Stalling — Page 158, Lesson 187

I want to walk you through the certification and design side of stalling. This is where we move from "what a stall is" to "what the regulations demand of an aeroplane near the stall, and how wing design shapes that behaviour." Let's start with the Basic Stall Requirements under EASA and FAR. These are the certification rules an aeroplane must meet before it's approved. The first requirement is about control authority. It must be possible to produce and to correct roll and yaw by unreversed use of aileron and rudder controls, right up to the time the aeroplane is stalled. "Unreversed" means the controls must do what you'd expect — push the stick left, the aeroplane rolls left; push the rudder, it yaws accordingly. There must be no reversal of effect as you approach the stall. Also, no abnormal nose-up pitching may occur, and the longitudinal control force — that's the force you feel through the control column — must be positive up to and throughout the stall. Positive means the stick is still pulling back, not pushing forward on its own. And finally, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls. Now the quantitative limits. For level wing stalls, the roll that occurs between the stall and the completion of the recovery may not exceed approximately 20 degrees. So if the wing drops during a level stall, you're allowed up to about 20 degrees of bank before you must have it under control. For turning flight stalls, the rules are about how violent the aeroplane's action may be. The action after the stall may not be so violent or extreme that it becomes difficult, with normal piloting skill, to effect a prompt recovery and regain control. Then there are specific bank angle limits during recovery. For deceleration rates up to 1 knot per second, the maximum bank angle may not exceed approximately 60 degrees in the original direction of the turn, or 30 degrees in the opposite direction. For deceleration rates in excess of 1 knot per second — a more aggressive deceleration — the limits relax: approximately 90 degrees in the original direction, or 60 degrees in the opposite direction. So the faster you're decelerating into the stall, the more bank excursion is tolerated. Now let's move to Wing Design Characteristics. We've established that stalling is due to airflow separation, characterized by a loss of lift and an increase in drag, which causes the aircraft to lose height. That's generally true. But there are aspects of aircraft behaviour and handling at or near the stall that depend on the design of the wing aerofoil section and planform. Planform is the shape of the wing as seen from above — its taper, sweep, and aspect ratio. The Effect of Aerofoil Section is our focus. The shape of the aerofoil section influences the manner in which it stalls. With some sections, the stall occurs very suddenly and the drop in lift is very marked. With others, the approach to stall is more gradual, and the decrease in lift is less disastrous. In general, an aeroplane should not stall too suddenly, and the pilot should have adequate warning, in terms of handling qualities, of the approach of a stall. That warning generally takes the form of buffeting — a vibration felt through the airframe — and a general lack of response to the controls. If a particular wing design stalls too suddenly, it becomes necessary to provide some sort of artificial pre-stall warning device, or even a stall prevention device. That's why you see stick shakers and stick pushers on transport aeroplanes — they're compensating for a wing that doesn't give natural warning. And here's the key principle to hold onto: a given aerofoil section will always stall at the same angle of attack. That's a fundamental fact. The stall is an aerodynamic event tied to angle of attack, not to airspeed. A given section, at a given configuration, will always separate at the same angle of attack — which is why we can set stall warning devices to trigger at a specific angle of attack regardless of speed, weight, or bank. So to tie it together: certification sets the handling and bank limits, and wing design determines whether the stall is sudden or gradual, and whether the pilot gets natural warning. The aerofoil section is the heart of that behaviour, and its stall angle is fixed.