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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.
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.
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.
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.
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.
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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