Fuselage, Wings and Stabilizing Surfaces — Page 28, Lesson 29

Let's pick up right where the bending moment discussion left off, because that's the key to understanding why the wing structure is built the way it is. We were just talking about how the wing root takes the maximum bending moment — that's the point where the wing attaches to the fuselage, and it's the most highly stressed part of the whole wing. That's why you'll see the structure get progressively heavier and stronger as you move inboard toward the root. Now, let's look at how the wing is actually built to handle those stresses. The mainplanes — that's the proper term for the wings themselves — can be constructed in a few different ways. You might have a single spar, a twin spar, or a multi-spar construction. Let me define what a spar is, because it's the backbone of the wing. A spar is the main longitudinal structural member that runs spanwise, from the root out toward the tip, and it's the primary beam that carries the bending loads. In a conventional structure, you'd have a front spar and a rear spar. Attached to those spars are the spar booms — those are the flanges, the top and bottom caps of the spar that actually take the tension and compression from bending. Then you have the metal skin, the ribs, and the stringers. Now here's the key point: these four main component parts — the spars, the skin, the ribs, and the stringers — together form what's called the 'torsion box'. That's the closed structural box that gives the wing its strength against twisting, or torsion. There's also an alternative form of construction where instead of having two big main spars, you use a series of smaller spars to replace them. That's the multi-spar approach I mentioned. Let me walk you through the other mainplane components, because each one has a very specific job. The skin takes the loads due to differences in air pressure across the wing surface, and it also carries the mass and inertia of the fuel if there's any in the wing tanks. Now, the skin generates direct stresses in a spanwise direction — that means along the length of the wing — as a response to bending moments. And it also reacts against twisting, which is torsion. So the skin is doing double duty: bending and torsion. The stringers are spanwise members — again, running along the wing's length — and their job is to give the wing rigidity by stiffening the skin in compression. Think of them as the longitudinal stiffeners that stop the skin from buckling when it's being squeezed. The ribs are the crosswise members. They maintain the aerofoil shape of the wings — that's the airfoil cross-section. They support the spars, stringers, and skin against buckling, and they pass concentrated loads from the engines, the landing gear, and the control surfaces into the skin and spars. So if an engine is hanging off the wing, the rib is what takes that concentrated load and spreads it out into the main structure. Now, what are these components made of? The major structural components of the wings are generally manufactured from aluminium alloys. But you'll also see composite materials used — and I want you to know these acronyms cold. GRP is glass reinforced plastic. CRP is carbon reinforced plastic. And there are honeycomb structures. These composites are typically used for the fairings, the control surfaces, the flaps, and so on — the secondary structure, not the primary load-bearing box. Now let's move to a completely different topic that's absolutely critical for you as a pilot: flutter and resonance. Flutter is an uncontrolled oscillation — an uncontrolled vibration — that can occur on fixed surfaces, such as the wing, or on control surfaces such as the ailerons or elevators. So it can hit both the fixed structure and the moving control surfaces. What causes flutter? It's caused by the interaction of three things: aerodynamic forces, inertia forces, and the elastic properties of the surface or structure. Let me unpack that. The air flowing over the surface produces aerodynamic forces. The mass of the structure produces inertia forces — the tendency of the structure to keep moving once it's disturbed. And the structure itself is elastic, meaning it flexes and springs back. When these three interact in the wrong way, the oscillation can build up rather than damp out. And here's the serious part: flutter can lead to the catastrophic failure of the structure. This is not a nuisance vibration — it can tear the wing off. Now, here's a practical point that might surprise you. Most wings are very flexible, and whilst on the ground they can easily be moved up and down by hand. That flexibility is normal — it's the elasticity I just mentioned. But it's exactly that flexibility that makes flutter a danger, because the structure can store and release energy in a way that sustains the oscillation. Let me bring this back to the bending moment note we started with, because it ties the whole lesson together. The note said this is particularly important at high All Up Mass — that's AUM, the total weight of the aircraft — when the outer wing fuel tanks are full. Here's the logic: when the outer tanks are full, you have a lot of fuel mass out at the wingtip, which increases the bending moment at the root. As the fuel is used, the weight of the aircraft decreases, which reduces the required lift, and therefore reduces the bending moments and the mass. So the wing structure is sized for that worst case — maximum AUM with full outer tanks — and that's why the maximum bending moment occurs at the wing root. So to summarize where we are: the wing is a torsion box made of spars, skin, ribs, and stringers, each with a specific load-carrying role, built from aluminium alloys with composites for secondary parts, and the whole structure is designed around the bending moment that peaks at the wing root — while flutter remains the dynamic danger that can destroy that structure if the aerodynamic, inertia, and elastic forces ever couple together.