Fuselage, Wings and Stabilizing Surfaces — Page 11, Lesson 18

I want to walk you through the fuselage, wings, and stabilizing surfaces now, and we're starting with a few foundational ideas that everything else builds on. First, the stress concentration factor. A stress concentration is simply the point on an object where stress is concentrated. Think of it this way: an object is strongest when the force is evenly distributed over its entire area. The moment you reduce that area, you get a localized increase in stress at that spot. That reduction in area can be produced by something as small as a crack. And here's the critical part — materials can fail via a propagating crack, meaning a crack that grows and spreads through the material. Now, here's a sobering reality of aircraft materials: most materials already contain small cracks or contaminants, and those are exactly the points where stress concentrates. Fatigue cracks will start at these points. That's why removing any defects will increase the fatigue strength of the part. So the inspection and maintenance philosophy we'll talk about later is directly tied to this — you're hunting for those tiny defects before they become propagating cracks. Next, we need a way to locate components on the aircraft so maintenance and repairs can actually be carried out. That's where station numbers come in. The manufacturer establishes reference lines and station numbers for the fuselage, wings, empennage, and so on. For the fuselage, station lines are determined by reference to a zero datum line — that's fuselage station 0.00 — located at or near the forward portion of the aircraft, as defined by the manufacturer. Station numbers are given in inches, and they're either forward of that zero datum, which is negative and gets a minus sign, or aft, which is positive and gets a plus sign. Wing stations are measured differently. They're measured from the centre line of the aircraft, given in inches left or right of that centre line. And for vertical position, we use what's called a Water Line, abbreviated WL, given as a dimension in inches from the horizontal datum. So you have three reference systems: fuselage stations fore and aft of the zero datum, wing stations left and right of the centre line, and water lines for vertical position. Now let's move into the aircraft structures themselves, starting with the fuselage. The fuselage is the main structure or body of the aircraft. It carries the aircraft payload — that's the passengers and/or freight — as well as the flight crew and cabin staff, in safe, comfortable conditions. It also gives the flight crew an effective position for operating the aircraft, and space for controls, accessories, and other equipment. And it transfers loads to and from the main planes, which are the wings, the tailplanes, the fin, the landing gear, and in certain configurations, the power plants. Now, if the aircraft is pressurized, the structure has to do more. It must be capable of supporting two specific types of stress imposed by the pressurization forces: axial stress and hoop stress. Axial stress, also called longitudinal stress, is set up in the fuselage when it's pressurized, and it tends to elongate the fuselage — pull it lengthwise. Hoop stress, also called radial stress, is set up in addition to axial stress, and it tends to expand the fuselage cross-section area — push it outward like a balloon. And these internal pressures can be as high as 65.5 kilonewtons per square metre, which is 9.5 pounds per square inch. So the fuselage skin is being pulled in two directions at once every time you pressurize. Finally, let's look at fuselage design and cross-sections. The fuselage can be built in a number of cross-sections, and they all have advantages and disadvantages. The first one we'll consider is the rectangular cross-section. Many non-pressurized aircraft use this shape due to cost constraints. They're easier to construct, but they do have a high weight-to-strength ratio — meaning for the strength you get, you're carrying more weight than you'd like. That's the trade-off you'll see repeated across all the cross-section designs.