General Principles - Cruise — Page 256, Lesson 313

Let’s pick this up right where the range graphs left off. We’ve already seen how weight affects the speed for best range on a propeller aeroplane. Now I want to walk you through what happens when we change the aeroplane’s configuration — that is, when we put the gear and flaps out. Look at Figure 5.23, which is the range graph for a propeller aeroplane with gear and flaps deployed. The story is much the same as what we saw with weight. With gear and flaps down, more power is required. That means the engine has to burn more fuel to produce that extra power, so fuel flow increases. And because we’re burning more fuel per hour, the range — the distance we can fly on a given amount of fuel — decreases. But here’s the extra detail to notice: the speed for best range, which we call VMD, is lower when the gear and flaps are out. VMD stands for the speed for minimum drag — the speed at which total drag is at its lowest, and therefore the speed that gives the best range. With the extra drag from the gear and flaps, that best-range speed shifts down. Now, there’s a point I always want you to keep in the front of your mind during cruise: any increase in parasite drag will be detrimental to range and endurance. Parasite drag is the drag that comes from the parts of the aeroplane that don’t produce lift — the fuselage, the gear, the antennas, the misaligned panels. It’s called parasite because it’s drag that gives us nothing useful in return. And the key word there is any increase. Parasite drag can go up for any number of reasons — damaged surfaces, misaligned surfaces. And here’s the specific term I want you to remember: the extra drag created by misaligned or misrigged airframe surfaces is called excrescence drag. That’s a professional term you’ll see in the performance world. Excrescence drag can be more than 4% of the aeroplane’s total drag. That might not sound like a lot, but in performance terms, 4% of total drag is significant — it’s fuel you’re burning for nothing. A careful preflight inspection should reveal misaligned or misrigged surfaces, so that’s one of the reasons the walk-around matters. Next, there’s an important point over which the pilot has direct control, and that’s aeroplane trim. A pilot must periodically check the aileron and rudder trim, the spoiler misfair, and the trailing edges, to ensure the aeroplane is “in trim” and in balanced flight. Let me unpack that. Aileron trim and rudder trim are the small adjustable surfaces that take the control loads off the pilot — they keep the aeroplane flying straight and level without you having to hold pressure on the controls. The spoiler misfair is when a spoiler — that’s a panel on the wing that can be raised to spoil lift and increase drag — isn’t sitting flush with the wing surface. And the trailing edges are the rear edges of the wings and control surfaces. If any of these are out of position, the aeroplane is out of trim, and that means unbalanced flight. Monitoring the control surfaces helps reduce the extra drag and extra fuel consumption that out-of-trim and unbalanced flight can cause. So this is something you actively manage in the cruise — you’re not just a passenger watching the instruments. Finally, there’s contamination on the airframe from airframe icing, and this can affect fuel flow. Icing does three things at once. First, it changes the shape of the wing, making it less efficient at producing lift — the smooth airflow gets disturbed. Second, it increases weight — ice is mass you’re carrying. Third, it increases drag. All of that is detrimental to aircraft performance and will reduce the aeroplane’s range. So icing isn’t just a lift problem — it’s a range problem too, because you’re burning more fuel to push a heavier, draggier, less efficient aeroplane through the air. So to tie it all together for the cruise: configuration changes like gear and flaps raise power required and lower VMD; parasite drag — especially excrescence drag from misrigged surfaces — hurts range and endurance; trim management is your direct lever to avoid extra drag and fuel burn; and icing hits you on lift, weight, and drag all at once, cutting your range. These are the things you watch in the cruise to protect your fuel and your distance.