PP-Design of Aircraft Structures

Chapter 12: Design of Aircraft Structures

Abstract

  • Discusses important aspects in the design and analysis of aircraft structures.

  • Key aspects include:

    • Material selection

    • Structural configuration

    • Loads evaluation

    • Static strength and deflection estimation

    • Static stability evaluation

    • Fatigue and fracture effects

    • Aeroelastic considerations

    • Influence of dynamic loadings

  • Manufacturing and testing not covered here.

Keywords

  • Aircraft structures

  • Material properties

  • Stiffened shells

  • Buckling

  • Landing gear

12.1 Introduction

  • Expertise required in four areas:

    • Structural analysis

    • Design (configuration and material selection)

    • Manufacturing (assembly and testing)

    • Flight testing

  • Considerations include cost, operational and maintenance issues, environmental impact, safety, and comfort.

  • Design philosophies:

    • Fail-safe design: Structural integrity maintained between inspections; repairs done when damage is found.

    • Safe-life design: Components function without failure during specified lifetimes.

12.2 Major Structural Components of an Aircraft

  • Structural components resist and transmit forces while providing aerodynamic shape and protection.

  • Lightweight structures have material weight at the surface, leading to thin plates and shells.

  • Types of structures:

    • Monocoque: No stiffening members, lightly loaded.

    • Semi-monocoque: Includes stiffening members, more efficient in high-load scenarios.

12.2.1 Semi-monocoque Structures

  • Fuselage structures include longitudinal stringers and transverse frames.

  • Spars are main longitudinal members in wings and empennage.

  • Multiple spars needed for fail-safe design.

12.2.2 Functions of Semi-monocoque Structural Components

  • Skin or cover transmits aerodynamic loads to cross members.

  • Resists torsional moments and maintains hoop stress.

  • Transverse frames enhance buckling resistance and cross-sectional shape retention.

  • Lightweight design constrains safety factors for high-performance aircraft.

12.3 Materials for Aircraft Applications

  • Distinction in materials design:

    • Military: Performance-driven (speed, manoeuvrability).

    • Civil: Safety and fuel economy.

  • Key materials include:

    • Aluminium alloys: Primary choice for airframes.

    • Titanium alloys: Used for high-performance applications.

    • Composite materials: Increasingly used due to strength and lightweight benefits.

Strength and Stiffness

  • Key material properties for semi-monocoque construction:

    • High specific strength and stiffness.

    • Unidirectional CFRP composites outperform metallic alloys but require multiaxially aligned fibers for optimal performance.

Fatigue and Fracture Toughness

  • Aircraft components are prone to fatigue and must resist stress damage, especially CFRP composites.

Corrosion Resistance

  • Critical for aluminium alloys and high-strength steels; protective coatings are necessary.

Elevated Temperatures

  • Material limits are defined:

    • Aluminium alloys: Up to 130 °C

    • Titanium alloys: Up to 500 °C

    • Specialist steels: 400–450 °C

    • Superalloys: Used at much higher temperatures.

12.3.1 Experimental Methods for Material Characterization

  • Importance of testing: Determines properties under various conditions.

  • Testing methodologies include:

    • Uniaxial tension and compression tests for metallic materials.

    • Flexural response studies for various materials.

  • Classical failure theories primarily predict yielding (ductile) or fracture (brittle) based on experimental data.

12.4 Idealization of Thin Stiffened Shell Aerospace Structures

12.4.1 Idealization of Structures

  • Simplifying assumptions are necessary to manage structural analysis complexity.

  • Idealizations must preserve elastic characteristics like area and moment of inertia.

12.4.2 Buckling in Aerospace Structures: Design Motivation

  • Understanding primary loads on structural members:

    • Wing structure experiences axial stress (compressive due to lift).

    • Skin experiences shear loads due to twisting moments.

12.4.3 Role of the Frames, Spars, and Ribs

  • These rigid elements maintain aerodynamic shapes and provide rigidity to prevent bending and torsion.

12.4.4 Aircraft Landing Gear Design

  • Landing gear serves multiple roles:

    • Absorbs impact energy during landing.

    • Provides directional control and stability.

    • Should be retractable to avoid drag during flight.

  • Design considerations cover:

    • Strength, stability, damping, and weight.

12.5 Aeroelastic Considerations

  • Aeroelasticity: Interaction of aerodynamic, elastic, and inertial forces.

  • Problems faced include:

    • Flutter

    • Control reversal

    • Load redistribution due to structural deformation.

12.6 Conclusions

  • Overview of structural design covering idealization, material selection, and stability issues.

  • Important structural considerations include manufacturability and maintenance.