GRADES 9-12
AERODYNAMICS OF CYCLING

BACKGROUND INFORMATION: Drag (Continued) Form drag is easy to demonstrate using a hand out the window of a moving car. If the hand is held flat and horizontal to the ground, it is basically a streamlined object, and the observer feels only a small tug, or drag. If he or she turns the hand so that the palm is facing forward, the drag force is greatly increased, and the hand is pulled backwards! If the observer could see what was happening in the invisible air, he or she would see many little swirls of air, or eddies, behind the hand. The streamlines around the hand would have to travel around these eddies, the pressures would be higher along the streamlines, and the drag is increased dramatically. An interesting offshoot of the discussion of form drag is in the design of large banners like those used to advertise school fairs, church picnics, or museum openings. If a large cloth or plastic banner is strung between two buildings across a street, and the wind blows against it, it has a high form drag. This form drag can be so large that the banner rips and tears, and it may be destroyed. Banner designers now know that they must include slits in the fabric to allow the air to move through the banner, so that the drag on the banner is decreased. These first two types of drag are often added together and called the profile drag by aerodynamicists. Pilots call the sum of these two drags the parasite drag. In either case, these drags are primarily a function of the shape of the body and the smoothness of the surfaces. All objects moving through a fluid will have these drag forces. Airplanes, because of the amount of lift generated and the velocities at which they travel, are also subject to two additional drag forces, the induced drag and the wave drag. Induced drag is sometimes called the drag due to lift. As the lift force is generated along a wing, a small amount of excess force can be generated in the direction opposing the motion, or in the direction of drag. This excess is called the induced drag, and because of its direction, it causes a decrease in the plane's motion. Therefore, it is considered a type of drag force. It is one of the odder concepts that an aerodynamicist must consider during the design of an airplane. Any change that he or she can make to increase the lift is a positive change. In the design of a better wing a designer will optimize the generation of the lift so as to minimize the generation of induced drag; a well designed wind will generate the needed lift, while minimizing the induced drag. However, other factors such as structural strength, overall weight, cost and complexity will also control the design of the "optimal" wing. The last of the four types of drag is the wave drag. This generally only occurs when the airplane is flying faster than the speed of sound in supersonic flight. It is caused by the interactions of the shock waves over the surfaces and the pressure losses due to the shocks. Wave drag can also occur at transonic speeds, where the velocity of the air is supersonic locally. Since most commercial jets today fly at transonic speeds, wave drag is an important part of the total drag.