are identical. In both examples, the only difference is the  relationship of the airfoil with the oncoming airstream  (angle). A paper airplane, which is simply a flat plate, has a  bottom and top exactly the same shape and length. Yet, these  airfoils do produce lift, and “flow turning” is partly (or fully)  responsible for creating lift.  As an airfoil moves through air, the airfoil is inclined  against the airflow, producing a different flow caused by the  airfoil’s relationship to the oncoming air. Think of a hand  being placed outside the car window at a high speed. If the  hand is inclined in one direction or another, the hand will  move upward or downward. This is caused by deflection,  which in turn causes the air to turn about the object within  the air stream. As a result of this change, the velocity about  the object changes in both magnitude and direction, in turn  resulting in a measurable velocity force and direction.  To this point, the discussion has centered on the flow across  the upper and lower surfaces of an airfoil. While most of the  lift is produced by these two dimensions, a third dimension,  the tip of the airfoil also has an aerodynamic effect. The high- pressure area on the bottom of an airfoil pushes around the tip  to the low-pressure area on the top.  [Figure 4-8]  This action  creates a rotating flow called a tip vortex. The vortex flows  behind the airfoil creating a downwash that extends back to  the trailing edge of the airfoil. This downwash results in an  overall reduction in lift for the affected portion of the airfoil.  Manufacturers have developed different methods to  counteract this action. Winglets can be added to the tip of  an airfoil to reduce this flow. The winglets act as a dam  preventing the vortex from forming. Winglets can be on the  top or bottom of the airfoil. Another method of countering  the flow is to taper the airfoil tip, reducing the pressure  differential and smoothing the airflow around the tip.  Modern general aviation aircraft have what may be considered  high performance characteristics. Therefore, it is increasingly  necessary that pilots appreciate and understand the principles  upon which the art of flying is based. For additional  information on the principles discussed in this chapter, visit  the National Aeronautics and Space Administration (NASA)  Beginner’s Guide to Aerodynamics at www.grc.nasa.gov/  www/k-12/airplane/bga.html.  Chapter Summary  A Third Dimension  Figure 4-8.  Tip vortex.  v  o  r  t  e  x  Tip  4-9 