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7.2. Aircraft Characteristics
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.
7.2.1. Aircraft Design: Background
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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