Sunday, June 1, 2008
Blades and Composites: Staying Sharp
Manufacturers are honing rotor blade and structural performance with improved designs and more composites, and repair shops are keeping pace.
Think hard. Is there any part of a helicopter that’s more important than the rotor blades? Even if the fuel is gone and the engines quit, it’s those big, spinning planks that will get you back to Earth in one piece when nearly everything else fails.
Rotor blades, by necessity, have to be strong, but light. They have to bend, but not too far. They have to twist, but not too much. Ever since the birth of the first helicopter prototypes, engineers have searched for the perfect mix of qualities. In time they found what they sought: the composite rotor blade, a mainstay of rotary-wing flight to this day.
Composite blades — or any composite components, for that matter — are built from a composition of two or more materials working together to form a more capable component. Composite manufacturers bond various textiles and metals together for just that purpose, especially for rotor blades.
Glass fiber is a popular material from which to make rotor blades, primarily because of its ability to bend and twist throughout a controlled range of movement. Steel, aluminum and titanium are common blade skins, because they are strong, offer less aerodynamic friction, and can maintain lift-friendly shapes without corroding as quickly as exposed glass fiber. The materials are pressure- and heat-bonded into one component using super-strong epoxies, then coated with special paint designed to seal the entire structure from the elements.
In spite of their strength and flexibility, rotor blades can still suffer from fatigue, mistreatment or foreign object damage that can make them unsafe. This means they must be submitted to inspections by qualified maintenance personnel, and sometimes repaired by specially trained technicians.
Short of a routine or scheduled inspection, the first notification a pilot gets that a blade has a problem is a vibration felt through the cyclic. When a blade is damaged, it struggles to do what it should be doing, resulting in a noticeable thump down the control path. When that happens, it’s time to call a maintenance professional.
Upon notification of a blade-related vibration problem, the mechanic will usually perform a series of in-house tests prescribed by the manufacturer to determine the nature of the problem. Regardless of the cause of the damage, maintenance manuals include general guidelines on how much damage to a rotor blade is acceptable, how much requires repair, and how much makes the blade unsafe to ever use again.
Composite rotor blades create particular problems. "Metal blades are simple sheet metal work," said Jeff Range, director of sales for Composite Technology, Inc. They can be repaired by cutting out a deformed section, inserting a new piece, securing it and sanding it down to conform with the rest of the component.
Composites are another matter. They are more complex than metal components. Each separate, carefully designed material must effectively do its individual job and work with the material to which it is mated.
When repairing a composite blade is deemed feasible, there are several companies dotting the globe that will inspect, diagnose, repair and return to service nearly any rotor blade in existence. They all employ a variety of tests and repair techniques, from the surprisingly simple to the technologically complex.
When a blade first arrives at a composite repair facility, technicians review the accompanying paperwork, checking to see what kind of work is needed.
If the blade is in for a routine, scheduled inspection, the technician will send it through a battery of tests prescribed by the manufacturer. If the blade is in because it needs repair, the technician will review the notes made by the mechanic who sent the blade, then set about confirming the suspected problem.
Whether an inspection or a repair is the task at hand, composite blade technicians begin by looking closely at every inch of the blade, checking it for obvious deformities. They then perform a tap test across the surface using a small metal hammer to detect any unusual reverberation, a tip-off that debonding — a condition where something is no longer in proper contact with something else — has occurred.
After the initial review of documentation, visual examination and tap testing, it’s on to other diagnostic techniques designed to pinpoint trouble spots.
"We do all kinds of non-destructive testing," said Dana Kerrick, vice president of International Aviation Composites of Fort Worth, Texas. One non-destructive testing (NDT) technique is ultrasonic inspection, or ultrasound, which he was eager to describe.
A properly calibrated machine "tests the thickness of the skin to ensure that is has not been sanded too thin," said Kerrick. Sanding occurs when abrasive substances such as sand, dirt and even rain — the most common abrasive substance in an aircraft’s world — scour away at the blade’s surface, an action that can result in debonding.
The results of the ultrasound test appear on an oscilloscope-like screen for interpretation. The location of an abnormal fluctuation marks the point of the damage, even if it can’t be seen with the naked eye.
Rotor Blades Inc (RBI) also uses a wide variety of NDT techniques to find problem spots in composite rotor blades. A sister company of Bell Helicopter’s completions center Edwards Associates, RBI is especially adept at working on tail rotor blades from Bell’s 430, 222 and 206 models.
Eddy-current testing is a common diagnostic procedure RBI uses on any number of the 2,000 blades it inspects annually at its headquarters in Broussard, La. (just outside Lafayette), its RBI Hawker Ltd partnership with Hawker Pacific in Dubai and its Rotor Blades Ltd facility in Warminster, United Kingdom. (It plans to open a facility in Calgary, Alberta later this year.)
Eddy-current "can detect minute defects up to one-eighth of an inch in depth in metallic material," said Troy Penny, RBI’s director of operations. "It takes a couple of hours to inspect a set of tail rotor blades."
Penny explained that the device’s probe emits an electrical current around its tip. That current fluctuates or eddies as it passes over a crack, causing an analog meter to jump and an audible alarm to sound. Eddy-current testing equipment is expensive — anywhere from $10,000 to $15,000 a unit — but takes much of the guesswork out of crack detection.
Another diagnostic procedure popular among blade repair stations is fluorescent penetrant testing (FPT). Rotor-Tech International in Stockton, Calif. is an authorized blade repair station for nearly every helicopter blade. It does a lot of S-61 and H-3 rotor blades, which require FPT as part of their Sikorsky Aircraft-prescribed inspections.
"You spray a penetrant onto the blade," explained Mike Rojas, Rotor-Tech’s director of maintenance. "It sits for 5-10 minutes, then it’s wiped off. Then you spray the developer on." If a crack is present, said Rojas, the developer will seep into the crack, where it will react with the penetrant that escaped the surface wipe-down. The reacting chemicals will glow under fluorescent light, clearly identifying the crack. "We use it a lot," he said.
As with most blade repair companies, the people at Composite Technology, Inc uses thermal imaging at its headquarters in Grand Prairie, Texas, as well as its locations in Singapore, Brazil, England, Canada and its sister operation, Helitech Industries, in Brisbane, Australia. Composite Technology is a Sikorsky Aerospace Services subsidiary.
"Rain will wear that resin down to the fiber material and it will wick that water into the layers" of glass fiber, said Composite Technology Vice President Jeff Range. "That causes delamination."
Delamination occurs when the layers of fibers separate from one another, weakening the composite and, therefore, the entire blade.
The thermal imager reads heat signatures the same way forward-looking infrared — a type of thermal imager — does when used by a soldier on a battlefield at night.
"Whenever we aim that camera, you’ll see a darker image indicating where the water is." explained Range, who said the company uses it on blades from all manufacturers, not just Sikorsky’s.
With moisture being a serious enemy of composite rotor blades, one company decided to be proactive in preventing water seepage into one particular brand of rotor blades: those for Robinson Helicopter aircraft.
According to Jonny Quest, the technical director for Airwolf Aerospace in Middlefield, Ohio, a few Robinson operators began complaining that their stainless steel main rotor blades were delaminating due to water seepage into the blades’ core. (The delamination also occurs when the blade’s the internal aluminum tip cap corrodes.) But while other manufacturers allow debonding to be repaired, Robinson requires blades damaged in that way to be replaced. With the price of new R22 and R44 blades running in the neighborhood of $26,000 and $40,000 a set, respectively, that problem can be an expensive one. Airwolf came up with a supplemental type certificate (STC) to prevent delamination. It uses a kit that consists of specially developed polymer tape that seals blades from the elements.
Airwolf Aerospace has won FAA and European Aviation Safety Authority approval for the tape’s installation on R22s and R44s. The FAA also recognizes the tape installation as an alternative means of compliance with Airworthiness Directive 2007-26-12, which targets the Robinson blade delamination problem. Use of the tape eliminates the need for daily preflight visual checks of the blades that are otherwise required by the AD.
Quest added that the Army uses the same system on its helicopters stationed in the Middle East, which resulted in blade lives jumping from 20 to 200 flight hours.
Robinson Helicopter’s position is that delamination is not a problem if operators follow the maintenance manual’s requirement to ensure that all blade surfaces remain completely painted, thus sealing out moisture. Quest counters that Airwolf’s tape kits, which sell for $1,200 for the R22 and $2,500 for the R44, are more cost-effective than repainting a blade when sections become exposed through regular wear and tear.
Vendors are pushing the state of the art in composites inspection and repair. Thermal Wave Imaging of Ferndale, Mich., for instance, has worked with the U.S. Navy to develop a portable thermal inspection system for detecting subsurface flaws in metal and composite structures. According to the company, its ThermoScope system uses a unique approach to processing infrared image data to detect delamination, corrosion and small amounts of fluid infiltration "that were previously undetectable by thermal inspection methods." The system has been used on components for U.S. Marine Corps Bell/Boeing V-22s and Boeing CH-46s and U.S. Army Boeing AH-64s and CH-47s and Sikorsky UH-60s. (A CH-46 main rotor blade root crack is shown in the ThermoScope image on page 32.)
In the end, whether the issue is trying to detect, repair or prevent problems, rotor blades take very good care of people. It’s very important that we take good care of them, too.
Life-Cycle Costs a Key Design Concern for BERP 4
AgustaWestland’s design, development and production of its new BERP 4 main rotor blades have pushed the boundaries of the practical use of composite materials in helicopter components. AgustaWestland’s efforts are only the latest in that regard, the development and fielding of the NH90 transport helicopter still being the most notable. A product of the partnership of AgustaWestland, Eurocopter and Stork Fokker Aerospace, the NH90 airframe is made entirely of composites. But the BERP 4 effort is noteworthy, too.
The British Experimental Rotor Program had seven goals for the fourth-generation blades: reduced initial and life-cycle costs, reduced rotor vibration at high and low speeds, improved hover and forward flight performance, improved damage tolerance, increased erosion resistance and reduced signatures. Achieving the first two concerning costs required rethinking how rotor blades are produced and maintained throughout their service lives, which drove a number of innovations that AgustaWestland officials say will filter down to blades for all their helicopters. One example? AgustaWestland developed removable blade caps to simplify repair of damage when blade tips strike objects.
The largely composite BERP 4 blades are entering service this year on AW101s operated by the U.K. Royal Air Force. The test AW101 that achieved certification of the BERP 4 blades is shown below. AW101s fitted with BERP 4 blades have also been flown at weights up to 36,376 lb (16,500 kg) — 4,189 lb (1,900 kg) more than the normal Merlin gross weight.