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• Turbulent boundary layer—dominated by eddies and irregular turbulent flow. [Figure 3-7] 

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Turbulent boundary layers generate 5 to 10 times more skin friction drag than the equivalent laminar boundary layer. [Figure 3-8] Therefore, glider designers try to maintain laminar flow across as much of the aircraft as possible. Figure 3-9  shows why this turbulent transition occurs. 

There is a point that is referred to as the separation point, in which the boundary layer breaks away from the surface of the wing due to the magnitude of the positive pressure gradient. Beneath the separated layer, bubbles of stagnant air form, creating additional drag because of the lower pressure in the wake behind the separation point. 

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These bubbles can be reduced or even eliminated by shaping the airfoil to move the separation point downstream or by adding a turbulator. Turbulators are aerodynamically positioned in a spanwise line along the wing and are used to trip laminar flow air into turbulent flow air at a desired location on the wing. This is beneficial because the turbulent boundary layer contains more energy, which will delay separation until a greater magnitude of negative pressure gradient is reached, effectively moving the separation point further aft on the airfoil and possible eliminating separation completely. A consequence of the turbulent boundary layer is increased skin friction relative to a laminar boundary layer, but this is very small compared to the increase in drag associated with separation. 

In gliders, the turbulator is often a thin zig-zag strip that is placed on the underside of the wing and sometimes on the fin. [Figure 3-10]  For a glider with low Reynolds numbers (i.e., where minimizing turbulence and drag is a major concern), the small increase in drag from the turbulator at higher speeds is minor compared with the larger improvements at best glide speed, at which the glider can fly the farthest for a given height. 

