AFH 4-3

Managing Energy is a Balancing Act Since the airplane gains energy from engine thrust (T) and loses energy through aerodynamic drag (D), energy flows continuously into and out of the airplane while in flight. Usually measured as Specific Excess Power (PS), or rate of energy change, the net energy flow is a direct function of the difference between thrust and drag. PS = (T – D)V/W Where, T = Thrust D = Drag V = velocity (airspeed) W = aircraft weight More importantly, there is a fundamental relationship between changes in the airplane’s total energy resulting from this net energy flow on one hand, and changes in the energy stored as altitude and airspeed on the other. This fundamental relationship can be summarized through the airplane’s energy balance equation. [Figure 4-2] Figure 4-2. The energy balance equation. The left side of the energy balance equation represents the airplane’s net energy flow, while the right side reflects matching changes to the energy storage. Thus, changes to the airplane’s total energy affect the left side of the equation, while the right side shows possible changes in energy distribution between altitude and airspeed. Note that a change in total energy resulting from the difference between thrust and drag (left side) always matches the change in total energy redistributed over altitude and airspeed (right side). Although rate of energy change, expressed as specific excess power (PS), varies during flight—becoming positive, negative, or zero—both sides of the equation are inexorably balanced regardless of whether the airplane is accelerating, decelerating, climbing, descending, or maintaining constant altitude and airspeed. (Note: This simplified balance equation does not account for long-term changes in total mechanical energy caused by the reduction in aircraft weight as fuel is gradually burned in flight. Although the effect of weight loss on total energy becomes critical when solving long-term aircraft performance problems such as range and endurance, it is negligible when considering short-term flight control problems.) Of course, the pilot controls the change in total energy on the left side of the equation, as well as the distribution of any changes in energy over altitude and airspeed on the right side. How the pilot coordinates the throttle and elevator to achieve and maintain desired altitude and airspeed targets as well as avoid energy "crises" is at the core of energy management and is elaborated in the rest of the chapter. Role of the Controls to Manage Energy State An energy-centered approach clarifies the roles of the engine and flight controls beyond the simple “pitch for airspeed and power for altitude” by modeling how throttle and elevator inputs affect the airplane’s total mechanical energy. From an energy perspective, the problem of controlling vertical flight path and airspeed becomes one of handling the airplane’s energy state—the total amount of energy and its distribution over altitude and speed. Thus, rather than asking what controls altitude and what controls airspeed, a pilot can now ask what controls total energy and what controls its distribution over altitude and airspeed.