General Principles - Cruise — Page 251, Lesson 302

Let’s pick up right where we left off — we’ve just seen how gear and flaps affect drag and fuel flow. Now I want to finish that thought, because there’s a hard number attached to it that you need to remember. In the landing configuration — that is, with gear down and flaps extended — fuel flow can increase by 150% compared to a clean configuration. Clean means gear up, flaps up, the aeroplane’s most streamlined shape. So a 150% increase means the engine is burning two and a half times as much fuel per hour as it would clean. That is a massive penalty. The practical lesson is obvious: do not deploy gear or flaps too early, because every minute you fly dirty is money and fuel you’re burning unnecessarily. Unnecessarily increasing the fuel costs for the flight is exactly what you’re doing. Now let’s move to a new topic within cruise — wind and altitude, and how each affects endurance. Wind is simple. It has no effect on endurance at all. Think about why. Maximum endurance is about minimizing fuel flow — the rate at which fuel is consumed per hour. Wind does not change the fuel flow into the engine. Endurance is time in the air, not distance covered. So whether you have a headwind or a tailwind, the aeroplane will remain airborne only as long as it has usable fuel in its tanks. The wind changes how far you get in that time, but not how long you stay up. So for endurance — time — wind is irrelevant. Altitude, however, does affect endurance, and here it gets a little complicated because it depends heavily on engine type. Let me take each engine type in turn. For jet aeroplanes, efficiency generally improves as altitude increases. There are two reasons given here. First, the decreasing ambient temperature — colder air is denser, and the engine works more efficiently in it. Second, the increasing rpm required to maintain thrust. So theoretically, the maximum endurance of a jet is achieved when flying at or above the tropopause — that’s the boundary layer in the atmosphere where the ambient air temperature is lowest. Above the tropopause, temperature stops decreasing with altitude, so that’s where you get the coldest air and the best jet endurance. Turbo-propeller aeroplanes function in a similar way to a jet, because in essence they are jet engines with a propeller attached to a geared shaft. The turbo-propeller engine does gain efficiency with altitude, just like a jet. But here’s the catch — the power required increases due to the rising TAS, true airspeed, and that rising TAS offsets the efficiency gains. So the two effects fight each other. The net result is that for the majority of modern turbo-propeller aeroplanes, maximum endurance is achieved at around 10 000 ft or less. Not higher — the efficiency gain is cancelled out by the power required to fly faster through thinner air. Finally, piston engine aeroplanes. These are most efficient at sea level, when the manifold pressure is high and rpm is low, provided that the mixture has been leaned correctly. Manifold pressure is the pressure in the intake manifold — think of it as the throttle setting, how much air the engine is drawing in. At sea level you get high manifold pressure, and you run at low rpm, and that combination is most efficient — but only if you’ve leaned the mixture properly, meaning you’ve adjusted the fuel-to-air ratio for the conditions. So to summarise the altitude picture: jets want to be high, at or above the tropopause. Turbo-props want to be around 10 000 ft or less. Piston engines want sea level. Three different engine types, three different answers — and that’s exactly the kind of contrast the examiners love. Now, I want to show you the drag and power curves that back all this up. Let me bring up the figures.