GRADES 9-12
AERODYNAMICS OF CYCLING

BACKGROUND INFORMATION: Drag Drag is a resistance force to the forward motion of any object, including airplanes, a runner and sports equipment like a baseball or a bicycle. There are four types of drag: friction drag, form drag, induced drag, and wave drag, and they are functions of the shape of the body, the smoothness of the surfaces, and the velocity of the object. All four sum together for the overall drag force. Since the drag force resists the forward motion of the plane, its direction is opposite the thrust force on a moving airplane or bicycle. Forces On An Airplane In Straight And Level Flight Drag Opposes Thrust With Permission and Courtesy Of Cislunar Aerospace, Inc. If the thrust force is greater than the drag force, the object, like an airplane goes forward, but if the drag force exceeds the thrust, the object will slow down and stop. The friction drag is sometimes also called the skin friction drag. It is the friction force at the surfaces of the plane caused by the movement of air over the whole plane. In the boundary layer along each surface, a skin friction coefficient can be computed as a function of the velocity of the air and the surface roughness. The summation of all the local skin friction coefficients is used to compute the friction drag. Aerodynamicists design the outside of the airplanes to be smooth surfaces so that the friction coefficients and therefore the friction drag will be small. If a person were to look at the surface of a wing, for example, he or she would see that all the sheets of metal join smoothly, and even the rivets are rounded over and are as flush with the surface as possible. Sometimes the aerodynamicist will design small tabs to be placed along a wing surface to trip the boundary layer for transition to turbulence. While this increases the friction drag slightly, the increase in lift and control of the airplane are judged worth the cost. The form drag, or pressure drag, as it is sometimes called, is directly related to the shape of the body or airplane. A smooth, streamlined shape will generate less form drag than a blunted or flat body. The term streamlined comes from the idea that a shape is designed so that streamlines above and below the body barely change and rejoin smoothly right behind the body. A thin, relatively sharp-nosed airfoil is a perfect example of streamlining. Any object that moves through a fluid can get a decrease in form drag by streamlining. Automobiles are streamlined, which translates into better gas mileage; there is less drag, so less fuel is required to "push" the car forward. Buses, vans, and large trucks are less streamlined, and this is one reason (the additional weight is another) why they use more fuel than smaller, streamlined cars. Laminar Flow Over A Wing With Permission and Courtesy Of Cislunar Aerospace, Inc.