AskELI5

everyday-engineering

How Airplanes Fly

An airplane stays up by pushing air down as it moves forward.

Hold your hand outside a moving car and tilt it slightly. The air pushes your hand upward because your hand turns some of that moving air downward.

  1. An airplane surrounded by arrows for upward lift, downward weight, forward thrust, and backward drag.VerticalLift goes upWeight goes downHorizontalThrust goesforwardDrag goes backward

    Step 1

    Four forces pull and push

    Weight pulls down, lift pushes up, thrust moves forward, and drag pushes backward.

  2. An engine pushing air backward while the airplane moves forward.Push waterbackward=Move a boatforwardPush air backward=Move a planeforward

    Step 2

    Engines create forward thrust

    Propellers or jet engines push air backward, and the airplane is pushed forward.

  3. Air flowing around the curved top and lower surface of a moving wing.Moving airWingAir turns down

    Step 3

    Moving air meets the wing

    Forward motion makes air flow around the wing's curved shape and tilted angle.

  4. A wing directing air downward and receiving an upward lift force.AirTurns downwardGains downwardmomentumWingFeels pressureMoves upward

    Step 4

    The wing turns air downward

    The wing changes the air's direction. The resulting pressure forces push the wing upward as lift.

  5. Balanced scales showing lift against weight and thrust against drag.Less forceMore forceLiftWeightThrustDrag

    Step 5

    Enough lift balances weight

    For steady level flight, lift balances weight while thrust balances drag. Changing that balance changes the motion.

  6. Smooth airflow over a wing compared with separated airflow during a stall.Attached airflowModerate angleSteady liftSeparated airflowAngle too largeLift drops

    Step 6

    Too much tilt can stall

    At too large an angle of attack, airflow separates from the wing and lift drops sharply.

Common trap

Air must travel over the wing's top and bottom in equal time.

There is no equal-time rule. Wing shape and angle create a pressure pattern and turn air downward, producing lift.

Quick check

What must lift balance during steady level flight?

  1. The airplane's weight
  2. The airplane's color
  3. The runway length
Show the answer

When lift balances weight, the airplane can maintain altitude instead of accelerating up or down.

Sources

  1. Four Forces on an AirplaneNASA Glenn Research Center
  2. What Is Lift?NASA Glenn Research Center
  3. Pilot's Handbook of Aeronautical KnowledgeFederal Aviation Administration