High Lift Devices — Page 212, Lesson 265

We're starting a new topic now: high-lift devices. These are the flaps on the trailing edge of the wing, and I want to walk you through the different types, how each one works, and what it does to your lift, drag, and stalling behaviour. Let's begin with the split flap. This flap forms part of the lower surface of the wing's trailing edge. The key point here is that when you lower it, the upper surface contour is completely unaffected. So you're only changing the shape of the underside. Now, at low angles of attack, the split flap gives about the same increase in lift as the plain flap. But at higher angles of attack, it gives slightly more lift. Why? Because the upper surface camber is not increased, and so separation of the airflow is delayed. However, the drag is higher than for the plain flap, and that's due to the increased depth of the wake behind it. Next, we have the slotted flap. When this flap is lowered, a slot, or gap, opens up between the flap and the wing. The purpose of that slot is to direct higher-pressure air from the lower surface over the flap, and that re-energizes the boundary layer. That re-energizing delays the separation of the airflow on the upper surface of the flap. The result is that the slotted flap gives a bigger increase in CLMAX—that's the maximum lift coefficient—than the plain or split flap, and it produces much less drag. The trade-off is that it has a more complex construction. Now, the Fowler flap. This one moves rearwards and then down. So initially, as it moves back, it gives an increase in wing area. Then, as it moves down, it gives an increase in camber. And importantly, the Fowler flap may be slotted. Because of the combined effects of increased area and camber, the Fowler flap gives the greatest increase in lift of all the flaps we've considered. It also gives the least drag, because of the slot and the reduction of the thickness-to-chord ratio. But there's a penalty: the change in pitching moment is greater, because of the rearward extension of the chord. There's also the triple slotted Fowler flap, which is exactly what it sounds like—a Fowler flap with three slots. Now, let's compare these trailing edge flaps. If we look at the lift curves for the same angle of flap deflection, we can see the differences. But I want to stress one important note: the different types of flap do not all give their greatest increase in lift at the same flap angle. So you can't just pick one deflection angle and assume it's optimal for all of them. Let's also look at how lift and drag increments vary with flap angle. As you deploy the flaps, the increment in lift is decreasing, and the increment in drag is increasing. And here's a critical point to remember: any amount of flap increases drag. There's no flap setting that doesn't add drag. Finally, let's talk about CLMAX and the stalling angle. When the flap is lowered, CLMAX is increased, but the stalling angle is reduced. Why? Because lowering the flap increases the effective angle of attack. It's conventional to plot the CL versus alpha curve using the angle of attack for the basic section. So, as shown in the figure, at the stalling angle of attack for the section with the flap lowered, the basic wing section is actually at a reduced angle. That's the key relationship to hold onto: more flap means more lift capability, but a lower stalling angle.