Unit 7, lesson 35. About 15 minutes.

Air pressure and flight

How can a plane heavier than a house stay up in the sky?

Play

Getting the experiment ready…

Explain

Fast air lifts wings up.

Planes need speed to fly.

Air presses on everything around you. We do not notice, because it presses from all sides.

A wing moves fast through the air and pushes the air down. The air pushes the wing up. That upward force is called lift.

A plane flies when lift is as big as its weight. Faster planes and bigger wings have more lift.

Lift can be explained two ways: pressure is lower above the wing than below, and the wing pushes air down, as in Newton's third law.

L = ½ρv²AC. Doubling the speed makes four times the lift.

Worked example: at 50 m/s with a 15 m² wing, L = ½ × 1.2 × 2500 × 15 = 22 500 N.

Engines give speed
Wings push air down
Air pushes wings up

Want the full story?

The usual school story, that air on top must speed up to catch up with the air below, is wrong. Air above the wing does go faster, but not for that reason. The wing number C depends on shape and angle, and is taken as 1 in the sim.

The Formula

lift = half × air density × speed × speed × wing area × wing shape number

L = \tfrac{1}{2} \rho v^2 A C

$L$: the lift force, measured in newtons (N)
$\rho$: the air density, about 1.2, measured in kilograms per cubic metre (kg/m³)
$v$: the speed through the air, measured in metres per second (m/s)
$A$: the wing area, measured in square metres (m²)
$C$: the wing shape number, taken as 1 here, which has no unit

Try the numbers

Speed: 50 m/sWing area: 15 m²

Lift: 22,500 N

Try it at home

You need a strip of paper.

You need: a strip of paper.

Blow over a paper strip. It lifts!

Hold a strip of paper just under your lower lip.
Blow hard across the top of it.
Watch the strip rise.

What's happening: the moving air curves down around the strip and pulls it up, a bit like air around a wing.

Physics is life

Your body

You breathe with air pressure.

Your lungs pull air in by lowering the pressure inside your chest.

Daily life

Straws use air pressure.

When you sip through a straw, air pressure pushes the drink up.

Nature

Birds fly with wings.

A bird's wing is shaped to give lift, and many birds soar on rising air.

Space

Spacecraft cannot use wings in space.

Wings need air, so rockets must carry their own thrust into space.

Quick check

Question 1 of 3

What is lift?

Words to know

Lift: The upward force on a wing as it moves through the air.

Lift: The upward force on a wing as it moves through the air.
Pressure: How much force presses on each bit of area.
Air resistance: The force from air pushing against something moving through it.

Next up

Hot and cold: temperature vs heat