NOSE-DOWN — Page 219, Lesson 270

We're looking at the tail end of the high-lift devices chapter, and I want to start with the overall pitch change you get when you extend flaps. When you put flaps down, two things happen at once. The wing produces more lift, and that changes the downwash behind it. The tailplane sits in that downwash. If the downwash increases, it hits the tailplane at a different angle, and that creates a pitching moment of its own. So you have the wing's lift increment trying to pitch the aircraft one way, and the tailplane's reaction to the changed downwash pitching it the other way. The resultant aircraft pitching moment depends on which of those two effects is dominant. Now, what decides which one wins? The type of flap, the position of the wing, and the relative position of the tailplane. The result can be a nose-up moment, a nose-down moment, or almost zero. Here's the concrete example from the text: if the tailplane is mounted on top of the fin, it sits higher, so it's less influenced by the change in downwash. That means the wing's effect dominates, and you get an increased aircraft nose-down pitching moment on flap extension. Then there's the attitude question. In steady flight, lift must equal weight. If you lower the flaps but keep the speed constant, lift increases. To bring lift back down to its original value, you have to decrease the angle of attack. So the aircraft flies in a more nose-down attitude when the flaps are down. On approach to landing, that's an advantage, because it gives you better visibility of the landing area. Now let's move to the leading edge high lift devices. There are two forms commonly in use: the leading edge flap, and the leading edge slot or slat. Let's take the leading edge flap first. On high speed aerofoil sections, the leading edge may have very little camber and a small radius. That can cause flow separation just aft of the leading edge at quite low angles of attack. The remedy is a leading edge flap, which increases the leading edge camber. There are two types of leading edge flap. The first is the Krueger flap. It's part of the lower surface of the leading edge, and it can be rotated about its forward edge. On a swept wing, Krueger flaps are used on the inboard section, because they're less efficient than the variable camber type, and that helps promote root stall. The second type is the variable camber leading edge flap. To improve efficiency by giving a better leading edge profile, the camber of the flap is increased as it's deployed. Here's a key contrast with trailing edge flaps: trailing edge flaps can be selected to intermediate positions, but leading edge flaps are usually either fully extended, which is deployed, or retracted, which is stowed. No in-between. Finally, the effect of leading edge flaps on lift. The main effect is to delay separation, and that increases the stalling angle and the corresponding CLMAX — that's the maximum lift coefficient. There's also some increase of lift at lower angles of attack, because of the increased camber of the aerofoil section. So you get a lift curve that's shifted up and extended to a higher stalling angle compared to the basic wing section.