The airflow outside of the boundary layer reacts to the  shape of the edge of the boundary layer just as it would  to the physical surface of an object. The boundary layer  gives any object an “effective” shape that is usually slightly  different from the physical shape. The boundary layer may  also separate from the body, thus creating an effective shape  much different from the physical shape of the object. This  change in the physical shape of the boundary layer causes a  dramatic decrease in lift and an increase in drag. When this  happens, the airfoil has stalled.  In order to reduce the effect of skin friction drag, aircraft  designers utilize flush mount rivets and remove any  irregularities that may protrude above the wing surface. In  addition, a smooth and glossy finish aids in transition of  air across the surface of the wing. Since dirt on an aircraft  disrupts the free flow of air and increases drag, keep the  surfaces of an aircraft clean and waxed.  The second basic type of drag is induced drag. It is an  established physical fact that no system that does work in the  mechanical sense can be 100 percent efficient. This means  that whatever the nature of the system, the required work  is obtained at the expense of certain additional work that is  dissipated or lost in the system. The more efficient the system,  the smaller this loss.  In level flight, the aerodynamic properties of a wing or rotor  produce a required lift, but this can be obtained only at the  expense of a certain penalty. The name given to this penalty  is induced drag. Induced drag is inherent whenever an airfoil  is producing lift and, in fact, this type of drag is inseparable  from the production of lift. Consequently, it is always present  if lift is produced.  An airfoil (wing or rotor blade) produces the lift force by  making use of the energy of the free airstream. Whenever  an airfoil is producing lift, the pressure on the lower surface  of it is greater than that on the upper surface (Bernoulli’s  Principle). As a result, the air tends to flow from the high  pressure area below the tip upward to the low pressure area  on the upper surface. In the vicinity of the tips, there is a  tendency for these pressures to equalize, resulting in a lateral  flow outward from the underside to the upper surface. This  lateral flow imparts a rotational velocity to the air at the tips,  creating vortices that trail behind the airfoil.  When the aircraft is viewed from the tail, these vortices  circulate counterclockwise about the right tip and clockwise  about the left tip.  [Figure 5-9]  As the air (and vortices) roll off  the back of your wing, they angle down, which is known as  downwash.  Figure 5-10  shows the difference in downwash at  Downwash points the relative wind downward, so the more  downwash you have, the more your relative wind points  downward. That's important for one very good reason: lift is  always perpendicular to the relative wind. In  Figure 5-11,  you  can see that when you have less downwash, your lift vector  is more vertical, opposing gravity. And when you have more  downwash, your lift vector points back more, causing induced  drag. On top of that, it takes energy for your wings to create  downwash and vortices, and that energy creates drag.  altitude versus near the ground. Bearing in mind the direction  of rotation of these vortices, it can be seen that they induce  an upward flow of air beyond the tip and a downwash flow  behind the wing’s trailing edge. This induced downwash has  nothing in common with the downwash that is necessary to  produce lift. It is, in fact, the source of induced drag.  Figure 5-9.  Wingtip vortex from a crop duster.  Figure 5-10.  The difference in wingtip vortex size at altitude versus  near the ground.  5-7  Induced Drag 