High Lift Devices — Page 227, Lesson 275

Let’s pick up right where the high-lift devices story gets interesting — the sequence of operation. For some aerofoils, the order in which you deploy the flaps is critical, not just a matter of preference. Here’s why. When you lower a trailing edge flap, it does two things at once: it increases the downwash and it increases the upwash. Downwash is the air deflected downward behind the wing; upwash is the air deflected upward ahead of the wing. Now, on a high-speed aerofoil, that leading edge already has a fairly high angle of attack. If you suddenly increase the upwash at the leading edge while the angle of attack is already high, you can push the wing over the edge and cause it to stall. So the rule is: the leading edge device must be deployed before the trailing edge flap is lowered. And when you retract them, you reverse the order — the trailing edge flap must be retracted before the leading edge device is raised. Think of it as protecting the leading edge from that extra upwash at the wrong moment. Now let’s talk about asymmetry of high lift devices. Deploying these devices produces large changes in lift, drag, and pitching moment — all three. If the movement isn’t symmetrical on the two wings, the unbalanced forces can cause severe roll control problems. On many flap control systems, the deflection on the two sides is compared while the flaps are moving. If one side fails, movement on the other side is automatically stopped — that’s the protection built into sophisticated systems. But on less sophisticated systems, a failure of the operating mechanism could lead to an asymmetric situation. Here’s the consequence: the difference in lift causes a rolling moment, which must be opposed by the ailerons. The difference in drag causes a yawing moment, which must be opposed by the rudder. Whether the controls will be adequate to maintain straight and level flight depends on two things — the degree of asymmetry and the control power available. Let me tie that together with the figure. shows the lift coefficient curves for the basic section, the basic section with a slot, the trailing edge flap, and the flap plus slot combination. The key point is that each device shifts the lift curve — the slot delays the stall, the flap increases the maximum lift, and together they give you the highest CL. That’s why the sequence matters: you want the slot working before the flap adds that upwash, so you get the benefit without the stall. So, to sum up the two big ideas: first, the deployment order protects the leading edge from stalling — leading edge device first on deployment, trailing edge flap first on retraction. Second, asymmetry is a real hazard — the systems compare both sides and stop if one fails, but if you do get asymmetry, you’re fighting a rolling moment with ailerons and a yawing moment with rudder, and whether you can hold straight and level depends on how bad the asymmetry is and how much control authority you have.