General Principles - Climb — Page 188, Lesson 216

Let's pick up right where the climb performance story gets interesting — the factors that change your best angle-of-climb speed, VX. We've already met the idea of Excess Thrust — that's the difference between Thrust Available and Thrust Required, the leftover thrust you can spend on climbing rather than just holding speed. The bigger that Excess Thrust, the steeper your climb gradient. Now let's look at what happens when the weight goes up. Here's the chain of cause and effect. More weight means the wing has to generate more Lift just to hold the aircraft up. More Lift means more Induced Drag — that's the drag that comes as a by-product of making lift. So the Total Drag curve moves up on the graph. But it also moves to the right, meaning the speed at which drag is minimum shifts to a higher IAS. That minimum-drag speed is VMD — the speed for minimum drag. So with more weight, Thrust Required is increased, and VX — your best angle-of-climb speed — becomes a faster IAS. And because Thrust Required has gone up, Excess Thrust has gone down, so the maximum climb gradient is reduced. Let me give you the formula that makes this crystal clear. Climb gradient in percent equals (T minus D) divided by W, all times 100. So Gradient % = (T - D) / W × 100. T is Thrust Available, D is Thrust Required — so T minus D is your Excess Thrust — and W is weight. Just by looking at that formula, two facts are self-evident. First, for a given Weight, the greater the Excess Thrust, the more times Weight divides into that bigger value, so the steeper the climb gradient. Less Excess Thrust means a more shallow climb gradient. Second, for a given Excess Thrust, the greater the Weight, the fewer times Weight divides into the same value, so the more shallow the climb gradient. Less weight means a steeper climb gradient. So the bottom line for weight: increased weight reduces maximum climb gradient and increases VX. Now let's look at the second factor — configuration. Configuration means whether the flaps or gear are extended or not. If flaps and gear are retracted, the aircraft is in the clean configuration. If flaps or gear are extended, Parasite Drag increases, but there's no significant change in Induced Drag. Here's the key contrast with weight. When you extend flaps or gear, Parasite Area increases, so Parasite Drag is greater. That moves the Total Drag curve up, but this time to the left — not to the right like weight did. So Thrust Required is increased, and VX becomes a slower IAS. Because Thrust Required has increased, Excess Thrust is decreased, so maximum climb gradient is decreased. So the bottom line for configuration: flaps or gear reduce maximum climb gradient and decrease VX. Notice the difference — weight pushed VX faster, flaps and gear push VX slower. Two different mechanisms, two different directions. So it seems a very good idea to retract the gear as soon as possible after lift-off, once a positive rate of climb is achieved, and also not to use flaps during a climb, so the climb angle is as large as possible. But you may recall the purpose of flaps is to decrease the take-off and landing run. If it's necessary to use flaps for the take-off run, retract them in stages after take-off as soon as it's safe to do so. The regulatory flap retraction schedule will be discussed later. So to sum up this whole picture: weight and configuration both reduce your maximum climb gradient, but they pull VX in opposite directions — weight makes it faster, flaps and gear make it slower. And the formula Gradient % = (T - D) / W × 100 ties it all together.