Stalling — Page 153, Lesson 185

Let's pick up right where the stall warning story gets interesting. We've already covered the natural stall warning — the aerodynamic buffet you feel through the controls when airflow separates. But here's the problem: that natural buffet isn't always strong enough or early enough to be a reliable warning. So we fit artificial devices. On small aircraft, that artificial warning is usually a buzzer or a horn. Simple, audible, gets your attention. But on modern large aircraft, we use something much more sophisticated: a stick shaker, working together with lights and a noisemaker. Let me explain the stick shaker properly, because it's a beautiful piece of engineering. The stick shaker represents what it is replacing — it literally shakes the stick, and that makes it a tactile warning. You feel it through your hands. Now, think about a critical scenario: the aircraft is on autopilot, and your hands are not on the controls. A very quiet stick shaker would be useless as a stall warning because you wouldn't feel it. That's why a noisemaker is added in parallel — so the warning works whether your hands are on the stick or not. So how does the stick shaker actually work? It's a pair of simple electric motors, one clamped to each pilot's control column. Each motor rotates an out-of-balance weight. When the motor runs, that unbalanced rotation creates vibration, and it shakes the stick. That's the tactile warning you feel. Now, let me bring in the figures that show you the bigger picture of stall warning and protection. Look at Figure 7.4 — that's the aircraft without a stick pusher. And Figure 7.5 — the aircraft with a stick pusher. These figures show the relationship between the speeds involved. Here's the key contrast. On the aircraft without a stick pusher, we have a margin of 5 knots or 5% between the stall warning speed and the reference stall speed. On the aircraft with a stick pusher, that margin is tighter — 2 knots or 2% for the stick shaker, and 3 knots or 3% for the stick pusher itself. Let me define these speeds precisely. VSR is the reference stall speed — that's the baseline stall speed the aircraft is certified against. VSW is the stall warning speed — the speed at which the warning activates. And VCLMAX is the maximum lift coefficient speed, the speed at which the wing achieves its maximum lift coefficient before stalling. So on the aircraft without a stick pusher, the stall warning comes on at VSW, which is set 5 knots or 5% above VSR. On the aircraft with a stick pusher, the stick shaker activates at 2 knots or 2% above VSR, and the stick pusher itself operates at 3 knots or 3% above VSR. The stick pusher is the device that physically pushes the control column forward to prevent the stall — that's why it needs a slightly larger margin than the shaker, so it acts before the shaker would be overwhelmed. Now, let me show you the vortex generators in Figure 7.13 — these are small vanes on the wing that energize the boundary layer and delay separation. And Figure 7.14 shows the lateral axis, and Figure 7.16 shows the wing. These all tie into how we manage and delay the stall. So to summarize what we've covered: natural buffet is often insufficient, so we fit artificial warning. Small aircraft use a buzzer or horn. Large aircraft use a stick shaker — a pair of electric motors with out-of-balance weights, one on each control column, shaking the stick as a tactile warning, backed up by lights and a noisemaker in parallel for when your hands aren't on the controls. And the margins differ: 5 knots or 5% without a stick pusher, versus 2 knots or 2% for the shaker and 3 knots or 3% for the pusher with one.