text: One example of interference between air- 
  planes in flight is shown first in figure 6.10 with 
  the effect of lateral separation of two airplanes 
  flying in line abreast. A plane of symmetry 
  would exist halfway between two identical air- 
  planes and would furnish a boundary of flow 
  across which there would be no lateral com- 
  ponents of flow. As the two airplane wing 
  the effect is to reduce the 
  tips are in proximity, 
  strength of the tip or trailing vortices and re- 
  duce the induced velocities in the vicinity of 
  wing tip. Thus, each airplane will experience 
  a local increase in the lift distribution as the 
  tip vortices are reduced and a rolling moment is 
  developed which 
  tends to roll each airplane 
  away from the other. This disturbance may 
  provide the possibility of collision if other air- 
  planes are in the vicinity and there is delay in 
  If the wing 
  control correction or overcontrol. 
  tips are displaced in a fore-and-aft direction, 
  the same effect exists but generally it is of a 
  lower magnitude. 
  
  The range sf the reciprocating powered air- 
  plane can be augmented by the use of ground 
  effect. When the airplane is close to 
  the 
  ground or water surface the reduction of in- 
  duced drag increases the maximum lift-drag 
  ratio and causes a corresponding increase in 
  range. Of course, the airplane must be quite 
  close to the surface to obtain a noticeable in- 
  crease in (L/D),., and range. The difficulty in 
  holding 
  the airplane at the precise altitude 
  without contacting the ground or water will 
  preclude the use of ground effect during ordi- 
  nary flying operations. The use of ground 
  effect to extend range should be reserved as 
  a final measure in case of emergency. Because 
  of the very detrimental effect of low altitude 
  on the range of the turbojet, ground effect will 
  not be of a particular advantage in an attempt 
  to augment range. 
  
  The most outstanding examples of the use 