The 4 Forces of Flight
By Hanna Moon
Weight is the force of gravity. It acts in a downward direction, toward the center of the earth. Weight opposes lift. Weight and lift are equal when a plane flies level at constant velocity. Because excess weight requires more lift, and therefore more thrust, heavy planes are more difficult to get off the ground as compared to lighter planes. Planes with less weight require less thrust. Thus, planes are designed to be as light as possible.
Lift is the force that acts at a right angle to the direction of motion through the air. Lift is created by differences in air pressure. Lift opposes weight. Newton's Laws and Bernoulli's Principle generate lift. A plane that sits on a runway doesn't have any lift, but it does have weight. Lift is proportional to the square of the velocity of an airplane and as a plane goes faster, its lift increases. As a plane moves forward, its lift force increases until it equals its weight. When lift equals weight, the plane can fly. In level flight, lift equals weight as the plane flies at constant velocity.
Thrust is the force that propels a flying machine in the direction of motion. Engines produce thrust. Thrust opposes drag. The engine creates thrust and moves the plane forward. (Gravity provides the thrust for a glider.) The engines push air back with the same force that the air moves the plane forward; this thrust force-pair is always equal and opposite according to Newton's 3rd Law. When thrust is greater than drag, the plane accelerates according to Newton's 2nd Law When the plane flies level at constant velocity, thrust equals drag. When the plane flies level at constant velocity, all opposite forces of flight are equal: drag = thrust and weight = lift.
Drag is the force that acts opposite to the direction of motion. Drag is caused by friction and differences in air pressure. Drag opposes thrust. Legs of birds and wheels of planes are tucked in to reduce drag. Drag is unwanted because it makes the plane or bird inefficient. Planes with more drag require more thrust to fly successfully. To reduce drag and increase efficiency, planes are streamlined. Planes also use camber and high aspect ratios to reduce drag.