• Aerospace Engineering and Technology
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• v.7 no.1
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• pp.24-29
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• 2008

Aircraft fuel efficiency is one of main concerns to aircraft manufacturers and to aviation companies because jet fuel price has tripled in last ten years. One of simple and effective methods to increase fuel efficiency is to reduce aircraft induced drag by using of wingtip devices. Induced drag is closely related to the circulation distribution, which produces strong wingtip vortex behind the tip of a finite wing. Wingtip devices including winglets can be successfully applied to reduce induced drag by wingtip vortex mitigation. Winglet design, however, is very complicated process and has to consider many parameters including installation position, height, taper ratio, sweepback, airfoil, toe-out angle and cant angle of winglets. In current research, different shapes of winglets are compared in the view of vortex mitigation. Appropriately designed winglets are proved to mitigate wingtip vortex and to increase lift to drag ratio. Also, the results show that winglets are more efficient than wingtip extension. That is the reason B-747-400 and B-737-800 chose winglets instead of a span increase to increase payload and range. Drag polar comparison chart is presented to show that minimum drag is increased by viscous drag of winglet, but at high lift, total drag is reduced by induced drag decrease. So, winglets are more efficient for aircraft that cruises at a high lift condition, which generates very strong wingtip vortex.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.6
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• pp.422-428
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• 2019

A tailless or Blended-Wing-Body(BWB) shaped configuration is highlighted for UCAV with low RCS characteristics. The BWB configuration is characterized by its directional static instability and low controllability. To control the directional movement of the BWB configured vehicle, directional thrust vectoring equipment or drag rudder typed control surfaces which utilize the drag differences of the wing can be considered. This paper deals with a BWB shaped configuration using a spoiler and describes the lateral-directional aerodynamic characteristics of the vehicle. In addition, it is shwon that the lateral-directional motion can be controlled effectively by using the classical PI control structure. This control law is verified by flight test and showed adequate for the tailless BWB shaped UAV.

• International Journal of Aeronautical and Space Sciences
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• v.18 no.2
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• pp.215-221
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• 2017

An electromagnetic (EM) wave absorber reduces the possibility of radar detection by minimizing the radar cross section (RCS) of structures. In this study, a radar absorbing structure (RAS) was applied to the leading edge of a blended wing body aircraft to reduce RCS in X-band (8.2~12.4GHz) radar. The RAS was composed of a periodic pattern resistive sheet with conductive lossy material and glass-fiber/epoxy composite as a spacer. The applied RAS is a multifunctional composite structure which has both electromagnetic (EM) wave absorbing ability and load-bearing ability. A two dimensional unit absorber was designed first in a flat-plate shape, and then the fabricated leading edge structure incorporating the above RAS was investigated, using simulated and free-space measured reflection loss data from the flat-plate absorber. The leading edge was implemented on the aircraft, and its RCS was measured with respect to various azimuth angles in both polarizations (VV and HH). The RCS reduction effect of the RAS was evaluated in comparison with a leading edge of carbon fabric reinforced plastics (CFRP). The designed leading edge structure was examined through static structural analysis for various aircraft load cases to check structural integrity in terms of margin of safety. The mechanical and structural characteristics of CFRP, RAS and CFRP with RAM structures were also discussed in terms of their weight.

• International Journal of Naval Architecture and Ocean Engineering
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• v.10 no.4
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• pp.439-449
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• 2018

In this paper, multi-objective optimization of a multi-bubble pressure cabin in the underwater glider with Blended-Wing-Body (BWB) is carried out using Kriging and the Non-dominated Sorting Genetic Algorithm (NSGA-II). Two objective functions are considered: buoyancy-weight ratio and internal volume. Multi-bubble pressure cabin has a strong compressive capacity, and makes full use of the fuselage space. Parametric modeling of the multi-bubble pressure cabin structure is automatic generated using UG secondary development. Finite Element Analysis (FEA) is employed to study the structural performance using the commercial software ANSYS. The weight of the primary structure is determined from the volume of the Finite Element Structure (FES). The stress limit is taken into account as the constraint condition. Finally, Technique for Ordering Preferences by Similarity to Ideal Solution (TOPSIS) method is used to find some trade-off optimum design points from all non-dominated optimum design points represented by the Pareto fronts. The best solution is compared with the initial design results to prove the efficiency and applicability of this optimization method.

• Journal of The Korean Astronomical Society
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• v.36 no.2
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• pp.55-60
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• 2003

In Lee, Kang & Byun (2001) the discovery of Raman scattered 6545 A feature was reported in symbiotic stars and the planetary nebula M2-9. The broad emission feature around 6545 A is formed as a result of Raman scattering of He II n = 6 $\to$ n = 2 photons by atomic hydrogen. In this paper, we introduce a method to compute the equivalent width of He II $\lambda$ 1025 line and present an optical spectrum of the symbiotic star RR Telescopii as an example for a detailed illustration. In this spectrum, we pay attention to the broad H$\alpha$ wings and the Raman scattered He II 6545 feature. The broad Ha wings are also proposed to be formed through Raman scattering of continuum around Ly$\beta$ by Lee (2000), and therefore we propose that the equivalent width of the He II $\lambda$ 1025 emission line is obtained by a simple comparison of the strengths of the 6545 feature and the broad H$\alpha$ wings. We prepare a template H$\alpha$ wing profile from continuum radiation around Ly$\beta$ with the neutral scattering region that is supposed to be responsible for the formation of Raman scattered He II 6545 feature. Isolation of the 6545 feature that is blended with [N II] $\lambda$ 6548 is made by using the fact that [N II] $\lambda$ 6584 is always 3 times stronger than [N II] $\lambda$ 6548. We also fit the 6545 feature by a Gaussian which has a width 6.4 times that of the He II $\lambda$ 6527 line. A direct comparison of these two features for RR Tel yields the equivalent width $EW_{Hel025} = 2.3{\AA}$ of He II $\lambda$ 1025 line. Even though this far UV emission line is not directly observable due to heavy interstellar extinction, nearby He II lines such as He II $\lambda$ 1085 line may be observed using far UV space instruments, which will verify this calculation and hence the origins of various features occurring in spectra around H$\alpha$.