Composite aircraft evolution

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

Learjet 85

Structure of the Learjet 85 is over 95% composite. The wings are made in Belfast, Northern Ireland with the balance being made in Mexico.

The process was a game changer and Bombardier enlisted Grob's assistance when it decided to develop the new Learjet 85 as a fully composite aircraft. Unfortunately, Grob lost its major investor in late 2008, at a time when it was impossible to find alternate funding, and was forced to shutter the entire factory.

While other Grob products have found buyers, the SPn is still looking for a white knight. This obviously threw a monkey wrench into Bombardier's plans too, and the Learjet 85 program had to find an alternative. Management at Bombardier realized that, with 31,000 employees in a number of plants in various places around the world, they had plenty of talent and experience to draw on.

For example, the former Short Brothers facility in Belfast, Northern Ireland has been making the fully composite horizontal stabilizer for the Global Express. Bombardier also has relationships with a number of educational institutions. One such university is the National Aeronautic University Querétaro (UNAQ), in Querétaro, Mexico where Bombardier had established an incubator for new technologies and processes.

According to Alan Young, vp of Learjet operations in Wichita KS for Bombardier, the Learjet 85 will be the first fully composite business jet to achieve certification. The main structure is built in Mexico and the wings are being built in Belfast.

In Mexico, they are using CYCOM 5320 from Cytek, which is based in New Jersey. CYCOM 5320 is a carbon fiber cloth that has been preimpregnated with uncured resin (called a "prepreg"). The main advantage of CYCOM 5320 is that it does not require an autoclave to cure. Other composite systems require an autoclave to provide both high temperature and vacuum to cure the resin in the carbon fiber matrix.

This achieves a high fiber/ resin ratio and helps eliminate voids (which reduce the strength). An autoclave that is big enough to take major airframe components is a very expensive piece of equipment, so a nonautoclave process that can produce equivalent fiber/ resin ratios and strengths is very desirable.

The Learjet 85 fuselage pressure vessel starts in 3 sections that are eventually joined together to form the whole. Similarly, the nose and tail cones are built in 2 sections each, as are the 2 stabilizers. Typically, the individual sections have a wide area at the joint that is thinner than that desired for the final joined part. As the parts are joined, additional layers spanning the joint are added and the whole is then recured, resulting in the desired thickness and strength across the joint.

Once the process has been finalized, production is more straightforward than aluminum construction. The parts count is dramatically reduced (from several thousand for a fuselage to a few dozen) and manufacturing control and repeatability are very high when compared with traditional processes.

However, the trick is getting it right the first time. It's not simply a matter of putting it in the oven at 350 and testing it with a straw after an hour. A complex temperature profile is required, much like a stepped climb, first ramping up to 200°F and then to 290°F with a final rest at 350°F.

The total cure time of approximately 12 hrs depends on the complexity of the part. It takes a number of tests with each part to get the time and profile right. This requires a lot of time at the front end to get the "recipe" right. Once done, however, production proceeds much faster than conventional construction.

The wing for the Learjet 85 comes from the Bombardier plant in Belfast. While mostly carbon fiber throughout, it is built using a very different process. Both the wing planks and the spar are hand-laid using a dry carbon fiber cloth. "Dry" means that there is no resin, as opposed to a prepreg. The mold with the dry cloth is placed in an autoclave and, under the influence of heat and vacuum, resin is pressure injected into the part. A similarly complex temperature profile is required to get the desired result.

Asked about anticipated certification difficulties, Young indicates that, while no certification program was easy, Boeing's efforts in certifying the 787 had smoothed the way and he expected no unusual difficulties as a result of the composite structure.

Benefits of composite construction

Grob SPn might have been the first fully composite business jet, had the company not closed.

As mentioned above, there are many aerodynamic benefits to composite construction. The ability to create compound curves for fairings and other complex parts improves aerodynamics, and the smooth skin reduces parasitic drag and even improves wing performance by delaying the onset of turbulence as the air flows to the rear of the wing.

The resulting structure is lighter than a conventional aluminum structure of equivalent strength. It's stiffer, which means that the aircraft shape is better preserved under dynamic loads which, in turn, means that the airflow over the structure in real life is closer to what the aerodynamicist intended in theory.

Obviously, corrosion is not a factor. In fact, Boeing will allow a much higher cabin humidity for the 787 as pockets of condensation in hidden areas will not cause the structural degradation that has plagued aging aluminum aircraft. This will increase passenger comfort and reduce maintenance.

On the subject of repairs, Young states that there are mixed opinions and that, as yet, there is no clear advantage to composite structures.

Certainly, composite structures respond very differently to impact than aluminum. While aluminum will show the damage (such as a dent from an impact), composites can be severely damaged and not show it on the outside. However, once damaged, the repairs are easier with composites.

The bottom line is that operators must have all impact areas inspected properly by someone familiar with composite construction.

While the fiberglass used in early composite aircraft construction would not conduct electricity, carbon fiber does, but not as well as aluminum. This means that bonding to protect against lightning strikes still requires embedding a conductive mesh (usually copper) in the matrix. Provisions must also be made for a conductive ground plane underneath radio antennas.

From the passengers' perspective, composites are all good. The increased strength allows more cabin volume for a given fuselage outer diameter and the structure's lighter weight provides for more payload. Note, however, that a composite structure is not necessarily lighter than an aluminum one. It takes careful design to achieve the expected weight savings.

From an aesthetic point of view, composites win hands down. The superior finish, free of joints and rivets, produces eye-popping ramp presence. This is further enhanced by the lack of "quilting," where an aluminum skin bulges slightly between rivet lines.

The future of composites

Composites are here to stay. Starting with fiberglass in the 1960s, glass resins in the 1970s and carbon fiber in the 1980s, the advantages far outweigh the disadvantages. The transition may be difficult in terms of developing manufacturing processes, inspection methods and repair techniques but the long-term benefits make it worthwhile for designers, fabricators, passengers and owners.

Mike Venables is an aviation consultant and freelance writer. The principal at TriLink Technologies Group, Venables has been involved in the aerospace industry for more than 40 years, including aero engine, airframe, avionics and simulator manufacturers.


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