Science Questions

Boomerang Physics

Sun, 13th Jan 2008

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Question

Anand, Colchester asked:

How does a boomerang work and what is the principle behind that?

Answer

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

A typical wooden boomerang

Thatís a really good question.  A boomerang goes around on more or less a circular path.  The motion is really just fantastic.  Itís a combination of various physical principles for example, aerodynamic lift and circular motion.  Youíve 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.

Thereís one more important physical principle.  The non-uniform lift generates torque or a moment, a twisting force Ė 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 Newtonís laws of motion.

Thereís one more little catch, and thatís 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.

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I think it's a combination a wing shape and precession, but I'm only guessing.

By some coincidence I was give one for Christmas and there was me thinking "not more coat hangers" before I opened it. turnipsock, Tue, 8th Jan 2008

Perhaps a more intriguing question would be, why doesn't any old stick come back if thrown with the technique as a boomerang? sciencebase, Wed, 9th Jan 2008

A Boomerang is shaped like a plane wing, one wing longer and fatter that the other.... As it spins it receives a lift, just as the plane wing. However due to the lack of symmetry one wing is lagging compaired to the other, which as a result will glide quicker through the air thus causing the curved trajectory. IF the boomerang is well balanced and thrower is experienced, he or she can cause the length of the flight of the boomerang to complete a full circle, thus landing back into the thrower's hand. sosjay, Thu, 10th Jan 2008

One can get quite strange behaviour from matches and rulers by employing the spin technique, if you take a match and like clicking your fingers but with a match between them the match actually climbs after dipping, just trying to remember how to get the ruler, to do the same, but as a result of spin.
it also sort of flies.
The boomerang when thrown, should have the leading edge pointing towards you: U< AlphBravo, Tue, 12th Feb 2008

I thought the reason was that as the boomerang moves through the air, 1 wing is moving forwards (relative to the boomerang's direction of travel) while the other is moving backwards. The wing on the inside - the 1 that is moving backwards - will consequently get less air travelling over it and, hence, less lift. As that wing revolves and gets to the outside, it will then be travelling forwards and the other will be travelling backwards. It is always the outside wing, the 1 moving forwards, that generates more lift. That will cause the boomerang to travel at an angle with the outside wing always slightly higher than the inside wing. That means the overall lift is not vertical, but slightly inclined towards 1 side. That will cause its trajectory to curve in that direction.

The principle is similar to a helicopter's rotor, but that uses variable attack angles (or pitch) to generate vertical lift. Without variable pitch the helicopter would behave like a boomerang & fly round in circles. DoctorBeaver, Thu, 14th Feb 2008

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