CENTRIPETAL FORCE (YR12)

The Yak 'planes looping over New Plymouth demonstrate circular motion.
Circular motion requires a centripetal force, which is a force directed towards the centre of the circular motion. This force is provided by the control surfaces of the aircraft pushing against the air, and by the aerofoil effect (lift) of the wings
At the top of the loop, gravity will provide some of the centripetal force, so the pilot will feel some "weightlessness"; but at the bottom of the loop, the 'plane has to have enough lift to overcome gravity and provide the necessary centripetal force and the pilot will experience a strong upward force from the seat, making him feel "squashed down".

Looping is also interesting from an energy perspective.
As the 'plane climbs into the loop it loses kinetic energy and gains gravitational potential energy. It needs enough kinetic energy at the bottom, to supply the potential energy at the top, and still have enough kinetic energy to keep flying.