General Principles - Cruise — Page 251, Lesson 300

Let's pick this up right where the weight effect leaves off, because there's a subtle point I want to make sure you carry forward. At higher weights, we said endurance drops and the best-endurance speeds rise. But there's one more effect that's seldom mentioned: at higher weights, your operating altitude is lower. For a jet, that matters because at lower altitudes the jet engine is less efficient, so the specific fuel consumption actually increases. So you get a double penalty — more fuel burned per unit of thrust, and less endurance overall. Now let's move to the next factor that shapes endurance: the aeroplane's configuration. This is about the position of the gear and the flaps. In normal cruise you'd never have them out, obviously. But think about a real operational scenario: you're stacked with other aeroplanes in a holding pattern over the destination. As the aeroplane at the lowest level exits the hold to land, everyone above has to descend to a lower hold, and eventually they prepare for landing by deploying gear and flaps. So configuration changes are a genuine part of holding, not just a theoretical point. Here's the key physics. Deploying the flaps and the undercarriage increases parasite drag. That pushes the total drag curve — and for a propeller aeroplane, the power required curve — up and to the left. Let me unpack that. "Up" means more drag at any given speed, so you need more thrust and therefore greater fuel flow. "Left" means the minimum of the curve shifts to a lower speed. And that minimum is exactly your maximum-endurance speed. Look at Figure 5.15, which is for a jet aeroplane using the drag curve. With gear and flaps deployed, the aeroplane has more drag and therefore requires greater fuel flow — endurance drops. But notice the speed for maximum endurance, VMD, is now lower. So the dirty configuration not only costs you fuel, it changes the speed you should fly to get the best endurance out of what you have. Figure 5.16 shows the propeller aeroplane, and it's much the same story. With gear and flaps deployed, more power is required, fuel flow increases, endurance decreases — and the speed for best endurance, VMP, is lower. So for both engine types, the pattern is identical: configuration drag raises the fuel burn and pulls the best-endurance speed down. Let me make sure the terminology is locked in. VMD is the speed for minimum drag — that's the jet's maximum-endurance speed. VMP is the speed for minimum power — that's the propeller aeroplane's maximum-endurance speed. Two different names because the two engine types optimise different things, but the concept is the same: fly at that speed and you stretch your fuel the furthest. So the complete picture for this section: weight and configuration both shift your endurance. Higher weight raises your best-endurance speed and lowers your endurance, and for jets it also raises specific fuel consumption because you're forced lower. Deploying gear and flaps raises drag, raises fuel flow, lowers endurance, and lowers your best-endurance speed. Both effects move you away from the efficient cruise you'd want in clean configuration.