Page 14 - March 2023

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hub and related blade retention hardware. By designing the lightest blades with the best vibration dampening qualities, the structural requirements of the hub and retention hardware can be more easily addressed.
The trend from metal, to structural composite, and finally natural composite blade construction shows the clear vibration and damping characteristic improvements gained with each respective technologi- cal advancement. The above figure shows the vibration and damping characteristics associated with various blade designs.
With respect to durability, all major propeller manufacturers have certified the use of composite blades to the latest 14 CFR Part 35 re- quirements, or the equivalent CS-P certification requirements, to take advantage of their benefits. Bird strike testing is a major activity in the certification of these blades. The testing involves firing hundreds of birds at the blades in a series of tests, under strict conditions defined by the certification authority, to demonstrate survivability of such impacts.
The following photos show the damage to the propeller blade (left) and the wing leading edge (right) from an in-service incident where an 8-pound stork impacted a turboprop airplane during takeoff.
In addition to the bird strike requirements for the certification of composite blades, lighting strike can be a major challenge in the certification process. All composite blades must meet lightning strike requirements, with the ideal design practice being that the load carrying structure is not conductive. In the situation where the load carrying structure is conductive, a lightning strike would necessitate scrapping of the blade. In the case where a natural composite is used as the load bearing structure, after lightning strike, the blade can be repaired, in many cases with a field-repair and/or overhauled.
The photos below provide an example of a lightning strike on a natu- ral composite blade. The picture on the left shows where the lighting strike hit the blade, and the picture on the right where the lighting left the blade.
In addition to lower weights and the corresponding structural and vi- bration advantages, composite blades have the advantage of longev- ity. Unlike metal propeller blades, composite blades are resistant to corrosion. Additionally, composite blades are easier to maintain and repair. Unlike composite propellers, the performance of metal propel- lers begin to deteriorate from the day they are installed as a result
of the grinding away of surface blemishes, gouges and other FOD damage, permanently removing material during overhaul and repair. During their limited service life, metal blades surfaces deviate further and further from their designed outer mold line (OML) until ultimately they must be replaced. On the other hand, composite blades are repaired by removing damage, then adding material back onto the blade, thereby restoring its OML. In this way, many composite blades have unlimited life and use. The oldest composite blades have now been in service for 80 years and are still airworthy.
Improvements in design methodology, analysis tools and new ma- terials technologies have allowed propeller designers to push their designs closer and closer toward the maximum possible efficiencies by adding more propeller blades, going beyond what was possible with the limitations of classis designs and materials. The byproducts of these efficiencies, in the form of lower noise, lower vibration and improved durability and longevity clearly align with the desire of op- erators to lower operating costs and improve the overall experience for their crew and passengers. Higher blade counts are propelling the industry into the future.
12 • KING AIR MAGAZINE
MARCH 2023