General Principles - Climb — Page 175, Lesson 205

Let’s start with the big picture, because this is the foundation for everything we do in climb performance. When an aeroplane climbs, it’s not the engine’s raw thrust that lifts it — it’s the excess thrust, the thrust left over after drag has been overcome. And the easiest way to see how an aeroplane can maximise that excess thrust, and therefore its climb angle, is with a simple graph of thrust against drag. So I want to walk you through the two halves of the drag curve first: parasite drag and induced drag. Let’s take parasite drag first. Look at Figure 3.15. Parasite drag is the drag that comes from the aeroplane’s shape pushing through the air — skin friction, form drag, interference drag, all that. The key relationship here is that parasite drag increases with the square of the indicated airspeed. In other words, parasite drag is proportional to IAS squared. So if you double your IAS, parasite drag doesn’t just double — it goes up by four times. That’s the square relationship. At low speed, parasite drag is small; at high IAS, it reaches its maximum. And one more thing: parasite drag also increases with what we call “parasite area” — that’s the frontal area of things like flaps, undercarriage, or speed brakes. Put those out, and you’ve added to the parasite drag. Now the other half of the curve, Figure 3.16: induced drag. This is the drag that comes from producing lift — the wing’s tip vortices, the downwash, the price you pay for generating lift. And here the relationship is the exact opposite. 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. At low IAS, induced drag is at its highest; as IAS increases, it comes down. And induced drag varies directly with lift production — if you increase weight, or bank the aircraft, you’re producing more lift, and that pushes induced drag up. So now you’ve got the two curves that make up the total drag curve: parasite drag rising with the square of speed, induced drag falling with the square of speed. And the point where those two balance — where total drag is at its minimum — is exactly where you’ll find the best conditions for climb. That’s where we’re headed next.