To fully understand what makes a boomerang work, it is necessary to understand the basic principles of lift and flight as well as two other laws of physics. Lift is important because a boomerang is simply two airplane wings joined in the middle. The other two principles involve the spinning of the boomerang and how that spinning creates the return.
Aborigines are credited with the invention of the returning boomerang and the oldest one found in Australia is around 14,000 years old. The best theories surrounding the subject are that returning boomerangs accidentally evolved from the non-returning hunting throw sticks. Returning boomerangs were most likely use for sport and recreation, but a large returner could easily injure a small animal, take out a bird in flight (with an incredibly lucky throw) or be used as a decoy to draw in birds and then net them.
Boomerangs are a wonderful fusion of Stone Age man and Space Age Technology.
Main Forces of Flight:
Lift - Thrust - Drag
1st law governing lift is NEWTONS 3RD LAW OF MOTION:
For every action there is an equal and opposite reaction.
This law implies that a wing is just an incline plane. So when you give it thrust, that deflects the air down, which in turn deflects the wing up. The air hitting the underside of the wing creates 30% of the lift of a wing. In the case of an airplane the thrust is obviously produced by the engines. In the case of boomerang the throw produces the thrust.
2nd law governing lift is BERNOULLI'S LAW:
An increase in air speed reduces the static pressure.
Hold a strip of paper in your hand, let it hang down then blow over the top. Air produces roughly 14 pounds of pressure on every object from every direction. The paper rises, because the air pressure on top is reduced and the air pressure pushing up on the bottom is the same.
As the air moves across the wing of a plane it has farther to go over its curved surface than it does it's flatter underside. The air needs to meet at the same time on the other side, so it speeds up over the top. As it moves faster over the top than it does on the bottom it creates less downward air pressure. Bernoulli's law produces 70% of the lift of a wing.
You may be asking why the air needs to reach the back of the airfoil at the same time. Air is a fluid and occupies space - if you push a basketball down into a tub of water, the water level throughout the tub rises because the ball is taking up space. An airfoil is creating a similar effect in the air.
The preceding laws plus gyroscopic stability and gyroscopic precession complete the science of a returning boomerang.
Gyroscopic stability is something everyone has experienced. Try balancing on a bicycle without rolling forward and you understand the stability of spinning wheels. We have all spun a gyroscope or top, the simple act of spinning it creates stability. That gyroscopic motion keeps rockets on course and bicycles upright. It is the same spinning motion that gives the boomerang stability in the air.
Gyroscopic precession is definitely more involved and the short version (without any math) is:
As the boomerang spins, the wing going over the top creates more lift.
This increased lift at the top of the circle begins turning the boomerang and it begins its return. As the speed of the flight decreases these forces decrease which result in finally laying the boomerang flat as it hovers in for an easy catch. For a quick simple example take a bicycle wheel off your bike and hold it by the axle while spinning the wheel. Turn it slightly and feel the force.
The very thing that creates lift, also creates drag. Flying an airplane especially landing and taking off, is a delicate balance of controlling lift and drag. At boomerang tournaments you will see boomerangs with holes drilled in them, rubber bands wrapped around the wings, spoiler flaps and coins taped on the wings. These things create drag to produce different effects in the flight like reducing the hover or keeping the flight lower. Controlling the lift using drag on a boomerang is also a delicate balance and something of an art. To boomerang fanatics DRAG is not a four letter word, it's a necessity.
The throw of a boomerang is an overhand throw exactly like throwing a football and at the moment of release the boomerang should be more or less straight up and down. The rate of spin of the boomerang is much more important to a good flight than a hard toss. To create lots of spin think of the action as an exaggerated knock on a door, or cracking a whip. The uninitiated thrower always thinks it's a sidearm toss, but because of the physics principles we just discussed, a boomerangs flight is perpendicular to the plane of the boomerang so it must be an overhand throw to produce the round returning flight. A sidearm throw will produce a straight up flight with a crashing return, usually resulting in a broken boomerang.
So why does a boomerang, boomerang?
The throw produces the thrust. Wings produce lift. The spinning produces gyroscopic stability. The lift and the spinning create gyroscopic precession that turns it until it is gently hovering in.
So next time you are throwing boomerangs and making momentary sculptures in the sky, consider the science involved, and envision Stone Age man working through technology that wouldn't be fully understood for thousands of years.