1. Research Scientist, Wind Energy Division, Building 118, Risø DTU, Roskilde, 4000, Denmark.
2. Associate Professor, Institute of Advanced Aerospace Technology, School of Mechanical and Aerospace Engineering, Building 301, Room 1418, Seoul National Univ., Seoul, 151-742, Korea (corresponding author).
3. Research Engineer, Rotor Dept., Korea Aerospace Research Institute, 45 Eoeun-dong, Yuseong-gu, Daejeon, 305-333, Korea.