Stalling — Page 173, Lesson 203

Let’s start with the heart of this section: stalling is caused by exceeding the critical angle of attack, not by slow speed. I want you to hold onto that, because it’s the single most important idea here. The critical angle of attack can be exceeded at any aircraft speed — you can stall fast, you can stall slow. Speed is not the cause. Now, why do we talk about stall speed at all? Because of a practical test. If an aircraft is flown in straight and level flight, and the speed is reduced at a rate not exceeding 1 knot per second, the calibrated airspeed at which the stall occurs can be identified. That calibrated airspeed is called the reference stall speed, and its symbol is VSR. I want you to note the subscript: VSR, with R for reference. This VSR is the foundation for the recommended take-off, manoeuvre, approach, and landing speeds. Those speeds are set as multiples of VSR — like 1.05VSR, 1.1VSR, 1.2VSR, 1.3VSR — to give an adequate margin from the stall during normal operations. Now, what can change VSR? The excerpt lists eight factors. Let me go through them: changes in weight; manoeuvring the aircraft, which means increasing the load factor; configuration changes, which change CLMAX and pitching moment; CG position; engine thrust and propeller slipstream; Mach number; wing contamination; and heavy rain. Each of these shifts where the stall occurs, and we’ll see how in the coming pages. Let’s now define the 1g stall speed. In straight and level flight, the weight of the aircraft is balanced by the lift. That balance is expressed by the load factor, symbol n, sometimes called ‘g’. The load factor is defined as Lift divided by Weight. So n = Lift / Weight. While n is the correct symbol, the relationship between lift and weight has for years been popularly known as ‘g’. One g corresponds to the force acting on us in everyday life — that’s the normal force of gravity we feel on the ground. Here’s the key relationship: if more lift is generated than weight, the load factor, or ‘g’, will be greater than one. That means the force acting on the aircraft and everything in it — including the pilot — will be greater than normal. So if you pull up and generate more lift than weight, you feel heavier; you’re pulling more than 1g. If Lift equals Weight, the load factor is one. And from the lift formula — L = ½ ρ V² CL S — we can see that lift will change whenever any of the other factors change. Let me unpack that formula. L is lift. ρ, the Greek letter rho, is air density. V is the true airspeed, and it’s squared. CL is the lift coefficient, which depends on angle of attack. S is the wing area. For this example, we consider density and wing area constant. So here’s the practical consequence. If the engine is throttled back, drag will reduce speed V, and from the formula, lift would decrease. To keep lift constant and maintain 1g flight at a reduced speed, CL must be increased — and CL is increased by increasing the angle of attack. That’s the connection: as you slow down, you raise the nose to keep lift equal to weight, and you’re increasing angle of attack. Keep doing that, and eventually you reach the critical angle of attack — and that’s the stall. So the stall speed is really the speed at which, in 1g flight, you can no longer increase CL enough to maintain lift, because you’ve hit the critical angle. That’s the core of this section. We’ve defined VSR, the reference stall speed; we’ve defined load factor n and its popular name ‘g’; we’ve seen the lift formula and how throttling back reduces speed, requiring more angle of attack to maintain 1g. Next, we’ll look at how each of those eight factors actually shifts VSR.