Stalling — Page 166, Lesson 200

Let's talk about a dangerous condition called the super stall, or deep stall. This is one of those things you absolutely need to understand, because it's a situation where the aeroplane can trap itself in a stall that you might not be able to recover from. To understand it, we first need to look at what happens with a swept-back wing. A swept-back wing tends to stall first near the tips. Think about that — the outboard sections of the wing, the tips, stall before the root does. Now, here's the critical part. On a swept-wing aeroplane, those wing tips are situated well aft of the centre of gravity, the CG. So when the tips stall and lose lift, that loss of lift happens behind the CG. And what does that do? It causes the pitch attitude to increase rapidly. The nose pitches up. And when the nose pitches up, the angle of attack increases even further. So we have this chain reaction: tip stall causes pitch-up, pitch-up increases angle of attack, which stalls more of the wing. That's what Figure 7.19 shows us — the pitch-up caused by tip stall. Now, this "automatic" increase in angle of attack, caused by that pitch-up, stalls more of the wing. As more of the wing stalls, drag increases rapidly, lift reduces, and the aeroplane starts to sink. And here's the key — it sinks at a constant, nose-high pitch attitude. That nose-high attitude means the angle of attack keeps increasing rapidly. So we're in a vicious cycle, and this is what Figure 7.20 illustrates — the super stall. Now, why can't the pilot just push the nose down and recover? That's where the tailplane comes in. Look at Figure 7.20 again. The separated airflow from the stalled wing will immerse a high-set tailplane in low energy turbulent air. So if the tailplane is mounted high on the fuselage, the turbulent, separated air from the stalled wing flows right over it. The tailplane is sitting in that low-energy, turbulent wake. What does that do to the elevator? Elevator effectiveness is greatly reduced. The elevator can't generate enough force to push the tail down and decrease the angle of attack. It becomes impossible for the pilot to decrease the angle of attack. The aeroplane will become stabilized in what is known as the "super stall" or "deep stall" condition. So let me summarise the whole chain for you. Swept wing stalls at the tips first. Tips are aft of the CG, so tip stall pitches the nose up. Pitch-up increases angle of attack, stalling more wing. Drag increases, lift reduces, the aeroplane sinks nose-high. The stalled wing's separated airflow blankets a high-set tailplane, killing elevator effectiveness. The pilot can't reduce angle of attack, and the aeroplane is stuck in the deep stall. That's the super stall — a self-sustaining, nose-high, sinking condition where the elevator has lost its authority because the tailplane is immersed in the stalled wing's turbulent wake.