Figure 3-10.  Glider wing on the left shows the airflow with a turbulator installed and the glider wing on the right shows how the airflow  is disturbed without the turbulator installed. The boundary layer can also be tripped into a turbulent flow  at any point by discontinuities on the wing’s surface. It is  important to keep wings clean and avoid rain and icing to  prevent premature transition, and the increase in drag that it  causes. As the boundary layer is only 1.0 millimeter thick at  the leading edge, objects, such as rivets, splattered insects,  rain drops, ice crystals, and dust, are all large enough to  cause localized turbulent transition to occur.  [Figure 3-11] Interference drag occurs when varied currents of air over a  glider meet and interact. Placing two objects adjacent to one  another may produce turbulence 50–200 percent greater than  the parts tested separately. An example of interference drag  is the mixing of air over structures, such as the wing, tail  surfaces, and wing struts. Interference drag can be reduced  on gliders with fairings to streamline the intersection of air. Induced drag is generated as the wing is driven through the  air to develop the difference in air pressures that we call lift.  As the higher pressure air on the lower surface of the airfoil  curves around the end of the wing and fills in the lower  pressure area on the upper surface, the lift is lost, yet the  energy to produce the different pressures is still expended.  The result is drag because it is wasted energy. The more  energy the glider requires to fly, the greater the required rate  of descent is to supply sufficient energy to convert into thrust  Induced Drag Interference Drag Figure 3-11.  Turbulence due to insects, ice crystals, and dust. 3-5 A. B. Normal turbulent transition point Splattered insects