We’re starting a brand-new topic now: stalling — Page 145, Lesson 172

We’re starting a brand-new topic now: stalling. This is one of the most important safety subjects in the whole of flight training, so I want you to think of it as a pilot’s problem, not just a theory problem. A stall is a potentially hazardous manoeuvre involving loss of height and loss of control. That’s the blunt reality of it. So a pilot must be able to clearly and unmistakably identify an impending stall so that it can be prevented. Different types of aircraft exhibit various stall characteristics, some less desirable than others, and the airworthiness authorities specify minimum stall qualities that an aircraft must possess. So there’s a regulatory floor beneath all of this. Let’s get into the cause of the stall, because everything hinges on this. The lift coefficient, which we write as CL, increases with angle of attack up to a maximum value, which we call CLMAX. That’s the peak of the lift curve. Any further increase in angle of attack above this stalling angle — and that stalling angle is also called the critical angle of attack — will make it impossible for the airflow to smoothly follow the upper wing contour. The flow will separate from the surface. And when that happens, CL decreases and drag increases rapidly. So the lift collapses and the drag shoots up, all at once. Now, why is CLMAX such an important point of reference? Because the CLMAX of an aerofoil corresponds to the minimum steady flight speed — that’s the 1g stall speed. So when we talk about stall speed, we’re really talking about the speed at which the wing is operating at its maximum lift coefficient. That’s the reference point we build the whole stall envelope around. Let me be very precise about what a stall actually is. A stall is caused by airflow separation. And separation can occur when either the boundary layer has insufficient kinetic energy, or the adverse pressure gradient becomes too great. So there are two contributing factors, and I want you to hold both of them in your head. The boundary layer is that thin layer of air right next to the wing surface, and it needs kinetic energy — energy of motion — to keep flowing along the surface. The adverse pressure gradient is the pressure rising along the chord towards the trailing edge, which works against the flow. If the pressure rise is too steep, or the boundary layer is too sluggish, the flow can’t stay attached. Let me walk you through what happens as angle of attack increases, because this is a sequence you need to visualise. At low angles of attack, virtually no flow separation occurs before the trailing edge. The flow is attached over the rear part of the surface in the form of a turbulent boundary layer. So the flow stays stuck to the wing all the way back. As angle of attack increases, the adverse pressure gradient increases, and that reduces the kinetic energy of the boundary layer. So the boundary layer will begin to separate from the surface at the trailing edge. Notice where it starts — at the trailing edge, not the leading edge. That’s the key point. The separation begins at the back of the wing. As you further increase angle of attack, the separation point moves forward, and the wing area that generates a pressure differential becomes smaller. So less and less of the wing is doing useful lift work. At angles of attack higher than approximately 16 degrees, the extremely steep adverse pressure gradient will have caused so much separation that insufficient lift is generated to balance the aircraft weight. So that 16-degree figure is a rough threshold for a typical aerofoil — beyond that, the wing simply cannot produce enough lift to hold the aircraft up. Now here’s the crucial operational point, and I want you to remember this for the rest of your flying career. The angle of attack is the angle between the chord line and the relative airflow. The chord line is the straight line from the leading edge to the trailing edge of the aerofoil. The relative airflow is the direction the air is moving relative to the wing. So the angle of attack is purely a geometric relationship between those two. And that leads to the most important sentence in this whole passage: if the angle of attack is increased up to or beyond the critical angle, an aeroplane can be stalled at any airspeed or flight attitude. Let me say that again, because it’s the takeaway. An aeroplane can be stalled at any airspeed or attitude. The stall is not about speed — it’s about angle of attack. You can be going fast, you can be going slow, you can be climbing, descending, turning — if you push the angle of attack past the critical angle, the wing will stall. Speed and attitude do not protect you from the stall. Only angle of attack control does. I want to show you the flow separation picture, because it makes this sequence concrete. That figure shows exactly what I’ve been describing — at low angle of attack the flow stays attached, and as the angle increases, the separation point creeps forward from the trailing edge until the lift collapses. One more thing before we move on. There’s a note at the start of this chapter that I want you to be aware of. Throughout this chapter, reference will be made to EASA Certification Specifications — CS23 and CS25 — stall requirements, and so on. But it must be emphasised that these references are for training purposes only and are not subject to amendment action. So when we talk about certification stall requirements, treat them as teaching material, not as regulation you need to amend or act upon. That’s the framework we’re working in. So to summarise where we are: a stall is airflow separation, driven by either a boundary layer short on kinetic energy or an adverse pressure gradient that’s too steep. The lift coefficient peaks at CLMAX, which corresponds to the critical angle of attack and the 1g stall speed. Beyond that angle, lift collapses and drag rises. And the golden rule — the stall is an angle of attack phenomenon, so it can happen at any airspeed or attitude. That’s the foundation. Next we’ll build on it with the stall characteristics and the certification requirements.