11.1 Structural Loads and Stress Analysis

Aircraft structures face complex loads during flight. Aerodynamic forces like lift and drag combine with inertial loads from the aircraft's mass and acceleration. Engineers must analyze these forces to design safe, efficient airframes.

Stress analysis is crucial in aircraft design. Different types of stress—tensile, compressive, and shear—act on various components. Understanding material properties through stress-strain curves helps engineers select the right materials and design structures that can withstand flight loads.

Types of Structural Loads

Aerodynamic and Inertial Loads

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• Aerodynamic loads result from air flowing over aircraft surfaces during flight
  • Include lift, drag, and side forces acting on wings, fuselage, and control surfaces
  • Vary with aircraft speed, altitude, and maneuvers
• Inertial loads arise from aircraft mass and acceleration
  • Encompass weight force and loads during maneuvers or turbulence
  • G-forces experienced during turns, climbs, and descents contribute to inertial loads
• Combined aerodynamic and inertial loads determine overall structural requirements
  • Engineers must account for worst-case scenarios (maximum load factors)
  • Load distribution changes throughout different flight phases (takeoff, cruise, landing)

Bending Moment and Torsional Stress

• Bending moment occurs when forces create a curve in structural elements
  • Wings experience bending due to lift forces and their own weight
  • Fuselage bends under the weight of payload and equipment
  • Tail surfaces bend from aerodynamic forces during flight control inputs
• Torsional stress results from twisting forces applied to structural members
  • Wings twist under aerodynamic loads, especially at high speeds
  • Fuselage experiences torsion during asymmetric maneuvers
  • Control surfaces undergo torsion when deflected
• Both bending and torsion can lead to structural failure if not properly managed
  • Aircraft designers use specialized materials and structures to resist these forces
  • Composite materials often employed to provide strength in multiple directions

Stress Analysis

Types of Stress and Their Effects

• Tensile stress stretches materials along their length
  • Occurs in components like wing spars under positive G-forces
  • Measured as force per unit area (Pascals or psi)
• Compressive stress squeezes materials, reducing their length
  • Found in landing gear struts during touchdown
  • Can lead to buckling in thin structural members
• Shear stress causes adjacent parts of a material to slide past each other
  • Present in riveted joints and bolt connections
  • Critical in wing-fuselage attachments and control surface hinges
• Multiple stress types often act simultaneously on aircraft structures
  • Engineers must consider combined effects when designing components
  • Finite element analysis used to model complex stress distributions

Stress-Strain Curve and Material Properties

• Stress-strain curve graphically represents material behavior under load
  • X-axis shows strain (deformation), Y-axis shows stress (force per area)
  • Elastic region where material returns to original shape after unloading
  • Plastic region where permanent deformation occurs
  • Ultimate strength point represents maximum stress before failure
• Key points on the curve inform material selection and design limits
  • Yield strength marks transition from elastic to plastic deformation
  • Young's modulus (slope of elastic region) indicates material stiffness
  • Area under the curve represents material toughness
• Different materials exhibit unique stress-strain characteristics
  • Metals typically show ductile behavior with large plastic regions
  • Composites often display more brittle failure modes
  • Material choice balances strength, weight, and cost considerations

Structural Design Considerations

Factor of Safety and Design Margins

• Factor of safety represents the ratio of ultimate load to design load
  • Typically ranges from 1.5 to 2.0 for aircraft structures
  • Higher factors used for critical components or when uncertainties exist
  • Lower factors possible with advanced materials and analysis techniques
• Design margins account for variations in manufacturing and operating conditions
  • Include allowances for material property scatter
  • Consider potential degradation over aircraft lifetime
  • Incorporate uncertainties in load predictions and analysis methods
• Balancing safety factors with weight and cost optimization
  • Higher factors increase reliability but add weight and cost
  • Modern design approaches use probabilistic methods to refine safety margins
  • Regulatory requirements set minimum acceptable safety levels

Fatigue and Structural Longevity

• Fatigue results from repeated loading and unloading of structures
  • Occurs even at stress levels below the material's yield strength
  • Microscopic cracks form and propagate over time
  • Critical in areas of stress concentration (holes, corners, joints)
• Fatigue life prediction essential for aircraft structural design
  • S-N curves relate stress levels to number of cycles before failure
  • Cumulative damage models (Miner's rule) assess fatigue under variable loading
  • Fracture mechanics approaches predict crack growth rates
• Strategies to enhance fatigue resistance in aircraft structures
  • Material selection (high fatigue strength alloys, composites)
  • Design features (smooth transitions, avoiding sharp corners)
  • Surface treatments (shot peening, cold working) to induce compressive stresses
  • Inspection and maintenance programs to detect and address fatigue damage