Subsonic Airflow — Page 54, Lesson 60

We're starting a brand-new topic: Subsonic Airflow. This is where we define the vocabulary of the wing itself, and I want to walk you through it carefully because every single term here is going to come back in later chapters. First, the aerofoil. An aerofoil is a shape capable of producing lift with relatively high efficiency. That's the definition we work from — it's not just any curved shape, it's a shape specifically designed to generate lift efficiently. Now let's build the geometry of that shape. The chord line is a straight line joining the centres of curvature of the leading and trailing edges of the aerofoil. Think of the leading edge as the front of the wing and the trailing edge as the back. The chord is simply the distance between those two edges, measured along the chord line. Next, the angle of incidence. This is the angle between the wing root chord line and the longitudinal axis of the aircraft. That's the fore-and-aft axis of the fuselage. This angle is fixed for the wing, but it may be variable for the tailplane — that's how the tailplane can be adjusted. Now the camber. The mean line, or camber line, is a line joining the leading and trailing edges, equidistant from the upper and lower surfaces. So it's the midline of the wing section. Maximum camber is the maximum distance of that mean line from the chord line. It's expressed as a percentage of the chord, and its location is given as a percentage of the chord aft of the leading edge. When the camber line lies above the chord line, we call it positive camber. If it's below, negative camber. And a symmetrical aerofoil has no camber at all, because the chord line and camber line are coincidental — they lie on top of each other. Next, the thickness/chord ratio. This is the maximum thickness or depth of the aerofoil section, expressed as a percentage of the chord, with its location as a percentage of the chord aft of the leading edge. The thickness and the thickness distribution have a great influence on the airflow characteristics of the section. Then the leading edge radius — the radius of curvature of the leading edge. The size of this radius can significantly affect the initial airflow characteristics of the aerofoil. Now, the crucial concept: relative airflow. It has three qualities, and all three must be present. First, direction — the air is parallel to, and in the opposite direction to, the flight path of the aircraft. In fact, the path of the centre of gravity. The direction the aircraft is pointing is irrelevant — what matters is the actual flight path. Second, condition — the air is close to, but unaffected by, the presence of the aircraft. Its pressure, temperature, and velocity are not affected by the passage of the aircraft through it. Third, magnitude — the magnitude of the relative airflow is the TAS, the true airspeed. If the airflow does not possess all three of these qualities, it is referred to as effective airflow. That's a key distinction — relative airflow is the undisturbed, free-stream flow; effective airflow is anything that falls short of that. Finally, we have total reaction. This is the resultant of lift and drag acting on the aerofoil. Lift acts perpendicular to the relative airflow, drag acts parallel to it, and total reaction is the vector sum of the two. Let me show you this on the figure. There you can see the chord line, the mean camber line, the leading edge radius, the maximum thickness and its location, the maximum camber and its location, and the angle of attack between the chord line and the relative airflow. And you can see the total reaction as the combination of lift and drag relative to the aircraft flight path. So the whole picture is this: the aerofoil's geometry — chord, camber, thickness, leading edge radius — defines its shape. The relative airflow defines the flow it sees. And the total reaction is the aerodynamic force that results. That's the foundation we'll build on.