text: The forces acting on the airplane during a 
  climb are shown by the illustration of figure 
  2.21. When the airplane is in steady flight 
  with moderate angle of climb, 
  the vertical 
  component of lift is very nearly the same as the 
  actual lift. Such climbing flight would exist 
  with 
  the lift very nearly equal to the weight. 
  The net thrust of the powerplant may be in- 
  clined relative to the flight path but this effect 
  will be neglectec! for the sake of simplicity. 
  Note that the weight of the aircraft is vertical 
  but a component of weight will act aft along 
  the flight path. 
  
  depends on the difference between thrust and 
  or excess thrust. Of course, 
  drag (T-D), 
  or 
  when the excess thrust is zero (T-D=0 
  T=D), 
  the inclination of, the flight path is 
  zero-and the airplane is in steady, level flight. 
  When the thrust is greater than the drag, the 
  excess thrust will allow a climb angle depend- 
  ing on the value of excess thrust. Also, when 
  the thrust is less than the drag. the deficiency 
  of thrust will allow an angle ~of descent. 
  
  If it is assumed that the aircraft is in a steady 
  climb with essentially small inclination of the 
  flight path, the summation of forces along the 
  flight path resolves to the following: 