Environmental Engineering Reference
In-Depth Information
Local regulations
Local regulations are becoming more commonplace. Most major airports have noise
limits, which, if contravened, can result in fines levied against offending aircraft
operators. Some airports, such as Washington, simply do not allow aircraft above a
certain noise threshold to land. More recently, Zurich Airport has started to apply
NO
x
limits. While these regulations limit noise emissions near the airport, there are
some concerns over their influence on the goal of reducing the total impact of avia-
tion over all flight stages. Increasingly stringent local regulations may have the effect
of driving airframe and engine design away from the optimum configuration for
fuel efficiency, as airlines request aircraft that do not incur local fines for either noise
or emissions.
C
URRENT
DESIGN
LIMITS
The dominant engine/airframe configuration, a classical swept wing with podded
underwing engines, first emerged in the form of the Boeing B-47 Stratojet bomber
in 1947. Since then, advances in materials, structures and aerodynamics have improved
and refined the design to what we commonly see today. However, some limits to future
developments are now being approached.
The three main environmental issues that result from engines are fuel burn
(and, hence, CO
2
and water vapour emissions), other gaseous engine emissions (most
notably NO
x
) and noise. These issues need to be considered alongside optimum
and/or desired range, cost and aircraft payload.
Within the environmental issues alone, trade-offs between fuel burn and NO
x
exist. In simplistic terms, increasing the combustion temperature and operating pres-
sure leads to conditions that give increased NO
x
formation. The converse of this,
combustors designed to reduce NO
x
through efficient mixing of fuel and air prior to
combustion, can impede increases in overall pressure ratio (OPR) and turbine entry
temperature (TET) aimed at increasing fuel efficiency. Noise and fuel efficiency also
conflict; reducing noise through the development of extremely large by-pass ratios
increases fuel burn through increased weight and drag, and reducing noise by noise
shields increases OEW and, thus, fuel burn. An additional issue to consider is the
effect of engine exhaust temperatures on contrail formation where there are indica-
tions that modern, efficient aircraft may be more likely to form contrails (Schumann,
2000).
Other issues need to be resolved. For example, the optimum configuration for a
long-range aircraft would be for minimum fuel consumption and weight to achieve
range targets that can exceed 8000nm. Good climb performance, an advantage in
crowded airspace, can be achieved but at the expense of cruise fuel consumption or
high maintenance costs. Some aerodynamic losses are inevitable, and the potential
for improving lift to drag ratio (L/D) with fully turbulent boundary layers for the
current aircraft configuration is thought to be limited.
