Flying high on composite wings

Buffeted by boom/bust business cycles and some painful shakeouts for most of the past 50 years, the general aviation (GA) market is currently enjoying an upturn. Sales have been on the rise since 1994, despite a slight retraction from 2001 to 2004, and last year they reached an all-time high (see our GA market report on p. 70). What’s driving the boom? A significant factor is the exponential increase in the number of GA planes made with composites. “Of the 2,675 piston-engine-powered aircraft produced in 2007, 1,376 were made with composite airframes,” says Jens Hennig, VP of operations for the General Aviation Manufacturers Assn. (GAMA, Washington, D.C.). That’s 51.4 percent. A decade ago, less than 5 percent were fabricated with composites.”

Much of that increase can be attributed to new certified aircraft manufacturers Cirrus Design Corp. (Duluth, Minn.), Diamond Aircraft (London, Ontario, Canada), Liberty Aerospace (Melbourne, Fla.) and the former Columbia Aircraft (Bend, Ore.), which is now owned by Cessna Aircraft Co. (Wichita, Kan.), all of whom started significant production of composites-intensive aircraft within the last few years. Additionally, Hawker Beechcraft Corp. (Wichita, Kan.) has employed automated advanced fiber placement techniques on its jet fuselage barrels for seven years. Companies that are currently in the process of certifying composite designs for personal/business jets include Cirrus, Diamond, Epic Aircraft (Bend, Ore.), Embraer (Sao Jose dos Campos, Brazil), Honda Aircraft Co. (Greensboro, N.C.), Spectrum Aeronautical (Carlsbad, Calif.), Farnborough Aircraft Corp. Ltd. (Farnborough, U.K.) and Bombardier Aerospace (Dorval, Quebec, Canada), which is working with partner Grob Aerospace (Zurich, Switzerland) on the new Learjet 85. This list doesn’t include the many kit plane companies who sell composite components for customers to assemble. HPC surveyed a representative group of these manufacturers to get a glimpse of the materials and fabrication methods that are driving this market segment.

A metal-to-composites leap

Known for its all-aluminum aircraft, Cessna became, in late 2007, a full-blown composite aircraft OEM when it purchased the assets of bankrupt Columbia Aircraft. Cessna inherited the composites legacy of aircraft designer Lance Neibauer, who spun off Lancair Certified and later Columbia Aircraft in the 1990s to produce certified aircraft based on his popular experimental kit plane designs. Noted for speed and good looks, the fixed-gear piston aircraft, now dubbed the Cessna 350, and its turbocharged piston sibling, the Cessna 400, have an enthusiastic following in the pilot community: “The 400 is the fastest certified production piston aircraft made. This was a great addition to the Cessna line,” notes Rod Holter, VP/GM of Cessna’s Independence, Kan. plant and interim GM of the Bend facility.

Holter explains that both planes were certified to meet more stringent-than-normal “utility” aircraft category rules under the U.S. Federal Aviation Admin.’s (FAA) Federal Aviation Regulations (FAR) Part 23. Utility-certified planes have to meet a 4.4G positive load factor and 1.76G negative limit load, says Holter, which makes the aircraft more maneuverable, durable and survivable in the event of a crash.

Structural parts for the 350 and 400 are hand layed up using unidirectional and woven prepregs supplied by both Newport Composites and Adhesives Inc. (Irvine, Calif.) and Advanced Composites Group Inc. (ACG, Tulsa, Okla.). Key structural parts, including wing spars, fuselage longerons, horizontal stabilizer, control surfaces and other parts subjected to high loads are made with carbon fiber/epoxy. E-glass/epoxy prepreg makes up the majority of the remaining structure, including the fuselage and wingskins. According to Holter, about 15 percent of each airframe is carbon. All material allowables are company proprietary, says senior project engineer Tom Bowen, meaning the company conducted its own qualification testing rather than relying on prequalified materials (for information about prequalified, see “Learn More”).
The sandwich construction fuselages are fabricated in two halves, right and left, with longitudinal bondlines. Skins are fiberglass/epoxy, and the core is a unique, 3-lb density resin-impregnated aramid paper core, manufactured by Advanced Honeycomb Technologies Inc. (San Marcos, Calif.). Bowen explains that the core material has a unique cell design and a higher density than standard phenolic paper honeycomb core, delivering higher hot/wet compression strength — one of the key elements that ensured “utility” certification.

The wings are a one-piece assembly with a dual-carry-through spar, explains Bowen, which attaches to the fuselage with mechanical fasteners through a reinforced contoured composite fuselage saddle. The two C-section spars face each other and run the full length of the wing structure. Each spar carries an upper and lower spar cap built up from unidirectional carbon prepreg strips; webs are made with fiberglass/epoxy prepreg over honeycomb core. The upper spar caps, which are in compression during flight, taper from approximately 135 plies of uni carbon prepreg at the root to just two plies at the wingtip. The lower spar caps, in tension, taper from 72 plies at the root to two plies at the tip.
“The wing is a very diverse part because of the complex loads to which it is subjected. The bending moment is very small at the wingtip but ramps up to a substantial load at the root area,” notes Bowen. The cored fiberglass upper wingskin, with its higher compression loads, is a 5/core/2 construction at the root (that is, five skin plies on the upper side of the core layer, two skin plies below), and it tapers to 2/core/2 at the wingtip. The lower wingskin is 3/core/2. Embedded aluminum and copper wire mesh in the skins provides lightning strike protection.

After layup, parts are cured in an oven and then demolded and bonded where required. A second oven cure is required to cure the adhesive. Nonstructural parts, like interior panels, are made either with prepregs or in a wet layup process and are similarly oven-cured.

Bowen notes that production tools at the Bend plant are currently a mixture of aluminum, fiberglass and carbon/epoxy composite: “Columbia couldn’t afford to invest in more expensive high-rate metal tooling.” Bowen and Holter both say that with Cessna’s deeper pockets, it is likely that more robust production tools will be forthcoming, based on ongoing trade studies.

“Composites are the only material that can effectively capture the complex shapes and radically different flowing lines of these planes’ designs,” concludes Bowen. Holter adds that the entire 2008 production quotas of both the 350 and the 400 are sold out, which bodes well for the future.

The lion’s share

The number one general aviation OEM in the U.S. today, in terms of total deliveries of composite-airframe aircraft, is Cirrus Design. The company delivered 710 aircraft in 2007, or 26 percent of the total for piston aircraft deliveries, more than any other manufacturer in GAMA’s database.

Cirrus Design was started in 1984 as a kit plane designer/manufacturer by brothers Alan and Dale Klapmeier. In 1998, the duo certified the SR20, a lightweight, all-composite four-place airplane (see HPC July/August 1999, p. 9). Today, the company offers the SR20, SR22, a turbocharged version of the SR22, the SRS Light Sport Aircraft model (see “Learn More,” p. 69) and a “personal” jet named the-jet, which is currently undergoing certification. Jay Yeakle, Cirrus Design’s chief of airframe engineering in the company’s advanced development group, says, “Alan and Dale Klapmeier cut their teeth on a Glasair Aviation kit plane before they started the company. Composites are what we know.”

The SR22 Generation Three (G3) aircraft is the company’s highest-performance piston-engine offering and its latest design. The majority of the aircraft is fabricated using E-glass/epoxy prepreg, with some S-2 Glass/epoxy prepreg (incorporating S-2 Glass fiber manufactured by AGY, Aiken, S.C.) and carbon/epoxy prepreg used strategically for added stiffness and strength, says Eric Hartwig, chief of materials and process engineering. Glass prepregs are supplied by TenCate Advanced Composites USA (Morgan Hill, Calif.) and are prequalified through the National Center for Advanced Materials Performance (NCAMP) at the National Institute for Aviation Research (NIAR) at Wichita State University. (Cirrus performed qualification and design allowable testing for the TenCate BT250 prepregs, notes Hartwig.) The plane, like Cirrus’ other models, is qualified under FAR Part 23 regulations to “normal” certification standards.

The G3 incorporates a one-piece, monolithic (uncored), autoclave-cured carbon/epoxy C-shaped wing spar that saves 30 lb/13.6 kg over the previous spar design, translating to higher payload and greater range for the G3. The 35-ft/10.7m long spar, approximately 0.5-inch/12.5-mm thick, is produced for Cirrus by Applied Composite Technology (ACT) Aerospace (Gunnison, Utah). The material in the spar is NCAMP-qualified carbon/epoxy prepreg from Toray Composites America Inc. (Tacoma, Wash.), made with Toray’s T700 carbon fiber, says Hartwig. Both a 12K plain weave fabric and a 145 g/m2 unidirectional prepreg are used in the spar’s construction, which is in the form of the letter “C,” with the curve facing forward.

“Large area parts” are sandwich constructions hand layed up by Cirrus technicians. The fuselage is split lengthwise into right and left halves that include the vertical tail. Typical construction is 2/core/2, with E-glass/epoxy skins over Divinycell HT (aerospace-grade) closed-cell foam core supplied by DIAB Sales Inc. (DeSoto, Texas). Core thickness is typically 0.4 inch/10 mm. Yeakle explains that the number of skin plies increases as needed in key areas to increase buckling stiffness and strength around penetrations. Plies of uni S-2 Glass prepreg are incorporated into the fuselage roof to form the roll cage structure for rollover protection.

The same materials and methods are used for the wingskins and horizontal stabilizer skins, which incorporate, in selected areas, a surfacing film containing an expanded aluminum mesh for lightning strike protection. Tooling is a mix of billet steel and glass/epoxy and carbon/epoxy composites made with tooling prepregs from ACG.

After oven cure, the large parts are bonded together using PTM&W Industries Inc. (Santa Fe Springs, Calif.) ES6292 paste adhesive — including the fuselage/tail halves, the wingskins (over the spar and ribs) and the horizontal stabilizer skins. The horizontal stabilizer is then bonded to the fuselage, and the wings are bolted to the fuselage with stainless steel and titanium fasteners. Control surfaces are aluminum, notes Yeakle, and galvanic corrosion is not an issue because no carbon is in contact with the aluminum. Interior panels include natural fiber composite materials from Composite America LLC (Fargo, N.D.).

Approximately 1,700 labor hours are required to produce each SR22 G3 aircraft, which incorporates safety features like the trademarked Cirrus Airframe Parachute System (CAPS). Yeakle attributes the company’s current success and market dominance to continuous improvement: “We don’t allow our designs to stagnate — we’re always striving to make our aircraft better.”

From kits to jets

“We would have to have a compelling reason not to use composites,” states Dieter Koehler, VP of engineering and certification for Epic Aircraft. Started in 2004 by CEO Rick Schrameck — a dot-com entrepreneur and early Lancair kit-plane customer — Epic made news last year, setting a record for the most sales logged — $23 million — by any exhibitor during the annual Experimental Aviation Assn. (EAA) Sun ‘n Fun air show in Lakeland, Fla. The buzz was generated by the company’s latest experimental models — a new, single-engine very light jet (VLJ), the Victory, and the Escape single-engine turboprop, a takeoff on the company’s successful Epic LT turboprop kit plane. Although neither is yet certified, the company is working toward that goal. Its Dynasty turboprop, the certified version of the LT, is expected to complete certification in 2009. These planes are fast — the Escape turboprop has a maximum cruise speed of 365 knots/420 mph, which is as fast or faster than most of the VLJs on the market today (see “Learn More”).
The company uses carbon/epoxy prepreg materials for hand layup of all of the aircraft’s structural components, which were designed with NASTRAN design software. All production tooling is made with carbon/epoxy composite, fabricated in-house by Epic technicians from outsourced CNC-machined masters and plugs.

Prequalified prepreg materials are supplied by either Toray Composites America or Advanced Composites Group. A small percentage of fiberglass prepreg is employed for galvanic corrosion protection, notes Koehler.
The fuselage is designed to be pressurized to 6.5 psi/0.45 bar for flight at altitudes of up to 28,000 ft. This “pressure vessel” is a single, large component, complete from the firewall in the front to the pressure bulkhead in the rear, fabricated in a proprietary, nonautomated process. While Koehler would not reveal details, the process might involve producing two shells and bonding them together, then cocuring the bonded component in one cycle to produce a single, seamless part. A composite nose cone and rear fuselage cone complete the fuselage. Doors and elliptical windows are cut out after the part has been cured.

Notably, the fuselage originally was designed with an aluminum center post between the two front windows: “We’ve replaced that with a composite center post made with nearly 100 carbon prepreg plies,” says Koehler. “We’ve saved about $2,500 despite the higher raw material cost because we’ve eliminated CNC machined parts, fasteners and assembly — it’s a better design for manufacturability.”

The Escape’s one-piece “wet” wing (fuel is carried in the wing’s hollow interior) is made with upper and lower skins, two carry-through spars and eight supporting ribs per side. Unidirectional carbon prepreg plies make up the spar caps that support the upper and lower wingskins and prevent the skins from buckling. Says Koehler, “We can get away with very few ribs because of the composites’ stiffness.” Wing skins are typically 3/core/3, with slightly thicker skin buildups in the root area, and 2/core/2 toward the wingtips. Closed-cell Divinycell foam core material supplied by DIAB Sales Inc. is used rather than honeycomb core in the wingskins.

The fuselage is attached to the one-piece wing via four sets of aluminum brackets. The horizontal stabilizer and vertical tail are also composite and of aramid/phenolic honeycomb-cored sandwich construction. Epic uses embedded copper mesh in the outer skin laminate for lightning protection. The aircraft’s interiors also incorporate composites in the floor and side panels and in the base structure of the instrument panel. Some nonstructural parts (including the tooling) are made via vacuum-assisted resin transfer molding (VARTM) methods.

“Composite airplanes are stronger and safer than metal planes because of the worst-case hypothetical test conditions that are imposed by the certifying bodies,” concludes Koehler.

Two seats, same range

Nearly 10 years in development, startup Liberty Aerospace Inc.’s (Melbourne, Fla.) certified XL-2 has been featured previously in HPC (May 2002, p. 23). The snug and sporty two-seater is targeted to training schools and private touring pilots who want a smaller aircraft, says Adam Maxfield, Liberty’s composites manager. The XL-2 is designed with a carbon/epoxy fuselage but has aluminum wings, rudder, flaps and ailerons, a decision made primarily to keep certification costs as low as possible. The company delivered 38 aircraft in 2007 and has a development deal in process with a Chinese manufacturer, Anyang Angel Aero (Henan, China), to manufacture 600 XL-2s for the Asian market.

Carbon/epoxy prequalified prepreg from Toray Composites America and 5-lb density closed-cell foam core from Alcan Baltek (Northvale, N.J.) are used to fabricate the sandwich-construction fuselage, comprising an upper and lower half — a departure from the lengthwise splits common to other designs discussed in this article. Maxfield explains that the design goal was to keep the overall airframe weight as low as possible. As a result, the fuselage has relatively thin skins and buildups only where necessary at fastener location hard points. The two seats are integral to the lower fuselage part and are fabricated with a combination of both carbon and E-glass prepregs. On the upper fuselage, unidirectional carbon/epoxy is used to build up the strong and stiff rollover hoop structure that supports the windshield and provides protection to the occupants as well. At metallic fastener locations, a ply of 7781 woven E-glass/epoxy prepreg is layed over the carbon plies to prevent galvanic corrosion of the aluminum.

In areas where buildups are not required, the fuselage is typically a 1/core/1 sandwich. “These thin skins,” says Maxfield, “eliminated honeycomb core as a material choice due to the potential for print-through on the surface.” The company opted instead for the high-density Airex foam, which gives a smoother result. In addition, the foam requires no edge potting, does not absorb moisture and has temperature resistance high enough to stand up to the curing process.

Other composite parts include a belly fairing that covers the chassis (see below), upper and lower engine cowls, the doors, wingtips and wheel “pants” (the aerodynamic covers that reduce drag on the fixed gear). Interior parts made in composites include the instrument panel and bulkhead supports.

Layup is structured into a series of “steps” — for example, the approximately 20 plies that make up the rollover hoop structure would constitute one step — for tight quality control. When complete, parts are oven-cured and then assembled together with the metallic components. The fuselage sits atop a metal chassis structure, to which it bolts. The engine and wings also attach to the chassis. Maxfield notes that after bonding, drilling and finishing, the fuselage part weighs a mere 101 lb/45.8 kg, which helps contribute to the plane’s relatively long range and useful payload, which are both outstanding for a two-seat trainer.

. . . in the sky with Diamonds?

Diamond Aircraft began 25 years ago as Hoffmann Flugzeugbau in Friesach, Austria, building fiberglass gliders. Renamed HOAC AG, it began production of the DV20 Katana two-seat airplane in 1991. The next year, North American production was established when HOAC opened a facility in London, Ontario, Canada. The Katana won Flying Magazine’s Eagle Award for best light aircraft in 1995, and its use as a trainer, particularly by the U.S. Air Force Academy, helped propel the Canadian group, now Diamond, to prominence in the mid-1990s.

Composites were always preferred to aluminum, says the company, for their corrosion resistance and better strength-to-weight ratio, which translates to a lighter overall aircraft and greater fuel economy. While its piston aircraft — the two-seat DA20 Eclipse, the four-seat DA40 Diamond Star and the twin-engine DA42 Twin Star — are predominantly fiberglass-skinned sandwich construction, the company’s latest offering, the single-engine D-JET very light jet, is mostly carbon fiber. Fuselage parts are made in left- and right-hand halves, with longitudinal joints.

Diamond is somewhat unique in that it employs wet layup methods to make its aircraft. A custom saturator wets out the fiberglass or carbon cloth reinforcement in a resin bath, applying a precise amount of epoxy. The wet out cloth is placed on a sheet of peel ply and then lifted into the mold for layup. While the company does not reveal precise details about the laminate architecture, several plies make up the sandwich skins. Unidirectional carbon fiber prepreg strips are applied to highly loaded areas, such as the perimeters of windows and doors, for additional stiffness and strength. Core is closed-cell foam, about 0.5 inch/12.5 mm in thickness, which helps with noise damping and insulation for pilot and passenger comfort.

When layups are complete, the parts are vacuum-bagged then cured in an oven at 100°F/38°C. After removal from the molds and deflashing, parts are adhesively bonded. Major subassemblies are returned to the oven for a freestanding postcure at 175°F/80°C for 18 hours.

The company’s momentum is building: Progress is being made on certification of the D-JET, and the company opened the Shandong Bin AO Aircraft Industries (SBAAI) production facility in Binzhou, China. The purpose-built 400,000 ft2 (37,160m2) plant is designed to produce up to 1,000 aircraft per year and will deliver 80 airplanes in 2008 for the Asian market, says the company.

Advancing with automation

Although the days of the Beech Starship have passed, the innovations associated with that certified, all-composite aircraft are still evident at Hawker Beechcraft, the current incarnation of the historic Beech Aircraft Corp., founded in 1932. The company currently produces the Hawker 4000 and Premier 1A jets, using advanced automated fiber placement methods. Says Mike Mott, Hawker Beechcraft’s senior principal engineer for structural integrity, “We selected composite materials based on economics, strength-to-weight, manufacturability and maintainability. The benefits are increased cabin volume, overall weight reduction and lower part count, which reduces touch labor — and we achieve a near-perfect aerodynamic surface.”

The Premier 1A fuselage consists of two barrel shells, spliced at the aft pressure bulkhead, while the Hawker 4000, a larger business jet, is made with three barrels spliced at the wing attachments. The barrels are manufactured on an aluminum mandrel, using a Viper automated fiber placement system from MAG Cincinnati (Hebron, Ky.). Unidirectional prepreg tape used in the process is 12K intermediate modulus carbon tow (G40-800 and IM7), supplied by Toho Tenax America Inc. (Rockwood, Tenn.) and Hexcel (Dublin, Calif.), prepregged with epoxy resin, either 977-2 or E7K8, supplied by Cytec Engineered Materials Inc. (Tempe, Ariz.), says Warren Hatfield, senior materials and process engineer. “We use 80 miles of 12K carbon tow in each fuselage,” notes Mott.

Fuselage barrel parts are sandwich construction, with epoxy skins over an aramid/phenolic paper honeycomb core supplied by Hexcel. The minimum construction is 3/core/3, which is increased in certain areas up to the number of skin plies needed for specific loads, says Mott. Common ply orientation is 0°/+45°/-45°/90° but fiber architecture is optimized, area by area, depending on specific load requirements. Buildups are created around doors and windows for added strength.

The uni prepreg tape is machine placed, while fabric prepreg, core and film adhesives are laser tracked and hand layed on the male mandrel. The outermost (mold line) ply includes SurfaceMaster 905 material from Cytec for lightning strike protection, says Hatfield. After layup on the mandrel is complete, Hatfield reports that green parts are taken off the aluminum mandrel and placed in a female, carbon/epoxy two-part clamshell tool, vacuum-bagged and cured in an autoclave in a stepped cure process. Barrel splices are accomplished with a combination of bonding and mechanical fasteners. Adhesive films, foaming adhesive, paste adhesive and potting compounds are supplied by 3M (St. Paul, Minn.).

Concludes Hatfield, “We absolutely see our company continuing with composite designs in the future — we’ve focused our designs to provide true value to our customers.”