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Protective Skins for Composite Airliners

1 Introduction
Current design of composite structures requires overdesigning to provide capacity to absorb impact
and operate in hot, humid environments. Extrapolating progress in composites technology out three
aircraft generations to the year 2035 results in a significant weight reduction but does not support a step
change in weight reduction and fuel burned reduction to meet the requirements of NASA’s Fundamental
Aeronautics Program/Subsonic Fixed Wing Project N+3 research program.
As part of work in Phase I of the N+3 program, when pushed to find a way to get the last seven
percent fuel burn reduction, Cessna turned the problem around and asked how to meet the structural
requirements without overdesigning the structure. What if the primary structure is designed without any
weight penalties (“knock downs”), and protective skins are used to meet the impact and hot, humid
requirements? What if multiple requirements are met by one material in the protective skins? What if the
external impact absorbing material also provides the acoustical treatment? What if the external protective
skin can replace the thermal insulation? What if the impact damage is visible unlike many of today’s
composite structures? What if paint is replaced by an aesthetic film, allowing both attractive decorative
outer surfaces and smooth surfaces which facilitate natural laminar flow? Could the result be a step
change in weight and fuel burned reduction? NASA has provided the funding for Cessna to begin to
answer these questions through funding for this research contract (NNClOCA3 6C Protective Skins for
Composite Airliners).
The goal of this research is the development of potential concepts for protective skins which enable
natural laminar flow and perhaps significant weight reduction in the aircraft’s primary structure. The
primary structure carries the load. The protective skin is needed to absorb impact damage and to provide
environmental protection. The STAR-C2 concept should be responsible for smoothing out bumps or gaps,
providing thermal insulation, absorbing impact and acoustic energy, reflecting ultraviolet and infrared
radiation, conducting large amounts of electrical current (for lightning strike), and providing a cosmetic or
appealing surface.
The outcome of this research program is proof of the feasibility of the STAR-C2 concept,
recommendations to NASA on future materials research and development by material suppliers to support
the STAR-C2 concept, and recommendations on a potential research path to ensure that the STAR-C2
concept is capable of being applied as soon as it is ready.
The research program was structured in two halves. During the first half of the program, the following
activities were accomplished: defining the metrics and requirements for the protective skins; conducting
a material search and developing the material composition for the protective skins, writing the test plans,
conducting the tests, and analyzing the test data, and finally, using the collected test data, selecting the
best materials for an improved set of STAR—C2 protective skins. Test articles developed during the first
half of the program are known as first—generation test articles, and test articles developed in the second
half of the program are second-generation test articles. The second half of the program included building
and testing the second-generation test articles, analyzing the data, and developing conclusions and
recommendations concerning the feasibility of protective skins for composite airliners. Testing
conducted in both halves of the program included impact, electromagnetic effects, aesthetics and
smoothing, and thermal. Acoustic testing was also conducted for the second-generation test articles.
2 Background
Cessna Aircraft Company led the vehicle configuration development in a NASA N+3 Phase I research
project while partnered with General Electric Aircraft Engines (prime contractor) and the Georgia
Institute of Technology (NASA Contract NNC08CA85C). Aircraft and propulsion technologies that
enable efficient and environmentally friendly transportation aircraft for the 2030 to 2035 time period were
explored throughout this previous effort. A transportation system scenario and an advanced aircraft that
satisfies all of the NASA research objectives for N+3 aircraft was developed and presented in the Phase 1
contract report (Reference 1).
The penultimate 2035 advanced aircraft developed during the NASA N+3 Phase 1 research project
made use of advanced turboprop engines, an advanced composite airframe, and a vehicle shape that
promotes natural laminar flow. While meeting all of the NASA goals at that time for reductions in
nitrous oxide emissions (NOx), noise, and balanced field length, the configuration was short of the fuel
burn reduction goal by nearly seven percent. In order to fully satisfy the fuel burn reduction, a novel
protective skin concept was proposed. Table 1 shows the projected benefits of each of the technologies as
a function of time. The protective skin (new conductive skin) shown in the 2030-2035 column of Table 1
provides an additional seven percent weight reduction.
With the anticipated benefits of the protective skin, the final aircraft concept (see Figure l and Table
2) was able to nearly meet the fuel burn reduction requirement as shown in Table 3. The technologies
that enable this breakthrough performance and hence are relevant to NASA’s subsonic fixed wing
research include (1) advanced turboprop engines for reduced fuel burn, (2) laminar flow for drag
reduction, and (3) advanced composite structures and systems for a reduction in vehicle empty weight.