Stalling — Page 173, Lesson 201

Let's pick up right where we left off — we've established that the swept-back wing is the major contributory factor in the super or deep stall, and now I want to walk you through how designers fight that dangerous pitch-up tendency, and then the ultimate safety device. First, the design modifications. The problem with a swept wing is that the boundary layer — that thin layer of air flowing along the surface — tends to drift spanwise, from the wing root out toward the tip. That's bad, because it starves the tip of clean airflow and makes the tip stall first. And when the tip stalls, the center of lift moves forward, which pitches the nose up — that's the pitch-up we're trying to avoid. So engineers use devices to minimize that root-to-tip spanwise flow of the boundary layer. The three I want you to know are wing fences, vortilons, and saw tooth leading edges. Wing fences are vertical plates on the upper surface that physically block the spanwise drift. Vortilons are small surfaces ahead of the leading edge that generate vortices to energize the flow. And a saw tooth leading edge is a sharp discontinuity in the leading edge that creates a vortex, again to keep the flow attached. All three delay tip stall. Then there are vortex generators — small vanes mounted on the wing that create vortices to re-energize the boundary layer. They're frequently used on a swept wing to delay tip stall and improve the stall characteristics. Now, the other side of the coin: instead of delaying the tip stall, you can encourage the root to stall first. That's done three ways. One, modify the aerofoil section at the root itself. Two, fit stall strips — small devices that trip the flow and force the root to stall early. Three, fit less efficient leading edge flaps — specifically Kruger flaps — to the inboard section of the wing. A Kruger flap is a leading edge flap that deploys forward and down; it's less efficient than other designs, and that inefficiency is exactly what we want here, because it makes the root stall first. Now, let's talk about the aircraft types. The DC-9, MD-80, Boeing 727, and Fokker 28 all have swept-back wings and high mounted tailplanes — the 'T' tail. They also have rear, fuselage mounted engines. Here's the key point: the only contribution the rear mounted engines make is that they're the reason the designer put the tailplane on top of the fin in the first place. In and of itself, mounting the engines on the rear fuselage does not contribute to super stall. So don't blame the engines — blame the swept wing and the T-tail combination. Now, the critical part: super stall prevention. If an aircraft design exhibits super stall characteristics, it must be fitted with a device to prevent it from ever stalling. That device is a stick pusher. And here's the reasoning: once such an aircraft begins to stall, it's too late. The progression to super stall is too fast for a human to respond, and the aircraft cannot then be un-stalled. So you can't rely on the pilot — you need a machine that acts faster than a human. A stick pusher is a device attached to the elevator control system. When it actuates, it physically pushes the control column forward, which reduces the angle of attack before super stall can occur. The force of the push is typically about 80 pounds. That's regarded as high enough to be effective, but not so high that it can't be held in a runaway situation — meaning if the system malfunctions and pushes when it shouldn't, the pilot can hold against it. There's also provision to "dump" the stick pusher system in the event of a malfunction. Once dumped, the pusher cannot normally be reset in flight. So if it fails, you disable it for the rest of the flight. And finally, the automatic disengagement: once actuated, the stick pusher will automatically disengage once the angle of attack reduces below a suitable value. So it's not a one-shot — it pushes you out of the stall, and as soon as the angle of attack drops back to a safe value, it lets go. So the whole picture is: swept wing plus T-tail gives you the super stall risk; wing fences, vortilons, saw tooth leading edges, and vortex generators delay the tip stall; root modifications, stall strips, and Kruger flaps make the root stall first; and if the design still has super stall characteristics, the stick pusher is the mandatory last line of defense that physically forces the nose down before a human could ever react.