The Effect of Reducing Cruise Altitude on the Topology and Emissions of a Commercial Transport Aircraft

In recent years, research has been conducted for alternative commercial transonic aircraft design configurations, such as the strut- braced wing and the truss-braced wing aircraft designs, in order to improve aircraft performance and reduce the impact of aircraft emissions as compared to a typical cantilever wing design. Research performed by Virginia Tech in conjunction with NASA Langley Research Center shows that these alternative configurations result in 20% or more reduction in fuel consumption, and thus emissions. Another option to reduce the impact of emissions on the environment is to reduce the aircraft cruise altitude, where less nitrous oxides are released into the atmosphere and contrail formation is less likely. The following study was performed using multidisciplinary design optimization (MDO) in ModelCenterTM for cantilever wing, strut-braced wing, and truss-braced wing designs and optimized for minimum takeoff gross weight at 7730 NM range and minimum fuel weight for 7730 and 4000 NM range at the following cruise altitudes: 25,000; 30,000; and 35,000 ft. For the longer range, both objective functions exhibit a large penalty in fuel weight and takeoff gross weight due to the increased drag from the fixed fuselage when reducing cruise altitude. For the shorter range, there was a slight increase in takeoff gross weight even though there was a large increase in fuel weight for decreased cruise altitudes. Thus, the benefits of reducing cruise altitude were offset by increased fuel weight. Either a two-jury truss-braced wing or telescopic strut could be studied to reduce the fuel penalty.