Extending flaps increases both lift and drag, but the increase in drag is greater than the increase in lift. So when the flaps are extended in a descent the L/D Ratio reduces, causing both the descent angle and descent rate to increase.
This is typically used during the approach to land where the flap allows a steeper approach angle without increasing airspeed.
With flaps extended, the stalling speed is also reduced giving a greater safety margin during approach – we will learn more about this in the lessons on stalling.
Let’s add some power in to the descent now. The thrust from the propeller acts opposite to drag and the aircraft reacts as if the drag has been reduced. Assuming airspeed remains the same, the descent path becomes shallower.
In flight, the pilot typically uses the throttle to control rate of descent and the pitch attitude to control speed.
If the pilot continues to increase thrust until it is equal to drag, the aircraft would return to straight and level flight.
As we have learnt, the descent angle in a power off descent is determined by the L/D Ratio. Since it is the design of the aircraft and the angle of attack that determines the L/D Ratio, weight will have no effect on gliding distance.
If two of the same aircraft are gliding at different weights, they will glide the same distance providing they fly at the same angle of attack (and thus the same L/D Ratio). The heavier aircraft will glide at a faster speed at this angle of attack – gliding along the same path but at a higher airspeed.