It won’t recover by itself — Page 131, Lesson 157

Let’s pick up right where we left off — we were just talking about the speed unstable region, and I want to make sure you understand exactly what that means for you as a pilot. When an aircraft is speed unstable, it means that if a disturbance — say a gust of wind — reduces the aircraft’s speed, the aircraft will naturally continue to slow down on its own. It won’t recover by itself. Conversely, if a disturbance increases the speed, the aircraft will continue to accelerate until the thrust and drag are once more balanced. So the aircraft has no natural tendency to return to its original speed — it just keeps going in the direction of the disturbance. This is why, during the approach phase of flight, you must closely monitor your Indicated Airspeed, or IAS. Any tendency for the aircraft to slow down must be countered immediately by a ‘generous’ application of thrust — that’s the word the book uses, generous — to quickly return to the desired trim speed. You don’t wait; you act right away. Now, let’s look at Figure 6.19. If an aircraft maintains a constant IAS in the speed unstable region, and you add parasite drag — for example, by selecting the undercarriage down or by deploying flaps — this has the benefit of reducing VMD. VMD is the minimum drag speed, the speed at which total drag is at its minimum. By reducing VMD, you move the speed stable region to the left, which improves speed stability. So adding drag can actually help you stabilize the aircraft in this region. At speeds very close to VMD, an aircraft usually exhibits no tendency towards either speed stability or speed instability — this is called the neutral IAS region. So there’s a band of speeds right around VMD where the aircraft is neither stable nor unstable; it just sits there neutrally. Now, let’s shift gears and talk about Power Required. This is a new concept, so let’s build it up carefully. We’ve already established that Drag can be referred to as ‘Thrust Required.’ Now we’re going to see that a similar relationship exists with ‘Power Required.’ Both are important airframe considerations. First, let’s define our terms. Thrust is a FORCE — a push or a pull — and it’s used to oppose Drag. But Power is different. Power is the RATE of doing WORK. So we write: POWER = WORK ÷ TIME And WORK = FORCE × DISTANCE So if we substitute that in, POWER = (FORCE × DISTANCE) ÷ TIME Now, for Power Required, which force do we use? Drag. And distance divided by time is speed. Which speed? The only speed there is — the speed of the aircraft through the air, which is True Airspeed, or TAS. So therefore: POWER REQUIRED = DRAG × TAS Let me make sure that lands. Power required is the product of drag and true airspeed. Not indicated airspeed — true airspeed, because that’s the actual speed of the aircraft through the air. Here’s an important consequence. If an aircraft climbs at a constant IAS, drag will remain constant — because drag depends on indicated airspeed, which isn’t changing. But TAS must be increased as you climb, because the air gets thinner. So since power required equals drag times TAS, and TAS is going up while drag stays the same, power required will increase. That’s a key relationship to remember. Why do we need to consider power required in Principles of Flight at all? Because work must be done on the aircraft to “raise” it to a higher altitude when climbing. Logically, maximum work can be done on the aircraft in the minimum time when the power available from the engine(s) is greatest and the power required by the airframe is least. So the climb performance is all about the balance between power available and power required. For easy reference, associate the word POWER with the word RATE. For example, minimum rate of descent is achieved in a steady glide when the aircraft is flown at the minimum power required speed, which is called VMP. So VMP is the speed for minimum power required, and that gives you the minimum rate of descent. These considerations will be examined more fully during the study of Aircraft Performance in Book 6 and Flight Mechanics in Chapter 12 of this book. But for now, the core idea is: power required equals drag times true airspeed, and it increases during a climb at constant IAS because TAS increases.