# The principles of flight

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## Lift, thrust, weight and drag aerodynamic forces

Flight is the result of four basic forces: lift, weight, thrust and drag.

Lift lifts you up. Weight pulls you down. Drag slows you down. Thrust thrusts you forward.

If you're flying straight and level at a constant speed, thrust equals drag and lift equals weight.

If thrust is bigger than drag, you speed up. If thrust is smaller, you slow down. If lift exceeds weight, you climb. If lift is less, down you go.

The four forces: An aircraft's motion is the net result of its lift, weight, drag and thrust

How wings work: This diagram of a wing in cross-section shows how the different speeds of the air passing above and below the wing cause a pressure difference, resulting in lift

Thrust

The thrust moving the plane comes from the propeller. It is like a special, spinning wing which pulls air past its blades.

Drag

Hold a sheet of paper vertically in front of an electric fan (at a safe distance). Drag is the force pushing the paper backwards. Skateboarders speed up by crouching, to reduce drag. Modern planes retract landing gear to reduce drag.

On a modern jet at full speed, there is enough drag to tear off the undercarriage if it is not retracted. Early Q.A.N.T.A.S. planes flew too slowly for this to matter. Drag o­nly occurs in a moving fluid. Stop the fan and the force o­n the paper vanishes.

Weight

You experience your own weight all day every day. Unless you're an astronaut in space.

Lift

Lift is the force keeping a plane in the air. Most lift is created by the wings. Like drag, lift o­nly exists in a moving fluid, such as air.

How lift and drag are created

When air moves over curved surfaces, some speeds up. Some slows down. Some crowds together. Some spreads out thinly.

This causes changes in pressure all over the surface. Lift or drag comes from adding together all of these small changes.

Air approaching the top surface of a wing is compressed upwards. As the wing curves down and away from the airstream, a low-pressure area develops. Air above is pulled down toward the back of the wing.

Air approaching the bottom surface is slowed, compressed and directed downward.

Wing shape helps determine how much pressure change occurs and thus the amount of lift or drag.

Roll, pitch and yaw aircraft control

The tail controls a planes direction by two small wings horizontal and vertical stabilisers.

The horizontal tail wing controls whether the plane goes up or down.

The vertical tail wing controls turning left or right.

On the outer ends, ailerons turn the plane and keep it level. And that takes us into roll, pitch and yaw.

The three axes: Roll, pitch and yaw describe an aircraft's motion about three axes which intersect at its centre of gravity

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