General Principles - Climb — Page 175, Lesson 199

Let's pick up with the climb. We've looked at the forces in steady flight, and now we're going to see what happens when we actually want to go up. The key idea here is something called Excess Thrust. Think about it this way. In straight and level flight, your thrust just has to balance the drag. But the moment you pitch the nose up into a climb, part of your weight—which always acts straight down—now has a component pointing backwards along your flight path. It's trying to pull you back down the slope. So to keep a steady speed while climbing, you need extra thrust to balance that backward pull. That extra thrust is what we call Excess Thrust. Let me give you the precise definition. Excess Thrust is the thrust available from the engine(s) after aerodynamic drag is balanced. So imagine your engines produce a certain total thrust. First, you use some of it to overcome drag. Whatever is left over—that's your Excess Thrust, and it's the only thrust you have available to handle that backward component of weight. Look at Figure 3.4. You can see the forward-acting force in green is now equal to the sum of the two rearward-acting forces in red—that's the drag and the backward component of weight. When those balance, the aeroplane maintains a steady speed along its new flight path. So the rule is: to maintain a steady climb with no loss of speed, thrust must balance both the aerodynamic drag and the backward component of weight. Now, here's the catch. What if you don't have enough Excess Thrust? Look at Figure 3.5. The aircraft only has a small amount of Excess Thrust available, but the climb angle that's been set creates too large a backward component of weight. The result? That climb angle simply cannot be maintained. The aircraft can't hold it. So what do you do? You reduce the angle of climb. That gives you a smaller backward component of weight—one that actually matches the Excess Thrust you have available. That's Figure 3.6. And here's the general rule that ties it all together: the greater the Excess Thrust, the larger the backward component of weight you can balance. In other words, the more Excess Thrust you have, the steeper you can climb, or the greater the weight you can carry at the same climb angle. Now let's talk about what happens when weight changes, because weight has a big influence on climb performance. Look at Figure 3.7. Suppose the aircraft tries to use the same climb angle as before, but at a higher weight. Two things go wrong. First, the backward component of weight is now greater, so there's insufficient Excess Thrust to balance it. Second—and this is subtle—the higher weight generates increased aerodynamic drag, specifically induced drag. And that increased drag further reduces your Excess Thrust, because more of your thrust is now being used just to overcome drag. So increased weight decreases the maximum climb angle you can achieve. That's exactly what Figure 3.8 shows. Finally, let's define 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, that thrust available must equal the sum of two things: the aerodynamic drag, which we write as D, plus the backward component of weight, which we write as W sin γ. That γ is the climb angle, so W sin γ is the component of weight acting backwards along the flight path. So the equation for a steady climb is simply: Thrust Available equals D plus W sin γ.