As in climbs, the forces acting on the airplane go thorough definite changes when a descent is entered from straight and level flight. The analysis here is that of descending at the same power as used in straight and level flight.
When forward pressure is applied to the elevator control to start descending, or the airplane's nose is allowed to pitch down, the angle of attack is decreased and, as a result, the lift of the airfoil is reduced. This reduction in total lift and angle of attack is momentary and occur during the time the flightpath changes downward. The change to a downward flightpath is due to the lift momentarily becoming less than the weight of the airplane as the angle of attack is reduced. This imbalance between lift and weight causes the airplane to follow a descending flightpath with respect to the horizontal flightpath of straight and level flight.
When the flightpath is in a steady descent, the airfoil's angle of attack again approaches the original value, and lift and weight will again become stabilized. From the time the descent is started until it is stabilized, the airspeed will gradually increase. This is due to a component of weight now acting forward along the flightpath, similar to the manner it acted rearward in a climb. The overall effect is that of increased power or thrust, which in turn causes the increase in airspeed associated with descending at the same power as used in level flight.
To descend at the same airspeed as used in straight and
level flight, obviously, the power must be reduced as the descent is entered.
The component of weight acting forward along the flightpath will increase
as the angle of rate of descent increases and conversely, will decrease
as the angle of rate of descent decreases. Therefore, the amount of power
reduction required for a descent at the same speed as cruise will be determined
by the steepness of the descent.