Stalling — Page 195, Lesson 236

We are now moving into the high-speed end of the stall envelope. I want to walk you through what happens when a large jet transport, cruising at high altitude, gets going too fast — and why that produces a stall of a completely different character from the low-speed, high-angle-of-attack stall we've been discussing. Picture this: a big high-speed jet at high altitude is cruising at a speed marginally above its critical Mach number. That means the airflow over the wing has already reached Mach 1 locally, and there's a small shock wave sitting on the wing. Now, if that aircraft overspeeds — if the pilot lets it accelerate beyond the intended cruise speed — that shock wave will rapidly grow larger. And here's the key mechanism: as the shock wave grows, the static pressure increases sharply in the immediate vicinity of the shock wave. That locally increased adverse pressure gradient — the pressure rising in the direction of flow, which opposes the flow — causes the boundary layer to separate immediately behind the shock wave. That's the definition of a shock stall. The airflow no longer clings to the wing surface; it breaks away right behind that shock. Now, what does that separated airflow do? It engulfs the tail area in a very active turbulent wake, and that causes severe airframe buffeting. The whole structure shakes. This is a very undesirable phenomenon — I want you to hold onto that word "undesirable," because it's going to come back. Here's the operational danger: high-speed buffet, which is another name for shock stall, can seriously damage the aircraft structure. So the designers install an artificial warning device. This device alerts the pilot if the aircraft exceeds its maximum operational speed limit — that's VMO/MMO — even by a small margin. Let me define those terms precisely, because they're going to appear throughout your career. VMO is the maximum operating Indicated Airspeed. MMO is the maximum operating Mach number. These will be fully discussed in Chapter 14, but for now, understand that VMO is your airspeed limit in knots indicated, and MMO is your Mach number limit — the ratio of your true airspeed to the local speed of sound. The high-speed warning is aural — it's a sound. It could be a "clacker," a horn, or a siren. And it's easily distinguishable from the low-speed warning. The low-speed, high-angle-of-attack warning is the "stick shaker" — that's the device that physically shakes the control column as you approach the critical angle of attack. So you have two distinct warnings: a sound for overspeed, a physical shake for approaching the stall angle. Now, let's tie the two ends of the envelope together. We've already seen that approaching the critical angle of attack can cause airframe buffeting — that's the "low-speed" buffet. And now we've shown that flying too fast also causes airframe buffeting — the "high-speed" buffet. ANY airframe buffeting is undesirable. It can quickly lead to structural damage, and besides that, it upsets the passengers. So buffeting, from either end of the speed range, is something you want to avoid entirely. Here's a striking operational fact: at high cruising altitudes — and I want you to note the specific range, 36,000 to 42,000 feet — the margin between the high-angle-of-attack stall warning and the high-speed warning may be as little as 15 knots. That's a very narrow corridor. At those altitudes, you don't have much room between stalling on the low side and overspeeding on the high side. That's why the warnings matter so much. Now, there's an important design philosophy behind VMO/MMO. It is operationally necessary to fly as fast as economically possible — airlines want speed for efficiency. So designers are constantly trying to increase the maximum speed at which aircraft can fly, without experiencing any undesirable characteristics. During certification flight testing, the projected maximum speeds are investigated, and from that, maximum operating speeds are established. The maximum operating speed limit — VMO/MMO — gives a speed margin into which the aircraft can momentarily overspeed and be recovered by the pilot before any undesirable characteristics occur. So it's not a hard wall; it's a buffer. The aircraft can briefly exceed it, and the pilot has time to recover before things like tuck, loss of control effectiveness, and several stability problems set in. Those will all be detailed in later chapters, but for now, understand that VMO/MMO is designed to give you a recoverable margin. Let me also show you the answers to the practice questions from page 173, because they illustrate the low-speed side of the stall envelope beautifully. The question was: what's the stall speed in a 25° and 30° bank, given that VS1g equals 150 knots CAS? VS1g is the stall speed at 1g — that's the stall speed in straight-and-level flight, in calibrated airspeed. In a 25° bank, the stall speed is 158 knots CAS. That's a 5% increase above VS1g, and the lift required is 10% greater. In a 30° bank, it's 161 knots CAS — a 7% increase, with lift 15% greater. And just to complete the picture, in a 45° bank it's 178 knots CAS — a 19% increase, with lift 41% greater. In a 60° bank, it's 212 knots CAS — a 41% increase, with lift 100% greater. Notice the pattern: as you bank, you need more lift to maintain altitude, and that increased lift requirement raises your stall speed. The relationship isn't linear — a 60° bank doubles the lift required and raises the stall speed by over 40%. So here's the complete picture: at the low-speed end, banking raises your stall speed and brings you closer to the stall. At the high-speed end, overspeeding past VMO/MMO brings on shock stall and buffeting. And at high altitude, those two warnings can be only 15 knots apart. That's the envelope you're operating within.