General Principles - Take-off — Page 160, Lesson 188

We're now into the flap angle part of the take-off story. Let's look at what happens when you change the flap setting, because it's a trade-off between getting airborne quickly and climbing away strongly. First, the graph in Figure 2.14 shows the effect of flap angle on the take-off distance required. The key idea is that increasing the flap angle reduces the take-off distance required. Why? Because more flap gives you more lift at a lower speed, so you can rotate and leave the ground sooner. That's the benefit. But here's the catch, and it's the central tension of this whole section. The flap setting also affects the climb gradient. And the climb gradient is what determines your Maximum Mass for Altitude and Temperature. Let me unpack that. The maximum mass you can take at a given altitude and temperature is set by a climb gradient requirement, and also by the clearance of obstacles in the take-off flight path. So it's not just about getting off the ground—it's about being able to climb away steeply enough afterwards. Now, increasing the flap angle increases drag. More drag means a reduced climb gradient for a given aircraft mass. So if you need a certain gradient, the maximum permissible mass for that required gradient will be reduced. In other words, the more flap you use, the less mass you can carry if the climb gradient is the limiting factor. This becomes really important in hot and high conditions. Think about a high-altitude airport on a hot day. The air is thin, so you need more speed and more runway. If you use the flap setting that gives the shortest take-off distance, you might find that the Mass-Altitude-Temperature requirement—that's the climb gradient limit—becomes more limiting than the field length requirement. So the thing that stops you isn't the runway length, it's the climb gradient. In those conditions, you can actually obtain a greater take-off mass by using a lower flap angle. Less flap, less drag, better climb gradient, more mass allowed. Now look at Figure 2.15. It shows two curves. There's the flap for maximum brake release mass, and there's the optimum flaps for take-off mass. The point is that the flap setting that gives you the shortest take-off distance is not necessarily the one that gives you the maximum take-off mass. The optimum is a balance between the field length limit and the climb limit. And then there's the obstacle question, in Figure 2.16. If there are obstacles in the take-off flight path, the flap setting that gives the shortest take-off distance may not give the maximum possible take-off mass. This happens when your Take-off Distance Available is greater than your Take-off Distance Required. You have more runway than you actually need. So if close-in obstacles are not cleared, using a lower flap angle will use a greater proportion of the Take-off Distance Available—you'll use more of the runway—but it may give you a sufficiently improved gradient to clear those obstacles. You trade runway length for climb performance. So the whole picture is this: flap angle is a compromise. High flap gets you off the ground quickly but climbs poorly. Low flap climbs well but needs more runway. The best setting depends on whether your limiting factor is field length, climb gradient, or obstacle clearance.