Numerical and experimental studies on the over-the-wing-engine configurations aerodynamics

DOI: 10.34759/vst-2021-2-37-51

Аuthors

Bolsunovskii A. L.^*, Buzoverya N. P., Bragin N. N., Gerasimov S. V., Pushchin N. A., Chernyshev I. L.^**

Central Aerohydrodynamic Institute named after N.E. Zhukovsky (TsAGI), 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia

*e-mail: bolsun@progtech.ru
**e-mail: ivan.chernyshev@tsagi.ru

Abstract

Environmental requirements, such as limits on community noise and emissions, will play an increasingly important role in the future of civil aviation. The possibilities of noise reduction in state-of-the-art layouts are limited, thus, it may be necessary to switch to radically new schemes to meet the goals declared by NASA, ACARE, the Ministry of Industry and Trade of Russia and other organizations for the next generation of aircraft.
Engine noise is one of the main factors in the overall aircraft noise. Although the current trend to increase the bypass ratio turbojet leads itself to the noise reduction, the possibility of placing large engines under the wing is limited. The upper position of the engines may help to eliminate this problem and additionally reduce the noise on the ground due to the shielding effect. Besides, the engines diameter increasing does not lead to the chassis struts elongation, i.e. there is a possibility of installing engines with ultra-high bypass ratio. Air intakes are better protected from foreign objects, especially on runways of poor quality. There is no gap in the slat spanwise, as in the layouts with engines under the wing. The jets of the engines do not fall on the flaps. The disadvantages include a significant risk of adverse aerodynamic interference, especially at transonic speeds, and increase in the cabin noise, which may require installation of additional sound-absorbing structures. Moreover, the thrust of the engines creates an undesirable negative dive moment at takeoff and in cruising flight. Many questions arise concerning rational design of the pylon-wing-nacelle assembly and its aero-elastic characteristics. Finally, the engine maintenance becomes noticeably complicated.
Intensive research on «quiet» layouts has been initiated in the US and Europe to meet the stringent environmental requirements of NASA and ACARE for the decades to come. TsAGI also conducts systematic research in this direction, trying to make allowances for the development of necessary technologies in various disciplines, especially in aerodynamics and power plants, since aerodynamics is the main bottleneck hindering introduction of the top-mounted engine layouts. This problem solution with a positive result is possible only with a powerful set of aerodynamic design tools. The set should include a detailed direct analysis method that accounts for all geometric features, an optimization procedure, and a reverse method, allowing create the aircraft surface element according to a given pressure distribution. The authors use in their practice the original version of the residual correction method, in which the upper level is represented by the RANS method, and the inverse method based on the full potential method is used as a corrector.
The article discusses the aerodynamic design features of various aircraft layouts with the engines location above the wing. In general case, their aerodynamics are more complex due to the possibility of adverse aerodynamic interference manifestation caused by the increased speeds over the wing. Thus, it is necessary to search for such configurations in which this risk is minimal, or even there is a chance of positive interference. Several aerodynamic models were designed, manufactured, and tested in TsAGI’s large transonic tubes. These included:
— the regional aircraft layout with natural flow-around laminarization of the wing of a small sweep (χ¼ = 15°)  with the cruising Mach number of M = 0.78. Aerodynamic tests in the T-128 WT (Wing Tunnel) demonstrated satisfactory transonic aerodynamic characteristics, including the possibility of obtaining extended laminar sections on the wing consoles, as well as excellent load-bearing characteristics at low speeds;
— the layout of business aircraft with a drop shape of the fuselage called a «tadpole», with a maximum cruise Mach number of M = 0.82 and a small wing sweep (χ¼ = 6°), with a normal distribution of the relative thickness (`с = 14–10% at the root and at the end respectively). Tests in the T-128 WT fully confirmed the speed properties of the layout;
— the layout of the «flying wing» with the engine nacelles located above the wing center section, designed with account for the unfavorable aerodynamic interference of the wing-pylon-nacelle assembly.

Keywords:

aerodynamic design, over-the-wing-engine configuration, experimental studies

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