5.1 Structural Loads and Stress Analysis

Aircraft structural loads are crucial for safe and efficient flight. These loads include aerodynamic forces like lift and drag, inertial forces from mass and acceleration, propulsive forces from engines, ground forces during takeoff and landing, and environmental forces like gusts and temperature changes.

Stress analysis and load distribution are key to understanding how these forces affect aircraft components. Engineers use analytical and numerical methods to determine internal forces, stresses, and strains in structural elements. Load factors and dynamic loads are also considered to ensure structural integrity throughout the aircraft's lifespan.

Aircraft Structural Loads

Types of aircraft structural loads

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• Aerodynamic loads generated by airflow over aircraft surfaces
  • Lift forces support aircraft weight and enable flight
  • Drag forces oppose aircraft motion through the air
• Inertial loads caused by aircraft mass and acceleration
  • Gravitational forces act downward due to aircraft weight
  • Maneuvering loads occur during changes in aircraft attitude (pitch, roll, yaw)
• Propulsive loads generated by aircraft propulsion system
  • Thrust forces propel aircraft forward
  • Torque from engines creates twisting moments on aircraft structure
• Ground loads experienced during ground operations
  • Landing gear loads during takeoff and landing absorb impact forces
  • Taxiing loads result from aircraft motion on the ground
• Environmental loads caused by external factors
  • Gust loads due to atmospheric turbulence create sudden changes in aerodynamic forces
  • Thermal loads from temperature variations cause expansion and contraction of aircraft materials

Stress Analysis and Load Distribution

Stress analysis in structural components

• Identify critical load paths and structural elements that carry significant loads
• Determine internal forces acting on structural components
  • Axial forces act along the longitudinal axis of a component
  • Shear forces act perpendicular to the component's surface
  • Bending moments cause the component to bend about an axis
  • Torsional moments cause the component to twist about its longitudinal axis
• Apply equilibrium equations and free-body diagrams to analyze forces and moments
• Consider the effects of structural discontinuities and stress concentrations that amplify local stresses

Stress and strain distribution methods

• Analytical methods provide closed-form solutions for simple geometries
  • Beam theory for simple structural elements like wings and fuselage
    • Determine normal stresses due to bending using the flexure formula: $\sigma = \frac{My}{I}$
    • Calculate shear stresses due to transverse loads using the shear stress formula: $\tau = \frac{VQ}{It}$
  • Thin-walled structure analysis for more complex geometries like aircraft skin and stringers
    • Determine shear flow in closed and open sections to analyze load distribution
    • Analyze the effects of torsion on thin-walled structures to ensure structural integrity
• Numerical methods provide approximate solutions for complex geometries and loading conditions
  • Finite Element Analysis (FEA) divides structure into small elements and solves for displacements, stresses, and strains
    • Discretize the structure into finite elements (triangles, quadrilaterals, tetrahedra)
    • Apply loads and boundary conditions to simulate real-world conditions
    • Solve for displacements, stresses, and strains using numerical methods
  • Computational Fluid Dynamics (CFD) simulates fluid flow around aircraft to determine aerodynamic loads
    • Discretize the fluid domain into small elements (cells)
    • Apply boundary conditions and solve governing equations (Navier-Stokes)
    • Determine pressure and velocity distributions on aircraft surfaces

Load factors for structural integrity

• Load factors represent the ratio of the total load to the aircraft's weight
  • Determine the design load factors based on aircraft category (utility, acrobatic) and mission requirements
  • Analyze the effects of load factors on structural components during maneuvering (pull-up, push-over)
• Gust loads occur due to sudden changes in wind velocity and direction
  • Evaluate the impact of vertical and lateral gusts on aircraft structures using gust load factors
  • Apply gust load factors to static loads for structural sizing and design
• Dynamic loads vary with time and can cause vibrations and fatigue
  • Consider the effects of vibrations and aeroelastic phenomena on aircraft structures
    • Flutter analysis predicts the onset of self-excited vibrations that can lead to structural failure
    • Divergence analysis predicts the loss of structural stability due to aerodynamic forces
  • Assess the fatigue life of structural components subjected to cyclic loading using methods like the stress-life (S-N) approach