Lift/Drag Ratio, Forces Interaction and Use

Lift/Drag Ratio, Forces Interaction and Use



Gain an understanding of:

  • Lift and Drag forces and the relationship between them

Interaction of Forces

The atmosphere (the air) is the medium in which an aircraft operates. It is the properties of the air, changed by the shape of the wing, that generate the required lift force.

Typically, the forces created during aircraft operation are arranged as illustrated below. They do not act through a common point. The center of pressure lies a short distance aft of the center of gravity. Since lift and weight do not act in the same line, they make up what is known as a couple. This couple has a turning effect on the aircraft, pitching the nose downwards.

In addition, the line of action of thrust is normally below the drag’s line of action, because engines are normally positioned midway or lower on the aircraft body. The thrust and drag forces constitute an opposite couple, the effect of which is to pitch the nose upwards. Thus, the two couples generally cancel each other out.

Lift, Weight, Drag, Thrust, relationship

Lift is a force which opposes the downward force of weight. It is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flightpath through the center of lift.

Drag is a rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. Drag opposes thrust, and acts rearward parallel to the relative wind.

In steady and level flight:

The WEIGHT is balanced by the LIFT,

The DRAG is balanced by the THRUST

The airplane will continue flying at the same velocity, i.e. at the same speed and in the same direction.

If during steady and level flight, thrust is increased, then the aircraft will start to accelerate in the direction of thrust (will start to gain speed). The increase in speed will lead to increase in drag.

In the reverse situation, when during steady and level flight thrust is reduced, the aircraft will start to accelerate in the direction of drag (the speed will start to decrease). The decrease in speed will lead to a decrease in drag.

Lift/Drag Ratio

Drag is the price paid to obtain lift. The lift to drag ratio (L/D) is the amount of lift generated by a wing or airfoil compared to its drag. The lift/drag ratio is used to express the relation between lift and drag and is determined by dividing the lift coefficient by the drag coefficient, CL/CD. A ratio of L/D indicates airfoil efficiency. Aircraft with higher L/D ratios are more efficient than those with lower L/D ratios.

The shape of an airfoil and other lift producing devices (i.e., flaps) affect the production of lift which will vary with changes in the AOA (Angle of Attack (AOA)). The maximum lift/drag ratio occurs at one specific CL (Lift Coefficient) and AOA (Angle of Attack (AOA)). If the aircraft is operated in steady flight at Lift/Drag maximum ratio, the total drag is at a minimum. Any AOA lower or higher than that producing the maximum Lift/Drag ratio reduces the Lift/Drag ratio and consequently increases the total drag for a given aircraft’s lift.

Lift/drag ratio also determines the glide ratio and gliding range. Since the glide ratio is based only on the relationship of the aerodynamics forces acting on the aircraft, aircraft weight will not affect it. The only effect weight has is to vary the time that the aircraft will glide for. The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio. If two aircraft have the same L/D ratio but different weights and start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. Both aircraft will cover the same distance but the lighter one will take a longer time to do so.

From the practical point of view one should remember that although it is well known that winglets reduce drag and save fuel, their effect on speed control may not have been highlighted before. Crews therefore need to be aware that reduced drag makes speed control on the approach more difficult.

Drag will also increase if the landing gear or flaps are extended and the airspeed will then decrease unless the pitch attitude is reduced. When pitch is reduced, the glide angle increases and the distance traveled will reduce.

Quiz Questions:

Q1: In steady, level, flight:

  1. all the forces are in balance
  2. the lift force must be greater than the the weight since the aircraft is flying
  3. the forces are not related to each other
  4. the thrust force is greater than the drag force


Q2: In a steady level flight: the weight force is balanced by the:

  1. drag force
  2. lift force
  3. thrust force


Q3: The weight affects the glide ratio of an aircraft? TRUE or FALSE?


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