Stalling — Page 166, Lesson 193

Let’s pick up right where the stall behaviour gets dangerous — the swept wing. I want to walk you through why a swept wing stalls the way it does, and then the design fixes we bolt on to tame it. First, the big picture. A swept wing is fitted for one main reason: it allows a higher maximum speed. That’s the payoff. But it comes with a serious cost — an increased tendency to stall first near the tips. So the stall doesn’t start at the root, it starts out at the wingtip. Now, why does that matter? Think about where the centre of pressure, the CP, sits. When the tips lose lift, the centre of pressure moves forward. And a forward CP creates an unstable nose-up pitching moment. Let me unpack that. The CP is the point where the total lift force effectively acts. If lift is lost at the tips, the remaining lift is concentrated further forward, so the CP shifts forward. A nose-up pitching moment means the nose wants to rise. And that’s unstable — it’s the wrong direction at a stall. There’s a second contributor. With the tips stalled, effective lift production is concentrated inboard. That concentrated lift sends a strong downwash — the air deflected downward behind the wing — and this maximum downwash now impacts the tailplane. The tailplane gets hit by that downwash, and that adds to the nose-up pitching moment. So you have two things stacking up: the forward CP movement and the increased downwash at the tailplane. Now here’s the dangerous sequence, and it has a name: pitch-up. As soon as a swept wing begins to stall, both of those effects — forward CP movement and increased tailplane downwash — cause the aircraft nose to rise rapidly. And a rising nose further increases the angle of attack. That’s the vicious circle. More angle of attack means more stall, more stall means more nose-up, and the nose keeps rising. This is a very undesirable and unacceptable response at the stall. It can result in complete loss of control in pitch, from which it may be very difficult, or even impossible, to recover. That phenomenon is pitch-up, and it’s a very dangerous characteristic of many high-speed, swept-wing aircraft. Let me show you the geometry. So why does the swept wing tip-stall in the first place? It’s due to the induced spanwise flow of the boundary layer from root to tip. Let me explain that. The boundary layer is the thin layer of air right next to the wing surface that gets slowed down by friction. On a swept wing, that slow-moving boundary layer doesn’t just flow straight back — it drifts spanwise, from the root toward the tip. That outward drift thickens the boundary layer at the tip, and a thick, slow boundary layer separates from the surface more easily. So the tip stalls first. That’s the root cause. Now, the good news — there are design features we can incorporate to minimize this effect and give a swept-wing aircraft more acceptable stall characteristics. Let’s go through them one by one. First, wing fences, also called boundary layer fences. These are thin metal fences that generally extend from the leading edge to the trailing edge on the top surface of the wing. Their job is to prevent the outward drift of the boundary layer. They act as physical barriers, stopping that spanwise flow from reaching the tip. Second, vortilons. These are also thin metal fences, but they’re smaller than a full chordwise fence. They’re situated on the underside of the wing leading edge. Now, here’s a clever detail — the support pylons of pod-mounted engines on the wing act in the same way. So if you have engines hanging under the wing on pylons, those pylons are doing the same job as vortilons. How do they work? At high angles of attack, a small but intense vortex is shed over the wing top surface. That vortex acts as an aerodynamic wing fence — it’s not a physical barrier, it’s a spinning column of air that blocks the spanwise flow. Third, saw tooth leading edges. These will also generate a strong vortex over the wing upper surface at high angles of attack, minimizing the spanwise flow of the boundary layer. But note the caveat — they’re rarely used on modern high-speed jet transport aircraft. So to tie it all together: the swept wing gives you speed, but it gives you pitch-up at the stall. The cause is spanwise boundary-layer flow from root to tip, which makes the tip stall first, which moves the CP forward and increases tailplane downwash, which pitches the nose up, which increases angle of attack, which deepens the stall. And the fixes are wing fences, vortilons, and saw tooth leading edges — all designed to stop that spanwise flow and keep the stall behaviour under control.