Natural Born

Flyers

However clever our engineers become,

they will always struggle to match the

variety of flying techniques that birds

have developed through millions of years

of evolution.

UITE   naturally, a bird wants to be able

to cover as much ground as it needs to

while wasting as little energy as

possible. Birds flap their wings, using fast-twitch

muscle fibres to push them forwards to generate

lift but that requires a lot of energy. Once they

are in the air, birds can ‘soar’ to great heights on

rising currents of air, the height then allowing

them to glide long distances without having to

flap too hard. Birds of prey such as kestrels can

hover, turning into the wind and flapping their

wings to hold themselves stationary in the air

while they scan the ground for a tasty mouse or

other scurrying creature. Then, they swoop.

Q

To follow fast-moving prey on the ground,

a bird of prey might swoop very low, gliding

along at daffodil height with barely a twitch

of its wing. When they are doing this they are

demonstrating how well they have mastered

a phenomenon known as ‘ground effect’. We

have seen how the airflow over a wing creates

a difference in pressure that generates lift but

where the surface of the wing comes to an end at

the wing tip, a disturbance is created that causes

a swirling vortex of air. This normally acts as an

extra form of drag, reducing the lift generated

by the wing, but when the wing is close to the

ground, the vortex can’t form properly and

without its drag factor the wing generates lift

more efficiently.

116  HUMAN POWERED AIRCRAFT

The close proximity of the ground also

compresses the air between the bird and the

ground, increasing the pressure beneath the

wing even more than in normal flight and again

producing more lift. Flying low, then, is a good

way of covering ground using the least amount

of energy. Can this really work for aircraft as

well as birds? In the late 1980s, the Soviet navy

had a seaplane called a Lun Ekranoplan which

was one of the biggest aeroplanes ever built,

longer than a 747 jumbo jet and powered by

eight jet engines. It was designed as a missile

platform and cruised at a height of less than

4 metres (13 feet), using the ground effect ‘air

cushion’ created by its wings to keep it flying

just above the waves.

Below: The different stages involved in one flap

of a bird’s wings, showing how lift is generated.