Commercial aviation has seen many technology breakthroughs over the past 40 years. Over that period, propeller-driven aircraft were replaced by jet-powered aircraft of the early 1960s, then by turbofan-powered aircraft of the 1970s to 1990s. As more powerful and fuel efficient powerplants were developed, matching baseline airframe improvements in aerodynamics and net weight reductions were also achieved. The driving forces for these improvements were, and continue to be, demand for increased range, better fuel efficiency, greater capacity, and increased speed-all of which have positive impacts on aircraft markets and economics. In many cases, these same characteristics have direct and beneficial influences on the impact of aircraft on the environment.
Design of a subsonic transport aircraft begins by establishing its range requirements and the number of passengers it needs to carry. Economic and technical parameters have to be considered with projected market conditions to arrive at design goals. Having established these goals, the aerodynamic design can begin. One of the most important elements is the wing. Wing shape determines that lift is produced in the most efficient and stable manner for each flight mode. During take-off and landing, flaps on the leading and trailing edges of the wing are deployed to generate the extra lift required at the slower speed. As the airflow airspeed increases during the climb, these devices are retracted, and the wing assumes the optimum shape for higher cruise speeds. Forward flight generates "drag," which is manifest in several forms. One is the drag produced by the lift (induced drag). This induced drag varies directly with lift produced. Another is the resistance of the air as it flows over the outer surfaces of the aircraft (termed zero lift drag), which is independent of lift. Sub-components of zero lift drag include skin-friction drag, form drag, roughness or excrescence drag, and interference drag caused by interaction effects of various parts of the aircraft. These drag components, of course, are balanced by the thrust of the engines.
Lift and drag components also create other forces that are controlled by vertical and horizontal tail surfaces, thus enabling the aircraft to be flown accurately. These control surfaces also provide the means to trim the aircraft in level flight, minimizing control inputs during steady parts of the flight profile. In addition to these controls, sections of the trailing edge of the main wing are hinged to form movable control surfaces to control the lateral roll of the aircraft about its longitudinal axis.
All such surfaces and associated maneuvering add, in small measure, to the overall energy required to propel the aircraft forward. A detailed knowledge of the aerodynamic processes involved is therefore called for if this energy is to be minimized.
A well-established equation used in the design process is the Breguet Range Equation (Corning, 1977). The equation provides a basis for comparisons of competing designs by taking into account all of the principal variables-take-off and landing weights, thrust/fuel flow, aerodynamics and speed, as well as mission requirements and passenger load-and providing a figure of merit for the efficiency for each candidate design.
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