[L1] 1 Drag Force [L2] 1) Definition [L4] - The force exerted by a flowing fluid on an object in the direction of flow. [L4] - A force that acts in the opposite direction to an object's motion when the object moves relatively within a fluid, resisting the object's movement. [L5] * Drag is similar to friction and resists the motion of an object. [L2] 2) Characteristics [L4] - A fluid can exert forces and moments on an object in various directions. Specifically, drag is generated by the pressure difference in the opposite direction to the motion and the viscous force of the fluid in contact with the object's surface. [L4] - Drag and lift are related to the fluid's density, velocity, object's size, shape, and orientation. [L2] 3) Calculation [L3] ① Calculation of Drag [L4] - Applied when the fluid is in a steady state along streamlines. [L4] - A refers to the platform area of the object. [L3] ② Drag Coefficient (Dimensionless Number) [L1] 2 Lift Force [L2] 1) Definition [L4] - A force acting perpendicular to the direction of fluid flow. [L5] * The generation of lift can be explained by action-reaction, pressure differences, etc. [L5] * Taking an airplane as an example, there are theories: one where the faster flow above pushes the air below, generating lift, and another where it's due to the dynamic force resulting from the pressure difference between lower pressure above and higher pressure below. [L4] - A mechanical force generated by fluid flow when a solid exists within the fluid. [L5] * It originates from faster fluid flow creating lower pressure and slower flow creating higher pressure. [L2] 2) Characteristics [L4] - Lift is affected by AOA (Angle of Attack); typically, if it exceeds 15 degrees, a stall occurs due to reduced lift caused by wake generation. [L4] - Among the components of pressure force and shear force, the component perpendicular to the flow direction moves the object in the vertical direction. [L5] a. Rolling Moment: Moment about the direction of flow [L5] b. Yawing Moment: Moment about the direction of lift [L5] c. Pitching Moment: Moment about the direction of side force [L4] - To increase lift, methods include using flaps or controlling the boundary layer. [L5] a. Flaps (trailing edge flaps): Changing the airfoil shape at the rear to increase the angle of attack; above a certain speed, this becomes a factor in increasing friction drag. [L5] b. Boundary Layer Control: A method to prevent streamline separation on the upper surface of an airfoil by creating a slot in the middle of the airfoil to prevent boundary layer separation. [L2] 2) Stall Prevention [L4] - Lift, the force capable of lifting an aircraft's weight, must be greater than gravity. [L4] - To maintain an aircraft's altitude, lift must be greater than gravity, which can be achieved by reducing the aircraft's own weight or increasing lift. [L4] - Lift has a similar calculation formula to drag. (The relationship between the drag coefficient and lift coefficient is the same)