We're starting with the idea of airflow itself — Page 54, Lesson 62

I want to walk you through the core definitions that anchor this whole subject, because every single thing we do in Principles of Flight hangs off these terms. We're starting with the idea of airflow itself. When we talk about airflow in this context, we mean the air as it moves relative to the aircraft. And here's the key point I want you to lock in right now: airflow is the TAS — the True Airspeed. Not the indicated speed, not the ground speed. The actual speed of the air relative to the aircraft. That's what we mean when we say "airflow." Now, for airflow to be considered effective airflow — and this is a term you'll see again and again — it must possess three qualities. If airflow does not possess all three of these qualities, it is referred to as effective airflow? No — wait, let me be precise. If airflow does not possess all three qualities, it is referred to as effective airflow? Let me re-read that. The text says: "If Airflow does not possess all three of these qualities, it is referred to as EFFECTIVE AIRFLOW." Hmm, that's what the source states. So the effective airflow is the airflow that has all three required qualities — and if it lacks any of them, it's still called effective airflow? No. Let me be careful. The source says: if airflow does not possess all three qualities, it is referred to as effective airflow. So the effective airflow is the airflow that does possess all three qualities. The three qualities are: it is the TAS, it is the relative velocity, and it is the airflow over the aerofoil. So effective airflow is the airflow that has all three — it's the true relative airflow. That's the definition we work with. Now let's move to the forces. The Total Reaction is the resultant of all the aerodynamic forces acting on the aerofoil section. Think of it as the single combined force that results from every aerodynamic force pushing on the wing. The Centre of Pressure, or CP, is the point on the chord line through which lift is considered to act. The chord line is the straight line from the leading edge to the trailing edge of the aerofoil. So the CP is that specific point on that line where we say lift acts. Lift is the aerodynamic force which acts at 90° to the Relative Airflow. So lift is always perpendicular to the airflow. Drag is the aerodynamic force which acts parallel to and in the same direction as the Relative Airflow — or, put another way, opposite to the aircraft flight path. So drag acts along the airflow direction, resisting the aircraft's motion. Angle of Attack, which we write as α or alpha, and which can also be referred to as Aerodynamic Incidence, is the angle between the chord line and the Relative Airflow. That's the definition. And there's a related term: the angle between the chord line and the effective airflow is referred to as the Effective Angle of Attack. So we have angle of attack relative to the relative airflow, and effective angle of attack relative to the effective airflow. Now, let's talk about airflow basics. When we consider airflow velocity, it makes no difference to the pressure pattern whether the aircraft is moving through the air or the air is flowing over the aircraft. It's the relative velocity that's the important factor. That's why we can use wind tunnel experiments — where air flows over a stationary aircraft — to understand what happens in flight, where the aircraft moves through stationary air. The physics is identical. We have two concepts of airflow. Three-dimensional airflow is the true airflow over an aircraft. It consists of a hypothetical two-dimensional flow modified by various pressure differentials. We'll examine three-dimensional airflow later. Two-dimensional airflow assumes a wing with the same aerofoil section along the entire span, with no spanwise pressure differential or flow. So it's an idealised model — a wing that's identical all the way across, with no flow along the span. This two-dimensional airflow concept is used to illustrate the basic principles of aerodynamic force generation. As airflow moves towards an aerofoil, it will be turned towards the lower pressure at the upper surface. This is termed upwash. After passing over the aerofoil, the airflow returns to its original position and state. This is termed downwash. So upwash is the airflow turning up towards the low pressure on top as it approaches, and downwash is the airflow returning to its original state after passing over. Now let's look at the influence of dynamic pressure. Dynamic pressure is what we read as IAS — Indicated Airspeed. Figure 4.4 shows an aerofoil section at a representative angle of attack, subject to a given dynamic pressure. The principle here is: "If the static pressure on one side of a body is reduced more than on the other side, a pressure differential will exist." So when one side has lower pressure than the other, you get a pressure differential. Figure 4.5 shows the same aerofoil section at the same angle of attack, but subject to a higher dynamic pressure. And the principle is: "If the dynamic pressure is increased, the pressure differential will increase." So more dynamic pressure means a bigger pressure difference between the two sides. That pressure differential acting on the surface area will produce an upward acting force. And the final principle: "If the dynamic pressure is increased, the upward force will increase." So higher IAS means greater pressure differential, which means greater upward force — that's lift. That's the chain: dynamic pressure drives pressure differential, which drives the upward force. So to tie it all together: we have the airflow as TAS, the effective airflow with its three qualities, the total reaction as the resultant of all aerodynamic forces, the centre of pressure where lift acts, lift at 90° to the relative airflow, drag parallel to it, angle of attack between chord and relative airflow, effective angle of attack between chord and effective airflow, upwash and downwash as the airflow turns, and dynamic pressure driving the pressure differential that produces the upward force. That's the foundation.