Stalling — Page 158, Lesson 186

I want to walk you through the stall warning systems now — the devices that tell the pilot a stall is coming before the aircraft actually stalls. These are all angle-of-attack detectors, and they all work on the same basic principle: they sense the angle of attack and trigger a warning at the appropriate moment. Let me start with the flapper switch, which is also called the leading edge stall warning vane. Look at Figure 7.6. The key to understanding this device is the stagnation point. As angle of attack increases, the stagnation point moves downwards and backwards around the leading edge. The flapper switch is located so that, at the appropriate angle of attack, the stagnation point moves to its underside. The increased pressure there lifts and closes the switch, which activates the warning. Now, the second device is the angle of attack vane, shown in Figure 7.7. This one is mounted on the side of the fuselage. The vane streamlines with the relative airflow, and the fuselage rotates around it. So as angle of attack increases, the vane rotates relative to the fuselage. The stick shaker is activated at the appropriate angle of attack. The third device is the angle of attack probe. It's also mounted on the side of the fuselage. It consists of slots in a probe, which are sensitive to changes in angle of relative airflow. Now here's the important part — why these systems are so valuable. All of these sense angle of attack, and therefore they automatically take care of changes in aircraft mass. That's a big deal, because a heavier aircraft stalls at a higher speed, but the angle of attack at the stall is essentially the same. So by sensing angle of attack rather than speed, these devices don't need to be told how heavy the aircraft is. The majority of these devices also compute the rate of change of angle of attack. That means they give earlier warning in the case of faster rates of approach to the stall. If you're pulling into the stall quickly, you get warned sooner. There are a few more design details worth noting. The detectors are usually datum compensated for configuration changes — that means they adjust for flap and gear position. They are always heated or anti-iced, because ice on the sensor would give false readings. And there are usually sensors on both sides of the aircraft to counteract any sideslip effect — if the aircraft is slipping, one side sees a different angle of attack than the other, so having both sides gives a more accurate picture. So to sum up: three types of detector — the flapper switch, the angle of attack vane, and the angle of attack probe. All sense angle of attack, all automatically account for mass changes, most compute rate of change for earlier warning, and they're all datum compensated, heated, and duplicated on both sides.