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.