As an airplane moves through the air it experiences several forces acting on its structure. The pattern of the airflow around the aircraft and the magnitude of the aerodynamic force depend on various variables which have to be taken into account. These variables are:
- shape of the aircraft
- angle of attack (attitude to the freestream airflow)
- speed of the aircraft (true airspeed)
- size of the aircraft (frontal area)
- air density
- viscosity (stickyness or thickness)
In general there are four main forces acting on the airplane during flight (figure 1.1). Like all other objects (whether moving or not), an airplane has weight. Weight is the force of gravity acting through the Centre of Gravity* (CoG) of the airplane in a vertical direction toward the centre of the earth.
|Figure 1.1 - Forces acting on an airplane, all in equilibrium
While the airplane is on the ground, its weight is supported by the force of the ground on the airplane which acts upward through the wheels. As the wings generate lift during take-off roll, the task of supporting the airplanes weight is transferred from the ground to the wings. In straight and level flight the weight of the airplane is supported by the lift force. Therefore lift and weight are equal and act in opposite directions. When an airplane moves forward through air it will experience a retarding force known as drag , which will cause the airplane to decelerate, unless counteracted. In straight and level flight, the engine creates thrust, which neutralizes drag. In a balanced flight, drag and thrust are equal and act in opposite directions. From this we can conclude that when an airplane is in equilibrium, all forces balance eachother out. As said, during unaccelerated straight and level flight the resultant total force is zero. During take-off for example this is not the case since thrust has to exceed drag in order to become and remain airborne.
The changing pressure distribution around the aircraft and/or an airfoil illustrates the way in which this force is produced. The relevant priciples are as follows:
These principles will help explain forces acting on an aircraft during different flight phases.
We'll get more into detail with the relevant subsections.