We're starting a brand-new chapter now — Chapter 6, Drag — Page 111, Lesson 130

We're starting a brand-new chapter now — Chapter 6, Drag. This is a big one, and it's going to tie together a lot of what you've already learned about airflow and lift. Let's begin at the very beginning. First, the definition you need to lock in: Drag is the force which resists the forward motion of the aircraft. It acts parallel to, and in the same direction as, the relative airflow — which means it's acting in the opposite direction to the flight path. So as the aircraft moves forward, drag is the force pulling it back. Now, here's a really important concept about airflow velocity that I want you to remember, because it comes up again and again. When we consider airflow velocity, it makes no difference to the airflow pattern whether the aircraft is moving through the air, or the air is flowing past the aircraft. It's the relative velocity that's the important factor. So whether you're in a wind tunnel with air rushing over a stationary model, or flying through still air, the aerodynamic picture is identical — it's all about the relative motion between the aircraft and the air. Now, every single part of an aeroplane that's exposed to the airflow produces different types of resistance to forward motion, and all of these contribute to what we call Total Drag. Total Drag is sub-divided into two main types, and this is the fundamental split of the whole chapter. The first is Parasite Drag — and the key thing here is that it's independent of lift generation. It's the drag that exists just because the aircraft is pushing through the air, whether or not it's producing lift. The second is Induced Drag — and this one is the result of lift generation. It's a by-product of producing lift. We'll come back to induced drag later in the chapter, but for now, just hold that contrast: parasite drag is there regardless of lift, induced drag is caused by lift. Now, parasite drag itself is further sub-divided into three elements. Let me give you their exact names: - Skin Friction Drag - Form (Pressure) Drag - Interference Drag And there's a note here that's worth its weight in gold for exams: Skin Friction Drag and Form Drag, when taken together, are known as Profile Drag. So profile drag is the sum of skin friction plus form drag — it's a collective term. Let me show you how this all fits together, because it's a hierarchy. Total Drag splits into Parasite Drag and Induced Drag. Parasite Drag splits into Skin Friction, Form, and Interference. And Skin Friction plus Form together are called Profile Drag. That's the structure of the whole chapter laid out for you. We're going to spend the next while on the elements of parasite drag — skin friction, form, and interference — before we ever get to induced drag. So let's start there.