High Lift Devices — Page 219, Lesson 268

Let’s pick this up with the drag comparison between flap types, because that’s where the text starts. I want you to picture Figure 8.8, which shows the drag polar curves for the different flap designs. A drag polar is just the plot of lift coefficient against drag coefficient, and here we’re comparing four flap types: the plain flap, the split flap, the slotted flap, and the Fowler flap. The key point is that for a given flap deflection, the drag produced varies considerably between types. The split flap gives the highest drag, and the Fowler flap gives the least. So if you’re choosing a flap for a particular job, that spread matters. Now, why does that drag difference matter in operation? During take-off, drag reduces the acceleration, so the flap should give as little drag as possible. You want to get up to speed quickly, so you don’t want extra drag fighting you. For landing, though, the situation flips. Drag adds to the braking force, so flap drag is beneficial. And there’s a third point: the addition of drag during approach also improves speed stability. That means the aircraft is less sensitive to speed disturbances on the approach path, which is a good thing for a stable landing. One more thing about drag increments. Just like the lift increments we discussed, the drag increments with increasing flap angle are not constant. The increments in drag get larger as the flap angle increases. So each additional degree of flap deflection adds more drag than the previous one did. Now let’s move to the lift-to-drag ratio, which is a big one. Lowering flap increases both lift and drag, but not in the same proportion. Although lift is the larger force, the proportional increase in drag is greater. So the maximum obtainable lift-to-drag ratio decreases when you lower flap. The maximum lift-to-drag ratio occurs where the tangent from the origin of the drag polar touches the curve, and the gradient of that tangent line is a measure of the maximum lift-to-drag ratio. That’s Figure 8.9. The lift-to-drag ratio is a measure of aerodynamic efficiency, and it affects performance in areas like range, climb angle, and glide angle. With flaps lowered, range will be decreased, climb angle reduced, and glide angle increased. So you trade efficiency for the ability to fly slower and steeper on approach. Finally, let’s talk about pitching moment, because flap movement, up or down, will usually cause a change in pitching moment. This is due to two things: Centre of Pressure movement and downwash at the tailplane. First, Centre of Pressure movement. Moving a trailing edge flap modifies the pressure distribution over the whole chord of the aerofoil, but the greatest changes occur in the region of the flap. When flap is lowered, the Centre of Pressure moves rearwards, giving a nose-down pitching moment. That’s Figure 8.10a. In the case of a Fowler flap, the rearward movement of the flap also causes the CP to move aft, resulting in an even greater increase in the nose-down pitching moment. So the Fowler flap gives you a stronger nose-down tendency. Second, the change of downwash. The tailplane’s effective angle of attack is determined by the downwash from the wing. If the flaps are lowered, the downwash will increase, and the tailplane angle of attack will decrease, causing a nose-up pitching moment. That’s Figure 8.10b. So you have two opposing effects: the CP movement gives a nose-down moment, and the downwash change gives a nose-up moment. The net pitching moment depends on which effect dominates, and that’s something you’ll need to manage in the aircraft. So to sum up the whole picture: flap selection is a trade-off between lift, drag, and pitching moment. Split flaps give you the most drag, Fowler flaps the least. Lowering flap reduces your lift-to-drag ratio, which hurts range and climb but helps with glide angle and speed stability. And the pitching moment changes come from both CP movement and downwash effects, with the Fowler flap giving a stronger nose-down tendency.