How Airplanes Stay in the Air: The Four Forces of Flight
Every flight comes down to four forces: lift, weight, thrust and drag. See what each one does, how pilots change the balance for takeoff, cruising and landing, and why engines do not lift the plane.
In this article
An airplane stays up because four forces are in play at once: lift, weight, thrust and drag. Two of them push the plane up and forward, and two pull it down and back. Everything a pilot does with the controls and the engines is a way of changing how those four forces compare. This article explains each one, using material from NASA's Glenn Research Center and the Smithsonian's National Air and Space Museum.
Four forces, two pairs
The forces come in two opposing pairs. Weight is opposed by lift, and drag is opposed by thrust. NASA's beginner's guide to aeronautics puts it simply: the engines overcome drag, and they do not provide lift. That surprises many people, because engines are the loud, obvious part of a plane. Their job is to push the aircraft through the air. The lifting is done by the wings.
- Weight pulls the aircraft toward the center of the Earth.
- Lift acts at a right angle to the direction of travel and holds the aircraft up.
- Thrust pushes the aircraft in the direction it is moving.
- Drag acts against the direction of travel.
Weight: gravity never stops
Weight is the force of gravity on the aircraft, pulling downward toward the center of the Earth. NASA explains that its size depends on the mass of everything on board: the structure of the airplane, the fuel, and the payload, meaning passengers, baggage and freight. Weight also changes during a flight, because fuel is burned. As the plane gets lighter, pilots adjust the controls to keep it trimmed.
Lift: the wings do the work
Lift is an aerodynamic force, which means it is created by the aircraft moving through the air. It acts perpendicular to the flight direction and opposes weight. The Smithsonian's How Things Fly pages say lift is created by differences in air pressure. NASA adds that the amount of lift depends on the shape, size and speed of the aircraft, and that most of it comes from the wings.
Thrust and drag: pushing against the air
Drag is the air's resistance to the aircraft's motion. It acts along the flight path, in the opposite direction to travel. According to NASA it depends on the shape of the aircraft, on the air's viscosity (its stickiness) and on speed. The Smithsonian notes that drag is caused by friction and by differences in air pressure.
Thrust comes from the engines, and its size depends on the type of engine, how many there are, and the throttle setting. In a jet, NASA explains, thrust is a reaction to the exhaust gas leaving the engine nozzle, an example of Newton's third law: the gas goes backward and the engine, and so the plane, is pushed forward.
Takeoff, cruising and landing
The Smithsonian sets out what has to be true at each stage of a flight.
- Takeoff: thrust must be greater than drag, and lift must be greater than weight.
- Level flight: lift must equal weight, and thrust must equal drag.
- Landing: thrust must be less than drag, and lift must be less than weight.
This is why a plane accelerates down the runway before it leaves the ground. While the wings are not yet producing more lift than the plane weighs, it stays on the ground. Once the speed is high enough, lift exceeds weight and the plane climbs.
When the forces are not balanced
When all four forces are balanced, the aircraft cruises at a constant speed in a steady direction. NASA's guide notes that if the forces are unbalanced, the aircraft accelerates in the direction of the largest force. Following that rule, thrust greater than drag speeds the plane up, and thrust less than drag slows it down. The Smithsonian adds that the balance shifts whenever an airplane climbs, descends, speeds up, slows down or turns.
Why a spacecraft is different
Lift and drag both depend on air. In the near-vacuum above the atmosphere, the Smithsonian points out, a spacecraft still has weight and still uses thrust, but lift and drag are absent. Only two of the four forces remain. Without air, wings have nothing to work with, which is why a spacecraft beyond the atmosphere does not stay up by lift at all.
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