lift.  Figure 5-6  depicts the L/D$_{MAX}$ by the lowest portion of  the blue line labeled “total drag.” The configuration of an  aircraft has a great effect on the L/D.  drag is the easiest to reduce when designing an aircraft. The  solution is to streamline as many of the parts as possible.  Drag is the force that resists movement of an aircraft through  the air. There are two basic types: parasite drag and induced  drag. The first is called parasite because it in no way functions  to aid flight, while the second, induced drag, is a result of an  airfoil developing lift.  Interference drag comes from the intersection of airstreams  that creates eddy currents, turbulence, or restricts smooth  airflow. For example, the intersection of the wing and the  fuselage at the wing root has significant interference drag.  Air flowing around the fuselage collides with air flowing over  the wing, merging into a current of air different from the two  original currents. The most interference drag is observed when  two surfaces meet at perpendicular angles. Fairings are used  to reduce this tendency. If a jet fighter carries two identical  wing tanks, the overall drag is greater than the sum of the  individual tanks because both of these create and generate  interference drag. Fairings and distance between lifting  surfaces and external components (such as radar antennas  hung from wings) reduce interference drag.  [Figure 5-8]  Parasite drag is comprised of all the forces that work to slow  an aircraft’s movement. As the term parasite implies, it is the  drag that is not associated with the production of lift. This  includes the displacement of the air by the aircraft, turbulence  generated in the airstream, or a hindrance of air moving over  the surface of the aircraft and airfoil. There are three types of  parasite drag: form drag, interference drag, and skin friction.  Form drag is the portion of parasite drag generated by the  aircraft due to its shape and airflow around it. Examples include  the engine cowlings, antennas, and the aerodynamic shape of  other components. When the air has to separate to move around  a moving aircraft and its components, it eventually rejoins  after passing the body. How quickly and smoothly it rejoins is  representative of the resistance that it creates, which requires  additional force to overcome.  [Figure 5-7]  Notice how the flat plate in  Figure 5-7  causes the air to swirl  around the edges until it eventually rejoins downstream. Form  Skin friction drag is the aerodynamic resistance due to the  contact of moving air with the surface of an aircraft. Every  surface, no matter how apparently smooth, has a rough,  ragged surface when viewed under a microscope. The air  molecules, which come in direct contact with the surface of  the wing, are virtually motionless. Each layer of molecules  above the surface moves slightly faster until the molecules  are moving at the velocity of the air moving around the  aircraft. This speed is called the free-stream velocity. The area  between the wing and the free-stream velocity level is about as  wide as a playing card and is called the boundary layer. At the  top of the boundary layer, the molecules increase velocity and  move at the same speed as the molecules outside the boundary  layer. The actual speed at which the molecules move depends  upon the shape of the wing, the viscosity (stickiness) of  the air through which the wing or airfoil is moving, and its  compressibility (how much it can be compacted).  Figure 5-8.  A wing root can cause interference drag. Figure 5-7.  Form drag.  5-6  Drag  Interference Drag  Parasite Drag  Form Drag  Skin Friction Drag  FLAT PLATE  SPHERE  SPHERE WITH  A FAIRING  SPHERE INSIDE  A HOUSING 