading edge of the prop. It is basically an edge with witness holes ground in it to ensure that when placed on the leading edge of the blade the mating surface beneath is dimensionally correct. On Hartzell blades, adhesive is applied and the edge is put in place and secured with a vacuum bag until cured. For the Hamiltons, the process is a little more involved. Epoxy is applied to the blade edge, and a special pressure bag is placed around it. This ensures that uniform computer-controlled pressure and temperature is applied during the curing process. Special paint is then re-applied to each prop blade. This involves applying multiple layers of cross coats as required. Each propeller blade is then balanced to a calibrated master, reassembled, documented, and carefully packaged. It is then ready to be shipped back to the customer.
In the end, by following the manufacturer's recommended maintenance and inspection procedures, these sturdy props will last a long time. In fact, neglect is the most common factor leading to higher costs at overhaul and a shortened propeller life. Considering the extreme damage that can be caused by the loss of a propeller blade in flight, there is no other choice but to pay close attention to these critical aircraft components.
Aircraft Propeller Service, Inc.
290 Larkin Drive
Wheeling, IL 60090