## Drag Classification

**Introduction: **Drag force is the summation of all forces that resist against aircraft motion. The estimation of the drag of a complete airplane is a difficult and challenging task, even for the simplest configurations. We will consider the separate sources of drag that contribute to the total drag of an aircraft. The real shape of drag force as a function of speed is parabola. It means that there are some parameters that will decrease drag as the velocity increases and there are some other parameters that will increase drag as the velocity increases. This observation shows us a good direction for drag classification. Although the drag and the drag coefficient can be expressed in a number of ways, for reasons of simplicity and clarity, the parabolic drag polar will be used in all main analyses. Different references and books use different terminology, so it may confuse students and engineers. In this section, a list of definitions of various types of drag is presented, and then a classification of all of these drag forces is described.

**Induced Drag**: The drag that results from the generation of a trailing vortex system downstream of a lifting surface of finite aspect ratio.

**Parasite Drag:**The total drag of an airplane minus the induced drag. Thus, it is the drag not directly associated with the production of lift. The parasite drag is composed of many drag components, the definitions of which follow.

**Skin Friction Drag**: The drag on a body resulting from viscous shearing stresses over its wetted surface. Frequently, the drag of a very streamlined shape such as a thin, flat plate is expressed in terms of a skin friction drag. This drag is a function of Reynolds number.

There are mainly two cases where the flow in the boundary layer is entirely laminar or entirely turbulent over the plate. The Reynolds number is based on the total length of the plate in the direction of the velocity. In a usual application, the boundary layer is normally laminar near the leading edge of the plate undergoing transition to a turbulent layer at some distance back along the surface__.__

**Form Drag (sometimes called Pressure Drag):**The drag on a body resulting from the integrated effect of the static pressure acting normal to its surface resolved in the drag direction. Unlike the skin friction drag that results from viscous shearing forces tangential to a body’s surface, form drag results from the distribution of pressure normal to the body’s surface. In an extreme case of a flat plate normal to the flow, the drag is totally the result of an imbalance in the normal pressure distribution. There is no skin friction drag present in this case.

**Interference Drag**: The increment in drag resulting from bringing two bodies in proximity to each other. For example, the total drag of a wing-fuselage combination will usually be greater than the sum of the wing drag and fuselage drag independent of each other.

**Trim Drag:**The increment in drag resulting from the aerodynamic forces required to trim the airplane about its center of gravity. Usually this takes the form of added induced and form drag on the horizontal tail.

**Profile Drag**: Usually taken to mean the total of the skin friction drag and form drag for a two-dimensional airfoil section.

**Cooling Drag:**The drag resulting from the momentum lost by the air that passes through the power plant installation for purposes of cooling the engine, oil, and accessories.

**Base Drag:**The specific contribution to the pressure drag attributed to the blunt after-end of a body.

**Wave Drag:**Limited to supersonic flow, this drag is a pressure drag resulting from non canceling static pressure components to either side of shock wave acting on the surface of the body from which the wave is emanating.

**Induced drag**: The induced drag is the drag directly associated with the production of lift. This results from the dependency of the induced drag on the angle of attack. As the angle of attack of the aircraft (i.e. lift coefficient) varies, this type of drag is changed. The induced drag is itself consists of two parts. The first part originates from vortices around wing, tail, fuselage and other components.

**Zero-lift drag**: The Zero-lift drag includes all types of drag that does not depend on production of lift. Every aerodynamic component of aircraft (i.e. the components that are in direct contact with flow) generates zero-lift drag. Typical components are wing, horizontal tail, vertical tail, fuselage, landing gear, antenna, engine nacelle, and strut.