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The Power of the Power CurveLearn why the power required by the aircraft varies with airspeed and load factor
By Steve Pomroy
When learning basic (and advanced!) aircraft handling, it helps to know and understand how much of a power reserve you have, and what that means to you as a pilot. It's also useful to understand how the power required by the aircraft varies with airspeed and load factor. Building this understanding is where the power curve comes in. Learning the nuances of the power curve helps us develop an understanding of aircraft behavior and control response while we're still on the ground. This ultimately makes our in-flight training much more efficient and effective.
The power curve is built on the assumption that the aircraft is in equilibrium. So it doesn't tell us a whole lot about the dynamic response of the aircraft. But since we spend such a large portion of our flight time in or near equilibrium, it is still quite useful.
Ok, So What Is It?
The power curve is actually two curves plotted on the same axis: the Power-Required curve, and the Power-Available curve. Both represent power (required or available) as functions of airspeed.
Power required is defined as the power we need to be providing (from the engine) in order for the aircraft to maintain a constant airspeed and constant altitude. The power required is a function of the drag being produced by the airframe and our true airspeed.
Power available is the maximum power that we can produce with the engine. Although engines are normally rated for a fixed maximum brake-horsepower, the power available curve doesn't show us constant power available—there is significant variation with airspeed. This is because the curve accounts for the efficiency of our propeller—which changes with airspeed—and as such represents thrust horsepower, not brake horsepower.
The difference between our power-available and power-required is our excess power, power margin, or power reserve. It's possible for this value to be negative at high speeds and/or altitudes, but it is normally positive—indicating that we have access to more power than we need to maintain a constant airspeed and altitude. Our power reserve is important, since it's an indication of how well we can accelerate and/or climb.
The shape and placement of both power-available and power-required curves can change according to conditions. For a given aircraft/engine/prop design, our power-available curve is determined primarily by our density altitude. Our power-required curve changes with altitude, but is also influenced by weight, CG, load factor, and configuration. For the sake of brevity, I won't get into the details of the 'how' and 'why' of these variables, but you should take some time to read up on them or discuss them with yor instructor.
Interpreting the Power Curve
Where we are on the power curve is determined by two variables: our airspeed and our power setting. Our power setting will always be at or below the power-available, and is controlled by our throttle and prop settings (or throttle alone if we're flying an aircraft with a fixed-pitch prop).
During steady cruise flight, we will always be on the power-required curve. The speed at which this occurs will depend on our power setting.
During a steady climb, we will always be above the power-required curve. Our rate of climb will be determined by how much excess power we are producing.
During a steady descent, we will always be below the power-required curve. Our rate of descent will be determined by how much of a power deficiency we have.
It's also possible to be above or below the power-required curve and to be holding a constant altitude. When this happens, we will be accelerating (with excess power) or decelerating (with a power deficiency). The rate of acceleration/deceleration is determined by our power reserve/deficiency.
Cruise Speed Range
In the normal cruise speed range, an increase in speed results in an increase in power required. So if we speed up without adding power, we end up with a power deficiency. The power deficiency will create a tendency to return to the original airspeed or to descend. The opposite happens when we slow down—we end up with an excess of power, resulting in a tendency to accelerate back to our original airspeed or to climb. This speed stability—the tendency to return to a particular reference airspeed after a disturbance—is important, as it makes our lives much easier.
When operating at low speeds, an understanding of the power curve is especially important, since the power-required trend reverses and we see an increasing need for power at progressively lower airspeeds. If we slow down, we get a power deficiency—resulting in further deceleration or a decent. If we speed up, we get an excess of power—resulting in further acceleration or a climb. This is the exact opposite of the aircraft behavior we see in cruise flight. The problem is exacerbated by the fact that we are operating close to the stall. The speed instability that results from the power curve trend can actally lead us into a stall if we aren't careful.
Clearly, this new behavior will influence our control methodology during low-speed operations, such as during approach. But it's also worth noting that the changed aircraft handling can serve as a warning of an approaching stall.
And So Much More ...
Even if you're not interested in number crunching (and really, the engineers have already done that for us), the power curve is a powerful conceptual tool. It can help us understand aircraft performance and how it might change with conditions. Information that can be derived from the power curve includes: range, endurance, cruise speeds (including maximum cruise), climb rates and angles, glide rates and angles, etc. Understanding how and why some of these numbers might change can help us anticipate aircraft performance under varying flight conditions.
So spend some time with your preferred textbook or favorite flight instrucor and learn the ins and outs of the power curve. The time spent can pay dividends in the quality, efficiency, and safety of your flying.