content_image/526613.jpg content_image/526614.jpg
<img src='content_image/526613.jpg'>

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 

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 

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 

<img src='content_image/526614.jpg'>

