What makes a plane fly can be explained by the four forces acting on an aircraft in straight and level flight: lift, gravity, thrust, and drag. The opposing forces balance each other, with lift equaling gravity and thrust equaling drag. Here is a brief explanation of each force:

• Thrust: The force that moves an airplane forward through the air. Thrust is created by a propeller or a jet engine.

• Drag: The air resistance that tends to slow the forward movement of an airplane.

• Gravity: The force that pulls all objects towards the earth.

• Lift: The upward force that is created by the movement of air above and below a wing. Air flows faster above the wing and slower below the wing, creating a difference in pressure that tends to keep an airplane flying.

The shape of an airplanes wing is designed to make air move faster over the top of the wing. When air moves faster, the pressure of the air decreases. So the pressure on the top of the wing is less than the pressure on the bottom of the wing. The difference in pressure creates a force on the wing that lifts the wing up into the air.

An airplane wing is also shaped so that the air is deflected downward as the wing passes. Because air is a fluid, both the top and the bottom surface of the wing deflect the air. This is very different than dealing with solid pellets for which only the bottom surface would deflect.

The faster an airplane travels, the more lift is generated. Inclining the wing to the wind also produces more deflection and more lift. The wings of an airplane have adjustable flaps that can be extended or retracted. When extended, the flaps increase the deflection of the air and provide greater lift for takeoff and landing.

In addition to the four forces, a pilot of a plane has special controls that can be used to fly the plane. There are levers and buttons that the pilot can push to change the yaw, pitch, and roll of the plane. To roll the plane to the right or left, the ailerons are raised on one wing and lowered on the other. The wing with the lowered aileron rises while the wing with the raised aileron drops. Pitch makes a plane descend or climb. The pilot adjusts the elevators on the tail to make a plane descend or climb. Lowering the elevators causes the airplanes nose to drop, sending the plane into a down. Raising the elevators causes the airplane to climb. Yaw is the turning of a plane. When the rudder is turned to one side, the airplane moves left or right. The airplanes nose is pointed in the same direction as the direction of the rudder. The rudder and the ailerons are used together to make a turn.