What Makes a Boomerang Keep Coming Back?

This question was answered by Dr Hugh Hunt, Dept. Engineering, Cambridge University:

That's a really good question. A boomerang goes around on more or less a circular path. The motion is really just fantastic. It's a combination of various physical principles for example, aerodynamic lift and circular motion. You've got to get these physical principles just right when you throw the boomerang which explains why boomerangs are a bit tricky to throw. Think of the two arms of the boomerang as being just like the wings of an aeroplane. The faster they move through the air, the more lift they generate. Unlike an aeroplane a boomerang spins as it moves through the air and the combination of spin and forward speed means that some parts of the boomerang are moving faster than others. This means that the aerodynamic lift is not uniform over all parts of the boomerang. The wings of an aeroplane are horizontal so the lift is upwards. The wings of a boomerang are sideways so the net lift is towards the centre of the circle that you see the boomerang move on.

There's one more important physical principle. The non-uniform lift generates torque or a moment, a twisting force - whatever you like to call it and this causes the gyroscopic effect to come into play. A spinning boomerang is really no different to a spinning gyroscope and the gyroscopic effect makes the boomerang turn around nicely, at just the right rate. It really is magical and the best bit of all this is that the entire explanation rests on Newton's laws of motion.

There's one more little catch, and that's to do with gravity. You need a bit of lift force directed upwards. Otherwise the boomerang will just drop down to the ground in no time. This explains why you need to throw a boomerang with a little bit of a tilt. It also explains why a good boomerang will increase its tilt angle as it slows down. This is called laying over. The boomerang that has slowed down and laid over is really easy to catch.