MODERN DESIGN TREND OF METAL AIRCRAFT FUSELAGE STRUCTURE

International Science Postgraduate Conference 2012 (ISPC 2012) © Universiti Teknologi Malaysia

MODERN DESIGN TREND OF METAL AIRCRAFT FUSELAGE STRUCTURE MOHAMED P.HASSAN 1*, MOHD RAFIE A. S.2 AND FAIRUZ L. ROMLI 3 Abstract. Throughout the years, development of aircraft design has already accomplished several major milestones. However, as can be observed from existing aircraft models, the fuselage structure remains still, mostly like it was in 1930s. In this paper, the trend of fuselage designs introduced within last few decades is studied through patents and published researches, particularly with regards to the development in metal fuselage structure. All in all, it can be concluded that there seems to be a big push towards more integral structures where the structural requirements are satisfied through pre-manufactured panels. Based on these findings, several potentials and opportunities for improvement are identified and discussed. It is believed implementing new and better fuselage designs can have a significant effect on aircraft industry. Thus, making air transportation more accessible to most people, by reducing the production time and cost. Keywords Fuselage design, very light jet, design trend, aircraft industry Abstrak. Sepanjang tahun, pembangunan reka bentuk pesawat telah mencapai beberapa kejayaan besar. Walau bagaimanapun, seperti yang dapat diperhatikan daripada model pesawat yang telah sedia , struktur fiuslaj masih kekal, kebanyakannya seperti adalah pada tahun 1930-an. Dalam kertas ini, trend reka bentuk fiuslaj yang diperkenalkan dalam tempoh beberapa dekad yang lalu dikaji melalui paten dan kajian yang diterbitkan, khususnya dari segi pembangunan dalam struktur fiuslaj logam.Dalam semua, ia boleh membuat kesimpulan bahawa terdapat seolah-olah menjadi dorongan besar ke arah struktur yang lebih penting yang mana keperluan struktur berpuas hati melalui panel pra-pembuatan. Berdasarkan penemuan ini, potensi dan beberapa peluang untuk penambah baikan dikenal pasti dan dibincangkan. Adalah dipercayai melaksanakan reka bentuk fiuslaj yang baru dan lebih baik boleh mempunyai kesan yang besar ke atas industri pesawat. Terutamanya, menjadikan pengangkutan udara lebih mudah bagi kebanyakan orang, dengan mengurangkan masa dan kos pengeluaran. Kata kunci Reka bentuk fiuslaj, jet sangat ringan, trend reka bentuk, industri pesawat

1,2,3

Aerospace Department, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia * Corresponding author: [email protected]

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1.0

INTRODUCTION

From dawn of Aircraft design up till now, major milestones have been accomplished. Yet it’s of most importance to realize, we are still at dawn of real industry evolution. Currently not each person can have an airplane; however same person could easily get a car. Using relevant articles and patents, we first started with new VLJ aircraft considering it being a similar example to the traditional private car. We then investigated proposed designs of metal fuselage structure, in general, looking for defined milestones; to be used in predicting the design trend. Relevant history and Airframe structure books have been reviewed as well.

1.1

History

Figure 1 Fuselage structure inside a turboprop military transport aircraft (top) and an engine enclosure of a jet fighter; comprising portion of its fuselage (bottom), both having semimonocoque structure

Metal aircraft fuselage structure main design concept hasn’t changed since 1930s. One might argue modern aircrafts include latest gadgets and use more sophisticated materials for their structure. However, we shouldn’t get past fact we are using same metal semi-monocoque structures (comprising: skin panels, frame members and stringers, attached together by fasteners) up till now, in both civil and military aircrafts, as shown in Figure 1. John Cutler criticized a similar issue in his book ‘Understanding Aircraft Structures’: “A glance at these post-1955 designs shows very clearly that the structures of big aircraft are not just little aircraft structures scaled up (or the opposite way around). In fact, whatever the size of the aircraft, the fuselage frames are always. about 500mm (20in.) apart and have between 75mm (3in.) and 150mm (6in.) deep cross-sections”. Cutler pointed-out in same book, modern development has considered only minor optimizations i.e. reducing rivets number and effects of structural damage [1].

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MODERN DESIGN TREND OF METAL AIRCRAFT FUSELAGE STRUCTURE

A recent study concluded; aircraft industry compared with automobile’s, is lagging 4 phases from a total of 8 phases automobile industry had already fulfilled to reach today’s mature level.

2.0

VERY LIGHT JET

Very Light Jet or simply VLJ is a class of three to eight passenger turbofanpowered aircraft that entered service in 2006. The price of models introduced in 2006 broke previous minimum price tag which was around 4 million American dollars and were sold for about 1 million dollar. Current VLJ aircrafts need just less than 700m to takeoff [3]. Searching for “very light jet” we obtained about 500 article and patent, where 27 had VLJ as their main topic.14 of these were concerned with future traffic models after introducing the new breed (VLJs), 2 with environment expected changes, 2 with marketing VLJs and 1 with engine configuration. While only 8 were found to be concerned with design. The 8 researches had: 1 discussing general theories for development of VLJs, 1 defining and giving an approach aimed at change of external design configuration in order to achieve a cheaper VLJ, 1 optimizing external design for better performance, 1 about audio system, 1 developing model for engineering students, 1 study concerned with interior passenger continence facilities and 2 specifying general guide model for VLJ specs and design.

2.1

VLJ Development

Since VLJ is “recent", it was to use all advancement developed, to produce this new segment of industry. However, need to mass produce the vehicle, was faced by manufacturing techniques limitations. We can easily recall Citation Mustang VLJ, “world's first fully certified entry-level business jet” which required Cessna Aircraft Company almost 5 years to roll out 400 of [4]. It was hence suggested in 2008, these vehicles to be produced out of composite materials and to contain as little as 200 structural parts. Moreover along “reduced number of parts” goal, a highly automated process is to be used for manufacturing and assembly [5]. Another research in 2010, considered revising arrangement of members within semi-monocoque structure where it was to choose between different concepts [6]. These later concepts dealing mainly with number of frame members related to that of stringers, where these arrangements (concepts) were tested in order to choose best combination.

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MODERN DESIGN TREND OF METAL AIRCRAFT FUSELAGE STRUCTURE

However more researches in 2009 and 2010, still considered it was essential to improve aircraft’s aerodynamic profile and performance [7], [8], [9].

2.2

Innovation importance

Since big aircrafts’ manufacturers might not benefit as much of such development. It was of great importance to start off with this particular new category of small aircrafts. Hence help visualize significant effect, a cheaper and faster to manufacture developed design of current structure, might have on facilitating a possible mass production of these small aircrafts.

3.0

NEW IDEAS INTRODUCED

Revolutionary designs were looked past, while minor ones got implemented Searching for “airframe structure” and “fuselage structure”; about 4500 results were obtained. Through first 250 results that appeared and after filtration by title 50% of articles and patents were relevant to research topic. Through second 250 results only 12% relevance within same category was obtained. Through rest of first 1000 reachable results within Google scholar search (within next 500 results), only 2.6% was found to be relevant. On other hand, searching for “integral airframe structure” only returned 1% relevance. Researches in 2001 and 2004, related to fuselage development, were more concerned with replacing traditional metal structures with carbon fiber [10], [11]. However in 2006, one research under same topic was investigating as titled: “challenges of the metallic fuselage” [12].

3.1

Patents timeline

As early as 1948, Richard H. Prewitt, filed for a patent titled “Method and apparatus for fabricating Airframes”. In which he explained a revolutionary design stating that: “The entire unit may be adhesively assembled in one curing operation”. Prewitt wasn’t granted the patent until 11th of September 1956. The invention comprised a structure made out of spiral member, running along length of fuselage, which was to be assembled on a mandrel with respective spiral recesses [13]. The spiral member to have a C-channel cross section, with leaps projected outwards at each edge. These leaps were to be adjacent to each other, with almost

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no space in between. Thus comprising a closed structure even before skin was applied. The section to be opened towards outer skin, which is later, bonded to the spiral structure. The patent was very detailed that it introduced solution to a possible undesired deformation of outer skin while force is being applied during bonding. Ten years later, in year 1965, an inventor, at state of Ohio in United States, proposed another innovative design for fuselage that would only require a spiral coil acting as “frames” along length of fuselage. The coil to be welded to members extended along length of fuselage and angularly spaced apart (longerons), thus comprising main structure which would later be fitted with stressed outer skin [14]. Although later design looked logically acceptable and provided a relatively crash resistant fuselage (due to spiral coil ability to absorb shocks, even patent was entitled “Aircraft Safety Body”). The design wasn’t structurally sound i.e. it didn’t comprise enough strength against torsional stresses (one of main functions of traditional fuselage peripheral frames). Unfortunately it didn’t change way planes are designed or manufactured, at least not yet. In 1981 a patent was granted for a new approach in “Airframe assembly”. The patent concluded using diffusion bonding and superplastic forming could eventually make it feasible to produce airframe comprising i.e. fuselage and wing, in a simultaneous operation. Where a blank or blank sheet is superplastically formed in a mold, after/during which fusion bonding is carried out to bond the newly formed structure to other components of same nature. Thus surpassing need for conventional machining or welding [15]. In 1991, some inventors, working for a Japanese company in Tokyo, proposed as they indicated “A plurality of ring-shaped members”. These were to be first prepared, to have recesses along there peripheral. After which a counter parts to these recesses with extended length are to be inserted so as to group these rings together thus concluding “stringers”. The “frames” are concluded from a lip that extrudes along peripheral of pre-prepared “rings”. Finally, the assembled structure would be much similar to traditional semi-monocoque fuselage in terms of structural worthiness [16].

3.2

NASA Report

In May 2000, NASA in a report performed by Boeing investigated Integral Airframe Structures. The report title says it all: “Validated Feasibility Study of Integrally Stiffened Metallic Fuselage Panels for Reducing Manufacturing Costs”. It goes on adding; although integral structures are cost effective there still some unresolved issues regarding i.e. Structure durability. The report mentions testing 812

MODERN DESIGN TREND OF METAL AIRCRAFT FUSELAGE STRUCTURE

different manufacturing concepts, some use forming and others use machining. One significant conclusion was, although forming (i.e. by extrusion) is cheaper, post machining required sums up total to be greater than using machining process performed alone. The report later mentions alternative processes: “Shot peening, roll forming, and stretch forming do not appear to be physically possible as forming options”. Where, Shot peening is a cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals [17]. Concepts pursued in report were experimentally tested and other tests carried on prototypes. As a result; roll forming only, roll forming then five-axis machine, extrusion only, extrusion then three-axis machine and casting only all got low rating, while three-axis machine then forming got high rating; as indicated by report. Report later suggested based on tested concepts; priority should be to: Three-axis machine then forming and Extrusion then three-axis machine. As for material forms to be further investigated report suggested: Extrusions and Machined Plates. It also defined two forming methods for further study: bump formed and Age creep forming. At one point, study estimated moving to IAS panels would reduce parts number by 91% and cost by 58%. Based on the concept, proposed IAS (Integral Airframe Structures) panels would only need to have skins and frames in comparison to built-up panels which comprise: Skins, frames, shear ties, stringer clips and stringers. The report however clarified later concepts mentioned were just "a starting point" [18].

Figure 2 Fuselage panels design development, where those to right are expected future fewer integral skin panels comprising stiffeners i.e. stringers (adapted from previously mentioned NASA’s report) [18]

3.3

More integral structures

In 2007, an inventor named Cord Haack, proposed an integral structure which comprises a frame and connection elements for connecting to skin fitted with stringers. In addition to that, a cross beam is attached to circumferential frame, structure to be pre-fabricated to form a single piece for each integral unit. Along with this patent Haack filed for another patent (on same day) concerning an idea

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to comprise fuselage of plurality of shells, the skin to be added later to integral structure so as to form a complete integral unit, which basically then forms a transverse portion of the fuselage [19], [20]. Another patent granted in 2007, claimed fuselage could contain premanufactured integral units, where portion of main deck cross members would merge into frame section creating a cord in its circle cross-section, as shown in Figure 3 [21].

Figure 3 Integral fuselage frame structure comprising lower portion of frame and adjacent part of floor member (figure adapted from patent WO 2007/141291) [21]

3.4

Latest

In 2009, some German inventors working for Airbus redesigned traditional fuselage frame. The new frame is to accommodate windows within in an innovative design. Traditionally windows act as stress raisers beside frames. The new design can now be seen implemented in Airbus 380 [22]. After same idea introduced by NASA report in 2000, Sergio M. O. Tavares among others, discussed in an article for Key Engineering Materials journal in 2011, “Impact of Integral Structures in the Design for Manufacturing and Assembly of Airframes”. He stated: “the huge amount of fasteners used in airframes appears to be the inverse of what the design for manufacturing should seek”. The article then pushed towards integral joints manufactured via welding, proposing this as a solution for “lightweight reinforced structures” [23]. That said it’s of some importance to realize not only load efficiency of weld over fasteners but also its advantageous effect on better aerodynamics. Especially considering it was proven in 2006 that RSW is more cost effective than riveting [24]. The whole development timeline is summarized in Figure 4.

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Figure 4 development timeline of fuselage structure

4.0

RESULTS

Surveying patents concerned with fuselage design, we found a rather shy number of those. Moreover only one patent within little number found was distinctively implemented, although being a minor optimization compared with others. Notably it was as well assigned by a company (Airbus) rather than being an individual’s initiative. Overviewing different designs, where some were very innovative. We could split them in two categories: 1. Type A; dealing with converting traditional structure, comprising many assembly parts, into integral ones having fewer parts or ones with enhanced features 2. Type B; dealing with completely new structures that should perform same job as traditional structures, however with better features This is within those designs concerned with metal fuselage structure; however some other designs proposed non-metal structures i.e. carbon fiber structures. Table 1 Time (in years) designs of type A and B were introduced Type A Type B

1981 1956

1991 1965

2000

2007

2009

2009

2010

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From simple data presented in Table 1, we could see there have been tendency and more concern towards improving current semi-monocoque structure (Type A), in one way or another, rather than introducing a completely new better design (Type B).

5.0

CONCLUSION

There is lack of interest to improve current structure or solve major issues for good. Moreover recent patents are in fact more concerned with solving minor issues and improving current semi-monocoque structure i.e. through integral structures, rather than inventing a new structure altogether. New designs or optimizations might have a real chance getting implemented when being assigned by a company rather than being an individual initiative. Designs introduced recently within patents along related articles and researches concerned with metal aircraft fuselage development are pushing forward: “Integral structures”, where fuselage would be formed out of few premanufactured panels comprising all structural requirements. It is thus expected future development would be more concerned with these new easier to assemble integral structures, rather than sticking with fixing issues arising in old “piece by piece” assembled structures.

ACKNOWLEDGMENTS We would like to thank “Universiti Putra Malaysia” for providing facilities that helped with this paper, specially having provided both military and civil aircrafts in full form.

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