PRINCIPLES OF FLIGHT
The
Basic Principles Behind Flight
In order for an aircraft 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 aircraft will speed up. Similarly, if the amount of lift drops below the weight of the aircraft, the aircraft will descend. By increasing the lift, the pilot can make the aircraft climb.
Thrust
Thrust is an aerodynamic force that has to be created by an aircraft in order to
overcome the drag. Aircraft create thrust using propellers, jet engines or
rockets.
Drag
Drag is an aerodynamic force that resists the forward motion of an aircraft
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
aircraft!
Weight/Gravity
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 produced by a lower pressure created on the
upper surface of an aircraft's wing compared to the pressure on the wing's lower
surface, causing the wing to be "lifted" upward. The special shape of
the wing (airfoil) is designed so that air flowing over it will
have to travel a greater distance 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).
An aircraft in flight is the centre of a continuous tug of war between four forces: lift, gravity force or weight, thrust, and drag. Lift and Drag are considered aerodynamic forces because they exist due to the movement of the aircraft through the air. The weight pulls down on the aircraft opposing the lift created by air flowing over the wing. Thrust opposes drag caused by air resistance to the frontal area of the aircraft. During take off, thrust must overcome drag and lift must overcome the weight before the aircraft can become airborne. In level flight at constant speed, thrust exactly equals drag and lift exactly equals the weight or gravity force. For landings thrust must be reduced below the level of drag and lift below the level of the gravity force or weight.