Materials 'keep 'em flying'—longer

With heavy defense budget cuts and commercial aircraft sales suffering, the aerospace industry and its suppliers have turned their endeavors to materials that last longer and are easier to repair. Such efforts can be equally rewarding from a design standpoint. In fact, you will find many of them wending their way into new aircraft concepts and components.

Some of this design work is already paying big dividends. The recent wreck of a Raytheon Starship provides an excellent case-in-point. So do activities taking place at aircraft maintenance facilities and by component makers that cater to the aerospace industry.

Even with this increased emphasis on keeping aircraft flying longer, new design projects have not come to a standstill. For instance, a new design will employ "smart" materials to de-ice helicopter rotors. In time, such designs will let the aerospace industry soar to new heights. Let's take a look at some recent happenings in the world of aerospace materials.

Sterling Starship. The increasing use of composites in aircraft got a major test last year when Starship N-35, in an aborted takeoff in Denmark, skidded off an icy runway on its belly for nearly 1,000 feet. The nose landing gear of the composite aircraft collapsed, causing considerable damage to both the left and right main gear. The starboard wing, which absorbed some of the impact when the aircraft turned sideways during its slide, sustained minor damage, while lesser damage occurred to the port wing when the main gear tore loose.

During the impact, the fuselage remained structurally intact, and the pressure cabin sustained no punctures. No fuel was spilled or glass broken. There were no injuries to the pilot or the four passengers.

Of equal interest to aerospace engineers, however, was the fact that repairs took just 24 weeks, and, despite an original estimate of $1.2 million, the rebuild totalled only $806,000, Consider, also, that the repair work was done with hand tools in a hanger by Beechcraft Scandinavia personnel. The only non-ordinary items involved the use of heat lamps for curing the carbon fiber/epoxy composite materials. All the tools were readily available in Denmark.

During repairs, the aircraft was supported by conventional plywood forms cut to fit the fuselage shape. "This shows that you don't have to have all the specialized equipment that people think of as necessary in handling composite materials," says Jack Marinelli, Starship program manager at Raytheon Aircraft, Wichita, KA.

Marinelli has briefed insurance companies on the ease of the repair and the durability of composite aircraft. Most of the damage to Starship NC-35, he points out, was to forged or machine-turned fittings and other non-composite parts. The repairs did add about 48 pounds to the aircraft, due mainly to material used to splice new sections to old.

"Nobody wants to see an aircraft accident," Marinelli adds, "but we were pleased that all the features we built into the aircraft worked. We will incorporate the lessons learned in this incident in the design of future aircraft." No doubt composites will be an integral part of that design.

Cure for composites. Easy-to-handle, rapid-cure materials also help speed the repair of damaged aircraft composite parts. To make the process go even faster and extend the life of the composites, American Composites, an FAA-approved repair station at the Miami International Airport, turned to Accustick™ epoxy syntactic from Ciba Aerospace Products, Los Angeles. American Composites specializes in maintaining flight-control surfaces, such as flap vanes, as well as nacelles, doors, fuselage fairings, and radomes.

To apply Accustick, technicians simply cut the desired amount of dough-like resin and hardener, knead it briefly into a uniform color, and install the material in the seam between pieces of honeycomb. "Accustick has saved us a lot of time on repair projects," says Jorge Sabido, American Composites president. "The compound cures in less than 10 minutes at 77F. It works well on composite components like wings and cowlings with its low cured density of 0.70 g/cc. And it has a compressive strength of 4,500 psi and tensile lap shear of 1,260 psi at room temperature. These values help us produce high-quality, durable repairs."

Depending on the extent of the damage, the aluminum/fiberglass/honeycomb composite components can be repaired by injecting delaminated sections with resin or with new composite materials.

"Aesthetics are of primary importance. Passengers dislike seeing blemishes on flying surfaces," Sabido explains. "With the Ciba materials, we can repair aircraft components quickly with no sacrifice to appearance. Fast, high-quality, long-lasting repairs are the key to success in maintaining aircraft components today."

Hoping to broaden the use of aerospace composites even more, ICI Fiberite, Tempe, AZ, has introduced a new graphite-reinforced epoxy material. The Fiberite® 970 composite is a 350F-cure, flow-controlled resin that can produce nonporous, void-free honeycomb sandwich structures, according to Fiberite's John McKnight. The material also exhibits excellent out-time and has a long shelf life, he adds.

Other properties of Fiberite 970 reported by McKnight include:

* Excellent tack characteristics.

* Outstanding durability.

* Cures at 45-100 psi.

* Easy to use for hand layup or fiber placement applications.

* Excellent self-adhesive properties.

The product comes in tape, fabric, and tow on a variety of fibers. McKnight expects the material to receive FAA approval shortly.

Alloy boosts engine performance. Boeing and Pratt & Whitney (P&W) established a strict timetable when they began a joint design effort to develop a powerplant for the new 777 twinjet. To meet that tight schedule and reduce uncertainties, P&W decided to extend its well-established PW4000 line of engines for the project. The result: the huge, high-thrust PW4084.

One key change, as reported by knowledgeable sources outside P&W and the material suppliers, involved selection of materials for the high-pressure-compressor (HPC) case to better control tip-gap clearance. The HPC experiences considerable dimensional changes, which, in turn change clearance margins. This occurs because centrifugal growth of the rotor takes place as speeds accelerate. The case heats up, as does the bore of the disc.

The material chosen had to exhibit expansion rates that are less than that of the rotor's discs and blades as temperatures increase. The winning materials: CTX-909 alloy and Thermo-Span®, the latter invented and patented by Carpenter Technology Corp., Reading, PA.

For the ring segments at the aft end of the compressor case, Thermo-Span best fit the bill. Though varying in mass, weight, and configuration, the rings must provide a predictable coefficient of expansion (COE) to mate with the moving and expanding components where temperatures range from 305 to 315C.

The alloy has a low and nearly constant COE between room temperature and its transition temperature of 320C, and improved elevated temperature oxidation resistance, according to James M. Dahl, a product application manager at Carpenter. It also has mechanical properties similar to 718 superalloy, used for the disc and blades. Nominal composition of Thermo-Span: Ni 24.5%, Co 29%, Cr 5.5%, Si 0.25%, Cb 4.8%, Ti 0.85%, Al 0.45%, B 0.004%, and the balance Fe.

"Thermo-Span has a transition temperature about 105C lower than CTX-909 used for rings in the fore part of the HPC," Dahl adds. "This low transition temperature, along with a low COE, proves desirable for compressor efficiency in the normal flight mode. And the alloy's higher expansivity above the transition temperature proves advantageous when the engine operates in its high-thrust mode."

The alloys aren't the only way that P&W controls the close tolerances between the engine components. It also calls on an active clearance control that extends from the high-pressure turbine along the entire length of the HPC. This dual-channel, full-authority, digital-engine control sends cooling air to shrink the case during the cruise mode. With this enhancement, clearances between moving parts can become even smaller.

Recent certification flights of the 777, as well as healthy initial sales of the twinjet, attest to the fact that P&W and Boeing made the right material choices.

Shapes of the future. In a somewhat more futuristic endeavor, Etrema Products, Inc., Ames, IA, has teamed with Northrop-Grumman Corp. to develop an Adaptive Wing. One concept under evaluation in the $3 million ARPA Smart Materials and Structures (SMS) program: a wing that will alter shape during flight to optimize performance at a range of speeds when cruising in the transition between subsonic and supersonic speeds.

The design would employ "smart" actuators made from Etrema's Terfenol-D®. The material is a giant magnetostrictive alloy that changes shape in proportion to an applied magnetic field. The material elongates 50% more than the best known commercially available materials, and exhibits 10 times the energy density, says Becky Jones, head of Etrema's customer development.

Currently, supersonic aircraft wings are designed for an optimal cruising speed. Consequently, at any other speed, they can produce shock waves that increase drag. Using Terfenol-D actuators, the Adaptive Wing would change its cross-sectional shape to compensate for various cruising speeds during transonic flight. Ultimately, the researchers feel, the wing would enable shock-free transonic flow to reduce drag, providing improved range and fuel use.

Etrema also has joined with researchers at San Diego State University in an effort to better detect cracks and delaminations in composite structures through the use of Terfenol-D. Here, micron-size particles of the material are embedded in the composite to provide constant monitoring of the structure. The particles generate magnetic fields that can be detected by sensing coils.

The application allows the internal stress field of the structure to be measured by a magnetic transducer. A defect or delamination would show up as an abnormally high magnetic field caused by the stress.

Another project based on shape-memory alloys will help ensure aircraft safety by electromechanically removing the ice that accumulates on the wings and tail. Innovative Dynamics, Ithaca, NY, introduced the technology, which it developed with a funding assist from NASA Lewis Research Center, at this year's Paris Air Show.

"This new technology enables--for the first time--de-icing systems on commercial and business helicopters of all sizes," says Joseph Gerardi, Innovative Dynamics president. "The system is lightweight, aerodynamically non-intrusive, has a low power requirement, and requires minimal supplemental hardware."

The nickel and titanium shape-memory alloy (SMA) is made into thin sheets and incorporated into the leading edge of the helicopter's rotorblade. The de-icer measures less than 1.5 mm thick.

Under icing conditions, the pilot or automatic cycling sensors electrically activate the SMA sheet. Heat from the power source causes the alloy skin to change shape slightly, which de-bonds the ice. The ice sheds into the airstream through the centrifugal force and vibration of the rotating blade; the alloy returns to its original shape.

"Only a small amount of power is needed to perform the de-icing action," explains Richard Ingram, lead project engineer. "That's significant in terms of generator requirements." Some large military helicopters use a de-icing system that melts the accreted ice, requiring a huge amount of energy and a large generator. Most helicopters don't have sufficient power for this system, and consequently have no de-icing alternative.

The special alloy has another key benefit in addition to its shape-change elasticity: It remains exceptionally resistant to corrosion and abrasion. This, says Gerardi, makes the de-icing system and the rotorblade durable and long-lasting, giving the system a high cycle life.

A rotorblade de-icer prototype underwent successful tests recently at NASA Lewis. The tests included a wide variety of ice types and thicknesses, and the entire icing temperature range. Every type of ice was de-bonded and shed. Based on these results, Innovative Dynamics will build a pre-production prototype as the next phase of the commercialization process.

What's next? According to Geradi, the use of SMA technology to "create an ice-phobic surface that never allows ice to accrete in the first place."

So, if the application is "out-of-this-world" or more "down-to-earth," materials can make the difference in optimizing aerospace designs. Stay tuned for further aerospace developments.