Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
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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