General Principles - Climb — Page 166, Lesson 198

Let's start the climb section of performance. I want to walk you through the very foundation of it, because everything we do in climb analysis builds on this. The climb phase of flight is defined as that stage from the end of the take-off phase to the beginning of the en route phase. So it's the segment between when you've finished your take-off and when you settle into cruise. And there are two distinct ways we have to examine a climb: the angle of climb and the rate of climb. Those are two different things, and we'll treat them separately. Let's begin with angle of climb. Look at Figure 3.1, which shows an aircraft in unaccelerated level flight. Unaccelerated means the speed is constant — no speeding up, no slowing down. In that condition, the forward acting force, Thrust, exactly balances the rearward acting force, Drag. Because they're equal, the aircraft maintains a steady speed. Now, a key question: how much Thrust is required for unaccelerated level flight? The answer is exactly the same as the aerodynamic Drag. And here's a term you'll hear constantly in performance work: a frequently used alternative name for Drag is "Thrust Required." So when you see "Thrust Required," think of it as the drag force that must be overcome to maintain that steady condition. Now, the crucial change. If we place the aircraft in a climb attitude, as in Figure 3.2, something new happens. A component of the aeroplane's Weight now acts backwards along the flight path. That backward component of Weight is added to Drag. So now we have two rearward acting forces: the Drag, plus this backward component of Weight. And here's the relationship you must remember: the larger the angle of climb, the larger the backward component of Weight. That's shown in Figure 3.3 — as you pitch up more steeply, more of the Weight vector points backwards along your flight path. Now, in both of those illustrations, it's apparent that the sum of the two rearward acting forces — Drag plus the backward component of Weight — is greater than the forward acting force, Thrust. If that situation were left unchanged, the aircraft would decelerate. So the whole point of climb performance is understanding that to actually climb, you need to deal with this extra rearward force that wasn't there in level flight. That's the core idea of angle of climb. We'll build on this to see how thrust and excess thrust come into play.