Stalling — Page 166, Lesson 197

Let’s start with the regulatory backbone, because that’s what the whole chapter hangs on. The EASA requirements for stall characteristics, up to the moment the aeroplane is actually stalled, are a set of five or six hard rules the design must meet. I’ll read them as they’re written, and fill in the blanks as we go. First: it must be possible to produce and correct a roll by unreversed use of the aileron and rudder. “Unreversed” means the control response stays normal—pull the stick the way you want to roll, and the aeroplane rolls that way; there’s no point where the controls flip and do the opposite. That has to hold right up to the stall. Second: no abnormal nose-up pitching may occur. In other words, the aeroplane must not pitch up violently on its own as it approaches the stall. Third: the longitudinal control force must be positive. That means the elevator or stick force must always require a definite, positive pull from the pilot—never a force that pushes back or goes neutral in a way that could confuse you. Fourth: it must be possible to promptly prevent a stall and recover from a stall by normal use of the controls. So a normal, standard recovery technique has to work, and it has to work quickly. Fifth: there should be no excessive height loss between the stall and completion of recovery. You shouldn’t lose a huge amount of altitude while you’re getting the wings flying again. And sixth, for turning flight stalls: the action of the aeroplane after the stall may not be so violent or so extreme as to make it difficult, with normal piloting skill, to effect prompt recovery and to regain control of the aeroplane. So in a turn, the post-stall behaviour must stay manageable for a competent pilot. Now let’s move to the aerofoil section effects, because these tell you how the shape of the wing decides the stall. An aerofoil section with a small leading edge radius will stall at a lower angle of attack, and the stall will be more abrupt. Small leading edge radius—think of a sharp, thin nose—gives you an early, sharp break. An aerofoil section with a large thickness-chord ratio will stall at a higher angle of attack and will stall more gently. Thickness-chord ratio is the thickness of the section divided by its chord, the distance from leading edge to trailing edge. A thick, fat section stalls later and softer. An aerofoil section with camber near the leading edge will stall at a higher angle of attack. Camber is the curvature of the section; if that curvature is concentrated near the front, the stall comes later. Now the planform—the shape of the wing as seen from above. A rectangular wing planform will tend to stall at the root first, the inboard section nearest the fuselage. And that’s why a rectangular wing usually has ideal stall characteristics. Those ideal characteristics are four: aileron control at the stall—you keep lateral control because the tips are still flying; nose drop at the stall—a gentle pitch-down; aerodynamic warning at the stall—buffeting or a natural cue that tells you it’s coming; and absence of violent wing drop at the stall. So the root stalls first, the tips keep flying, and you get a clean, predictable break. But a tapered planform—where the wing narrows toward the tip—doesn’t behave that way naturally. To give it the desired stall characteristics, designers can include several devices. First, washout, which is decreasing incidence from root to tip—the tip is twisted to a lower angle of attack so it stalls later. Second, an aerofoil section with greater thickness and camber at the tip, again to delay the tip stall. Third, leading edge droop at the tip. Fourth, stall strips fitted to the wing inboard leading edge—small devices that force the root to stall first. And fifth, vortex generators, which re-energize the boundary layer at the… and the text cuts off there, but you’ll see vortex generators again in a moment. Now, a swept-back wing—the wing swept rearward—has an increased tendency to tip stall because of spanwise flow of the boundary layer from root to tip on the wing top surface. The boundary layer is the thin layer of air right next to the skin; on a swept wing it drifts outward toward the tip, and that makes the tip stall first. That’s dangerous, and there are several methods to delay tip stall on a swept planform. First, wing fences—thin metal fences that generally extend from the leading edge to the trailing edge on the wing top surface. They block the spanwise flow. Second, vortilons—also thin metal fences, but smaller, and they’re situated on the underside of the wing leading edge. Third, a saw tooth leading edge, which generates vortices over the wing top surface at high angles of attack—those vortices re-energize the flow and delay separation. Fourth, engine nacelles of pod-mounted wing engines also act as vortilons—the engine pods themselves do the same job. And fifth, vortex generators are also used to delay tip stall on a swept wing. Now here’s the critical consequence. Tip stall on a swept wing planform gives a tendency for the aircraft to pitch-up at the stall. And the reason is this: it’s due to the centre of pressure moving forwards when the wing tips stall first. The centre of pressure is the point where the lift force acts; when the tips stall, the lift moves forward, and that forward shift of the centre of pressure pitches the nose up. That pitch-up at the stall is exactly the kind of abnormal nose-up pitching the EASA requirements forbid—so you can see why all those devices exist. One last note: the key facts with the word inserts for this chapter can be found on page 201 of the book, so when you want to revise, that’s where the summary lives.