PRODUCTION TECHNOLOGY

Composite aircraft evolution

After several false starts, fully composite airframes are finally acceptable for mainstream aircraft.

By Mike Venables
Principal, TriLink Technologies Grp


Bombardier Learjet 85 is expected to be the first composite business jet to achieve certification.

My first experience with plastic aircraft occurred on a birthday long ago when my grandmother gave me a model. It was a Harvard (aka AT6 Texan).

I must have used half a tube of glue to put it together and, for the radio antenna from a mast behind the cockpit to the top of the vertical stabilizer, I used a piece of string which, at the scale of the model, must have represented a cable some 3 inches in diameter. (I was 6 years old, after all.)

My next exposure to composite aircraft was in the early 1970s when I started towing gliders. The first composite (fiber-reinforced plastic) aircraft was a Glasflügel Libelle (Dragonfly) H301, a competition sailplane certified in both Germany and the US in 1964. Its performance was a paradigm shift compared with so-called conventional aircraft made of wood or steel tube and fabric or sheet metal.

The composite structure provided great strength and low weight. The manufacturing process allowed the designers free rein to use compound curves wherever needed to optimize the aerodynamics, particularly with fairings around the wing/ fuselage interface.

The resulting smooth skin, free of rivets, lap joints, etc, provided very low parasitic drag. The Libelle instantly made other sailplanes noncompetitive—and within a few years all competition ships were of composite construction.

Composite powered aircraft

In contrast to their rapid acceptance among soaring pilots, the powered aircraft community (with the exception of amateur builders) has long ignored composite aircraft. The first powered aircraft to have a fully composite structure was the Windecker Eagle (certified in 1969).

The aircraft was made of nonwoven glassfiber embedded in resin and the fuselage was molded in 2 halves and then joined together, much like my Harvard. (In a composite structure, the resin or epoxy filler holds the fibers in position and the fibers take the load.)

Due to FAA's unfamiliarity with composite structures at the time, the agency required a 20% strength increase that introduced a weight penalty. Despite the im­proved performance and very clean and pleasing aesthetics, the market voted with their feet and stayed away.

The economy at the time didn't help, but the main issue was lack of acceptance of a "plastic" airplane. Only a handful was ever produced, although the military did test 1 or 2 examples for their stealth characteristics.

Beechcraft Starship taxies for take-off at OSH (Oshkosh WI). The large winglets replaced the vertical stabilizer and rudder.

As mentioned above, amateur builders have embraced composite construction techniques. Burt Rutan kicked off the concept in the early 1970s with 2 unusual canard designs (which would become a Rutan trademark)—the VariEze and the slightly larger Long-EZ. Both models were built using resin and glass cloth laid up by hand over foam cores.

The Long-EZ had startling performance—with its Lycoming O235 producing 115 hp, the aircraft could travel along at 160 kts (max cruise) or, with the power reduced, fly more than 1700 nm. More than 1500 examples of the VariEze and Long-EZ models have been built.
Another attempt at a composite powered aircraft was initiated by Hawker Beechcraft (then Beech Aircraft Corp) in collaboration with Scaled Composites (Burt Rutan's then-new company).

The Starship 2000, which first flew in 1986, was spectacular for its unconventional configuration, speed and docile handling qualities. It was constructed of carbon fiber composite and, unfortunately, FAA imposed the same strength penalty that hobbled the Windecker Eagle.

This, combined with bad economic times, limited production to a few dozen. In 2003, Hawker Beechcraft tried to scrap all examples to eliminate the product support obligation, but a few owners were so enamored of their Starships that they refused to turn them over. These few examples are still flying with parts being cannibalized from a number of hangar queens.

Hawker Beechcraft currently has 2 partially composite aircraft in its line-up—the Beechcraft Premier and the Hawker 4000 (née Horizon). Both feature a carbon fiber/ epoxy honeycomb fuselage with a conventional metal wing.

Burt Rutan's Long-EZ was readily accepted by amateur builders. The startling design and performance were harbingers of the Starship.

After 32 years building composite gliders, motorgliders and single-engine trainers, Grob Aerospace began development of an 8-passenger business jet, the SPn, in 2003. As with its other models, the aircraft was entirely composite, with major sections being made of long strips of carbon fiber cloth sandwiched with strips of uncured resin and laid up by hand in metal molds.

The molds were then placed in an oven to cure the resin. In the case of the fuselage, it was built in 2 halves that were then joined and recured.

The Glasflügel Libelle H301 was the first fully composite aircraft when it was certified in 1964.

The amazing thing about this type of structure is that it gains strength with use. Certification authorities typically require 4 times life fatigue testing. In the case of the SPn, with a design life of 25,000 hrs, this meant that 100,000 simulated hours of load cycling were required.

At the end of the test, the ultimate load strength must be no less than 95% of the original value. Grob found that the strength of the carbon fiber structure was actually 104% of original! The accepted explanation is that the repeated cycling causes the fibers to align better and therefore gain strength.

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