GRADES 9-12
AERODYNAMICS OF CYCLING

BACKGROUND INFORMATION: Drag (Continued) Observing Drag Look at these two pictures below. Both are taken in a wind tunnel. The first is the airflow over a tennis ball; the second is the airflow over a bowling ball. Flow Separation Over A Tennis Ball Flow Over A Bowling Ball With Permission and Courtesy Of Cislunar Aerospace, Inc. The air is moving from left to right. The air strikes the front of the ball and part of the air moves up and around the top of the ball and some of it flows under the ball. Look at the wake on backside of the tennis ball. The size and flow of air in the wake is an indicator of the amount of drag. In the picture of the tennis ball, you can see that the flow separates from the tennis ball before it reaches the back side of the ball. This creates a large wake - and is an indicator of greater drag. You can see that the airflow around the bowling ball does not have a large wake. Here is a picture of an airplane wing. As the leading edge (front) of the wing is raised, the airflow no longer remains completely attached to the wing. Again this is a sign of drag. Flow Separation And A Large Wake Over The Wing With Permission and Courtesy Of Cislunar Aerospace, Inc. Examine the picture of the spinning baseball. In the wake on the lower side of the ball, you can even see that the flow is reversing and starting to move towards the ball again. Flow Over A Spinning Baseball With Permission and Courtesy Of Cislunar Aerospace, Inc. One of the things that a "tuft test" can show is the direction of the airflow. Short lengths of string or yarn are fastened to the body. Tufts that move around chaotically are a sign of drag. Tufts that stay in line with the airflow are a sign of lower drag. Tufts that move in the direction opposite to the airflow are a sign of high drag. Tufts Indicate The Direction Of The Airflow With Permission and Courtesy Of Kurt Long, US Navy