Fuselage, Wings and Stabilizing Surfaces — Page 1, Lesson 10

Let's start with the very basics of aircraft structures. I want you to think of the airframe as a skeleton that has to handle a whole set of different forces, and each force has a specific name and a specific way it tries to deform the material. First, tension. A tension load, also called a tensile load, is one that tends to stretch a structural member. Imagine pulling on both ends of a rope — that's tension. Now, the components that are specifically designed to resist these tensile loads are called ties. So a tie is a part that's built to handle being pulled apart. Next, the exact opposite: compression. Compressive loads tend to shorten structural members. Instead of pulling, you're pushing the ends together. The components designed to resist compressive loads are called struts. So a strut is built to handle being squeezed. Then we have shear. Shear is a force which tends to slide one face of the material over an adjacent face. Picture a stack of cards — if you push the top card sideways while holding the bottom card still, the cards slide over each other. That sliding action is shear. And here's a key practical point: riveted joints are designed to resist shear forces. That's why you see rows of rivets on aircraft skins — they're there to stop the panels from sliding past each other. Now, real aircraft structures rarely see just one of these loads in isolation. They get combination loadings. The first combination is bending. When a structure bends, it actually involves all three basic loadings at once: tension as the outer edge stretches, compression as the inner edge squeezes together, and shear across the structure as the forces try to split it. Think of bending a ruler — the top surface stretches, the bottom surface compresses, and internally the layers want to slide. Then there's torsion, which is twisting. Torsion forces produce tension at the outer edge, compression in the centre, and shear across the structure. So twisting a component creates the same three stress types, just arranged differently. Now let's define two terms that people often confuse: stress and strain. Stress is the internal force per unit area inside a structural part, and it's a result of external loads. So a tensile load sets up a tensile stress, a compressive load sets up compressive stresses. Stress is defined as force per unit of area, and it's measured in units of N/mm² — that's newtons per square millimetre — or MN/m², meganewtons per square metre. Strain is different. When an external force of sufficient magnitude acts on a structure, the structural dimensions change. That change is called strain. More precisely, strain is the deformation caused by the action of stress on a material. It's normally given as the change in dimension expressed as a percentage of the original dimensions of the object. So if a rod stretches from 100 mm to 101 mm, the strain is 1 percent. Now, here's the beautiful relationship: for an elastic material, the relationship between stress and strain is generally a constant, and that constant is known as Young's Modulus of Elasticity. It's a material property that tells you how stiff the material is — how much stress you need to produce a given strain. One more failure mode you need to know: buckling. Buckling occurs to thin sheet materials when they're subjected to end loads, and also to ties if they're subjected to compressive forces. This is critical — a thin sheet pushed from the ends will suddenly fold or wrinkle rather than compress cleanly. That's why aircraft skins are often stiffened with stringers; they prevent buckling. Now, the big picture. Aircraft components are subjected to some or all of these stresses, and these will tend to elongate, compress, bend, shear, or twist the component. But here's the safety principle: providing the resulting deformation is within the elastic limit of the material, the component will return to its original dimension once the deforming load has been removed. If any load takes the structure beyond the elastic limit, the deformation will be permanent — and that's when you have a structural problem. So the whole design philosophy is: keep every load within the elastic limit so the structure springs back to shape every time. That's the foundation for everything else we'll cover in this chapter.