General Principles - Cruise — Page 251, Lesson 300

Let’s pick this up right where the weight effect left off, because there’s one more consequence of higher weight that we haven’t touched yet, and it’s easy to miss. At higher weights, the operating altitude is lower. For a jet aeroplane, that means the specific fuel consumption increases, because at lower altitudes the jet engine is less efficient. So even beyond the drag penalty, a heavy jet burns more fuel per unit of thrust simply because it’s forced to fly lower where the engine doesn’t work as well. That’s the seldom-mentioned effect. Now let’s move to the next factor that affects endurance: the aeroplane’s configuration. This is about the position of the gear and flaps. In cruise, obviously, you wouldn’t deploy them, but there are real situations where you will — for example, when you’re stacked in a holding pattern over the destination. As the aeroplane at the lowest level exits the hold to land, the others descend to a lower hold and eventually prepare for landing by deploying gear and flaps. Here’s the key physics: deploying flaps and undercarriage increases parasite drag. That causes both the total drag curve and the power required curve to move up and to the left. Let me show you what that does to the speed for maximum endurance. For a jet aeroplane, look at Figure 5.15, which uses the drag curve. With gear and flaps deployed, the aeroplane has more drag and therefore requires greater fuel flow. But notice the important detail: the speed for maximum endurance, VMD, is now lower. So the drag curve shifts up, and the minimum point — the speed that gives you the best endurance — moves to a lower speed. For a propeller aeroplane, it’s much the same, as you can see in Figure 5.16, which uses the power required curve. With gear and flaps deployed, more power is required, so fuel flow increases and endurance decreases. And again, the speed for best endurance, VMP, is lower. So let me summarise the pattern we’ve built up. For a jet, the speed for maximum endurance is VMD, and for a propeller aeroplane it’s VMP. Higher weight raises both of those speeds — VMD and VMP go up. But deploying gear and flaps does the opposite: it lowers both VMD and VMP. In both cases, endurance decreases because fuel flow increases, but the speed at which you get the best endurance moves in opposite directions depending on whether the change is weight or configuration. That’s the complete picture for this part of the cruise chapter.