High Lift Devices — Page 227, Lesson 276

All right, let's get into the high lift devices chapter. We've already covered the basics of flaps and slats, so now I want to walk you through a critical safety system and then the operational logic of choosing flap settings. First, the Flap Load Relief System. On large, high-speed jet transports, there's a device fitted in the flap operating system. Its entire job is to prevent the flaps from deploying if the aircraft speed is too high. Think about the physics: if you extend flaps at too high a speed, the aerodynamic loads on them could exceed their structural limits. So this system acts as a guard. The pilot can select the flaps, but they will not extend until the airspeed is below the flap extend speed, which we abbreviate as VFE. Here's the key behavior: if a selection is made and the flaps do not run because the speed is too high, they will extend automatically as soon as the airspeed decreases to an appropriate value. So the system doesn't cancel your selection—it just delays the action until it's safe. Now, let's move to the operational side: the choice of flap setting for take-off, climb, and landing. This is a balancing act, and I want you to see the trade-offs clearly. Starting with take-off. The take-off distance depends on two things: the unstick speed—that's the speed at which the aircraft lifts off—and the rate of acceleration to that speed. Here's the conflict. The lowest unstick speed is possible at the highest CLMAX, and that maximum lift coefficient is achieved at a large flap angle. So big flaps give you a low unstick speed. But—and this is the catch—large flap angles also give high drag. That high drag reduces acceleration, which means it increases the distance required to accelerate to unstick speed. So you're trading one thing against another. A lower flap angle gives a higher unstick speed, but better acceleration, and therefore a shorter distance to unstick. So there is some optimum setting that gives the shortest possible take-off distance. And if leading edge devices are fitted, they will be used for take-off, because they increase the CLMAX for any trailing edge flap setting. Now climb. After take-off, a minimum climb gradient is required in the take-off configuration. Climb gradient is reduced by flap. So if climb gradient is the limiting factor, a lesser flap angle may be selected, even though it gives a longer take-off distance. You're sacrificing take-off performance to meet the climb requirement. Finally, landing. Landing distance depends on touchdown speed and deceleration. The lowest touchdown speed is given by the highest CLMAX, obtained at a large flap angle. And a large flap angle also gives high drag, which gives good deceleration. So for landing, a large flap angle will be used, and leading edge devices will also be used to obtain the highest possible CLMAX. Let me show you these relationships on the graphs. Look at Figure 8.18, which plots CL against angle of attack. You can see the curves for flaps up, 20 degrees, and 30 degrees. Notice how the 30-degree curve peaks higher—that's the higher CLMAX—and it's labelled "LANDING." The 20-degree curve is labelled "TAKE-OFF." Now Figure 8.19 plots CD, the drag coefficient, against angle of attack, with the same flap settings. Here you see the drag increasing with flap angle, which is exactly the penalty we discussed. So the take-off setting is a compromise—enough flap to lower unstick speed, but not so much that drag kills your acceleration. Landing uses the maximum flap angle to get both low touchdown speed and high drag for deceleration. That's the complete picture: the load relief system protecting the flaps, and the three-phase logic of flap selection.