Aircraft Motion
Physics of Aircraft
Weight and Thrust
Secondary Controls
Straight and Level
Aircraft Design Features
The Stall
Practice Exam

Factors Affecting the Descent

Flap in a Descent

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.

The increase in drag from flaps is greater than the increase in lift, so L/D Ratio will always decrease with flap extension.

Power in a Descent

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.

Weight in a Descent

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.

Earlier we mentioned your flight manual will state a maximum glide airspeed. This is typically the maximum glide speed at the maximum take off weight. As weight is reduced, the maximum glide speed will also reduce. However, the reduction in maximum glide speed in small training aircraft is typically only a few knots – most pilots will struggle to fly that accurately and so in light aircraft one airspeed is usually used and memorised, regardless of actual weight.