Aerodynamics of Flight

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