Failure is not an option in aeronautics. That’s doubly true in combat situations. For that reason, lightweight, composite components are usually
Composite materials are a combination of two or more material classes. They can be any mixture of metals, polymers, or ceramics. The resulting properties are based on the “rule-of-mixtures.”
A mixture of a strong, brittle material and a soft, ductile material will have properties somewhere between those two extremes. This ability to tailor the properties is great for making strong, lightweight materials, but it is bad for making repairs on damaged components. Until recently.
Aviation is one of the biggest consumers of lightweight composites, and the US Army has one of the largest aircraft inventories in the military. As noted in a recent Army press-release, “… the Prototype Integration Facility's, PIF's, Advanced Composites Lab has successfully designed and made repairs on damaged composite aircraft components for several years now.”
Composite materials, such as carbon fiber or Kevlar, are often restricted to specialty use due to their high cost. Replacing components is very costly, but repairing composites is difficult because the materials are not homogeneous. They cannot be simply bolted or welded like many metals. The orientation of the composite and repair layering must be done with precision.
Repairs are important because, the "pull-and-replace" approach was costing the Army up to six figures per helicopter stabilator replacement. A stabilator is a movable tail that aids in maneuvering and stability. The ability to repair a large component like a stabilator presents a significant cost reduction, but there are many other components that can benefit as well. That includes the engine cowling.
As the PIF Advanced Composites Lab lead, Kimberly Cockrell, explains, "When the aircraft is on the ground being maintained, the engine cowling folds out to become a maintenance stand. Two Soldiers can stand up there with a tool box and work on the engine. Unfortunately, minor damage to [the] hat channels can cause these (cowlings) to catastrophically fail and seriously injure the Soldiers."
To increase safety, engineers designed and built a composite “doubler” which can be added if any damage is visible. This restores strength to the cowling and adds less than one pound to the helicopter. The repair procedures being developed at the Advanced Composite Lab are the first approved for a primary composite structure on Army aircraft.
The lab is currently working on repairs for helicopter blades too, as well as just-in-time tooling for parts with complex curves or topography. The ability to simultaneously improve functionality and reduce cost is desirable in any industry. Military applications demand reliability. Combining desirable material properties with successful and economical repair procedures ensures technological success now and in the future.