1 Introduction

1.1 Introduction

This chapter aims at presenting the reader with aerospace structures, its basic structural parts and the transfer from aluminum to laminated composite materials, as a main way of saving weight. To be able to analyze those aerospace structures, which are thin-walled structures, basic elasticity equations will be derived and presented. The following chapters will present additional topics and equations regarding the behavior of thin-walled structures and how to analyze it.

1.2 Aerospace structures

Prior to defining the structures itself, one has to define the word AEROSPACE. As it is defined, aerospace combines the science, engineering and business of flying vehicles in the earth atmosphere, leading to the word AERONAUTICS, while flying in the surrounding space is called ASTRONAUTICS. The vehicles used for aerospace traveling are called Aerospace Structures.

The machine or the vehicle such as an airplane, helicopter, glider or any autonomous device capable of flying in the earth atmosphere is denoted as aircraft. Its main parts are schematically presented in Fig. 1.1.

Fig. 1.1: Main parts of a typical aircraft (adapted from www.clker.com/clipart-parts-of-an-airplane.html).

The vehicle, capable of flying in the space, like a satellite, space station or any other machine, would be named as spacecraft.

Figure 1.2 presents one of the old famous aircrafts, the Lockheed Vega, a Lockheed Corporation six-passenger high-wing monoplane, used to break flight records. The famous Amelia Earhart used this aircraft to fly the Atlantic. From structural point of view, one should note the wooden monologue fuselage and the plywood-covered wings [1]. With increasing the performance of the aircraft, the wood was replaced by aluminum, as the basic material to manufacture the structural parts of the aircraft. A typical metal skin aircraft fuselage assembly is shown in Fig. 1.3, depicting the frames and their clips and the longitudinal riveted stringers. The wings would be made of an assembly of spars giving the aerodynamic shape to the wing, ribs connecting the spars and all covered by skin panels. A space aircraft has similar structural design, like an aircraft (see Fig. 1.4) if it has to return to earth. If the space device is designed to spend its life in space, like satellites, solar panels and/or space antennas (see Figs. 1.5, 1.6), then its shape and the adjacent structure would be according to the loads expected to be applied on it during its space mission.

Fig. 1.2: Lockheed Vega aircraft.

Fig. 1.3: Typical metal skin aircraft fuselage assembly (from NASA CR4730, Ref. [2]).

Fig. 1.4: A spacecraft – the NASA Shuttle.

Fig. 1.5: The PEASSS (Piezoelectric-Assisted Smart Satellite Structure) nanosatellite structure launched on 15 February 2017 by an INDIAN PLSV rocket together with other 103 nanosatellites. Adapted from: Rockberger, D. and Abramovich, H.,” Piezoelectric Assisted Smart Satellite Structure (PEASSS) – An Innovative Low Cost Nano-Satellite”, SPIE’s Annual International Symposium on Smart Structures and Materials, Conference 9057: Active and Passive Smart Structures and Integrated Systems VIII, 9–13 March 2014, San Diego, CA, USA.

Fig. 1.6: Space antennas and solar panels.

The aerospace structures aim at providing operational demands and safety within a minimum weight. These structures comprise thin load skins, frames, stiffeners and spars, all made of high strength and stiffness materials to comply with the minimum weight criterion. To be able to design and calculate those structures, three basic structures, namely beams (or column for buckling and rods for tension), plates and shells have to be analyzed and their behavior under various types of loads be understood (Fig. 1.7). As shown, a beam is a 1D structure (h, b ≪ L), a thin plate is considered as a 2D structure (a, b ≫ t), while a cylindrical shell forms a 3D thin-walled structure.

Fig. 1.7: Schematic drawings of the three basic structures: a straight beam, a planar plate and a cylindrical shell.

Many books and articles have been written on aerospace structures, how to analyze and calculate their response to static and dynamic loads. Typical references and their contents can be found in [2, 3, 4, 5, 6].

1.3 Aerospace structures – transition to composite materials

The constant strive to improve efficiency, increase performance of the aircrafts in parallel with the need to reduce their development and operating costs is the moto of the aircraft industry [7]. Using composite materials for primary aircraft structures and thus reducing the aircraft weight may be the answer to improve aircraft efficiency and performance.

Intensive investigation had been performed on introduction of composite materials in commercial and military aircrafts [8, 9, 10, 11]. Figures 1.8 and 1.9 present the realization of a fuselage panel, originally manufactured from aluminum (see Fig. 1.3), using either laminated graphite epoxy stringers (Fig. 1.8) or laminated graphite epoxy sandwich faces (Fig. 1.9).

Fig. 1.8: Typical skin-stringer-frame side design composite concept (from NASA CR4735, ref. [11]).

Fig. 1.9: Typical sandwich-frame side design composite concept (from NASA CR4735 [11]).

One should note that the composite structure depicted in Fig. 1.9 does not have any stringers. The fibers have been oriented to support the loads normally carried by stringers, as in Figs. 1.3 and 1.8.

The reduction in the panel weight in comparison with the aluminum baseline model is presented in Fig. 1.10 from [8]. One can observe that using skin-stringer or sandwich structures would reduce the total weight by 24% or 13%, respectively.

Fig. 1.10: Weight comparison of side panel designs (from NASA CR4730 [8]).

It is interesting to note that already in the 1950s composite materials were first used on commercial aircraft, and 2% of the Boeing 707 was made of fiberglass (see Fig. 1.11). Airbus introduced composite materials in its aircraft in the 1980s, using 5% composites on the A310-300.

Fig. 1.11: Boeing 707-120 from PAN AMERICAN airlines (adapted from Pan Am: An Airline and Its Aircraft, by R.E.G. Davies, 1987).

This trend continued, and by the turn of the century, the advancements made in composite manufacturing allowed both Boeing and Airbus to significantly increase their use of composite. Boeing jumped from 12% on the Boeing 777 to 50% on the Boeing 787, better known as the “Dreamliner” (see Table 1.1), while Airbus moved from 10% on the A340 to 25% on the A380 (see Table 1.2) and finally to 53% on the A350XWB (Fig. 1.12).