Principles of Aerodynamics – Aerodynamic Drag

Drag is the force experienced by an object representing the resistance to its movement through a fluid. Sometimes called wind resistance or fluid resistance, it acts in the opposite direction to the relative motion between the object and the fluid. The example opposite shows the aerodynamic drag forces experienced by an aerofoil or aircraft wing moving through the air with constant angle of attack as the air speed is increased..

The Total Aerodynamic Drag is the sum of the following components:

• Induced Drag – Due to the vortices and turbulence resulting from the turning of the air flow and the downwash associated with the generation of lift. Increases with the angle of attack. Inversely proportional to the square of the air speed. Decreases as aircraft speed increases and the angle of attack is reduced. Induced drag associated with the high angle of attack needed to maintain the lift is dominant at low air speeds.
• Form Drag or Pressure Drag – Due to the size and shape of the aerofoil. Increases with the square of air speed. Streamlined shapes designed to reduce form drag.
• Friction Drag – Arises from the friction of the air against the “skin” of the aerofoil moving through it. Increases with the surface area of the aerofoil and the square of air speed.
• Profile Drag or Viscous Drag- The sum of Friction Drag and the Form Drag.
• Parasitic Drag or Interference Drag – Incurred by the non-lifting parts of the aircraft such as the wheels, fuselage, tail fins, engines, handles and rivets. Increases with the square of air speed. Parasitic drag becomes dominant at higher air speeds.

Form Drag is caused by the separation of the boundary layer from a surface and the wake created by that separation. 

Where the airflow interacts w 2 surfaces (instead of 1) creating turbulent airflow. Resultant combined drag can be greater than sum of independent drag values. Reduced by the use of fairings.

All the Drag that is not associated with the production of Lift.

Parasite Drag is defined as all drag that is not associated with the production of lift.