General Principles - Climb — Page 182, Lesson 208

Let’s start with the drag picture, because that’s the foundation for everything else in the climb. I want you to picture the aircraft in what we call the Clean configuration — that means both the flaps and the undercarriage, the gear, are fully retracted. When the aircraft is Clean, it has the lowest total drag for any given speed, and that’s our reference point. Now, the moment we lower either the flaps or the gear, we change the drag balance. Look at Figure 3.19 with me. The key idea is that Parasite Drag increases at a given Indicated Airspeed, or IAS, because the parasite area — the frontal area of the aircraft that creates form and skin-friction drag — has increased. Flaps and gear stick out into the airflow, so they add parasite area. Here’s the subtle part, and I want you to really get this. The increase in Parasite Drag is proportionally greater at the high-speed end of the graph. Why? Because Parasite Drag is proportional to IAS squared. So if you double the speed, parasite drag quadruples. At high speed, that extra parasite area from the flaps or gear produces a much bigger drag penalty than it does at low speed. Now look at where the Parasite Drag curve and the Induced Drag curve intersect. Induced Drag is the drag created by generating lift — it’s highest at low speed and falls off as speed increases. When we lower flaps or gear, that intersection point moves further towards the low-speed end of the graph. And the sum of Parasite Drag plus Induced Drag — the Total Drag — is greater at every speed. Here’s the consequence that matters for your flying: VMD — that’s the speed for Minimum Drag — becomes a lower IAS when either flaps or gear are lowered. Think about why. The parasite drag curve has shifted up, so the point where it crosses the induced drag curve happens at a lower speed. Total drag is higher, but the speed at which that total drag is minimised has dropped. That’s a classic exam point, so hold onto it: flaps or gear down means more total drag and a lower VMD. Now let’s move to Thrust. This is the force required to balance aerodynamic Drag, plus the backward component of Weight when the aircraft is in a steady climb. That second part is crucial — in a climb, part of your weight is acting backwards along the flight path, so thrust has to overcome both drag and that weight component. How does a turbojet generate thrust? By accelerating a mass of air rearwards. That’s the fundamental principle. And here’s the important characteristic: the variation of Thrust Available with forward speed is relatively small. The engine output is nearly constant with changes in IAS. That’s very different from a propeller aircraft, but for a turbojet, thrust stays roughly flat across the speed range. Let me give you the equation, because it ties it all together: Thrust Available = Mass Flow × Acceleration (Exhaust velocity − Intake velocity) So thrust is the mass of air flowing through the engine each second, multiplied by the change in velocity of that air — the exhaust velocity minus the intake velocity. That velocity change is the acceleration the engine imparts to the air. Now here’s where it gets interesting, and I want you to follow the logic carefully. As forward speed increases, the intake velocity increases — the air is already moving fast when it enters the engine. So to keep Thrust Available constant, you need either an increase in mass flow or an increase in exhaust velocity. At low forward speed, here’s what happens: any increase in speed reduces the velocity change through the engine — because intake velocity is going up — without a corresponding increase in mass flow. So the velocity change shrinks, mass flow stays roughly the same, and Thrust Available actually decreases slightly. That’s the low-speed behaviour. But at higher speed, a different effect takes over: the ram effect. The forward speed of the aircraft rams air into the engine, which helps to increase mass flow as speed increases. Now mass flow is rising, so Thrust Available no longer decreases — it actually increases slightly with speed. So the full picture for a turbojet: thrust is nearly constant overall, but it dips slightly at low speed and rises slightly at high speed, all because of how mass flow and the velocity change through the engine respond to forward speed. Let me pull it together for you. Lowering flaps or gear increases parasite drag, raises total drag, and drops VMD to a lower IAS. And thrust — the force that must balance drag plus the backward weight component in a steady climb — comes from accelerating air rearwards, and stays nearly constant with speed, with that slight low-speed dip and high-speed rise from the ram effect. That’s the drag and thrust foundation we’ll build the climb on next.