Stalling — Page 153, Lesson 177

Right, let's get into the stalling chapter. We're starting with how the aircraft actually behaves as it approaches the stall, and then the critical business of what you do with the controls. First, the general behaviour. Stall characteristics vary with different types of aircraft. But for a modern aircraft during most normal manoeuvres, the onset of stall is gradual. So it's not a sudden cliff edge — it builds up. The first indications of a stall may be provided by any or all of three things. One: unresponsive flight controls. Two: a stall warning or stall prevention device. Three: aerodynamic buffet — that's the vibration you feel through the airframe as the airflow starts to break away. Now, why do the controls feel unresponsive? At the low speeds normally associated with stalling, dynamic pressure is at a very low value. Dynamic pressure is the pressure you feel from the airflow hitting the aircraft — it's what the control surfaces need to generate force. At low speed, that pressure is weak, so a greater control deflection is required to achieve the same response. The flying controls will feel unresponsive, or "mushy" — that's the technical term for that spongy, dead feel. Here's the crucial part about control use close to the stall. Moving a control surface modifies the chord line, and hence the angle of attack. The chord line is the straight line from the leading edge to the trailing edge of the wing. When you deflect a control surface, you're effectively bending that line, which changes the angle at which the wing meets the airflow. Now picture this. An aircraft being flown close to the stall angle may have one wing that produces slightly less lift than the other. That wing will tend to drop. Your instinct is to use aileron to lift that dropping wing. But here's the trap: trying to lift a dropping wing with aileron will increase its angle of attack. And if you're already close to the stall angle, that extra increase can push that wing over the critical angle — it stalls completely. The result is that wing drops at an increased rate. So at speeds close to the stall, ailerons must be used with caution. So what do you do instead? On straight wing aircraft, the rudder should be used to yaw the aircraft just enough to increase the speed of a dropping wing, to maintain a wing's level attitude. Think about it — yawing the aircraft increases the speed of the wing that's moving forward, which increases its lift, and that's how you level it without risking the stall. Now, swept wing aircraft are different. Their basic stall requirements are designed to enable the ailerons to be used successfully right up to "stall recognition" — that's the point where the stall is actually detected. But even then, small amounts of rudder can be used, provided they're smoothly applied and co-ordinated with the ailerons. Let me show you the geometry of that aileron problem. That figure shows the angles involved — you can see how deflecting the aileron changes the chord line and pushes the angle of attack up from 15 degrees toward 22 degrees, which is right at the stall. That's the danger I described. So the key takeaway: close to the stall, your ailerons are a hazard, not a tool. On a straight wing, use rudder to manage wing drop. On a swept wing, you have more aileron authority up to stall recognition, but still co-ordinate with smooth rudder inputs.