General Principles - Climb — Page 195, Lesson 222

Let’s pick up with the climb phase and look at two things that change how well an aeroplane climbs: bank angle and wind. First, the effect of bank angle. When an aircraft is banked, any increase in bank angle beyond approximately 15 degrees will significantly increase the amount of Lift that needs to be generated. Why? Because in a bank, the lift vector tilts sideways, so part of it is used to turn the aircraft rather than hold it up. To keep climbing, you have to generate more total lift. Now, increased Lift will generate more Induced Drag. Induced drag is the drag created as a by-product of producing lift. So more lift means more induced drag. That extra drag reduces your Excess Thrust — the thrust available beyond what is needed to overcome drag. Since Excess Thrust is what drives the climb, reducing it means your maximum climb angle will be reduced. So the chain is: bank beyond about 15 degrees → more lift needed → more induced drag → less excess thrust → shallower maximum climb angle. Now, the effect of wind on climbing. Wind is the motion of a body of air over the ground. The effect it has depends on which climb gradient you are considering, because there are two types: Air gradient and Ground gradient. Air gradient is the one used by aviation authorities to lay down minimum climb performance limits. For example, a Class ‘A’ aeroplane: starting at the point at which the aeroplane reaches 400 ft (122 m) above the take-off surface, the available gradient of climb may not be less than 1.2% for two-engined aeroplanes. So that 1.2% figure is an air gradient requirement. Let me define Air gradient precisely. Air gradient is the vertical distance gained in a body of air divided by the horizontal distance travelled through the same body of air. The key point: the fact that the body of air might be moving over the ground is NOT considered. So wind has no effect on Air gradient. Think of it as your climb performance measured relative to the air itself, not the ground. Look at Figure 3.31: an aeroplane in the bottom left corner of a body of air, directly above the control tower on the ground. Then Figure 3.32 shows the body of air stationary relative to the ground — that is called “Zero Wind” or “Still Air”. The aeroplane has climbed to the top right corner of the body of air, and the Air gradient is shown as Gamma ‘a’. So gamma, the Greek letter, is the symbol for the climb angle here. One simplifying note for studying climbs: for climb angles less than approximately 20 degrees, it is considered that doubling the climb angle will double the climb gradient. So within that small-angle range, the gradient is roughly proportional to the angle. Now the Ground Climb Gradient. This is also known as the Flight Path Angle, or FPA, and also as the Climb Angle. The air gradient is not affected by wind, but the ground gradient is influenced by wind. Figure 3.33 shows the effect of a tailwind. Because the body of air is moving over the ground in the direction of flight, the Ground gradient is smaller than the Air gradient. So a tailwind does not change the Air gradient, but it decreases the Ground gradient. Your path over the ground is shallower because the air is carrying you forward. Figure 3.34 shows the effect of a headwind. Because the body of air is moving over the ground opposite to the direction of flight, the Ground gradient is larger than the Air gradient. So a headwind does not affect the Air gradient, but it increases the Ground gradient. The air is pushing back, so for the same climb through the air, you cover less ground — a steeper ground path. Finally, the practical rule: the only time wind is used to calculate climb gradient is when obstacle clearance is being considered. In all other cases of climbing, still air is used, even if a wind value is supplied. So for performance calculations and certification, you use still air; wind only enters when you are checking whether you can clear an obstacle on the ground.