General Principles - Climb — Page 175, Lesson 205

Let’s start with the big picture. When we talk about an aeroplane climbing, the real question is: how much Excess Thrust do we have? Excess Thrust is simply the thrust available from the engines minus the drag the aeroplane is fighting. The more Excess Thrust we have, the steeper our climb angle can be. So to understand climb performance, we need to understand drag — and the easiest way to see it is with a graph of Thrust versus Drag against Indicated Airspeed, or IAS. Now, drag isn’t one single thing. It splits into two main parts, and we’re going to study each curve in detail. The first is Parasite Drag. Parasite Drag is everything that isn’t producing lift — the friction of air over the skin, the form drag of the fuselage, the interference where parts meet. Look at Figure 3.15: Parasite Drag increases with the square of the IAS. That means Parasite Drag is proportional to IAS squared. So if you double your Indicated Airspeed, Parasite Drag doesn’t just double — it goes up by four times. That’s the square relationship. At low speed, Parasite Drag is small, but it reaches a maximum at high IAS. And here’s the kicker: Parasite Drag also increases with what we call Parasite Area. That’s the effective frontal area of everything that sticks out into the airflow — flaps, undercarriage, or speed brakes. Put the gear down, extend the flaps, or pop the speed brakes, and you’ve increased Parasite Area, so Parasite Drag goes up. Now the second curve — Induced Drag. This is the drag that comes from producing lift. Look at Figure 3.16: Induced Drag decreases with IAS squared. It’s inversely proportional to IAS squared. So if you double your IAS, Induced Drag drops to one quarter of its previous value. Notice the contrast with Parasite Drag — one goes up with speed squared, the other comes down with speed squared. Induced Drag is at its highest at low IAS, and it decreases as IAS increases. Induced Drag varies with Lift production. So if you increase the Weight of the aeroplane, or if you bank the aircraft, you’re demanding more lift — and that increases Induced Drag. So here’s how the two curves fit together. At low speed, Induced Drag dominates — it’s huge. At high speed, Parasite Drag dominates — it’s huge. Somewhere in between, the total drag curve has a minimum. And that’s where we get the maximum Excess Thrust, which gives us the best climb angle. That’s the whole point of studying these curves — to find where Excess Thrust is greatest. Let me make sure you’ve got the two relationships locked in, because they’re the heart of this. Parasite Drag: proportional to IAS squared — double speed, drag quadruples. Induced Drag: inversely proportional to IAS squared — double speed, drag quarters. And remember what feeds each one: Parasite Drag feeds on Parasite Area — flaps, gear, speed brakes. Induced Drag feeds on Lift production — more weight or more bank means more Induced Drag. That’s the foundation. Once you can picture those two curves crossing on the graph, you’ll see exactly why an aeroplane climbs best at a particular speed — the speed where Excess Thrust is maximised.