DRAG

As a child, it was fun to stick your hand out of the car window and feel the force of the moving, invisible air. To the aeronautical engineer, however, there is nothing very funny about aerodynamic drag. A continuing struggle for the practicing aerodynamicist is that of minimizing drag, whether it is for an airplane, missile, or ground-based vehicle such as an automobile or train. It takes power to move a vehicle through the air. This power is required to overcome the aerodynamic force on the vehicle opposite to its velocity vector. Any reduction of this force, known as the drag, represents either a direct saving in fuel or an increase in performance.

The estimation of the drag of a complete airplane is a difficult and challenging task, even for the simplest configurations. A list of the definitions of various types of drag partly reveals why this is so.

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 (see Equation 2.15).

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.

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 noncanceling static pressure components to either side of a shock wave acting on the surface of the body from which the wave is emana­ting.

With the exception of wave drag, the material to follow will consider these various types of drag in detail and will present methods of reasonably estimating their magnitudes. Wave drag will be discussed in Chapter 6.