1. The Basic Principles Behind Flight
In order for an airplane to be able to fly, thrust has to equal drag, and lift has to equal
weight. If, for any reason, the amount of drag becomes larger than the amount of thrust,
the plane will slow down. If the thrust is increased so that it is greater than the drag, the
plane will speed up. Similarly, if the amount of lift drops below the weight of the airplane,
the plane will descend. By increasing the lift, the pilot can make the airplane climb.
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.
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. Because of this
lower pressure, the airplane will be able to fly.

2. Thrust
Thrust is an aerodynamic force that has to be created by an airplane in order to
overcome the drag. Airplanes create thrust using propellers, jet engines or rockets.
Drag
Drag is an aerodynamic force that resists the forward motion of an airplane flying
through the air. To understand how drag works, stick your hand out of a car window
while moving. The amount of drag that your hand creates depends on a few factors,
such as the size of your hand, the speed of the car and the density of the air. If you
were to slow down, you would notice that the drag on your hand would decrease. This is
why, after takeoff, a passenger jet always retracts its landing gear. The amount of drag
produced by the landing gear of a jet is so great that, at cruising speed, the gear would
be ripped right off of the plane!
Weight
This one is the easiest principles of flight to understand. Every object on earth has
weight, and it is caused by the downward pull of gravity.
Lift
Lift is the aerodynamic force that holds an airplane in the air, and is created by air
moving over the wings. The air moving over the top of the wing is moving faster than the
air across the bottom, thus a lower pressure is created on the top of the wing. Because
of this lower pressure, the airplane will be able to fly.