Advanced Aircraft Structures for Engineers

This course presents some of the important aspects in the design and analysis of aircraft structures.  An aircraft is a device that is used, or intended to be used, for flight, according to the current Title 14 of the Code of Federal Regulations (14 CFR) Part 1, Definitions and Abbreviations. Categories of aircraft for certification of airmen include airplane, rotorcraft, glider, lighter-than-air, powered-lift, powered parachute, and weight-shift control. 14 CFR part 1 also defines airplane as an engine-driven, fixed-wing aircraft that is supported in flight by the dynamic reaction of air against its wings.These important aspects are related to material selection, structural configuration, loads evaluation, static strength and deflection estimation, static stability evaluation, fatigue and fracture effects, aeroelastic considerations, and influence of dynamic loadings. The key aspects in specific areas are combined to provide an overall perspective of aircraft structural design and analysis. Aircraft structural members are designed to carry a load or to resist stress. In designing an aircraft, every square inch of wing and fuselage, every rib, spar, and even each metal fitting must be considered in relation to the physical characteristics of the material of which it is made. Every part of the aircraft must be planned to carry the load to be imposed upon it.

The determination of such loads is called stress analysis. Although planning the design is not the function of the aircraft technician, it is, nevertheless, important that the technician understand and appreciate the stresses involved in order to avoid changes in the original design through improper repairs.

The term “stress” is often used interchangeably with the word “strain.” While related, they are not the same thing. External loads or forces cause stress. Stress is a material’s internal resistance, or counter force, that opposes deformation. The degree of deformation of a material is strain. When a material is subjected to a load or force, that material is deformed, regardless of how strong the material is or how light the load is.

There are five major stresses [Figure 1] to which all aircraft are subjected:

• Tension

• Compression

• Torsion

• Shear

• Bending

Aircraft structures are generally classified as follows in terms of criticality of the structure:

•critical structure, whose integrity is essential in maintaining the overall flight safety of the aircraft (e.g., principal structural elements in transport category aircraft);

•primary structure carries flight, ground, or pressurization loads, and whose failure would reduce the aircraft’s structural integrity;

•secondary structure that, if it was to fail, would affect the operation of the aircraft but not lead to its loss; and

•tertiary structure, in which failure would not significantly affect operation of the aircraft.

Aircraft structures are vulnerable to impact damage resulting from impact by hard or soft bodies, such as steel fragments, birds, burst tyre rubber or hail. Aircraft structures, being extremely flexible, are prone to distortion under load. When these loads are caused by aerodynamic forces, which themselves depend on the geometry of the structure and the orientation of the various structural components to the surrounding airflow, structural distortion results in changes in aerodynamic load, leading to further distortion and so on. The interaction of aerodynamic and elastic forces is known as aero-elasticity. Two distinct types of aeroelastic problem occur. One involves the interaction of aerodynamic and elastic forces of the type described above. Such interactions may exhibit divergent tendencies in a too flexible structure, leading to failure or, in an adequately stiff structure, converge until a condition of stable equilibrium is reached. In this type of problem, static or steady state systems of aerodynamic and elastic forces produce such aeroelastic phenomena as divergence and control reversal. The second class of problem involves the inertia of the structure, as well as aerodynamic and elastic forces. Redistribution of aerodynamic loads and divergence are closely related aeroelastic phenomena; they are, therefore, simultaneously considered. The flexibility of the major aerodynamic surfaces (wings, vertical and horizontal tails) adversely affects the effectiveness of the corresponding control surfaces (ailerons, rudder, and elevators).