Stalling — Page 195, Lesson 234

Let’s pick this up right where the wing’s behaviour in a turn leaves off, because this is where stall and recovery get genuinely demanding. I want to walk you through what happens in a level co-ordinated turn at a constant bank angle. The inside wing is moving through the air more slowly than the outside wing. Slower airspeed means less lift, so the inside wing generates less lift than the outside wing. If you hold the ailerons neutral, the aircraft has a tendency to continue to roll in the direction of bank — that’s the over-banking tendency. So rather than returning the ailerons to neutral once you’ve reached the required bank angle, you must hold aileron opposite to the direction of bank. And here’s the key relationship: the lower the airspeed, the greater the aileron input required. Now, the inner, lower wing may have a greater effective angle of attack due to the lowered aileron. That means it may reach the critical angle of attack first — it stalls first. So the rudder must be used at all times to maintain co-ordinated flight, which we express as keeping the ball in the middle. Let’s move to a climbing turn. In a climbing turn, airspeed will be lower. In a single-engine propeller aircraft, the rolling and yawing forces generated by the propeller and its slipstream add their own requirements for unusual rudder and aileron inputs. Take a concrete example: an aircraft with a clockwise rotating propeller in a climbing turn to the left at low speed. It may be necessary for the pilot to be holding a lot of right roll aileron and right rudder. If an aircraft in this situation were to stall, those gross control deflections could make the aircraft yaw or roll violently. That’s why correct co-ordination of the controls is essential in all phases of flight — to prevent the possibility of an accidental spin. Now the conclusions. In whatever configuration, attitude or power setting a stall warning occurs, the correct pilot action is, first, to decrease the angle of attack below the stall angle to un-stall the wing. Second, apply maximum allowable power to minimize altitude loss. Third, prevent any yaw from developing to minimize the possibility of spinning. And that’s pretty much in that order. The practical reminder: keep the ball in the middle. Let’s turn to high speed buffet, also called shock stall. When we explain the basic principles of flight, we consider air to be incompressible at speeds less than four tenths the speed of sound — that’s Mach 0.4. Incompressible means pressure is considered to have no effect on air density. At speeds higher than Mach 0.4, it is no longer practical to make that assumption, because density changes in the airflow around the aircraft begin to make differences to the behaviour of the aircraft. That’s the threshold where compressibility starts to matter. That figure shows the shockwave and the separated airflow — the two things that define the shock stall condition. The shockwave forms on the wing, and behind it the airflow separates, which is what produces the buffet you feel. So to tie it together: in the turn, the inner wing stalls first because of its higher effective angle of attack, and you manage that with opposite aileron and constant rudder co-ordination. In the climb, propeller forces demand unusual control inputs, and a stall there with gross deflections can lead to a violent yaw or roll and a possible spin. And at high speed, once you pass Mach 0.4, compressibility changes the airflow enough that a shockwave and separated airflow produce the high speed buffet.