High Lift Devices — Page 227, Lesson 281

We’re now looking at what happens when you actually extend the flaps on the approach to land. This is the practical payoff of everything we’ve built up about the lift curve. Let me set the scene. You’re in level flight in the terminal area, before landing. On the lift curve — that’s the graph of coefficient of lift, CL, against angle of attack — you’re sitting at point ‘A’ on the flaps-retracted curve. That’s your current trimmed condition, your current angle of attack, your current CL. Now here’s the key idea. If you extend the flaps and do nothing else — no change to angle of attack — the coefficient of lift jumps up to point ‘C’ on the flaps-extended curve. The same angle of attack now produces more lift. And what does more lift do in level flight? The aircraft gains altitude. We call that ballooning. That’s the first thing to understand: flap extension, at constant angle of attack, increases CL and makes the aircraft climb. So the procedure to manage this is a two-step sequence, and I want you to hold onto it because it’s exactly what you’ll do in the aircraft. Step one. From point ‘A’, as the flaps extend, you decrease the angle of attack down to point ‘B’. Why? To keep the coefficient of lift constant. You’re trading the lift increase from the flaps for a lift decrease from reducing angle of attack, so CL stays put and you don’t balloon. Step two. From point ‘B’ on the lift curve, you decelerate the aircraft down to point ‘C’. So now you’re slowing down, and as speed decreases you need more CL to maintain lift — that’s why you’re moving up the curve toward ‘C’. Point ‘C’ is your final approach condition: flaps fully extended, slower speed, higher CL. So the whole sequence is: extend flaps, reduce angle of attack to hold CL, then slow down to the approach speed. Each step is deliberate, and each one is about managing that lift curve so you don’t get an unwanted climb or an unwanted sink. One more thing — if you ever need to refresh your memory on reading the lift curve itself, that’s covered back on page 76, the interpretation of the lift curve. But for now, the essential picture is: flaps retracted curve, flaps extended curve, and the three points A, B, C marking your path from cruise condition to landing condition.