Subsonic Airflow — Page 54, Lesson 65

Let's start fresh with subsonic airflow and the influence of angle of attack. This is where lift really begins to make sense. I want you to picture an aerofoil — a wing cross-section — sitting in an airflow. We're holding the indicated airspeed, or IAS, constant. That means dynamic pressure is constant. Now, as we increase the angle of attack — the angle between the chord line and the relative airflow — up to about 16 degrees, something important happens. The pressure differential increases, but the pattern of pressure distribution across the surface changes too. The key idea is this: the aerofoil profile presented to the airflow determines the distribution of velocity, and hence the distribution of pressure on the surface. That profile is set by three things. First, the aerofoil geometry — its thickness and how that thickness is distributed, which is fixed. Second, the camber and its distribution, which we're assuming is fixed for now. And third, the angle of attack, which is variable. Now let's talk about the stagnation point. This is where the greatest positive pressure occurs, and it's where the relative flow velocity is zero. It sits somewhere near the leading edge. Here's the fascinating part: as the angle of attack increases from minus 4 degrees, the leading edge stagnation point moves from the upper surface, around the leading edge, and onto the lower surface. It's at this front stagnation point that the flow divides — part goes over the top of the section, part goes underneath. The pressure at that stagnation point — the stagnation pressure — is static pressure plus dynamic pressure. That's a fundamental relationship you'll use again and again. Now, over the top of the section, the flow accelerates rapidly around the nose and over the leading portion of the surface. This induces a substantial decrease in static pressure in those areas. And here's a critical point: the rate of that acceleration increases as angle of attack increases, up to about 16 degrees. Anything that changes the accurately manufactured profile of that leading portion — ice, snow, frost, dirt, or dents — can seriously disrupt the airflow acceleration in this critical area. That's why pre-flight contamination checks matter so much. The pressure reduces continuously from the stagnation value, through the free stream value, to a position on the top surface where a peak negative value is reached. From there onwards, the flow continuously slows down again, and the pressure increases back to the free stream value in the region of the trailing edge. Now, at angles of attack less than 8 degrees, the flow under the section is accelerated much less. It reduces the pressure to a small negative value, also with subsequent deceleration and an increase in pressure back to the free stream value near the trailing edge. Here's where drag enters the picture. The pressure differential between the leading edge stagnation point and the lower pressure at the trailing edge creates a force acting backward. That force is called form drag, or pressure drag. We'll discuss it in more detail later, but for now, just know that name and what creates it. Finally, let's look at the special case of minus 4 degrees angle of attack. At this angle, the decrease in pressure above and below the section are equal — no differential exists. So there is no lift force. This is called the zero lift angle of attack. It's a specific, measurable condition, and it's worth remembering that it's not zero degrees — it's minus 4 degrees for this aerofoil. So to tie it together: the stagnation point is where flow divides and pressure is highest, the top surface accelerates flow and drops pressure to a peak negative value, the bottom surface barely accelerates at all, and the pressure difference between front and back creates form drag. And at minus 4 degrees, everything balances out to zero lift.