Stalling — Page 153, Lesson 179

We're starting a new topic now: stalling. And I want to begin with something that might surprise you — the formal definition of a stall isn't about the wing at all. The aeroplane is considered stalled when the behaviour of the aeroplane gives the pilot a clear and distinctive indication of an acceptable nature that the aeroplane is stalled. So it's defined by what the pilot feels and sees, not by a number on an airspeed indicator. Let me unpack that. There are three acceptable indications of a stall, and they can occur individually or in combination. The first is a nose-down pitch that cannot be readily arrested. That means the aircraft pitches forward on its own, and you can't easily stop that pitching motion. The second is buffeting — a shaking or vibration — of a magnitude and severity that is a strong and effective deterrent to further speed reduction. In plain terms, the airframe shakes so convincingly that you naturally don't want to slow down any more. The third indication is that the pitch control reaches the aft stop, and no further increase in pitch attitude occurs when the control is held full aft for a short time before recovery is initiated. So you've pulled the stick all the way back, it's against the stop, and the nose simply won't come up any further. Now, why do we care about stall speed? Because it's necessary to fly at slow speeds — which means high angles of attack — during take-off and landing, in order to keep the required runway lengths to a reasonable minimum. You can't land at high speed or you'd need a runway miles long. But there must be an adequate safety margin between the minimum speed allowed for normal operations and the stall speed. You need a buffer so normal flying never accidentally crosses into the stall. Here's how stall speeds get established. Prototype aircraft are stalled, and stall speeds are established for inclusion in the Flight Manual during the flight testing that takes place before type certification. So it's a real, measured quantity determined during the certification flight test programme. Now, the certification rules split aircraft into two categories. For "small" aircraft, under CS-23, they use VS0 and VS1 on which to base the stall speed. VS0 is the stall speed in the landing configuration, and VS1 is the stall speed in a specific configuration — you'll see these referenced throughout performance work. For "large" aircraft, under CS-25, a reference stall speed, VSR, is used instead. Let's look at VSR carefully, because it has a precise definition. The reference stall speed is a calibrated airspeed defined by the aircraft manufacturer. And VSR may not be less than a 1g stall speed. That's a key limitation — the manufacturer can't set it lower than the speed at which the aircraft stalls at one g, which is the normal straight-and-level load factor. VSR is expressed as a formula: VSR is greater than or equal to VCLMAX divided by the square root of nZW. Let me break down each symbol. VCLMAX is the calibrated airspeed obtained when the load factor corrected lift coefficient is first a maximum during the manoeuvre prescribed on page 149. So during a specific stall manoeuvre, you watch the load factor corrected lift coefficient, and VCLMAX is the speed at the moment that coefficient first reaches its maximum value. There's also an important addition. When the manoeuvre is limited by a device that abruptly pushes the nose down at a selected angle of attack — for example, a stick pusher — then VCLMAX may not be less than the speed existing at the instant the device operates. So if a stick pusher fires and forces the nose down, the speed at that instant becomes the floor for VCLMAX. And nZW is the load factor normal to the flight path at VCLMAX. So it's the load factor measured perpendicular to the flight path, taken at the same moment you captured VCLMAX. So the whole formula is saying: the reference stall speed must be at least the speed at which the lift coefficient peaks, divided by the square root of the load factor at that point. The square root of the load factor accounts for the fact that stall speed scales with the square root of load factor — at higher g, the stall speed increases. Let me show you a figure that illustrates the vortex generators, which are one of the devices used to influence stall behaviour. That's the core of stall recognition and the definition of stall speed. The key takeaway is that a stall is defined by pilot-observable behaviour, and the reference stall speed is a carefully defined, manufacturer-set calibrated airspeed with a hard floor at the 1g stall speed.