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In a steady descending flight (descent angle 3) equilibrium of forces acting on the aeroplane is given: (T = THRUST, D = DRAG, W = WEIGHT)
• A
T-W cos 3 = D
• B
T+W sin 3 = D
• C
T-D=W sin 3
• D
T+D= - W sin 3

Refer to figure.

In a steady descent, thrust is reduced. As a result, the aircraft slows down due to a reduced forward force (Thrust) in comparison to the backward force (Drag).

In order for the aircraft to maintain speed and be in a steady descent, the forward component of weight must be increased to balance the Drag. This is achieved by lowering the nose (increasing descent angle y) until this component of weight increases providing enough forward force to balance the drag.
At this point, the backward and forward forces are balanced. Drag (D) is balanced by the Thrust (T) and Weight component (W sin y) - "y" being the angle of descent.

Resulting on the following equation:
D = T + W sin y

In this case, Descent angle = 3. Therefore, balance is represented:
• D = T + W sin 3

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