Fuselage, Wings and Stabilizing Surfaces — Page 28, Lesson 35

Let’s start with the big picture. We’re looking at the fuselage, wings, and stabilizing surfaces, and the first thing I want you to understand is why modern aircraft are turning to composites for their structure. The reason is threefold: high specific strength, high specific stiffness, and the ability to retain those properties at elevated temperatures. Let me unpack that. Specific strength means strength per unit weight — how much load a material can carry for each kilogram of its own mass. Specific stiffness is the same idea but for stiffness, which is resistance to bending or flexing under load. So composites give you a lot of strength and a lot of stiffness for very little weight, and they keep those properties even when the structure gets hot, which matters a lot near engines or at high speeds. There’s another big advantage: you can tailor the strength to the direction of the load. That means you can lay the material fibres so the structure is strong exactly where the forces come from, and not waste weight where there’s no load. Now, the cost picture. Composites are not cheap to build with. The manufacturing costs are high because the process is labour intensive and often complex. But those costs are outweighed by the reduced operating costs. The classic example is the Boeing Dreamliner — the 787 — which is approximately 20% lighter because of composite construction, and that gives a large reduction in fuel consumption. So you pay more to build it, but you save continuously on fuel over the life of the aircraft. Next, let’s look at Sandwich Construction. This is used extensively on aircraft of all types. Where do you find it? Typically for flight control surfaces, flooring, fuselage panels, empennage skin — that’s the tail structure skin — and sound proofing for engines. It’s a laminar construction, meaning it’s built up in layers. The core is a honeycomb, and on each side you have skins made of composite material — either GRP or CFP — or aluminium alloy. GRP is glass-reinforced plastic, CFP is carbon-fibre plastic. The honeycomb core with those skins gives you rigidity and strength. It has a good strength-to-weight ratio, and it’s particularly strong in the direction of the honeycomb openings — so the cells act like little columns carrying load along their axis. One important limitation: parts made of sandwich material need additional provision to carry concentrated loads. If you bolt something heavy to it, the load doesn’t spread well through the honeycomb, so you have to add local reinforcement. Now let’s move to how we join materials together — Attachment Methods. There are many methods, but the common ones are five: riveting, welding, bolting, pinning, and bonding. Let me take riveting and welding in detail, because those are the two the syllabus wants you to know properly. Riveting has been the most common way of joining materials. The process: you place a rivet in a pre-drilled hole, then you deform the tail of the rivet, and that deformation clamps the material together. Sometimes you only have access to one side of the joint — you can’t get behind it — so there are a variety of blind rivets for that situation. Blind rivets can be set from one side only. Rivets may be set by hand or by a power-operated machine. And here’s the critical engineering point: all rivets are meant to be used in shear and have little strength in tension. Shear is a sliding force across the rivet’s cross-section — the two plates trying to slide past each other. Tension is a pulling force along the rivet’s axis, trying to pull the plates apart. Rivets are excellent in shear, but they’re weak in tension, so you design joints so the rivets carry shear loads, not pull-out loads. Finally, welding. This is a process where the two metals are fused to become one — the parent metals actually melt together into a single piece. The specific type called fusion welding uses a gas flame to heat the metal, and a filling material is used to fill the gaps between the pieces. There are many other types of welding too: forge welding, electric arc welding, and spot welding, and each of those has particular applications. So the key distinction is that riveting joins by mechanical clamping, while welding joins by actually fusing the metal into one continuous piece. Let me just make sure you’ve got the load path clear. Composites give you light, strong, stiff structure that can be tailored to load direction. Sandwich construction gives you rigid panels that are strong along the honeycomb cell axis but need local reinforcement for concentrated loads. And when you join it all together, rivets carry shear, welds fuse the metal, and you have bolting, pinning, and bonding as the other options. That’s the foundation for how the whole airframe goes together.