Subsonic Airflow — Page 63, Lesson 72

Let’s start with the big picture. The lift and drag your wing produces are not random — they depend on four things, and I want you to hold these in your mind as the skeleton of the whole chapter. First, angle of attack, which changes the airflow cross-sectional area. Second, the aerofoil shape — its thickness and camber — which also changes that cross-sectional area. Third, air density, which governs the mass flow of air and decreases as you climb in altitude. And fourth, velocity, which also governs mass flow and changes with your aircraft’s true airspeed, the TAS. Now, where does lift actually come from? The lift force is the result of a pressure differential between the top and bottom surfaces of the aerofoil. And here is the key point that surprises a lot of students: the greatest contribution to overall lift comes from the top surface, not the bottom. So when we talk about lift, we are really talking about the upper surface doing the heavy lifting. That is why the leading portion of the upper surface is so precious. Anything that changes the accurately manufactured profile of that leading upper surface can seriously disrupt the airflow acceleration in that area, and therefore the magnitude of the lift force is affected. The book names the culprits: ice in particular, but also frost, snow, dirt, dents, and even water droplets. So a perfectly clean wing is not just cosmetic — it is a lift-preservation requirement. Let me give you two direct relationships that you will use constantly. An increase in dynamic pressure — which you read on the airspeed indicator as IAS — will increase the lift force, and vice versa. And an increase in angle of attack will increase the lift force, and vice versa, within the range from 0° to 16°. That 16° is your warning line; beyond it, the story changes, but for now, within that range, more angle of attack means more lift. Now let’s talk about the centre of pressure, the CP. This is the point where the resultant aerodynamic force acts. For a cambered aerofoil, the CP moves forward as the angle of attack increases. But for a symmetrical aerofoil, the CP does not move under the influence of angle of attack — at least within the confines of the normal range. So camber gives you a moving CP; symmetry gives you a fixed one. Next, the aerodynamic centre, the AC. Throughout the normal range of angles of attack, the aerofoil’s nose-down pitching moment about the AC remains constant. And here is a precise locating fact: the AC is located at the quarter chord position for subsonic flow of less than M 0.4. So below Mach 0.4, you can put your finger at the quarter chord and that is your aerodynamic centre, and the nose-down moment about it does not change with angle of attack. We also have two coefficients that you will see everywhere. The coefficient of lift, written CL, is the ratio between lift per unit wing area and dynamic pressure. And the coefficient of drag, written CD, is the ratio between drag per unit wing area and dynamic pressure. So both are normalised quantities — they take the force per unit area and divide by dynamic pressure, giving you a number that describes the aerofoil’s behaviour independent of speed and size. Now, a subtle but important flow detail. As the angle of attack increases from -4°, the leading edge stagnation point moves from the upper surface, around the leading edge, to the lower surface. The stagnation point is where the flow comes to rest, and the greatest positive pressure occurs right there at the leading edge stagnation point, where the relative flow velocity is zero. So maximum pressure, zero velocity — that is the definition of stagnation. Finally, drag. Form drag, also called pressure drag, is the result of the pressure differential between the leading edge and the trailing edge of the aerofoil. And just like lift, an increase in dynamic pressure, your IAS, will increase form drag, and vice versa. So the whole picture ties together: lift and drag both scale with dynamic pressure, both are expressed as coefficients per unit wing area, and both are governed by that clean upper surface and the angle of attack range you fly within.