Airframes & Aircraft Systems #1 - Aircraft Structures - Loads Applied to the Airframe

Introduction to Airframe Structures

Overview of Aircraft Loads and Design

• The lesson series will cover various loads on aircraft structures, focusing on fuselage, wings, and tail unit design.

• Discussion includes materials used in construction and the impact of corrosion and fatigue.

• Effects of hard landings on airframes will be examined alongside failure statistics for safe aircraft design.

Types of Loads on Aircraft Structures

Understanding Load Types

• Tensile loads stretch structural members; components resisting these are called ties (e.g., stringers in fuselage).

• Compressive loads shorten structural members; components resisting these are known as struts (e.g., landing gear).

• Shear forces slide one face over another; riveted joints resist shear forces but are increasingly replaced by adhesive bonding.

Complex Load Interactions

Bending, Torsion, and Buckling

• Bending involves tension at outer edges, compression at inner edges, and shear across the structure.

• Torsion creates tension at outer edges while compressing the center; buckling occurs under end loads or compressive forces.

Stress and Strain Concepts

Definitions and Measurements

• Stress is the internal force resisting external load, measured in Newton's per square millimeter or mega Newton's per square meter.

• Strain measures deformation as a ratio of change in length to original length; example given with a 2-meter material stretching to 2.5 meters.

Design Limit Load vs. Ultimate Load

Safety Factors in Aircraft Design

• Design limit load is the maximum expected load during service; expressed as a load factor (lift/weight ratio).

• Different types of aircraft have varying design limit loads: 2.5 G for public transport planes up to 6 G for aerobatic aircraft.

Safe Life Concept in Aircraft Structures

Maintenance Considerations

• Safe life defines minimum operational life without catastrophic failure; critical components measured by flying hours or cycles.

• After reaching safe life limits, components must be replaced or overhauled to ensure safety.

Fail-Safe vs. Damage Tolerant Structures

Structural Integrity Principles

• Fail-safe structures use redundancy with multiple load paths to share loads among adjacent members if one fails.

• Damage tolerant designs spread loading over larger areas, reducing need for extra structural members while maintaining integrity under damage.

Fatigue Failure Mechanisms

Understanding Material Performance

• Continual reversals of loading lead to fatigue failures occurring at lower stress levels than steadily applied loads.

• SN curves graphically represent material performance against cycles to failure based on stress levels experienced.

Component Location Identification

Maintenance Accessibility

• Reference lines establish component locations for maintenance using fuselage station numbers relative to a zero datum line near the nose.

Fuselage Structure Functions

Role in Aircraft Design

• The fuselage carries payload including passengers/freight while providing space for crew operations and equipment.

• Modern pressurized fuselages must withstand axial stresses from pressurization that elongate between pressure bulkheads.