General Principles - Climb — Page 182, Lesson 206

Let’s start with the Total Drag curve, because it’s the foundation for everything else in the climb chapter. 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, the parts that don’t produce lift. The second is Induced Drag — that’s the drag created as a by-product of producing lift. The sum of Parasite Drag and Induced Drag is called Total Drag. And here’s an important convention: when we just say the word “Drag” in aviation, we always mean Total Drag. So if someone says “drag increases,” they mean total drag. Now look at the Total Drag curve. At any given Indicated Airspeed, or IAS, Total Drag is the sum of Parasite Drag and Induced Drag. The curve is U-shaped. At low speed, Induced Drag dominates; at high speed, Parasite Drag dominates. There’s a specific IAS where Parasite Drag equals Induced Drag exactly — and at that point, Total Drag is at its minimum. That speed is 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 slower than VMD and Total Drag increases. Fly faster than VMD and Total Drag also increases. So VMD is the sweet spot — the speed at which the aeroplane needs the least thrust to maintain flight. That’s why the vertical axis on the graph is labelled both “Drag” and “Thrust Required” — because in steady flight, thrust must equal drag. Now, a practical habit I want you to develop: whenever you’re thinking about drag, sketch the Total Drag curve along with the Parasite and Induced Drag curves. It’s a quick way to see how any variable — weight, bank angle, altitude — shifts the picture and lets you reach the correct conclusion. Now let’s look at what happens when weight or bank angle changes. Figure 3.18 shows the effect of increased weight and/or turning the aircraft. When the aeroplane is heavier, or when it’s turning, Lift must be increased. And because Induced Drag is a by-product of lift, Induced Drag increases at any given IAS. Here’s the subtle part: the increase in Induced Drag is proportionally greater at the low-speed end of the graph. Why? Because Induced Drag is inversely proportional to IAS squared. So at low speed, the denominator is small, and the effect of any increase in lift is magnified. Now, what happens to the curves? The intersection of the Induced Drag curve and the Parasite Drag curve shifts further towards the high-speed end of the graph. The sum of Induced and Parasite drag is greater. So Total Drag increases, and VMD becomes a faster IAS. In other words, a heavier aeroplane, or one in a turn, needs a higher speed to achieve minimum drag. Conversely, throughout flight, weight decreases because you’re burning fuel. As the aeroplane becomes lighter, Total Drag decreases, and the IAS for VMD also decreases. So VMD is not a fixed number — it moves with weight and bank angle. Let me tie that together. VMD is the speed for minimum total drag, where parasite equals induced. Heavier or turning means more induced drag, especially at low speed, so VMD shifts faster. Lighter means less drag and a slower VMD. That’s the core of the drag picture for the climb.