Drag on an Aircraft
Total Drag: this is the resistance to motion of the aircraft through the air, the resistance is made up of various Drag forces. The Drag forces that act on an aircraft can be split into two categories, Subsonic Drag and Supersonic Drag. Supersonic Drag comes more into play when an aircraft is moving at high speeds and so will be introduced in a later Hub. In Subsonic Drag you get both Profile Drag and Interference Drag, Profile Drag is made up of Form Drag and Skin Friction.
Types Of Drag
Skin Friction Drag as the name suggests is created by friction forces existing between an object and the air which it is moving. How much Drag is produced depends on surface area, roughness and laminar / turbulent flow. If an aircraft has a large surface area it will have a large friction force, as the whole surface area will experience skin friction. If a surface is rough then friction is increased. This can easily be overcome by the use of polish and flush riveting which both increase the smoothness. Lastly if the boundary layer near the surface is laminar or turbulent.
There is a layer of air flowing over the surface which is moving slower in relation to the relative airflow, this is called the boundary layer. Laminar boundary layer is seen with initial airflow over a smooth surface will show a very thin boundary layer with smooth laminations of air over one another. As this flow continues the friction forces in the boundary layer dissipate the energy of the airstream, thus slowing it down. The Laminar boundary layer will now increase in thickness as the flow gets further and further back, eventually this will produce a turbulent flow which will grow and end the smooth laminar flow. The Transition Point is the point at which laminar flow becomes turbulent.
There are some devices which are used to control the boundary layer in order to delay the onset of turbulent flow. When the Slats (flight control surface) are opened they create a gap between the Slat itself and the leading edge of the wing, this gap introduces high energy air into the boundary layer over the top wing.
Form Drag occurs when the airflow physically separates from the surface allowing a turbulent flow. This flow then creates a pressure that acts along the airflow. This will happen because the pressure of air at the front of the aircraft is increased and decreased at the back. Streamlining or can be used to reduce Form Drag. By Streamlining you decrease the curvature of the surfaces and so avoid a sudden change of area / shape.
Interference Drag can also be thought of as flow interference. Flow interference will occur at the joining points of surfaces like the wing and fuselage. This creates additional turbulence which in turn creates a larger pressure difference between the front and back surfaces of the aircraft. Streamlining again is one of the solutions to this.
Induced Drag isn't as simple as the others as it is produced during the production of lift. How the aerofoil is designed to produce lift also produces Induced Drag. You should note that the pressure above the wing is less than atmospheric pressure and the pressure below is is equal to or greater than atmospheric pressure. Fluid always moves from high pressure to low pressure; this will mean that there is a movement of air from the bottom of the wing travelling around the wing tip to the top of the wing. A vortex is created around the wing tip which increases the drag.
From the hub on aerofoils we know that lift is increased as the angle of attack is increased, this will mean that the Induced Drag is also increased. This in turn makes the pressure difference bigger and bigger which causes much more turbulent vortices to be created.Newton's Third Law Of Motion ('for every action there is an equal reaction and opposite reaction') So if an airflow on a wing will produce an upward flow, then there will be and equal and opposite reaction which will be downwards in this case.
To reduce Induced Drag we can use a narrow long wing (High Aspect Ratio). Compared with a short thick wing (Low Aspect Ratio) of the same area has less turbulent vortices and therefore less Induced Drag.