General Principles - Climb — Page 188, Lesson 216

Let's pick up with the climb — specifically, what changes the angle of climb. We've already seen that the climb gradient depends on Excess Thrust, which is Thrust Available minus Thrust Required, or Drag. Now I want to show you the two big factors that change that Excess Thrust and therefore change VX, the speed for best angle of climb. First, weight. Look at Figure 3.28. We plot Thrust Available and Thrust Required against IAS for a turbojet. When the aircraft is heavy, more weight means more Lift is needed, and more Lift means more Induced Drag. That moves the Total Drag curve up — so Thrust Required is higher — but it also moves the curve to the right. The result is that VX, the speed for best climb angle, becomes a faster IAS. And because Thrust Required has gone up, Excess Thrust goes down, so the maximum climb gradient decreases. Now, the formula ties this together. Climb gradient in percent equals (T minus D) divided by W, all times 100. T is Thrust Available, D is Thrust Required, W is Weight. Look at that formula and two facts are self-evident. 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 gradient. And for a given Excess Thrust, the greater the Weight, the fewer times Weight divides into the same value, so the more shallow the gradient. Less weight, steeper gradient. So the bottom line: increased weight reduces maximum climb gradient and increases VX. Now 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. Look at Figure 3.29. With flaps or gear extended, Parasite Area is increased, so Parasite Drag is greater. That moves the Total Drag curve up, but this time to the left. Thrust Required is increased, and VX becomes a slower IAS. Again, because Thrust Required has increased, Excess Thrust is decreased, so maximum climb gradient is decreased. So flaps or gear reduce maximum climb gradient and decrease VX. That's why 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 not to use flaps during a climb, so the climb angle is as large as possible. But remember the purpose of flaps — they decrease the take-off and landing run. So if you need 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 is something we'll discuss later.