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."
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