# Principles of Flight

FORCES ACTING ON A PLANE

There are four forces acting on the airplane all the time during airplane is flying.The four forces are
(1) Lift, (2) Gravity force or Weight, (3) Thrust, and (4) Drag.
Lift and Drag are considered aerodynamics forces because they exist due to the movement of the Airplane through the Air. Lift: is produced by a lower pressure created on the upper surface of an airplane’s wings compared to the pressure on the wing’s lower surfaces,causing the wing to be LIFTED upward. The special shape of the airplane wing (airfoil) is designed so that air flowing over it will have to travel a greater distance and faster resulting in a lower pressure area (see illustration) thus lifting the wing upward. Lift is that force which opposes the force of gravity (or weight).

Lift depends upon:

(1) shape of the airfoil,

(2) the angle of attack,

(3) the area of the surface exposed to the airstream,

(4) the square of the air speed,

(5) the air density.

Weight: The weight acts vertically downward from the center of gravity (CG) of the airplane.

Thrust: is defined as the forward direction pushing or pulling force developed by aircraft engine . This includes reciprocating engines , turbojet engines, turboprop engines.

Drag: is the force which opposes the forward motion of airplane. specifically, drag is a retarding force acting upon a body in motion through a fluid, parallel to the direction of motion of a body. It is the friction of the air as it meets and passes over an airplane and its components. Drag is created by air impact force, skin friction, and displacement of the air.

THE AXES OF ROTATION

An airplane has three axes of rotation, namely , the longitudinal axis, the vertical axis, and the lateral axis. see figure below and you will understand what we mean. The simplest way to understand the axes is to think of them as long rods passing through the aircraft where each will intersect the other two. At this point of intersection, called the center of gravity. The Axis that extends lengthwise (nose through tail) is call the longitudinal axis, and the rotation about this axis is called “Roll”

The axis that extends crosswise (wing tip through wing tip) is called the lateral axis, and rotation about this axis is called “Pitch”

The axis that passes vertically through the center of gravity (when the aircraft is in level flight ) is called the vertical axis, and rotation about this axis is called “Yaw”

Reference: #thai technics 