General Principles - Climb — Page 182, Lesson 206

I want to walk you through the Total Drag curve, because this is the foundation for understanding climb performance. Let's start with the basics. In flight, an aeroplane experiences two distinct types of drag. The first is Parasite Drag — that's the drag caused by the airframe itself, the skin friction, the form drag of the fuselage and wings, everything that isn't producing lift. The second is Induced Drag — that's the drag generated as a by-product of producing lift. The sum of Parasite Drag and Induced Drag is called Total Drag. And here's a convention you need to lock in: when someone just says "Drag" without any qualifier, they mean Total Drag. Not parasite, not induced — Total Drag. Now, look at the Total Drag curve. At any given IAS — that's Indicated Airspeed — Total Drag is simply the sum of Parasite Drag and Induced Drag at that speed. The curve has a very specific shape, and there's a critical point on it. The IAS at which Parasite Drag equals Induced Drag is the point that generates minimum Total Drag. That speed has a name — it's called the Minimum Drag Speed, and we abbreviate it as VMD. The "V" stands for velocity, and "MD" for minimum drag. Here's the key behaviour: fly at an IAS slower than VMD and you generate more drag. Fly at an IAS faster than VMD and you also generate more drag. So VMD sits at the bottom of a U-shaped curve — it's the sweet spot where total drag is at its absolute minimum. Anything either side of it, and drag goes up. I want to give you a habit right now, because this will serve you throughout your career. Whenever you're considering drag, sketch the Total Drag curve together with the Parasite and Induced Drag curves. It's an excellent habit. It lets you see the result of any variable you introduce, and it lets you draw the correct conclusion. We're about to introduce a variable right now, so let's see how that works. The variable is weight, or bank angle. Let's look at what happens when the aircraft gets heavier, or when it turns. In both cases, lift must be increased — more weight needs more lift to support it, and a turn needs more lift to provide the centripetal force. And here's the crucial point: Induced Drag is the drag of producing lift, so if lift goes up, Induced Drag goes up at any given IAS. Now, why does the effect look so much bigger at the low-speed end of the graph? Because Induced Drag is inversely proportional to IAS squared. Let me unpack that. Inversely proportional means as speed goes down, drag goes up. Squared means the relationship is not linear — it's dramatic. At low speed, the denominator is small, so the drag value is large, and any increase in lift produces a proportionally greater increase in Induced Drag. At high speed, the denominator is large, so the effect is much smaller. So what happens to the curves? The Induced Drag curve shifts upward, and its intersection with the Parasite Drag curve moves further towards the high-speed end of the graph. Remember, that intersection is where the two drags are equal — and that's the point of minimum Total Drag. So if the intersection moves to a higher speed, VMD becomes a faster IAS. And the sum of Induced Drag plus Parasite Drag is greater, so Total Drag increases overall. Let me state that clearly: when an aircraft operates at increased weight, or when it's turning, Total Drag increases and VMD becomes a faster IAS. Now the converse. Throughout flight, weight decreases because you're burning fuel. As the aircraft becomes lighter, the opposite happens — Total Drag decreases, and the IAS for VMD also decreases. So VMD is not a fixed number on the airspeed tape; it's a moving target that depends on your weight and whether you're turning. Here's how this connects to climb, which is where we're heading. Drag is the force you have to overcome with thrust. If drag goes up, you need more thrust to maintain speed, and less thrust is available to climb. So a heavier aircraft, or one in a turn, has more drag and a higher VMD — and that directly affects how well it can climb. Keep that Total Drag curve in your mind's eye, and you'll understand why climb performance changes with weight. Let me show you the curves so you can see exactly what I mean.