Just in time for copter’s revival, manufacturer seeks Labs’ help to evaluate simpler, better rotor system
In Afghanistan the US military relied on helicopters to insert troops into dangerous terrain, rain fire on adversaries, and rescue soldiers in peril.
As a war-fighting tool, the chopper is hotter than ever. And at least one major US manufacturer’s new birds feature technology tested and improved at Sandia.
Labs researchers at the FAA Airworthiness Assurance Center (AANC) near the Albuquerque Sunport have been working with Bell Helicopter since 1997 to evaluate rotor hubs made from composite materials rather than traditional aluminum and steel.
The new, stronger composites — essentially many layers of fiberglass stacked together and hardened with epoxy resins — are resistant to failures caused by the growth of cracks as the materials age. More important, they allow for simpler rotor hub designs that require no hinges, gears, or bearings.
Bell is using data gathered by the AANC to refine its new "bearingless rotor" design — now available on Bell military helicopters, including the Marines’ Super Cobra attack helicopter, the Army’s Kiowa Warrior gunship, and the multi-service Huey transport, as well as several commercial choppers — and to substantiate flight certification and maintenance procedures for the new designs.
Bell’s four-bladed bearingless system "provides unprecedented agility, substantially increased speed, a smoother ride, a more stable weapons platform, and excellent reliability. It will also reduce crew fatigue and enhance combat mission effectiveness," according to a company web site.
The AANC also is working with several other US helicopter manufacturers and operators to advance the use of composites and associated nondestructive inspection (NDI) procedures industry-wide.
Rotors take a licking
"Vibration causes helicopter components, particularly the rotor parts, to wear out faster than in fixed-winged aircraft," says project leader Dennis Roach (6252). "A stronger material and a simpler design provide both engineering and economic advantages."
Bell had been working on its bearingless rotor hub for several years when company officials approached Sandia in 1997, says Dennis. The company sought the AANC’s longtime experience with NDI technologies and in evaluating composite materials for aviation applications.
"They wanted help optimizing their blade designs and evaluating the composite material’s performance over millions of fatigue cycles," he says. "They also needed an inspection schedule for in-service helicopters to catch defects before they reach critical size."
Simpler rotor hub designs
Traditional helicopter rotors are supported by an orchestration of hinges, gears, and bearings that keeps the blades at their optimum angles during airborne maneuvers. Replacing these contraptions with two long composite planks, stacked perpendicularly at their centers and affixed directly to the rotor hub (to form an X), required that each plank’s thickness be tapered to achieve the needed droop at various rotation speeds.
For the project the AANC developed a custom "biaxial test facility" to evaluate blade samples with various center thicknesses and taper profiles, subjecting them to millions of cycles of bending and twisting under high-g centrifugal forces to simulate the punishing vibration environment a helicopter’s rotor endures during flight.
In particular, says Dennis, Bell wanted Sandia to fatigue the composites to accelerate the formation of twixt-layer cracks, called delaminations.
Sandia also developed a complementary ultrasonic technique to inspect the hub samples during the fatigue tests to determine how fast the defects grow, how quickly they become critical, and which designs and resin systems create the most rugged rotor hubs.
Safe inspection intervals
As a result of the project, Bell ranked the performances of various complete resin systems, optimized rotor hub designs, and determined safe inspection intervals for its composite rotor hubs.
The company also incorporated the AANC data into a new "damage-tolerance analysis" (DTA) methodology that can accurately predict the onset and growth of flaws in the composite materials.
DTA is an approach to rotorcraft maintenance — an alternative to the traditional "safe-life" maintenance practice (where parts are replaced conservatively before cracks are expected to appear) — that forecasts flaw initiation and growth in structures so that safe inspection intervals can be established to detect and eliminate flaws.
If adopted widely, says Dennis, the DTA approach would significantly reduce the cost of helicopter maintenance, extend helicopter life, and retain if not improve current levels of safety.
Industry to reap benefits
Widespread adoption of DTA, however, requires the widespread use of NDI techniques. The AANC’s rotorcraft program is working to develop and introduce NDI technologies that support DTA-based maintenance to the larger rotorcraft industry.
"We want to help bring both the big service providers and the small outfits up to speed on the advanced NDI technologies that are now available," says Dennis.
The data and methodologies produced as part of Sandia’s damage-tolerance analyses and inspection of composite rotor hubs will be made available to US rotorcraft manufacturers, as well as third-party companies and maintenance depots that inspect in-service helicopters.
The AANC is working directly with a team of helicopter manufacturers — including Sikorsky, Boeing, and Bell — as well as large operators, including several companies and the US Navy.
"Increasing niche applications, growing international markets, and the emergence of improved rotorcraft technology are expected to increase the number of helicopters over the next decade," says Dennis. "As this happens, new materials and the optimization of maintenance practices will become more attractive than ever for the rotorcraft industry.
"The main goal of our work is to provide the US rotorcraft industry with information that advances the cause of safety in US aviation," he says.