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7.3. Airframe Performance and Technology
Historically, efforts to improvement aerodynamic efficiency have been aimed
mainly at two phases of flight: Take-off/climb and cruise. To this end, significant
improvements in lift and drag performance have been achieved (Lynch et al.,
1996). A comprehensive range of detailed aerodynamic studies, examining all
aspects of the complex flows around the airframe, has been a major part of these
efforts. Such work, involving the development and use of high fidelity computational
fluid dynamics (CFD) prediction codes (Rubbert, 1994) coupled with improved
wind tunnel testing techniques (Lynch and Crites, 1996), has led to much better
understanding of the aerodynamic characteristics of new and proposed designs.
In turn, this work has led naturally to improved predictions of the effectiveness
of measures aimed at improving the performance of aircraft in general and reducing
fuel burn rates for future aircraft in particular.
7.3.1. Aerodynamic Improvement
Historically, efforts to improvement aerodynamic efficiency have been aimed
mainly at two phases of flight: Take-off/climb and cruise. To this end, significant
improvements in lift and drag performance have been achieved (Lynch et al.,
1996). A comprehensive range of detailed aerodynamic studies, examining all
aspects of the complex flows around the airframe, has been a major part of these
efforts. Such work, involving the development and use of high fidelity computational
fluid dynamics (CFD) prediction codes (Rubbert, 1994) coupled with improved
wind tunnel testing techniques (Lynch and Crites, 1996), has led to much better
understanding of the aerodynamic characteristics of new and proposed designs.
In turn, this work has led naturally to improved predictions of the effectiveness
of measures aimed at improving the performance of aircraft in general and reducing
fuel burn rates for future aircraft in particular.
7.3.2. Airframe Weight Reduction
The increasing availability of advanced lighter and stronger materials for
use in structural components of the airframe has also been a major factor in
the achievement of reduced fuel burn. Of particular note are the greater use
of new aluminum alloys, titanium components, and composite materials for secondary
(non-load-bearing) structures.
One of the important enabling technologies that has had a major impact on these
developments is high-fidelity finite element models (FEMs). FEMs are now extensively
used for strength analyses and to obtain better understanding of safety load-factor
margins. This work has already contributed to additional reductions in structural
weight.
7.3.3. Nacelle Efficiency
As engine bypass ratios (fan bypass airflow divided by engine core flow) have
risen over the past 2 decades, so too have the drag and weight of the nacelle
(aerodynamic casing surrounding the engine). Furthermore, integration of the
engine and the nacelle-which incorporates the air inlet, the engine, and the
exhaust nozzle-can be a source of significant interference drag problems. On
balance, however, high bypass ratio engines have provided a significant gain
for transport aircraft in terms of reduced fuel requirements for a given mission.
This development has led to greater performance flexibility for operators wishing
to optimize range and payload, hence take-off weights, compared with earlier
low bypass ratio engines. Improvements in the aerodynamics of engine-nacelle
flows and changes to the shape and length of the inlet section continue to reduce
local drag effects and increase efficiency. The current trend is toward higher
bypass ratio ducted fan engines having shorter and thinner lip inlets. This
approach may be limited in the future, however, by the need to meet more stringent
noise regulations. The development of lighter nacelle materials/ structures
has reduced operating empty weight (OEW). Increasing thrust reverser efficiency
for enhanced landing performance can also reduce nacelle package weight.
7.3.4. Propulsion/Airframe Integration (PAI)
Reduction of interference drag caused by flow interactions in the region of
the wing-pylon-nacelle during take-off/climb/ cruise conditions is a complex
design problem (Berry, 1994). Recent improvements in modeling localized airflow,
using CFD, have brought important benefits in terms of reduced interference
drag (Lynch and Intemann, 1994). There is an inevitable tradeoff between the
higher drag of high bypass ratio engines and the need to minimize interference
drag for a given mission fuel burn; a great deal of effort is aimed at achieving
an optimum balance. For example, if the nacelle can be located closer to the
wing without creating interference penalties, it is possible to reduce pylon
weight and drag and reduce landing gear height (and weight). Other tradeoffs,
such as noise impacts, also need to be considered.
7.3.5. Control Systems
Older technology aircraft use mechanical, hydraulic, and electrical systems
to control flight, propulsion, and environmental systems. Today's modern airframes
and airframes under design utilize much lighter fly-by-light (using fiber optics)
and fly-by-wire technology, with significant savings in OEW.
Changes to aircraft pressurization and air conditioning systems-particularly
increases in the amount of air, which is now recirculated-has reduced engine
bleed flow requirements. These measures have significantly reduced engine fuel
burn at cruise conditions. Cabin air quality requirements, however, might limit
these methods of achieving further fuel savings.
More detailed analysis of PAI/high-lift system interference is regarded as
a way to achieve weight reductions in low-speed/take-off drag. Again, CFD techniques
are invaluable tools in achieving such improvements. The design of a high-lift
system that can provide the same lift versus drag performance at a lower weight
is seen as another path towards overall aircraft system improvements that would
result in fuel savings.
Increasing use of databus (multiplexing of signals) technology has led to significant
reduction in the amount of wiring needed to support the numerous advanced electrical
systems in modern aircraft. Although increased wire shielding has become necessary,
the overall result has led to further reductions in airframe OEW.
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