Ukrainian Aerospace Developer Employs Clean Sheet Design
Antonov designs integral torsion boxes for highly loaded tail structures.
By Sara Black, Technical Editor | November 2003
Carbon fiber composites have played a key role in commercial and military aviation progress — a true statement wherever aircraft are built. In the case of the Antonov Aeronautical Scientific/Technical Complex located in Kiev, Ukraine, composites have been incorporated into aircraft designs since the early 1970s. Founded in 1946 by aircraft pioneer Oleg Antonov and headed today by general designer Piotr Balabuyev, the Antonov "design bureau" is a well-known developer of a line of durable transport planes, both turboprop and jet versions. All are high-wing designs (with the exception of the AN-2 biplane) for ease of loading, landing on unpaved runways and other operating benefits. Over the company's history, a large number of its aircraft designs have been manufactured, including the AN-225 six-engined jet transport — the largest aircraft on record, built to carry the Soviet Union's version of the Space Shuttle.
A team of Antonov engineers initially began designing and manufacturing composite components to replace noncritical metallic parts such as doors, trim tabs and panels, but in the few years that followed, comparative performance tests convinced the company that composites could meet design specifications, so they were put into production. In 1975, the AN-72 model carried approximately 980 kg/2,156 lb of fiber-reinforced polymer in the belly fairing, engine nacelles, radome and other areas. The AN-124 super heavyweight model, the world's largest series production aircraft, incorporated composite parts throughout the airframe for a total of 5,500 kg/12,100 lb.
"The experience accumulated during the first stage opened the way for the transfer from low-stressed and medium-stressed composite structural components to highly loaded structures," says Victor Kazurov, deputy general designer.
Not black aluminum
In the late 1980s, design work began on the AN-70 transport model. General designer Balabuyev made the decision to develop composite torsion boxes for the tail structure. According to Vladimir Tsarikovsky, team leader of composite structures design, an analysis of composite designs used by other aircraft OEMs for highly loaded structures demonstrated that all showed, to some degree, the influence of more traditional metallic designs. Antonov designers wanted a "clean sheet," or a completely new design concept to fully exploit the unique properties of composites, while eliminating stress concentrators, minimizing the potential for impact damage and making production as straightforward and automated as possible. Their concept was an "integral" torsion box structure for the vertical and horizontal stabilizers.
Source: Antonov
The AN-70 transport was developed in the early 1990s by
"We reduced the number of parts to be joined, and rejected, to the extent possible, the use of mechanical fasteners to eliminate stress concentrator sources that composites are so highly sensitive to," says Tsarikovsky. "Load-carrying structures created by nature are always integral, without discrete joint points."
The design for the 10m/32.5-ft high vertical stabilizer and the two 7m/22.75-ft long horizontal stabilizer torsion boxes essentially involves tape-wound, hollow rectangular spar sections with spar caps (five in the vertical stabilizer and four in the horizontal stabilizer), molded with each other and covered with a core and outer skins to form a sandwich. The walls of the hollow spars and the outer sandwich skins, loaded primarily in shear, are designed with a high percentage of ±45° fibers. The unconventional sandwich core consists not of honeycomb, but 15-mm/0.6-inch-square continuous carbon fiber prepreg tubing — wound with ±45° prepreg tape — bonded to the spars and oriented chord-wise (i.e., parallel to the air flow, or front to back). Spar caps are made mainly with unidirectional prepreg tape. Root and tip ends of the spar caps are reinforced and the parts are attached to the fuselage with metallic fittings at a flange joint.
Pavel Gorobets, team leader for structural strength analysis, explains that the deformation of the integral structure under load differs considerably from the deformation of a traditional ribbed and riveted structure, which required a new approach for stress and strain analysis. Analytical tools and an in-house software program were developed to allow accurate strain analysis and optimum use of the composite material for this unusual design.
"A major challenge for developing the composite torsion boxes was design for manufacture," says Vasily Bondar, chief specialist for composite structures production. "Automation was obviously preferred, given the size of the parts as well as their critical structural function," he stresses. Antonov obtained a tape winder, developed by PROGRESS Production Assoc. (Savelovo, Russia), capable of winding parts as large as 2.5m/8.1 ft in diameter and up to 12m/39 ft in length.
Tooling represented another hurdle. A matched set of tapered rectangular winding mandrels for the spars was designed from fiberglass, based on the similarity of its linear coefficient of thermal expansion (CTE) characteristics with the CTE characteristics of carbon. One kit of five mandrels was fabricated for the vertical stabilizer, and a second kit of four mandrels was made for the horizontal stabilizer. For the small, square tubing used as core, a cable braider was used to wind prepreg tape over long, hollow mandrels made with extruded PVC and silicon rubber.
Selected materials included unidirectional carbon fiber/epoxy prepreg tape, 0.08 mm/0.003-inch thick and 10-mm/0.4-inch or 20-mm/0.8-inch wide, used for the skins, spar webs and the tubular cores. Thicker 0.24-mm/0.009-inch (240 to 265 gm/m2) carbon/epoxy tapes were used for the spar caps. The 130°C-/265°F-cure epoxy was selected for its long out-time (three to four months). The ARGON facility (Balakovo, Russia) was the material supplier. All materials were certified by the All-Russian Aviation Materials Research Institute (VIAM, Moscow, Russia).
