The Helicopter is a Charleston dance move which creates the illusion that your leg turning in a way which looks impossible.
The key to mastering this move is not to think ‘I will make a full rotation with my lower leg’. Instead think of it as a flick of the leg as if you are brushing something off your shoe. You put a lot of energy into the flick, then relax and let the inertia take you all the way around.
The standing foot should also swivel but you can build up to this. Keep the knees close together and make sure there is room for the flicking foot to pass behind the foot of the standing legs. You can then add arms, classic positions are 'stirring the pot' or flat palm making a circle.
Work on your posture so you are well balanced and the upper body looks relaxed.