General Principles - Climb — Page 175, Lesson 199

Let's start with the core idea of this section: Excess Thrust. To maintain a steady speed along the flight path in a climb, additional Thrust is required to balance the backward component of Weight. That additional Thrust is called Excess Thrust. So, Excess Thrust is the Thrust available from the engine(s) after aerodynamic Drag is balanced. Think of it as the leftover thrust you have after overcoming drag. Look at Figure 3.4. The forward acting force in green is now the same as the two rearward acting forces in red. The aeroplane will maintain a steady speed along its new flight path. To maintain a steady climb with no loss of speed, Thrust must balance not only the aerodynamic Drag, but also the backward component of Weight. Now, in Figure 3.5, the aircraft only has a small amount of Excess Thrust available. Notice that there is too much backward component of Weight from the climb angle that has been set, so that climb angle cannot be maintained. The angle of climb must be reduced to give a smaller backward component of Weight that matches the Excess Thrust available, as shown in Figure 3.6. The greater the Excess Thrust, the larger the backward component of Weight that can be balanced. In other words, the more Excess Thrust available, the steeper the angle of climb or the greater the weight at the same climb angle. Now let's look at the effect of Weight on climb angle. Weight has an influence on climb performance. Figure 3.7 illustrates that if the aircraft tries to use the same climb angle as before, but at a higher weight, the backward component of weight will be greater and there is insufficient Excess Thrust to balance it. In addition, the higher weight will also generate increased aerodynamic Drag, specifically Induced Drag, which will further reduce Excess Thrust. Increased weight therefore decreases the maximum climb angle, as shown in Figure 3.8. Finally, let's talk about Thrust Available. Figure 3.9 shows that Thrust Available is the total amount of Thrust available from the engine(s). Under a given set of conditions and in a steady climb, the thrust available must be the same as the sum of the aerodynamic Drag (D) plus the backward component of Weight, which is W sin γ. So, the key equation here is: Thrust Available = Drag + (Weight × sin of the climb angle). That's the balance you need for a steady climb.