General Principles - Climb — Page 182, Lesson 208

Let's pick up right where the graph leaves off. I want to walk you through what happens to drag when you lower the flaps or the gear, and then we'll get into the heart of thrust. First, the big picture. When you lower flaps or gear, you are increasing what we call the parasite area. That's the total frontal area of the aircraft that isn't producing lift — the bits that just push through the air. Because that area goes up, Parasite Drag will be greater at any given IAS, any given indicated airspeed. Now, here's the key relationship to hold onto: Parasite Drag is proportional to IAS squared. That means if you double your speed, parasite drag doesn't double — it quadruples. So at the high-speed end of the graph, the effect of lowering flaps or gear is proportionally much greater. That's why the parasite drag curve kicks up so sharply on the right side. Now, the other half of total drag is Induced Drag — the drag that comes from generating lift. When you lower flaps or gear, the induced drag curve also shifts. Here's the important consequence: the point where the Parasite Drag curve and the Induced Drag curve intersect moves further towards the low-speed end of the graph. And the sum of the two — the Total Drag — is greater at every speed. That intersection point is exactly where VMD sits. VMD is the speed for minimum drag. So when you lower flaps or gear, VMD becomes a lower IAS. Let me make sure that's clear: total drag goes up, and the speed at which that drag is at its minimum shifts down to a slower indicated airspeed. Now, there's a special term you need to know. When both the flaps and the gear are fully retracted, the aircraft is said to be in the Clean configuration, or simply Clean. That's your baseline — the lowest-drag state of the aeroplane. Let's move on to Thrust. This is a force, and I want to give you the precise definition. Thrust is the force required to balance aerodynamic Drag, plus the backward component of Weight when the aircraft is in a steady climb. So in a climb, thrust isn't just fighting drag — it's also holding the aircraft up against the component of weight that's pulling it back down the flight path. How does a turbojet actually produce this thrust? It generates Thrust by accelerating a mass of air rearwards. That's the fundamental principle. And here's a useful characteristic: the variation of Thrust Available with forward speed is relatively small. The engine output is nearly constant with changes in IAS. Now, let's look at the equation that ties this together: Thrust Available = Mass Flow × Acceleration (Exhaust velocity − Intake velocity) Let me unpack each symbol. Mass Flow is the mass of air passing through the engine per unit time. Exhaust velocity is the speed of the air leaving the back of the engine. Intake velocity is the speed of the air entering the front — which, at any given moment, is essentially your forward speed. The acceleration is the change in velocity, so it's exhaust velocity minus intake velocity. Here's the subtlety. As your forward speed increases, the intake velocity increases. That means the velocity change through the engine — the acceleration term — gets smaller. To keep Thrust Available constant, you'd need to increase either the mass flow or the exhaust velocity to compensate. Now, what actually happens in practice? At low forward speed, if you increase speed, the velocity change through the engine reduces without a corresponding increase in mass flow. So Thrust Available will decrease slightly. The engine just can't keep up. But at higher speed, something different happens. The ram effect kicks in. The forward speed of the aircraft rams more air into the engine intake, which helps to increase mass flow as forward speed increases. Now Thrust Available no longer decreases — it actually increases slightly with speed. So you have two regimes: at low speed, thrust falls off a little as you accelerate; at high speed, the ram effect takes over and thrust starts to creep back up. That's the behaviour you'll see on the thrust-available curve. Let me just pull it together. Lowering flaps or gear increases parasite area, which increases parasite drag — and because parasite drag scales with IAS squared, the effect is biggest at high speed. Total drag goes up, and VMD drops to a lower IAS. And thrust, in a steady climb, must balance drag plus the backward component of weight. A turbojet makes that thrust by accelerating air rearwards, and the thrust available stays nearly constant with speed — dipping slightly at low speed, then rising slightly at high speed thanks to the ram effect.