High Lift Devices — Page 219, Lesson 274

Let's pick this up with the automatic slot, because that's the first new idea here. On some aircraft, the pilot doesn't control the slot at all — it operates automatically. And the trigger for that movement is the change in pressure around the leading edge as the angle of attack increases. Think about what happens at low angles of attack. The high pressures around the stagnation point — that's the point on the leading edge where the airflow actually comes to rest and splits — those high pressures keep the slat pushed closed. The slat stays shut. Now as the angle of attack increases, that stagnation point moves underneath the leading edge. And once it does, you get what we call "suction" pressures on the upper surface of the slat. Those suction pressures pull the slat forward, and that forward movement creates the slot. So the slot opens itself, purely from the aerodynamic pressure changes, with no pilot input. Now, where do we actually find this system? Mainly on small aircraft, and it serves as a stall protection system. Because the slot delays the stall, it protects the aircraft from stalling. On larger aircraft, though, the slats are pilot-selected — the pilot chooses when they're needed, and the movement is controlled either electrically or hydraulically. Now let's look at the disadvantages of the slot, because there's an important trade-off here. The slot can give increases in CLMAX — that's the maximum lift coefficient — of the same magnitude as the trailing edge flap. So both can add a similar amount of maximum lift. But here's the key difference: the trailing edge flap gives its CLMAX at slightly less than the normal stalling angle. The slot, on the other hand, requires a much increased angle of attack to give its CLMAX. What does that mean in flight? It means the aircraft will have a very nose-up attitude at low speeds. And on the approach to land, that nose-up attitude could restrict your visibility of the landing area. So you get the lift, but you pay for it with a high deck angle and reduced forward visibility. Now, drag and pitching moment — compared to trailing edge flaps, the changes in drag and pitching moment from leading edge devices are small. So leading edge devices are relatively benign in those two respects. And finally, the big picture: most large transport aircraft employ both trailing edge and leading edge devices together. That combination is what gives you the full lift curve benefit — the leading edge devices keep the flow attached at high angles, and the trailing edge flaps add the lift increment. Figure 8.17 shows the effect on the lift curve of both types of device together.