Tham khảo tài liệu 'kỹ thuật máy bay chiến đấu episode 2 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 262 AIRCRAFT DESIGN strength of the shocks around it resulting in wave-drag-due-to-lift. This drag is fairly small and is usually ignored in early conceptual design. Two-dimensional 2-D airfoil drag or profile drag is a combination of skin-friction drag and viscous separation drag. There is no drag-due-to-lift for the 2-D airfoil because the lift force is perpendicular to the freestream direction. However the profile drag increases as the angle of attack is increased leading to some confusion. This increase in 2-D airfoil drag is due to an increase in viscous separation caused by a greater pressure drop on the upper surface of the airfoil as the angle of attack is increased. This increase in profile drag with increasing angle of attack is not technically caused by the generation of lift but does vary as the lift is varied. Most preliminary drag-estimation methods do not actually use the airfoil profile drag data to determine total wing drag. Instead the drag for an idealized wing with no camber or twist is determined and then a separate camber drag is estimated. Often the camber drag term is included statistically in the drag-due-to-lift calculation even though it is not technically caused by the generation of lift Changing the lift on the wing changes the velocities above and below it. This change in local airflow velocity causes a small change in skin-friction drag. Sometimes called a supervelocity effect this is minor and is usually ignored. AERODYNAMIC COEFFICIENTS Lift and drag forces are usually treated as nondimensional coefficients as defined in Eqs. and . The wing reference area Sref or simply s is the full trapezoidal area extending to the aircraft centerline. The dynamic pressure of the freestream air is called q as defined in Eq. . L qSCL D qSCD where q VzpV2 By definition the lift force is perpendicular to the flight direction while the drag is parallel to the flight direction. Remember that the 2-D airfoil .
