Understand the conditions for Equilibrium

The four forces acting on an aircraft


Source: FAS

An aircraft in powered flight can be said to be under the influence of the four main forces:

Lift (L)

Lift of main planes, acting vertically upwards through the Centre of Pressure. The main purpose of lift is to keep the aircraft airborne.

Lift is a mechanical force whereby it is generated by the interaction and contact of a solid body with a fluid (liquid or gas). In order for lift to be generated, its solid body must be in contact with the fluid. When there is no fluid, there is no lift. It does not apply for a Space Shuttle.

Lift is generated by different velocity between the solid object and the fluid. There must be motion between the fluid and the object. If there is no motion, there will be no lift. Lift acts perpendicular to the motion whereas drag acts in the direction opposite to the motion. (National Aeronautics and Space Administration)

Weight (W)

Weight , acting vertically downwards through the Centre of Gravity of aircraft.

Drag (D)

Drag, acting horizontally backwards opposing forward motion.

There are 2 components under drag:

- Induced drag, which is caused from the creation of lift and increases with the angle of attack. When the wing is not producing lift, induced drag is zero and it decreases with airspeed.

- Parasite drag, which is all drag not caused from the production of lift. Parasite drag is created by displacement of air by the aircraft, turbulence generated by the airfoil, and the hindrance of airflow as it passes over the surface of the aircraft or components. It increases with the speed and includes skin friction drag, interference drag and form drag.

Thrust (T)

Thrust, of engine(s) pulling(or pushing) the aircraft horizontally forwards. It is a force provided by the engines which is required to overcome drag (D).

Understanding the difficulties in balancing the four forces

While weight always acts vertically downwards, lift acts vertically upwards only during level (horizontal) flight. Lift is inclined backwards during climb and forwards during descent. Furthermore, the centre of pressure and centre of gravity also change in the course of flight. Line of action of thrust and drag are also similarly affected by the attitude of aircraft.

The role intended for the aircraft determines the relative position of its main planes, engines, etc. Consider a low-winged aircraft with fixed landing gear and engine mounted on its nose. It is very likely to have its drag centreline below that of thrust, resulting in nose down pitch (moment) during level flight. Lift and weight are deliberately coupled to provide nose down pitch for safety reasons.

Conditions for equilibrium

Maintaining a steady flight requires a balance and is often described as an equilibrium of all the forces acting upon an aircraft.

In a steady flight, the sum of all the opposing forces equals to zero. There can be no unbalance forces when the aircraft is flying level or when it is climbing or descending (Newton's Third Law). This does not mean that all the four forces are equal. It means that the opposing forces are equal.

  • L = W
  • D = T

If the lift is greater then the lift, the aircraft will accelerate downward.
When the thrust is greater than drag, the aircraft will accelerate forward.
If the drag is greater then the thrust, a deceleration will occur. (Source: Mansfield Connecticut)


National Aeronautics and Space Administration Retrieved on 28th August 2012
FAS Retrieved on 28th August 2012
Mansfield Connecticut Retrieved on 28th August 2012

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