High Lift Devices — Page 219, Lesson 272

We're starting a new topic now: high lift devices, and specifically the leading edge slot and the leading edge slat. Let's get into it. First, let's be precise about the difference between a slot and a slat, because they're closely related. A leading edge slot is a gap that runs from the lower surface to the upper surface of the wing's leading edge. It can be fixed, meaning it's always open, or it can be created by moving part of the leading edge forwards. That moving part is the slat. So, a slat is a small auxiliary aerofoil attached to the leading edge of the wing. When it's deployed, it forms that slot. The slot allows air to pass from the high pressure region below the wing to the low pressure region above it. Now, here's the clever part: the slat forms a convergent duct, which means the passage narrows. That adds kinetic energy to the airflow passing through the slot. Why does that matter? Because when slats are deployed, the boundary layer is re-energized. If you add kinetic energy to the boundary layer, you delay boundary layer separation to a much higher angle of attack. The graph in Figure 8.15 shows this clearly: the slatted wing's lift curve extends far beyond the un-slatted wing's. The un-slatted wing stalls around 15 degrees, but the slatted wing keeps producing lift up to about 25 degrees. At approximately 25 degrees, the increased adverse pressure gradient once again overwhelms the kinetic energy of the boundary layer, and separation occurs. Now, why not just have a fixed slot all the time? Because a permanently open slot creates extra drag at high speed, which is an unnecessary disadvantage. So most slats in commercial use are opened and closed by a control mechanism. The slot is closed for high speed flight and opened for low speeds, usually in conjunction with the trailing edge flaps, and actuated by the same selector on the flight deck. Let me explain the mechanism in more detail. When the slat is deployed at high angles of attack, the slat itself is generating a high lift coefficient because of its marked camber. The action of the slat is to flatten the marked peak of the low pressure envelope at high angles of attack, changing it to one with a more gradual pressure gradient. This flattening means the boundary layer doesn't undergo the sudden thickening that would occur from negotiating a very steep adverse pressure gradient immediately behind the former suction peak. So it retains much of its kinetic energy, enabling it to penetrate almost the full chord of the wing before separating. Figure 8.16 shows this alleviating effect on the low pressure peak. Although the peak is flatter, the area of the low pressure region, which is proportional to its strength, is unchanged or even increased. And importantly, the suction peak does not move forward, so the effect of the slot on pitching moment is insignificant. So to summarize: the slat re-energizes the boundary layer, delays separation to a higher angle of attack, flattens the pressure peak, and does so without affecting pitching moment. That's the leading edge slot and slat in a nutshell.