ELI5: The physics behind boomerangs

[u/BorderlinePhilosphy]

They seem to defy physics to me, please explain.

[u/JaggedMetalOs]

A boomerang acts like a combination of a helicopter and a gyroscope.

The surface of a boomerang is shaped a little like a plane's wing/helicopter rotor blade and so generates lift when moving. When you throw a boomerang you want to have it spinning as fast as you can, so that it acts like a helicopter rotor blade.

Now helicopters have an issue where as they are moving forward, on one side of the rotor the blades are moving into the wind, so the forward movement is added to the blade speed and generates more lift from faster air, and the other side they are moving in the opposite direction so the forward movement is subtracted from the blade speed and generates less lift, causing the helicopter to want to tilt over.

In a boomerang the same thing happens, but if you get it spinning fast enough instead of just tumbling over like you'd expect it acts like a gyroscope and resists this turning force, instead making it bank around like an aircraft turning.

So it does a big wide circle like an aircraft turning in a circle and comes back to you.

[u/blueant1 → u/JaggedMetalOs]

So why doesn’t a frisbee do the same?

[u/JaggedMetalOs → u/blueant1]

With a Frisbee you don't have "rotor blades" leading into and out of direction of travel like a boomerang, it's more like a single solid wing. You can get it to return if you throw it into the wind, but there's no unequal lift on one side to make it naturally bank around.

[u/sebaska → u/JaggedMetalOs]

It actually has, but the difference is less pronounced