General Principles - Cruise — Page 256, Lesson 313

Let's pick this up right where the drag and range story left off. We've already seen how weight affects a propeller aeroplane's range graph, and now I want to show you what happens when we change the aeroplane's configuration — that is, when we put the gear and flaps down. Look at Figure 5.23, which is the range graph for a propeller aeroplane. You'll see matters are much the same as they were for the jet. With gear and flaps deployed, more power is required. That's the key relationship: more power required means the engine burns more fuel, so fuel flow increases, and therefore range decreases. But there's a second, very important detail hiding in that graph — the speed for best range, which we call VMD, is lower. So not only does your range shrink when you're dirty, but the speed at which you achieve that best range also drops. You're flying slower and getting less distance for your fuel. Now, I want to give you a rule to carry with you through every cruise phase of your career: any increase in parasite drag will be detrimental to range and endurance. Parasite drag is the drag that doesn't produce lift — it's the friction and pressure drag from the airframe itself. And what causes it to increase? Any number of things, such as damaged and misaligned surfaces. Here's where I want you to learn a specific term: the extra drag created by misaligned or misrigged airframe surfaces is called excrescence drag. And this isn't a trivial amount — it can be more than 4% of the aeroplane's total drag. That's a significant penalty for something you can often catch on the ground. A careful preflight inspection should reveal misaligned or misrigged surfaces before you ever take off. Next, let's talk about something over which you, the pilot, have direct control: aeroplane trim. You must periodically check the aileron and rudder trim, the spoiler misfair, and the trailing edges to ensure that the aeroplane is "in trim" and in balanced flight. Let me unpack that. Aileron trim controls roll, rudder trim controls yaw, and the spoiler misfair refers to the spoilers not sitting flush with the wing surface. Monitoring these control surfaces will help reduce the extra drag and extra fuel consumption that out-of-trim and unbalanced flight can cause. So this is a practical, hands-on task — you're actively managing drag in the cruise. Finally, there's contamination on the airframe from airframe icing. This affects fuel flow, and it does so through three distinct mechanisms. First, icing changes the shape of the wing, making it less efficient at producing lift. Second, it increases weight. Third, it increases drag. All of this is detrimental to aircraft performance and will reduce the aeroplane's range. So icing isn't just a take-off hazard — it's a cruise performance killer too. So to tie it all together for the cruise phase: configuration changes cost you range and lower your best-range speed, parasite drag from any source hurts range and endurance, excrescence drag from misrigged surfaces can exceed 4% of total drag, trim management is your direct lever to minimise drag, and icing degrades the wing, adds weight, and adds drag. Every one of these is a range penalty you need to anticipate and manage.