Young-Min Kwon, Sung-Boo Hong, Jae-Sang Park and Yu-Been Lee
The purpose of this study is to use the individual blade pitch control (IBC), reduce actively both the rotor hub vibratory loads and airframe vibration responses for the…
Abstract
Purpose
The purpose of this study is to use the individual blade pitch control (IBC), reduce actively both the rotor hub vibratory loads and airframe vibration responses for the lift-offset compound helicopter at a high-speed flight condition.
Design/methodology/approach
The Sikorsky X2 technology demonstrator (X2TD) is used as the lift-offset compound helicopter. The X2TD lift-offset rotor is modelled and its rotor hub vibratory loads at a flight speed of 250 knots are predicted using a rotorcraft comprehensive analysis code, CAMRAD II, and the airframe structural dynamics is represented with a finite element analysis code, MSC.NASTRAN. When the propulsive trim methodology is applied for rotor trim, the best input condition for IBC using multiple harmonic inputs is searched to reduce the rotor vibration, while the rotor aerodynamic performance (the rotor effective lift-to-drag ratio) is improved or maintained at least. Finally, the reduction in airframe vibration responses is investigated when the best input condition for IBC with multiple harmonics is applied to the lift-offset rotor.
Findings
When the IBC with the single harmonic input using the 2/rev actuation frequency, amplitude of 2° and control phase angle of 120° (2P/2°/120°) is considered for X2TD rotor, the rotor vibration is reduced by about 26.37% only and the rotor effective lift-to-drag ratio increases slightly by 0.98%. When X2TD rotor uses the IBC with multiple harmonic inputs (2P/2°/45° + 5P/1°/90°), the rotor hub vibratory loads and airframe vibration responses are reduced by 44.69% and from 0.48 to 79.10%, respectively, while rotor effective lift-to-drag ratio is improved by 0.77%, as compared to the baseline without IBC.
Originality/value
This study is the first study to use the 2/rev actuation for IBC to the four-bladed lift-offset coaxial rotor and to investigate to obtain simultaneously the rotor vibration reduction, rotor performance improvement and airframe vibration reduction, using IBC with multiple harmonic inputs.
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Young-Min Lee and Sung-Eun Cho
The purpose of this paper is to ascertain the status of recent Korean four-year college students’ acquisition of vocational qualification certificates (hereinafter “certificates”…
Abstract
Purpose
The purpose of this paper is to ascertain the status of recent Korean four-year college students’ acquisition of vocational qualification certificates (hereinafter “certificates” or “qualifications”), the factors that influence certificate acquisition, and the resulting employment and wage effects.
Design/methodology/approach
Regression analysis was conducted on the data from the Graduates Occupational Mobility Survey.
Findings
The results showed that females and medicine and education majors had higher rates of vocational qualifications, as did the students in provincial as compared to metropolitan areas and students with high grades. In terms of effects on labor market outcomes, vocational qualifications exert a moderate degree of positive influence on employment rate, wage at the time of initial employment, and current wage (i.e. there is a persistent wage effect).
Originality/value
This study will provide policy implications to decrease youth unemployment in Korea and worldwide as well as to build employment services supporting the acquisition of vocational qualifications across various disciplines.
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Jun Sik Kim and Sol Kim
This paper investigates a retrospective on the Journal of Derivatives and Quantitative Studies (JDQS) on its 30th anniversary based on bibliometric. JDQSs yearly publications…
Abstract
This paper investigates a retrospective on the Journal of Derivatives and Quantitative Studies (JDQS) on its 30th anniversary based on bibliometric. JDQSs yearly publications, citations, impact factors, and centrality indices grew up in early 2010s, and diminished in 2020. Keyword network analysis reveals the JDQS's main keywords including behavioral finance, implied volatility, information asymmetry, price discovery, KOSPI200 futures, volatility, and KOSPI200 options. Citations of JDQS articles are mainly driven by article age, demeaned age squared, conference, nonacademic authors and language. In comparison between number of views and downloads for JDQS articles, we find that recent changes in publisher and editorial and publishing policies have increased visibility of JDQS.