text: the wing 
  field where higher relative 
  downwash exists. Thus, a decrease in stability 
  would take place. 
  
  flow 
  
  Certain changes in the flow field behind the 
  wing at high angles of attack can produce large 
  changes in the tail contribution 
  to stability. 
  If the wing tips stall first, the vortices shift in- 
  board and increase the local downwash at the 
  tail for a given airplane C,. Also, the fusel~age 
  at high angle of attack can produce strong 
  cross flow separation vortices which increase 
  the local downwash for a horizontal tail placed 
  above the fuselage. Either one or a combiua- 
  tion of these downwash influences may provide 
  a large unstable contribution of the horizontal 
  tail. 
  
  generally applies to 
  the static longitudinal 
  instability encountered 
  by certain configurations at high angle of 
  attack. The condition of pitch-up is illustrated 
  by the graph of CM versus C, in figure 4.33. 
  Positive static longitudinal stability 
  is evident 
  at low values of Cs by the negative slope of the 
  curve. At higher values of Cs the curve changes 
  to a positive slope and large positive pitching 
  moments are developed. This sort of 
  in- 
  stability 
  implies that an increase in angle of 
  attack produces nose up moments which tend 
  to bring about further 
  increases in angle of 
  attack hence the term “pitch-up” 
  
  The pitch-up instability 