Aircraft

The four forces

Every airplane in flight balances four forces: weight pulling it down, lift from the wings holding it up, thrust from the engines pushing it forward, and drag holding it back. When the forces balance, the plane cruises at a constant velocity. When they don’t, it accelerates in the direction of the largest force.^[2]

The main parts and what they do

• Wings generate the lift that carries the plane, its fuel, passengers and cargo, and they also create drag. Many modern airliners have winglets at the tips to reduce drag.^[1]
• Engines, either turbines or propellers, provide the thrust that pushes the plane through the air.^[1]
• The fuselage is the body that holds everything together: cockpit, passengers, cargo and sometimes fuel.^[1]
• The tail has a fixed vertical stabilizer that stops the nose swinging side to side; its hinged rudder controls that motion (yaw). A fixed horizontal stabilizer stops the nose bobbing up and down; its hinged elevator controls that motion (pitch).^[1]
• Ailerons, hinged sections on the outer part of each wing, roll the plane from side to side.^[1]
• Flaps and slats extend during takeoff and landing to increase the force the wing produces. Spoilers are small plates that disrupt the flow over the wing to reduce lift when needed.^[1]

Flying without an engine

Engines overcome drag, not weight: holding the plane up is the wings’ job. That’s why some aircraft, such as gliders, can fly with no thrust at all.^[2] Lift still needs motion through the air, though, so a glider keeps moving to stay aloft.^[3]