report
1.2.2 Application: Airframe

1.2.2.1 Short application description

Airframes consist of components such as the wing upper and lower, the fuselage, spars, frames, ribs, landing gears and control surface. Modern aircraft structures are designed using a semi-monocoque concept consisting of a load-carrying shell (reinforced by frames and longerons in the bodies) and a skin-stringer construction supported by spars and ribs in the surfaces [15].
Airframes development is split between serving two market views. One view expects an increase of direct flight between local airports, thus requiring a large number of smaller (and safter) aircarft. This is the so-called "fragmentation" approach. The other view (so-called "consolidation") is based on further development of "hub" airports thus requiring large high-capacity aircrafts [2].
From business jets to civil aircrafts, airframes come in a large variety of shapes and sizes with material selection depending on the required performance and cost boundary conditions. From (very) light aircrafts costing less than $ 500.000 and with an empty weight of around 1.000 Kg to Large Civil Aircrafts costing more than $ 100 million and an empty weight of more than 150 Tons.
A very general estimate of the ratio of empty weight to gross weight of most airplanes is about 50%, though there is a modest tendency to increase the structural weight fraction with size. As the aircraft size increases there are components more strongly affected by the stress increases due to structural weight increases (e.g. lifting surfaces) and those that are little affected (e.g. fuselage) [15].
Fail-safe design is achieved through material selection, proper stress levels and multiple load paths structural arrangements that maintain high strength even in the presence of a crack or damage. Typically, the primary structure for civil aircrafts is designed based on average expected operational conditions and average fatigue test results for 120.000 hours that, after applying scatter factors between 2 and 4, leads to crack-free structural life ranges from 30.000 to 60.000 hours [15].
Materials used in airframes
Materials that best match the requirements of airframe components are aluminium, titanium, steel and composites. Over the last decades composite materials have been gaining importance in airframe construction leading to less use of aluminum and much less use of steel (nowadays around 10% in weight). Titanium use in civil aircarft has basically remained flat and represents around 10-12% of the overall airframe weight though it is expected to increase in new large aircarfts [35].
What it is probably more important than the present mix of materials is the trend to keep developing multi-material airfarmes in the future. As stated by a senior structural engineer at Airbus, aircraft manufacturers keep their options open and, "as far as they can see it, it will always be a mixture of materials" [17].
Aluminum alloys have a low density (aprox 2.7 g/m3) are the most widely used material in civil transport; however, their low melting temperature (aprox. 660ºC) impedes their application in areas near the engine or as skin materials for supersonic aircrafts. The 2XXX and 7XXX age hardening alloys are the most commonly used in airframes. Age hardening (or precipitation hardening) increases the strenght of the alloy by distorting the lattice and creating resistance to dislocation motion.
On the one hand, 2XXX (alloyed with Copper) have lower crack growth rates (thus better withstanding fatigue than 7XXX) are used in the lower wing and the fuselage. To avoid overaging (leading to degradation of mechanical properties), the skin temperature must be kept below 150ºC. Creep may also occur at high temperatures (e.g. supersonic aircrafts) and special alloys are required. On the other hand, 7XXX (alloyed with Zn) are the alumium alloys offering the greatest potential for age hardening, though copper is usually added to improve stress corrosion cracking (with the drawback of difficulting weldability). The resistance Stress Corrosion Cracking (SCC) has been the main limiting factor for these alloys and decreases with increasing Zn:Mn ratio.
Composites have been used as floor beams, doors, and aerodynamic fairings and for control surfaces (e.g. rudders, elevators and ailerons). Due to the possibility to design composite materials to fulfil a specific set of properties (e.g. stiffness, strenghth, density), carbon-fibre composite materials are gaining importance. This is thanks to the high specific strength and stiffness, tailored directional properties, non-corroding in salt environments, excellent fatigue performance, dimensional stability and the possibility to integrate different functions into a single part (reducing the parts' count). On the negative side, composites are susceptible to impact damage and lightening strikes, moisture pick-up and relatively high cost. Besides, do not yield plastically in regions of high stress concentrations and are subject to random variation of properties due to manufacturing techniques (namely RTM).
For instance, while the Boeing 777 contains 50% aluminum and 12% composites, the numbers for the new 787 Dreamliner are 15% aluminum, 50% composite (mostly carbon fiber reinforced plastic) and 12% titanium. The majority of its primary structure -including the tail, wing and fuselage- is made of advanced composite materials. The function integration benefit of composites is highly visible in the new 787 airliner where each fuselage barrel will be manufactured in one piece and the barrel sections joined end to end to form the fuselage thus eliminating the need for about 50,000 fasteners used in conventional airplane building and replacing 1.500 aluminum sheets. The Airbus 380 contains a 25% of composites but the planned A350 is claimed to contain up to 60% composites (using pannelled fuselage skins made of CFRP offering much easier maintenance and reparability) [7]. Whereas the use of composite fuselage structure is becoming the standard in the General Aviation sector (small planes), the A380 is expected to be the first large aircraft in which partially non-metal skin materials will be applied [13]
Looking into the future, the ALCAS project (Advanced Low-Cost Aircraft Structures) aimed at fully applying carbon-fibre composites into aircrafts' primary structures resulting in 20% weigth saving with a zero increase in recurring costs as compared to metallic structures for airliner platforms. For business jet platforms the objective was a 20-30% reduction in recurring cost with a 10% weight saving against the metallic structure [20].
Titanium alloys are stronger and stiffer than aluminum alloys. Besides, they are corrosion resistant and also have a low density (for pure Ti 4.5 g/m3 as compared to 2.7 g/m3 for Al). Thus, Ti components can be smaller in size than its aluminum counterparts and are used in components where volume is important (e.g. landing gears and attachment points) or in high temperature areas. Ti has a metling point around 1.100ºC above that of alumium and 200ºC above that of steel though its maximum operating temperature is around 600ºC above which creep (starting at 0.3-0.5 of the melting temperature) and rapid oxidation occur. The main limitation for Ti application is its high cost (approximately 7 times that of aluminum and steel). Applications of titanium alloys in airframes include undercarriage components and flap tracks while it can be used as skin materials for supersonic aircrafts above Mach 2.4.
Steel is also stronger and stiffer than aluminum alloys and is used in components where volume is important (e.g. landing gears, attachment points, gears and bearings) whenever Titanium alloys cannot provide the required tensile strenght. However, their use has been declining as it suffers from a higher density (7-8 g/m3).
Finally, magnesium alloys are being developed but cannot be found in aircrafts airframes yet. For instance, the IDEA R&D project has been investigating magnesium application (including coatings) on non-structural, semi-structural and structural casting parts 30% lighter than present solutions. This project was expected to produce demonstrators that would undergo all the full-scale testing procedures carried out by aircraft manufacturers. Moreover, the AEROMAG project is focused on the development of new magnesium wrought products (extrusion and sheets) that could replace aluminum alloys in fuselage parts, systems and interior components and aimed at 35% weight reduction in selected parts [20].

1.2.2.2 Functional requirements

The airframe is required to resist applied loads (originated from different sources), provide an aerodynamic shape and protect passengers, payload and equipment from external environmental conditions. As different parts of the airframe are subject to different mechanical, chemical and thermal property requirements, each component/system is subject to different selection criteria. Typical contraints include weight, stiffness, strength, fatigue performance (high/low cycle), corrosion resistance and cost [19]. The following picture visualises the critical requirements for different airframe components.

Figure 2‑3: Critical requirements of airframe components [19]
From a more general perspective, an airframe structure needs to fullfil a wide set of requirements both in terms of performance (through design & engineering) and manufacturing. Depending on the design approach and the material mix selected for the airframe structure, it would be possible to fulfil them all through an integrated design or auxilliary solutions may be needed. The following picture visualises the requirements both from a performance and manufacturing perspective.
Thus, the basic structure could provide the required stiffness, strength and stability while specific coatings would provide environmental protection (e.g. against corrosion). In terms of manufacturing, different parts may be produced and assembled or integrated into one bigger and more complex part.

Figure 2‑4: Design requirements for fuselage structures [13]

Large aircrafts lifetime in western countries is of around 30 years (with many of them then being sold to airlines in developing countries and kept in operation beyond that). Given this timeframe, different modes of failure due to corrosion (e.g. stress and exfoliation corrosion) and fatigue damage are important drivers of maintenance cost for metal aircrafts.
Possible damage either from plane's crash or (accidental) impacts during operation, maintenance or while in the ground are also critical requirements. Either through multiple load path structures, no-growth concepts (e.g. crack does not propagate and the structure's integrity is not compromised) or fail-safe designs, airframes (and specialy primary structural elements) must resist and tolerate damage and crashworthy. Beyond this, fire resistance must ensure to fulfil with the 90-seconds rule for evacuation in large aircrafts.
An important functional requirement for external parts is protection against lightning strikes. According to Boeing estimates, every commercial airplane is hit by lightning about twice a year. Moving from metallic parts to composites poses new challenges as they will not be able to conduct lighting away as metallic parts do (no airplane crash caused by lighting has been reported over the last 40 years). Most sensitive areas triggering lighting (e.g. while going through an electrically charged cloud) include the nose, the leading edges, the tail or the wings. Amongst them, wings pose special risks as they accommodate the main fuel tanks.
With regard to passengers' comfort, to avoid the need for using oxygen masks passengers' compartments are pressurized; however, the onboard air is equivalent to an altitude of 8.000 ft and results on passenegers tiredness. Using stronger materials could enable further pressure increases and increase passengers' comfort [23].

1.2.2.3 Boundary conditions

The aircaft industry is, by the very nature of this business (transporting human beings and a very high likelyhood of passengers' death in case of an accident), risk averse. Though this does not mean it's a conservative industry, if the (potential) reward is good enough, any measure will be taken to ensure that risk is kept at the very minumum through extensive test programmes and evolutionary - rather than revolutionary - development programmes.
From an aircraft industry perspective there are 4 critical requirements to be considered when introducing a new material: (1) stable material and material supplier; (2) materials and design database; (3) stable process and (4) demonstrated technology [14].
Due to the globalisation process this industry has gone through, airframes' production is largely distributed in different sites and organisations [16]. Thus, realising the benefits of new materials and/or technologies often requires reconsidering the way airframes are built and lead to reorganisation of the suppliers' networks. Being capable of operating in such a complex global industry is a critical factor to introduce innovative solutions into the market.
Regarding the parts' sizes and production volumes, there are of course differences between business jets/ small aircarfts and large civil aircrafts. While companies like Airbus and Boing may produce around 400 - 500 aircrafts per year, busines jet companies may produce two to three times this number.
In terms of sizes, different airframe components are much larger for large aircrafts but may not differ much in terms of thicknesses. On the other hand, they pose different challenges in terms of manufacturing and cost. For instance, tools accessibility may be more difficult for large aircrafts and these will also require much bigger equipment (e.g. for composites curing and consolidation) and, in the case of composites, ensure proper viscosity of the resin to be used in processing the larger parts.
Long development times required for the design and development of new large aircrafts (aprox. 10 years) and their production (10 years as well) makes it critical to exploit windows of opportunities to avoid even further delays in commercialising nanotechnology developments (and revenues generation). Initiatives such as the EU VIVACE project (led by Airbus) aimed at creating a virtual environment to support the entire design process in a virtual extended enterprise may reduce development times by 30% but will pose new requirements in terms of being able to adopt new organisational approaches and IT tools.
The introduction of new materials in airframes (as for other aircraft components) is highly dependent on factors such as cost but also materials' manufacturability and availability. Regarding cost, it is critical to understand the entire manufacturing/assembling processes to reliably estimate the cost impact of introducing a new material. Regarding availability, when new materials (e.g. nanomaterials) are introduced into the market they are rarely available for mass-production as their full introduction into the market normally requires further technological development.
Besides, the introduction of new materials needs to undergo a long and expensive certification process by the US and European airworthiness authorities. The EU R&D project like MUSCA (Multi-scale analysis of large aerostructures; led by EADS Corporate Research) project started in 2005 aims at developing, testing and validating technologies for more cost effective structural static analysis. The project aims also at providing recommendations to the relevant Airworthiness Authorities.
One important boundary condition is to ensure proper and economic maintence, repair and operation throughout the airframe lifetime. Thus, difficult access for inspection and repair will set much stronger requirements on the part (e.g. through fail-safe approaches) and possibly limit the potential benefits of introducing new materials (e.g the theoretical minimum thickness that would realize the required weight savings may not even be possible due to safety requirements).
For large aircrafts flying at high altitudes, fuselage pressurization poses more stringent requirements to the airframe structure so as to ensure passengers comfort (and safety) during flight.
Finally, the cyclic character of the aircraft business clearly affects the production capacity to match the requirements of the ups and downs. Therefore, managing the workload and level peaks and valleys is critical both for aircraft manufacturers and their suppliers. For instance, getting involved in the development of new aircraft when production of existing ones falls can be an approach to manage oscillations in workloads [15].

1.2.2.4 Product examples

There have not been nanotechnology applications reported in present commercial aircraft airframes until recent. However, a first application in the General Aviation sector was already announced in 2008, when Avalon Aviation's Giles G-200 (single engine fully acrobatic) flew with Unidym's carbon nanotubes incorporated into its carbon fiber composite engine cowling [59]. The material provided for increased strength and flexibility to combat the effects of aerodynamic stress and engine vibration.
Early in 2009, PPG Industries also introduced a chromate-free depaint/repaint process that includes a new exterior epoxy primer based on nanotechnology and an adhesion promoter PPG manufactures under license from Boeing [60].

1.2.2.5 Economic evaluation

This chapter provides an overview of possible market sizes that could be targeted by nanomaterials and the materials systems they are part of.
The fact that fuel is the largest cost for all airlines (30 up to 50% of direct operating cost) is probably the strongest driver for lighter aircrafts and cleaner propulsion systems. As important as the fuel economy, aircraft designs aim at maximising the payload in relation to cost. As an indication, the weight of a fully loaded aircraft taking off could be broken down as follows: 20% is payload, 40% is structural weight and 40% is fuel [19]. Airport landing fees are partially dependant on aircraft weight.
The value of weight saving in the airborne commercial transport has been estimated at around US$ 300 per pound saved (getting to as much as US$ 3.000 for fighter aircrafts) [6]. This comes from the cost savings realised throughout the complete Life Cycle of the vehicle or its components that justify higher investments. However, the realization of all potential benefits will depend on several factors (many of which uncertain) such as the evolution of oil prices (e.g. from 140$/barrel to 40 in few months), the actual use (e.g. varying projections for air traffic growth) or lifetime of the vehicles.
In terms of the airframes, the economic impact of nanotechnology can be roughly calculated based on the expected and potential use of nano-enabled materials (both bulk materials and coatings) in the production and the repair of airframe components. Nowadays, materials represent about 50% of the costs for manufacturing an aircraft structure in high-cost countries. The remaining costs are 35% labour and 15% overheads [5].
Airframes consist of a material mix that has changed its composition over time. Without nanotechnology being used at all, composites have gained importance in airframes designs and this trend is expected to continue. However, it must be taken into account that airframes are systems and not stand-alone components. Parts/components are produced and assembled to deliver the required overall performance. Thus, calculating the economic impact by assuming direct material replacement can be at least misleading and, at the same time, hide some of the potential benefits of new or improved materials (e.g. reduction of parts' count and assembly costs by using composites).
Considering the growing importance of composites in airframes design (more than 50% of the weight of the Boeing 787), nanocomposites will be one of the key areas for nanotechnology applications as long as oil prices are high and production costs similar to metallic counterparts [36]. Conductive composites (required for electric charges dissipation) are one of the lead markets [37]. Despite carbon-fibre composites only contributed to 3% of the 20% decrease in fuel consumption of the Boeing 787, the company has the conviction it is the technology for the future [42].
The Green Regional Aircraft ITD (part of the JTI Clean Sky / Composite materials) is specifically looking at demonstrating the applicability of nano-filled resin systems to produce composite panels with improved impact resistance and thermo-mechanical properties such as electrical conductivity and fire resistance. However, the generic planning of the Clean Sky initative does not foresee flight test before 2013 [55]. According to a material supplier, 90% of the outer structure of the new Boeing 787 consists of lightweight polymer-based composites and Boeing was looking towards adding carbon-nanotubes to improve structural integrity and provide lightening protection [57].
About 2.400 metric tonnes of composite aerostructures were produced in 2007 and it was estimated that about 70.000 metric tonnes of engineered composites would be produced over the next decade. About 13% would be used in regional aircraft whereas the success of the Boeing and Airbus twin-aisle aircraft will be responsible for most of the new demand for composites [36].
By 2020 more than 163 million Kg of nanomaterials, valued at $ 2 billion, would be used to produce nanocomposites, with demands for nanotubes alone exceeding $ 1 billion. Aircraft is a key market for nanoclay and nanotube polymer composites and will remain so accounting for 40% of demand by 2020 [34].
What's important to realize is that nanofillers may not fully replace established fillers (e.g. carbon or glass fibres) but reduce the load of these materials and make them more effective [21]. The overall demand for structural carbon materials in the aerospace and defense segment was estimated at around $ 1 Billion in 2008 and expected to increase up to $ 1.4 Billion in 2013 [45]. The use of carbon fiber reinforced plastics (CFRP) is expected to grow at around 17% annually by replacing aluminum. The market for CFRP is expected to reach $ 3.5 Billion in 2019 [46].
Despite most of the present demand is for thermoplastics, thermosets are expected to increase their share and reach around 20% of the nanocomposite market (not aeronautics specific) in 2020 [23]. In terms of value, the US nanocomposites industry in 2006 reached $ 860 million and was expected to grow 21 percent annually through 2011. The global nanocomposite market has been estimated at $ 1.5 billion in 2008 [47]. Higher-priced resins, such as engineering plastics used in less cost-sensitive applications, will lead gains [34].
Regarding nanostructured metal parts such as Titanium, Steel or Aluminium, research results promise further weight reduction yet to be realized. For instance, the company Metallicum Inc. (using Severe Plastic Deformation production processes) claims its nanostructured titanium could reduce by 5% the Titanium weight in a Boeing 747 (estimated at 100.000 pounds) [44]. The Airbus A320 and the Boeing 737 are expected to have a buy-in weight of titanium of 12 and 18 Tonnes respectively (including engines' components) and reaching up to 90 tonnes for the Airbus A350 and Boeing 787 [35]. The development of metal matrixes (e.g. aluminium) reinforced with nanotubes could reduce the weight of polymer composites by 68% in dry weight [58].
The replacement of aluminium parts by CFRP is also expected to increase the use of titanium, which is used instead of alumium for fasteners and joints [46]. The standard Boeing 747 jumbo jet contains about 75,000 kg of aluminium. Because the metal resists corrosion, some airlines don't paint their planes, saving several hundred kgs of weight [48].  
In terms of airframe coatings, they do influence aircraft drag and weight of all kinds of aircrafts. As an example, the weight of exterior coatings of the new A380 accounts for around 725 Kg (as compared to the 480 Kg in the Boeing 747). A reduction of about 12% in dry weight (80 Kg) in a large aircraft would lead to savings of around 20.000 liters of fuel (or 4.400 gallons) and about 50 tonnes of CO2 emmissions. This does not include interior structural coatings that would sum up another 725 Kg. The market for aeronautics' coating could be less affected by the economic recession and aircraft production slowdown because existing aircrafts need to be periodically repainted (e.g. decorative coatings usually last between 5 to 6 years) [39].
The total coating market for the aerospace sector had been estimated at about $ 150 million in 1999 [40], $ 215 million in 2001 and expected to increase by 50% until 2010 [41]. Other sources claim the paint market alone for commercial aircraft is around $ 300 million per year [49]. In the U.S., commercial aviation accounted for more than 70% (including general aviation) of the market. OEMs accounted for 65% and refurbishers for the remaining 35% in the commercial aviation sector [41].
Considering the characteristics of the aeronautics' business in general and the airfames in particular, this application is expected to stay a niche market for nanomaterials. Beyond this, the penetration of nanomaterials into this sector is expected to be slow especially due to the long lifetime of airframes and related components.

1.2.2.6 Selected key companies

Airbus / EADS
Boeing
Alenia Aeronautica S.p.a (Italy)
Embraer (Brazil)
NASA
Integran Technologies Inc. (Canada).
Héroux-devtek Inc (Canada).
Metallicum Inc. (owned by Manhattan Scientific)
MetaMateria Inc. (subsidiary of NanoDynamics Inc.)
PPG Aerospace (United States)
AkzoNobel Aerospace Coatings (The Netherlands)