Refer to figure.

CLIMB
There are four forces that act on an aircraft in a steady unaccelerated climb: Lift, Weight, Thrust and Drag. Lift, Drag and Thrust are aerodynamic forces which are defined relative to the flight path. Where Lift is perpendicular to the flight path, whereas Drag and Thrust act along it. The Weight of an aircraft is directed towards the centre of the Earth and is, therefore, along the vertical axis.

Weight will be resolved into two different components, as you can see on the attached figure: a force which is opposite to the lift (W cos y) and another force which acts in the same direction as drag (W sin y). In order for the aircraft to be in equilibrium in a steady climb, Thrust will be equal to Drag + backwards component of weight; and Lift must be equal to the opposing component of weight. Lift will be less than Weight because Lift only has to oppose a portion of Weight.

• It is the excess of thrust over drag that determines the climb angle and therefore the climb gradient.
  Climb Gradient = ((Thurst - Drag) / Weight) x 100