General Principles - Climb — Page 188, Lesson 214

Let’s pick this up right where the drag curve left us, because the climb lesson builds directly on it. I want to walk you through what happens to a turbojet aeroplane as we slow down, and then I’ll introduce the first of the two key climb speeds — VX, the Best Angle of Climb Speed. First, look at Figure 3.24. We’ve got a turbojet flying unaccelerated — that means straight and level, no speeding up, no slowing down — at 250 KIAS, which is faster than VMD. To hold that speed, the Thrust Available from the engines must exactly match Thrust Required, which is the drag. The point of the figure is this: to maintain unaccelerated flight at that lower speed of 250 KIAS, you must decrease Thrust Available, and the aircraft slows until Thrust Required reduces to the same value. So as you ease the power back, the drag falls with it, and the two lines meet again at a lower speed. Now Figure 3.25. Here Thrust Available has been reduced further, and the aircraft settles at VMD — the minimum drag speed. At VMD, drag is at its absolute minimum, so Thrust Required is at its lowest. That’s the bottom of the drag curve, the sweet spot where the aeroplane needs the least thrust to keep going unaccelerated. Here’s where it gets interesting, and this is the heart of the climb lesson. Figure 3.26 shows what happens if you want to fly slower than VMD — say, at 175 KIAS. To maintain unaccelerated flight at an IAS slower than VMD, Thrust Available must be increased. Why? Because at speeds below VMD, Thrust Required — that’s drag — increases again. You’re on the other side of the curve now, the back side, and the slower you go, the more drag you generate. This region slower than VMD has three alternative names, and you need all three because examiners love them. First, it’s called “the back-side of the Drag curve.” Second, it’s called “the speed unstable region.” And third — perhaps the most descriptive — it’s called “the region of Reverse Command.” That name comes from the counter-intuitive behaviour: to maintain unaccelerated flight at an IAS slower than VMD, you must increase thrust — the reverse of what is normally required. Normally, slower means less thrust. Here, slower means more thrust. That’s the reverse command. Now let’s move to Figure 3.27, which sets up VX. This graph shows Thrust Required — that’s aerodynamic drag — and Thrust Available from the engines, for a turbojet-powered aeroplane. The key new concept is Excess Thrust. Excess Thrust is the amount of thrust that exceeds aerodynamic drag. On the graph, you see it as the vertical distance between the Thrust Available line and the Thrust Required line. And you’ll recall the principle: to maximize the climb gradient, Excess Thrust must be a maximum. Maximum Excess Thrust is obtained by flying at the IAS where the distance between the Thrust and Drag lines is greatest. Now notice something crucial: maximum Excess Thrust is available only at one particular IAS, and that IAS is labelled VX. At any other speed — faster or slower — the distance between the two curves is smaller, so Excess Thrust is less. That means climbing at any IAS other than VX gives you a climb gradient less than the maximum possible. VX is defined as the IAS at which the aeroplane generates the greatest amount of Excess Thrust and is therefore capable of its steepest climb gradient. It’s called the Best Angle of Climb Speed. And here’s the turbojet-specific result, straight from Figure 3.27: for a turbojet aeroplane, VX is the same IAS as VMD. They coincide. But that’s not true for every powerplant. For a propeller aeroplane, VX is less than VMD, and at low altitudes it will be in the region of VMP — that’s the minimum power speed, which we’ll cover shortly. So hold that contrast: turbojet, VX equals VMD; propeller, VX sits below VMD, near VMP. So the whole picture is this: the drag curve has a minimum at VMD, the region below it is the back-side, speed-unstable, reverse-command region, and the steepest climb happens at VX, where excess thrust peaks — which for a turbojet is right at VMD.