text: ASYMMETRICAL POWER. The design 
  of a multiengine airplane must account for the 
  possibility of an engine failure at low airspeed. 
  The unbalance of thrust from a condition of 
  unsymmetrical power produces a yawing mo- 
  ment dependent upon the thrust unbalance 
  and the lever arm of the force. The deflection 
  of the rudder will create a side force on the tail 
  and contribute a yawing moment to balance 
  the yawing moment due to the unbalance of 
  thrust. Since the yawing moment coefficient 
  from the unbalance of thrust will be greatest 
  at low speed, the critical requirement will be 
  at a low speed with 
  the one critical engine 
  out and the remaining engines at maximum 
  power. Figure 4.26 compares the yawing 
  moment coeflicient for maximum rudder deflec- 
  tion with 
  the yawing moment coefficient for 
  the unbalance of thrust. The intersection of 
  the two lines,determines the minimum speed 
  for directional control, i.e., the lowest speed 
  at which the rudder control moment can equal 
  It is usually 
  the moment of unbalanced thrust, 
  specified that the minimum directional control 
  speed be no greater than 1.2 times the stall 