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Characteristics of Aircraft Structures and Materials

1.1 INTRODUCTION

An aircraft is a vehicle that is used for flight in the air. A vehicle like this is typically built by assembling many component structures such as wing, fuselage, landing gears, stabilizers, etc. Each component structure is typically built by assembling many substructures. Each substructure can be made out of different materials. The main difference between aircraft structures and materials and civil engineering structures and materials lies in their weight. The main driving force in aircraft structural design and aerospace material development is to reduce weight. In general, materials with high stiffness, high strength, and light weight are most suitable for aircraft applications.

Aircraft structures must be designed to ensure that every part of the material is used to its full capability. A typical aircraft design cycle involves three major steps – (i) conceptual design, (ii) preliminary design, and (iii) detail design. In any of these design stages, different factors such as aerodynamics, avionics, propulsion, and structural integrity are simultaneously taken into account. As such, aircraft structures are not designed for structural safety and integrity only; many nonstructural requirements impose additional restrictions in designing aircraft structural components. For instance, an airfoil is chosen according to aerodynamic lift and drag characteristics. As such, the size and shape of an aircraft structural component are usually predetermined. Such restrictions significantly limit the number of solutions for structural problems in terms of global configurations. Often, the solutions resort to the use of special materials developed for applications in aerospace vehicles.

The nonstructural and weight‐saving design requirements generally lead to the use of shell‐like structures (monocoque constructions) and stiffened shell structures (semimonocoque constructions). The geometrical details of aircraft structures are much more complicated than those of civil engineering structures. They usually require the assemblage of thousands of parts. Technologies for joining the parts are especially important for aircraft construction.

Because of their high stiffness/weight and strength/weight ratios, aluminum and titanium alloys have been the dominant aircraft structural materials for many decades. However, the recent advent of advanced fiber‐reinforced composites has changed the outlook. Composites may now achieve weight savings of 30–40% over aluminum or titanium counterparts. As a result, composites have been used increasingly in aircraft structures.

1.2 TYPES OF AIRCRAFT STRUCTURES

Most aircraft are built as fixed‐wing vehicles and are commonly known as airplanes. Other categories include rotorcrafts, glider, lighter‐than‐air vehicles, etc. Presence of air is essential for generating lift on these vehicles. As such, structural design of such vehicles depends on how airload is transmitted to the structural elements.

1.2.1 Fixed‐Wing Aircraft

A fixed‐wing aircraft is a kind of air vehicle that is heavier‐than‐air but can fly in the air by generating lift using the wings. An aircraft with a powered engine is generally called an airplane (Figure 1.1a). The unpowered version of fixed‐wing aircraft is called gliders (Figure 1.1b).

1.2.2 Rotorcraft

A rotorcraft (Figure 1.1c) or rotary‐wing aircraft is a heavier‐than‐air vehicle that generates lift using rotary wings or rotor blades, which revolve around a rotor. Depending on how rotor blades function, rotorcrafts are categorized as helicopters, autogyros, or gyrodynes. Recently, small‐scale multirotor rotorcrafts are widely used for surveillance or video‐capturing purposes. Designing blades for the rotorcraft is far more complex than designing a fixed‐wing aircraft because of the complex aerodynamic forces.

1.2.3 Lighter‐than‐Air Vehicles

Aircraft such as balloons, nonrigid blimps, and airships (also known as dirigibles) are designed to contain sufficient amount of lighter‐than‐air gases (typically helium) so that lift can be generated from the lifting gas (Figure 1.2).

1.2.4 Drones

Drones (Figure 1.3) are small‐scale air vehicles that can be fixed‐wing type or rotary‐wing type. The size of a drone is significantly smaller than a typical airplane or rotorcraft. As such, most drones are powered by electrical sources. Other than their size, the lifting mechanism of a drone is similar to the conventional fixed‐wing or rotary‐wing vehicles.

Fig. 1.1 (a) Powered fixed‐wing aircraft, (b) glider, and (c) rotorcraft.

Fig. 1.2 Various lighter‐than‐air vehicles: (a) hot‐air balloon, (b) blimp, and (c) dirigible.

Fig. 1.3 (a) Fixed‐wing drone; (b) multirotor rotary wing drone.

1.3 BASIC STRUCTURAL ELEMENTS IN AIRCRAFT STRUCTURE

An aircraft has many integrated parts, as shown in Figure 1.4. In general, these parts can be categorized into basic structural elements such as wing, fuselage, landing gears, tail units (horizontal and vertical stabilizers), and control surfaces such as aileron, rudder, and elevator.

1.3.1 Fuselage

The fuselage is the main structural element of a fixed‐wing aircraft. It provides space for cargo, control system and pilots, passengers and cabin crews, and other accessories and equipment. In single‐engine aircraft, the fuselage also carries the power plant. As shown in Figure 1.5, a fuselage can be constructed in various configurations such as truss, semimonocoque, and monocoque.

1.3.2 Wing

The main function of the wing is to pick up the air and power plant loads and transmit them to the fuselage. The wing cross‐section takes the shape of an airfoil, which is designed based on aerodynamic considerations. In general, wings are constructed based on monospar, multispar, or box beam configurations, as shown in Figure 1.6. These three design configurations are considered as the basic designs, and aircraft manufacturers may adopt a modified configuration. In the monospar wing configuration, only one main spanwise member is present. Ribs or bulkheads are used to provide the necessary aerodynamic contour or shape to the airfoil. The multispar wing configuration has more than one main longitudinal member in its construction. To attain the desired aerodynamic shape, ribs or bulkheads are often included. The box beam wing configuration has two main longitudinal members and connecting bulkheads to attain the required airfoil contour.

Fig. 1.4 Fixed‐wing aircraft parts.

Fig. 1.5 Various fuselage configurations: (a) truss type, (b) semimonocoque type, and (c) monocoque type.

Fig. 1.6 Various wing configurations: (a) monospar (b) multispar type and (c) box beam type.

1.3.3 Landing Gear

The landing gear is used to support an aircraft during landing and while it is on the ground. Small aircraft flying at low speeds generally have fixed gear. On the other hand, faster and more complex aircraft have retractable landing gear. To avoid parasite drag forces, the landing gear is retracted into the fuselage or wings after take‐off.

1.3.4 Control Surfaces

Since an aircraft is free to rotate around three mutually perpendicular axes (longitudinal, transverse, and vertical) intersecting at its center of gravity (CG), a pilot must be able to control rotation about each of these axes to control overall position and direction of the aircraft. Aircraft flight control surfaces are aerodynamic devices that allow a pilot to maneuver and control the aircraft's flight in midair. As shown in Figure 1.4, there are three basic control surfaces, namely aileron, rudder, and elevator. Rotation about the transverse axis, defined by the line that passes through an aircraft from wingtip to wingtip, is called pitch. The elevators are the major control surfaces for pitch. Ailerons control the rotation about the longitudinal axis, called roll. This axis passes through the aircraft from nose to tail. The rotation about the vertical axis is called yaw, and the primary control of yaw is done with the rudder.

1.4 AIRCRAFT MATERIALS

Traditional metallic materials used in aircraft structures are aluminum, titanium, and steel alloys. In the past three decades, applications of advanced fiber composites have rapidly gained momentum. To date, some new commercial jets, such as the Boeing 787, already contain composite materials up to 50% of their structural weight.

Selection of aircraft materials depends on many considerations that can, in general, be categorized as cost and structural performance. Cost includes initial material cost, manufacturing cost, and maintenance cost. The key material properties that are pertinent to maintenance cost and structural performance are as follows:

• Density (weight)
• Stiffness (Young's modulus)
• Strength (ultimate and yield strengths)
• Durability (fatigue)
• Damage tolerance (fracture toughness and crack...