As pointed out earlier, there are four elementary flight forces: Lift, Weight, Drag, and Thrust.
We have already dealt with lift and drag when we considered aerofoil sections, we now look at these two forces and two others, thrust and weight, in particular with respect to their effect on the aircraft as a whole.
For the aircraft to maintain constant height then the lift force created by the aerofoil sections must be balanced by the weight of the aircraft (Figure 7.1). Similarly for an aircraft to fly with constant velocity, or zero acceleration, the thrust force must be equal to the drag force that opposes it.


Figure 7.1 shows the four flight forces acting at right angles to one another with their appropriate lines of action:
• Lift of the main planes acts perpendicular to the relative airflow through the CP of the main aerofoil sections.
• Weight acts vertically downwards through the aircraft’s CG.
• Thrust of the engines works along the engine axis approximately parallel to the direction of flight.
• Drag is the component acting rearwards parallel to the direction of the relative airflow and is the resultant of two components: induced drag and profile drag. For convenience the total drag is said to act at a point known as the center of drag.
The aeroplane is under the influence of these four main forces. These elementary flight forces are manipulated to execute elementary phases of a flight operation, like:
• Takeoff
• Climb
• Straight & Level Flight (Cruise)
• Approach
• Landing
In fact, manipulation of these forces, controlling and balancing is the art of flying.
The longest of these phases is the CRUISE or LEVEL FLIGHT.