Let's work through it — Page 206, Lesson 253

We're now in the answers section for Chapter 7, and this is all about stall warning and stall characteristics. Let's work through it. First, the regulatory requirement. The stall warning must give sufficient margin to prevent inadvertent stalling, and it must be clear and distinctive to the pilot — and this applies in both straight and turning flight. So the warning has to be obvious whether you're wings level or banked in a turn. Now, what counts as an acceptable stall warning? It can be one of two things. Either the inherent aerodynamic qualities of the aeroplane itself — meaning the natural buffeting or handling cues the aircraft gives you as it approaches the stall — or it can be a device that gives clearly distinguishable indications under the expected conditions of flight. So either the airframe tells you naturally, or a dedicated device tells you. Here's the key numerical requirement. The stall warning must begin at a speed exceeding the stall speed by not less than 5 knots or 5% CAS, whichever is the greater. Let me unpack that. CAS is calibrated airspeed. So the warning has to come on at least 5 knots above the stall speed, or at least 5% above the stall speed — and you take whichever of those two margins is bigger. So if your stall speed is low, 5 knots might be the bigger margin; if your stall speed is high, 5% might be bigger. The warning must never be closer to the stall than that. Now, how is the warning actually presented? It depends on the size of the aircraft. On a small aircraft, the artificial stall warning is usually a horn or a buzzer. On a large aircraft, it's usually a stick shaker — that's a device that physically shakes the control column — and it works in conjunction with lights and a noisemaker. So on the big jet, you get the stick shaker plus visual and audible warnings together. How does the device get activated? There are three common sensing methods. A flapper switch, an angle of attack vane, or an angle of attack probe. The flapper is a small tab that gets deflected by the airflow as the wing approaches the stall. The vane and the probe both sense the angle of attack directly. And here's a refinement. Most angle of attack sensors compute the rate of change of angle of attack. That means they don't just look at the current angle — they look at how fast the angle is increasing. This gives earlier warning in the case of accelerated rates of stall approach. So if you're pulling hard and the angle of attack is rising quickly, the system can warn you sooner than it would if it only looked at the instantaneous value. Now let's move to the EASA required stall characteristics. These are the handling qualities the aeroplane must have, up to the time it is actually stalled. There are six of them, and they're all about controllability and safety. First, it must be possible to produce and correct yaw by unreversed use of the ailerons and rudder. "Unreversed" means the controls respond in the normal sense — push the rudder, the nose yaws the way you expect; move the ailerons, the roll response is normal. No reversed control effect. Second, no abnormal nose-up pitching may occur. The aircraft must not pitch up violently on its own as it approaches the stall. Third, the longitudinal control force must be positive. That means the force you feel on the column must be in the direction that helps you — typically a nose-down force that encourages you to lower the nose and recover. A positive force means the aircraft is naturally pushing you toward recovery. Fourth, it must be possible to promptly prevent stalling and recover from a stall by normal use of the controls. So a normal, standard recovery technique has to work — no exotic or extreme control inputs needed. Fifth, there should be no excessive roll between the stall and completion of recovery. The aircraft shouldn't drop a wing violently or roll uncontrollably during the stall and recovery. Sixth, and this is for turning flight stalls specifically — the action of the aeroplane after the stall may not be so violent or extreme as to make it difficult, with normal piloting skill, to effect prompt recovery and to regain control. So in a stall from a turn, the behaviour has to stay manageable for a competent pilot. Now, the excerpt starts to discuss aerofoil sections. An aerofoil section with a small leading edge radius will stall at a smaller angle of attack, and the stall will be more — and the text cuts off there, but the point is that a sharp leading edge stalls earlier and more abruptly. Then we get into design devices to give a wing with a tapered planform the desired stall characteristics. There are five of them. First, washout — that's decreasing incidence from root to tip. The wing root is set at a higher angle than the tip, so the root stalls first and the tip keeps flying, preserving aileron control. Second, an aerofoil section with greater thickness and camber at the tip. Thicker, more cambered sections at the tip stall later, again keeping the tip flying. Third, leading edge slots at the tip. These re-energize the airflow over the tip and delay the stall there. Fourth, stall strips fitted to the wing inboard leading edge. These are small devices that deliberately trip the flow at the root, forcing the root to stall first. Fifth, vortex generators, which re-energize the boundary layer at the tip. They keep the tip airflow attached longer. Now, a swept-back wing has an increased tendency to tip stall. Why? Because of the spanwise flow of the boundary layer from root to tip on the wing top surface. The boundary layer — that's the thin layer of air right next to the wing surface — drifts outward along the span on a swept wing, and it accumulates at the tip, making the tip stall first. There are five methods of delaying tip stall on a swept wing 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 and stop the boundary layer from piling up at the tip. Second, vortilons — also thin metal fences, but smaller, and they're situated on the underside of the wing leading edge. They generate vortices that help keep the flow attached. Third, a saw tooth leading edge — this generates vortices over the wing top surface at high angles of attack, which re-energize the flow. Fourth, engine pylons of pod-mounted wing engines also act as vortilons. So the engine installation itself helps break up the spanwise flow. Fifth, vortex generators are also used to delay tip stall on a swept wing. And finally, the consequence. Tip stall on a swept wing planform gives a tendency for the aircraft to pitch up at the stall. This is due to the centre of pressure — the CP — moving forwards when the wing tips stall first. When the tips stall, the lift is lost at the rear of the wing, so the centre of pressure shifts forward, and that forward shift of the CP creates a nose-up pitching moment. That's the dangerous pitch-up tendency you have to guard against on swept-wing aircraft. That's the full picture on stall warning and stall characteristics.