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X-55 Advanced Composite Cargo Aircraft (ACCA)

The ACCA was a modified Dornier 328J aircraft with the fuselage aft of the crew station and the vertical tail removed and replaced with completely new structural designs made of advanced composite materials fabricated using out-of-autoclave curing. The primary structural elements of large aircraft are typically made from metal. Fuselage shells for such aircraft, for example, are typically made from high-strength aluminum alloys. Aircraft manufacturers continually strive for ways to increase aircraft performance and reduce manufacturing costs. One well-known method for increasing aircraft performance is to reduce airframe weight through the use of composite materials having relatively high strength-to-weight ratios.

Although some composite materials may offer higher strength-to-weight ratios than aluminum alloys, there are often difficulties with manufacturing large shell structures from composite materials. For this reason, the use of composite materials for fuselage shells has mostly been limited to smaller aircraft, such as fighter aircraft, high-performance private aircraft, and business jets. Composite materials typically include glass, carbon, or polyaramide fibers in a matrix of epoxy or other resin.

The ACCA fuselage is wider and stronger to accommodate military standard 463L pallets and features a cargo door and cargo ramp. The vertical tail features integrally stiffened skin. Despite its larger size, the materials and processes used for the fuselage reduced the number of parts by an order of magnitude relative to the original metallic design (approximately 300 versus 3,000) and drastically reduced the number of mechanical fasteners (about 4,000 vs. 30,000), program officials said.

The ACCA isn't designed to be a prototype for a small airlifter or any other aircraft. It was a proof of concept technology demonstrator for composite manufacturing processes in a full-scale, certified aircraft. In an effort to demonstrate and test the technologies while keeping costs down and on schedule, the small team of Air Force and Lockheed Martin engineers elected to modify the high-wing Dornier jet, mating its existing engines, wing, landing gear and avionics systems to the new composite structure. The modified fuselage has enlarged rear cargo doors and can accommodate two standard size military pallets.

The new composite structure was manufactured without complex tooling and the bonding process yields a 90 percent reduction of structural components and fasteners. Lacking traditional fasteners like rivets, the composite structure was inherently aerodynamic. Composite structures will address many of the corrosion and aging issues associated with all-metal aircraft, reducing airframe lifetime maintenance. Lighter weight of composite materials can also contribute to increased cargo capacity, aircraft performance and lower operating costs. The real game changer; however, was the maturation of manufacturing processes which collectively dramatically reduce the cost and complexity of building large airframes.

One of the main advantages of composite materials over conventional materials is a high strength to weight ratio making such composite materials ideally suited for use in aircraft construction. There is a significant relationship between relative part count and major cost categories does exist. Specifically, a percentage reduction in part count drives a corresponding percentagereduction in the manufacturing hours. Furthermore, the impact of monolithic parts appears to permeate most of the major cost categories in developmentand production.

The contractor developed a Vehicle Concept design based on an advanced STOL transport that incorporates the following general characteristics relative to military utility. The configuration was a multi-mission, light transport aircraft biased for STOL performance and speed agility. General layout considered facilitating operation at small, austere bases and improvised landing zones. The concept design must account for low susceptibility to foreign object damage, benign outwash/exhaust environment, ease of maintenance access, system reliability, etc.).

The design implemented widespread use of composite materials for low airframe weight and low cost manufacture. Selection of airframe design concepts and features, materials, and manufacturing processes to be used would be based on adaptation to large scale production and scalability to larger airframes to maximize potential applications.

The ACCA was made possible by a 10-year Air Force Research Laboratory-led research and development investment called the Composite Affordability Initiative. Government labs worked collaboratively with industry to develop advanced materials and manufacturing technologies. The ACCA's large composite sections are essentially formed, cured and bonded together in room-sized ovens, instead of using expensive autoclaves, which use a combination of heat and high pressure. Out-of-autoclave curing of large, unitized and co-bonded structures minimizes part count. The "ripple effect" of this approach contributes to costs reduction across every aspect of airframe production, Mr. Shenk explained. Tooling, raw materials, fabrication man-hours, quality control and floor space utilization efficiencies are realized. Together, they combine to greatly reduce cost, design and manufacturing complexity.

Subsequent phases (beyond this effort) may focus on using this article as the basis for flight demonstration of other technologies critical for an AMC-X-based family of tactical mobility aircraft. A follow-on demonstration (or series of demonstrations) may utilize the aircraft as a host for system-level assessment of emerging tactical airlift technologies and to perform initial risk reduction efforts for implementation in future weapons systems. Therefore, to maximum extent possible, the baseline demonstrator was configured with this purpose in mind. Provisions for successive technology demonstrations, including integrated high lift and control systems, airframe-integrated propulsion systems etc. was factored into the overall moldline shape and structural configuration to the maximum extent possible. Other than “first principles” consideration regarding planform layout and general arrangement, signature-specific technology integration was not required.

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