High Lift Devices — Page 227, Lesson 279

I want to walk you through the management of high-lift devices — specifically, how the flight crew handles flap retraction after take-off. This is a real operational skill, not just theory, because getting it wrong means the aircraft sinks. Let's start with the lift curve. Imagine the aircraft has just taken off with flaps extended. We're at point 'A' on the lift curve — that's a high coefficient of lift, which is what got us airborne. Now, if we retract the flaps without changing anything else — no change to angle of attack, no change to indicated airspeed, IAS — the coefficient of lift drops to point 'C'. And here's the consequence: the aircraft will sink. That's the danger we're managing. So the correct procedure is a two-step sequence. Step one: from point 'A', accelerate the aircraft to point 'B'. Step two: from point 'B', as the flaps are retracted, increase the angle of attack to point 'C' — and that increase in angle of attack is what maintains the coefficient of lift constant. So we're trading airspeed for angle of attack to hold the lift steady while the flaps come up. The key rule: the pilot should not retract the flaps until the aircraft has sufficient IAS. And this same factor applies to any intermediate flap position between fully extended and fully retracted — every stage needs enough speed before you move the flaps. Now, as the configuration changes from flaps down to flaps up — which we call the "clean" configuration — three important changes take place. Let me walk you through each. First, the pitching moment. Two opposing effects happen at once. The change in pressure distribution on the wing generates a nose-up pitching moment. But at the same time, the reduced wing downwash increases the tailplane's effective angle of attack, and that generates a nose-down pitching moment. The actual pitching moment the aircraft experiences depends on which of these two is dominant. So it's a balance — you can't just say "flaps up means nose up" or "nose down"; it depends on the aircraft. Second, the drag. Retracting the flaps — going from point 'B' to point 'C' — causes a reduction in the drag coefficient. That drag reduction improves the aircraft's acceleration. That's why we can accelerate in the first place. Third, the timing. Flap retraction usually takes place in stages, and moving the flaps between stages takes a finite period of time. Remember what we said: as flaps retract, you need an increase in angle of attack to maintain the same lift coefficient. So here's the operational consequence: if the aircraft's acceleration is low throughout the flap retraction speed range, the angle of attack must be increased by an appreciable amount to prevent the aircraft from sinking. That situation is typical after take-off when gross weight and density altitude are high — heavy aircraft, hot and high airports. But here's the good news for modern operations: most modern jet transport aircraft have enough acceleration throughout the flap retraction speed range that the resultant rapid gain in airspeed requires a much less noticeable increase in angle of attack. So the technique is gentler, but the principle — never retract flaps without sufficient speed, and hold the lift coefficient by raising angle of attack — is exactly the same. Let me show you the lift curve so you can see points A, B, and C on it. That's the whole picture: accelerate, then retract while raising the nose to hold lift, and understand the pitching, drag, and timing effects that come with it.