text: effect on takeoff distance and is approximated 
  by the following relationship: 
  
  $2 v 2 
  ..-.= 
  13 
  Sl c 
  1 
  
  where 
  
  Si= zero wind landing distance 
  Sa=landing distance into a headwind 
  I’, = headwind velocity 
  Vi=landing 
  
  ground velocity with zero 
  wind or, simply, the landing airspeed 