Ground Effect

It is possible to fly an airplane just clear of the ground (or water) at a slightly lower airspeed than that required to sustain level flight at higher altitudes.  This is the result of a phenomenon, which is better known than understood even by some experienced pilots. 

Pilots Handbook of Aeronautical Knowledge FAA-H-8083-25

Let us develop a clear explaination of this effect. 

In the simplest description the wing planiform moving thru the air results in a lower pressure above the wing and a higher pressure below it.

In the simplest formula lift is calculated by multiplying a fixed coefficient by the angle of attack by the surface area of the wing element.   This generates a force normal to the relative flow of air across the airfoil.   If
we increase the angle of attack this normal force increases in direct proportion until it reaches a point of stall.

In most free body diagrams the pure vertical component of this normal force is labeled lift and the horizontal component of this normal force is labeled induced drag.   Induced drag results from the production of lift.   

This convention may have originated from the Wright Brothers and Dayton Ohio; this diagram is from a correspondence  with Octave Chanute:
  

                

However for this discussion we will use a very specific definition; lift is the force perpendicular to the chord line of the wing and induced drag is the force parallel to the chord line.  This is the engineers definition; as lift and drag are purely a function of the wing regardless of its relationship to the horizontal.
                                
 
If the wing had infinite span, the relative flow of air across the wing would follow the chord line thru the airfoil and the normal force would be perpendicular to the wing chord.

However, the aircraft wing does not have infinite span, and the high pressure air below steals over the wingtips to the top surface at each end.   This continuous flow forms a vortex.

  
                                                     


                                                          Picture courtesy of DANTEC Measurement Technology. 

These vortices have two immediate effects.  The airflow reduces the difference between low pressure air above the wing and high pressure air below it (reducing the total normal force that translates into lift).   The swirling vortices also deflect the trailing airstream downward.   The downwash deflection alters the effective path of the relative wind downward and tips the normal force back (moving the lift vector backwards). 


                            



The resultant lift vector has a smaller overall  vertical component and a slightly increased horizontal (backwards) component.   Thus the wing is producing less lift and more induced drag directly as a result of these effects at the wingtips.

However, when the wing is operating within half a span of the ground or water; the downwash is obstructed and these wing vortices are distrupted.  

In this way, the normal reduction in normal force (and lift) and the larger induced drag due to downwash is diminished.  

The wing in ground effect produces more lift for a given angle of attack, and less induced drag.

The venerable Pilots Handbook of Aeronautical Knowledge goes on to comment:

 

 

This simple statement glosses over the horizontal stabilizer as an airfoil and overlooks a potential grave misunderstanding of the consequences of center of gravity limits and weight and balance ....