Stalling — Page 145, Lesson 175

Let’s pick up right where the lift curve leaves off, because that’s the heart of understanding a stall. I want you to picture the lift curve graph. On the horizontal axis we have angle of attack in degrees, and on the vertical axis we have the lift coefficient, which we write as CL. Now, as the angle of attack increases from the zero-lift value, the curve is linear over a considerable range — meaning lift grows in a straight, predictable line. But as the effects of airflow separation begin to be felt, the slope of that curve starts to fall off. Eventually, lift reaches a maximum and then begins to decrease. The angle at which lift reaches that maximum and starts to drop is called the stalling angle, or the critical angle of attack. The corresponding value of lift coefficient at that point is called CLMAX — that’s the maximum lift coefficient. A typical stalling angle is about 16 degrees. So the stall isn’t about speed alone; it’s about exceeding that critical angle of attack where the airflow can no longer stay attached. Now, how do we recover? To recover from a stall, or to prevent a full stall from developing, the angle of attack must be decreased. Why? Because decreasing the angle of attack reduces the adverse pressure gradient — that’s the pressure rise along the chord that’s causing the airflow to separate. Recovery might be as simple as merely releasing back pressure on the control column, or it might require smoothly moving the pitch control forward, depending on the aircraft design and the severity of the stall. But here’s a caution: excessive forward movement of the pitch control may impose a negative load on the wing and actually delay recovery. For most modern jet transport aircraft, it’s usually sufficient to lower the nose to the horizon, or just slightly below, while applying maximum authorized power to minimize height loss. Now, wing drop — that’s when one wing stalls before the other and the aircraft rolls. On straight-wing aircraft, you should use the rudder to prevent wing drop during the stall and recovery. On swept-wing aircraft, the recommendation is different: use the ailerons to prevent wing drop, with a small amount of smoothly applied co-ordinated rudder. Why the difference? Because on swept wings, ailerons are more effective at controlling roll near the stall. But there’s a warning here: the rudder on modern high-speed jet transport aircraft is very powerful, and careless use can give too much roll, leading to what’s called pilot induced oscillation — PIO. That’s when the pilot’s control inputs cause the aircraft to oscillate, making things worse. Once the nose is down and airspeed is increasing, you recover lost altitude with moderate back pressure on the pitch control. But be careful — pulling too hard could trigger a secondary stall, or worse, could exceed the limit load factor and damage the aircraft structure. As the angle of attack reduces below the critical angle, the adverse pressure gradient decreases, the airflow re-attaches to the wing, and lift and drag return to their normal values. So the whole recovery is about getting the angle of attack back below that critical value, letting the airflow reattach, and then gently regaining altitude. Take a look at Figure 7.2 — it shows that lift curve with CLMAX and the stall point marked at the critical angle of attack. That’s the visual anchor for everything we just covered.