General Principles - Climb — Page 188, Lesson 214

Let’s pick this up right where the drag curve left us. I want to walk you through what happens when we fly slower than VMD, and then I’ll introduce the climb speed that matters most for gradient — VX. First, recall the picture we built: Thrust Required is the same thing as aerodynamic Drag. On a turbojet, that curve has a minimum at VMD — the minimum drag speed. Above VMD, drag increases as speed increases. Below VMD, drag also increases as speed decreases. That’s the key shape. Now look at Figure 3.24. To maintain unaccelerated flight at a lower speed — say 250 KIAS — Thrust Available must be decreased, and the aircraft slowed, until Thrust Required reduces to the same value. So as we slow down from a high cruise speed toward VMD, we’re reducing thrust to match the falling drag. Figure 3.25 shows Thrust Available reduced further, to hold unaccelerated flight right at VMD — the minimum drag speed. At VMD, drag is at its lowest, so the thrust needed to hold that speed is also at its lowest. Now here’s the twist — Figure 3.26. To maintain unaccelerated flight at an IAS slower than VMD — say 175 KIAS — Thrust Available must be increased. Why? Because at speeds below VMD, Thrust Required — that is, Drag — increases again. You’re on the other side of the curve now. This region slower than VMD has three alternative names, and you need all three. First, “the back-side of the Drag curve.” Second, “the speed unstable region.” And third — perhaps the most descriptive — “the region of Reverse Command.” It’s called that because to maintain unaccelerated flight at an IAS slower than VMD, thrust must be increased — the reverse of what is “normally” required. Normally, slower means less thrust. Here, slower means more thrust. That’s the instability: if you slow down, drag rises, so you need more thrust just to hold that slower speed. Now let’s move to the climb. Figure 3.27 shows Thrust Required — aerodynamic drag — and Thrust Available — from the engines — for a turbojet-powered aeroplane. Here’s the new concept: Excess Thrust. Excess Thrust is the amount of Thrust that exceeds aerodynamic Drag. On the graph, you see it as the distance between the Thrust Available line and the Thrust Required line. You’ll recall that 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 the Drag lines is maximum. Now notice something important: maximum Excess Thrust is available only at one particular IAS, labelled VX. At any other speed — faster or slower — the distance between the Thrust and Drag curves is smaller, so Excess Thrust is less. Therefore, climbing at an IAS other than VX gives a climb gradient less than the maximum possible. So 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. VX is referred to as the Best Angle of Climb Speed. And here’s the turbojet-specific result, visible right on Figure 3.27: for a turbojet-powered aeroplane, VX is the same IAS as VMD. That’s because the turbojet’s thrust available is roughly constant with speed, so the maximum gap between thrust and drag sits right at the drag minimum. But for a propeller aeroplane, it’s different. VX is less than VMD, and at low altitudes it will be in the region of VMP — the minimum power speed. That’s a contrast worth holding onto: turbojet VX equals VMD; propeller VX is below VMD, near VMP. So to tie it together: the region below VMD is the back-side of the drag curve, speed-unstable, reverse command — where slower means more thrust. And the speed that gives you the steepest climb, VX, sits right at VMD for a turbojet, but below it for a propeller aeroplane.