text: the airplane exhibits static directional 
  stability with rudder lixed, each angle of side- 
  the 
  slip requires a particular deflection of 
  rudder to achieve equilibrium. Rudder-free 
  directional stability will exist when the float 
  angle of the rudder is less than the rudder 
  deflection required for equilibrium. However 
  at high angles of sideslip, the floating 
  tend- 
  ency of the rudder increases. This is illus- 
  trated by the second chart of figure 4.25 where 
  the line of rudder float angle shows a sharp 
  increase at large values of sideslip. 
  the 
  floating angle of the rudder catches up with 
  the required rudder angle, the, rudder pedal 
  force will decrease to zero and rudder lock will 
  occur. Sideslip angles beyond this point pro- 
  duce a floating angle greater than the required 
  rudder deflection and the rudder tends to float 
  to the limit of deflection. 
  
  If 
  
  Rudder lock is accompanied by a reversal of 
  pedal force and rudder-free instability will 
  exist. The dorsal fin is a useful addition in 
  this case since it will 
  improve the directional 
  stability at high angles of sideslip. The re- 
  sulting 
  requires larger 
  deflections of the rudder to achieve equilibrium 
  at high sideslip and the tendency for rudder 
  lock is reduced. 