Figure 3-7.  Turbulent boundary layer. Figure 3-6.  Laminar boundary layer. Figure 3-5.  Layer of decelerated air called the boundary layer. Figure 3-8.  Skin friction increases due to the turbulent boundary layer. Figure 3-9.  Combinations of low and high pressure on the airfoil  causing a turbulent flow of air. 3-4 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.  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. 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. Boundary layer Wing surface Laminar boundary layer Wing surface Turbulant boundary layer Wing surface At this point, the air has: ● low pressure high speed At this point, the air has: ● higher pressure lower speed Air does not tend to flow into an  area of higher pressure. Hence,  the flow becomes unstable and  the friction from the wing’s  surface becomes capable of  tripping the boundary layer into  a turbulent flow. Turbulent transition point Turbulent transition point Laminar boundary layer Turbulent boundary layer •  Turbulent boundary layer—dominated by eddies and  irregular turbulent flow.  [Figure 3-7]