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Boomerangs were the world's first heavier than air
flying machines, used most famously by the Australian
Aborigines, but also found in other ancient cultures in
Egypt, stone age Europe and the Indian Sub Continent.
No-one knows how they were first invented and even
though they look simple, they use a very complex
combination of physics and aerodynamics to perform their
amazing returning flights.
A returning boomerang is basically two wings joined at
an angle of between 80o and 120o,
although it can have more than two wings. The wings are
arranged so they work best when the boomerang is
rotating rather than flying straight like an aeroplane.
The combination of spin with forward motion causes
uneven lift on the wings because at any given time, one
wing is rotating forward in the same direction as the
flight, while the other is rotating backwards, against
the direction of flight. This means the air flow over
the wing on one side of the disk of rotation has a
higher airspeed than the other wing and so generates
more lift. The uneven lift tries to tip the boomerang
over, but just like leaning a moving bike over makes it
turn, the boomerang's spin twists the tipping force at
right angles and gives the boomerang a curving flight.
Another tipping force, caused by the centre of lift
being forward of the centre of gravity, is also twisted
to make the boomerang "lie down" in flight.
The name for both these twisting motions is gyroscopic
precession. The illustration above shows how it all
comes together to make the boomerang fly in a circle and
(given some skill) come back. 
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