a bit depending on how the wing is to operate

(a supersonic jet fighter will have a different

cross-section to that of a glider) the upper

surface of the wing generally curves outwards

more than the lower surface, and the front of

the wing (known as the leading edge) is the

thicker part of the teardrop. The shape of the

wing’s cross-section forces air passing over

the upper surface to travel faster than the air

passing beneath the lower surface. At the thin

rear edge of the wing, the trailing edge, the air

flow that has been split by the leading edge

comes back together again, but because of the

curve on the upper surface the air that has

passed above the wing has further to go, so it

has to travel faster to keep up. As a result, the

air particles become more spaced out. This

creates an area of low pressure above the wing

and, with the air above the wing exerting less

pressure than the air below, the result is that

the wing is pushed upwards, creating lift.

Everything on Earth has weight (some
people have more than others!), which is a
combination of mass and the effect of gravity.

It is easy to confuse mass and weight, and for

the purposes of working out how an aeroplane

flies we have to think of weight as a downward

force in order to balance it with the other forces

(thrust, drag and lift) that are at work.

This is where Sir Isaac Newton crops up

again. According to Newton’s ‘Second Law

of Motion’ (the one we skipped over in the

first chapter) force (F) is a product of mass

(m) and acceleration (a). Force is measured in

Newtons, named after good old Sir Isaac, mass

is measured in kilograms, and the acceleration

involved is the effect of gravity. Gravity is the

thing that attracts everything in the universe

towards everything else. That means that

you are attracting the earth towards you but,

because the earth is so much bigger than you

and has so much more mass, its gravitational

pull is far greater than yours. If you jump in the

air, the earth pulls you back towards it, rather

than the earth being pulled towards you. The

effect of the earth’s gravitational pull causes

objects to accelerate towards it at 9.8 metres

per second per second. The figure of 9.8 metres

per second is about 22 mph, meaning that an

object falling towards the earth will accelerate

from rest to 22 mph after the first second,

44 mph after the next second and so on. The

scientists and engineers use metres per second

rather than miles per hour in their calculations,

which is why the acceleration is given as 9.8

metres per second (the speed) per second

(the rate of acceleration). To avoid writing 9.8

metres per second per second all the time, it

is normally written as 9.8 metres per second

squared – 9.8m/s2.

UNDERSTANDING WINGS AND THINGS   95