General Principles - Climb — Page 202, Lesson 231

Let’s start with a question I often get from students: how many speeds does an aeroplane actually have? And the answer I want you to lock in is: one. The True Airspeed, or TAS. That is the only speed there is — the actual speed of the aeroplane through the air. Everything else we read on instruments is just a reference to that. Now, from that single speed, we build a very important relationship. Power Required equals Drag multiplied by TAS. Let me say that again slowly: Power Required = Drag × TAS. So if you take a Thrust Required curve — which is really a Drag curve — plotted against Indicated Airspeed, IAS, in sea level ISA conditions, and then you multiply the Drag at each airspeed by the TAS and plot the result, you get the Power Required curve. And that’s exactly what Figure 3.43 shows you. Here’s the key point about the shape. The Power Required curve looks very similar to the Thrust Required curve, but it’s displaced to the left. That displacement is the whole story. Because it’s shifted left, the speed for minimum Power Required — which we call VMP — is slower than the speed for minimum Thrust Required, which we call VMD. So remember: VMP is slower than VMD. That relationship between the two curves and the two speeds is something you must be able to visualise, because we’ll build on it later. Let me show you one practical use of that formula. Suppose an aircraft climbs at a constant IAS. Drag stays constant, because IAS is constant. But as the aircraft climbs, air density decreases, so TAS must increase to compensate. And since Power Required equals Drag times TAS, and TAS is going up, Power Required increases. So climbing at constant IAS means increasing power required. That’s a direct consequence of the formula. Now let’s talk about rate of climb. Rate of climb is the vertical speed of the aeroplane, measured in feet per minute. In the cockpit, it’s displayed on the vertical speed indicator — the VSI. Another way to think of it: rate of climb is the TAS of the aeroplane along a gradient. So it’s not just how fast you’re going forward — it’s how fast you’re going upward. Look at Figure 3.44. Two identical aeroplanes, same angle of climb. The one on the right has a higher TAS along the gradient. In the same time, it climbs through a greater vertical distance. So it has a higher rate of climb. That tells you TAS is one important factor. Now Figure 3.45. Two identical aeroplanes, same TAS. But the one on the right is climbing at a steeper angle. Again, in the same time, it climbs through a greater vertical distance, so it has a higher rate of climb. That tells you angle of climb is also important. Put those two together, and you get the full picture: rate of climb is a function of both angle of climb and TAS along the achieved gradient. Both matter. Neither alone tells the whole story. So to summarise what we’ve covered: one speed — TAS. Power Required = Drag × TAS. VMP is slower than VMD. Climbing at constant IAS increases power required. And rate of climb depends on both angle of climb and TAS. That’s the foundation for everything we’ll do with climb performance.