Drag Reduction

Drag-Reduction

“Any physical body being pulled through air has drag associated with it”. [1] Drag is the force that resists forward motion in the atmosphere, and is parallel to the path of the free-stream velocity of the airflow. Usually drag is produced by 9 states associated with the motion of air particles over the aircraft. There are many different types of drag, including pressure, skin-friction, form, induced, parasite, and wave. My research focused more on induced drag, skin-friction drag, and pressure drag, and what effect they have on non-lifting bodies versus lifting bodies.

Skin-Friction drag is the force caused by “friction between the molecules of fluid and a solid boundary”. [3] For lifting bodies like airplanes, it is the aerodynamic drag due to the contact of moving air with the surface of the airplane.”[2] This type of drag happens because air particles hit the exterior of the aircraft, which can be equated to the friction between two objects or materials. Since skin-friction drag is between solid (airplane) and gas (air), the extent of the drag depends on these properties, and in order to decrease the skin-friction drag on the airplane and increase the speed of air, the surface should remain clean and smooth. [1] For the gas (air), the magnitude of the drag depends on the viscosity of the air, and a boundary layer of low energy is created on the surface of the airplane which the degree of the drag is totally dependent upon. In non-lifting bodies like trucks, skin-friction drag is caused by the contact of the bodywork and the airflow. The boundary layer is pulled with the air because of its viscosity, and shear forces are created. The sum of the shear forces over the whole exterior produces the skin-friction drag. In trucks, the drag increases with the actual length of the truck, and in order to keep skin-friction drag low, the surface of the truck should be even.

Induced drag is drag due to lift, and it occurs whenever a moving object redirects the airflow coming at it. For an airplane induced drag takes place when the wings redirect the air causing lift, and since there is such high pressure below the wing, it makes the airflow at the tips of the wings twist around from bottom to top in a circular motion; which results in a trailing vortex. [4] The angle between the airplanes reference line and the oncoming airflow is referred to as the angle of attack. When all other parameters are kept the same, the induced drag increases as the angle of attack increases.