Air, believe it or not, is sticky. Really, it's viscous - as it flows over the surface of your wing, it slows down due to friction. In fact, immediately above the surface of your wing, the air isn't moving at all.
Imagine you're flying at 100 knots in your Piper Cherokee. The air flows around your wing at around 100 knots - or somewhat faster due to your airfoil. However, if you measure the airspeed within an inch of the wing's surface, you'll find that the airflow slows down. As you reach the surface of your wing, the airflow's speed drops to zero. The area where friction slows down the airflow is called the boundary layer.
The boundary layer isn't very deep, maybe .02 to an inch thick, but it's important. It's the source of skin friction drag, and can actually decrease pressure drag.
Air flowing in the boundary layer travels in one of two states: laminar flow and turbulent flow.
Laminar Flow - It's Smooth
In lamanar flow, the air flows smoothly across a surface and the streamlines move parallel to each other. A lamanar-flow boundary layer is very thin - possibly only .02 inches thick.
As you move up and away from a surface, the airflow's speed smoothly increases in a laminar flow boundary layer until it reaches free-stream speed.
A laminar-flow boundary layer minimizes skin-friction drag - so engineers often optimize long, flat surfaces (like your wings) to preserve laminar flow. Any disturbances along the surface - even microscopic ones - can turn a laminar flow layer turbulent. So, on metal wings, you'll find flush mounted rivets with smooth filling on your leading edges to help preserve laminar flow.
Turning Turbulent
However, try as hard as you want, any laminar flow will quickly turn turbulent - often after it travels several inches back from the leading edge. That's why the trailing sections of your wings use round headed rivets. The laminar flow has already turned turbulent - so flush mounted rivets aren't necessary.
A turbulent layer is thicker than a laminar flow layer and it generates more skin-friction drag. While the speed increases evenly in a laminar flow layer, friction affects the airflow more in the lower region of a turbulent flow layer. Most of the airflow's speed reduction occurs right above the surface.
It turns out that the air's velocity combined with the distance it has traveled across a surface determine whether the boundary layer is laminar or turbulent. Engineers measure this using a "Reynolds Number" - named after Osborne Reynolds, who popularized its use. You take the velocity of the air times the distance it has traveled, and divide the product by the air's kinematic viscosity, which is approximately 1.46 meters2 per second at sea level on a standard day.
A low Reynolds number indicates laminar flow, and a high Reynolds number indicates turbulent flow.
Turbulent Layers Aren't All Bad
Turbulent flow boundary layers do have several upsides - even if they have more skin-friction drag. A turbulent flow boundary layer has more energy than a laminar flow layer, so it can withstand an adverse pressure gradient longer. That allows a turbulent boundary layer to remain attached to the surface longer.
Think of the air flowing over the top of your wing. As it moves back from the center of lift, it moves from an area of low pressure to higher pressure. The low pressure is trying to "suck" the airflow back, and it pulls energy out of the air. Once the airflow runs out of energy, it separates from the surface. That separation produces pressure drag.
Pressure drag is more significant than skin friction drag on large bodies - like your fuselage and nacelles. And since a turbulent boundary layer has more energy to oppose an adverse pressure gradient, engineers often force the boundary layer to turn turbulent over fuselages to reduce overall drag.
You also see this design on golf balls and tennis balls. The dimples on a golf ball and fuzz on a tennis ball develop a turbulent boundary layer, and minimize pressure drag behind the ball. If the balls were smooth, they wouldn't travel nearly as far or as fast.
What Can You Do About It?
Not much, since you can't re-design the wing. But, you can remove those bugs baked on to your leading edges before flight. All the flush-mounted rivets in the world won't keep a boundary layer laminar if dried insects get in the way. And, if you're flying a 1973 Cessna 172 with sun-baked chipped paint on the leading edge, you're out of luck - can you feel the extra skin-friction drag now?
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