How do they work? How do they mostly return to the throwing point? There are so many shapes and sizes.
I always found it interesting. Enlighten me with your boomerang knowledge
In: Physics
I’ll start by getting this out of the way first: Contrary to what you might initially think, the iconic bent shape of a boomerang is *not* the feature that makes it return to you. It *is* important, and it is indirectly helping, but it’s not the cause. A straight stick that has all the other features of a boomerang, thrown correctly, will also exhibit that behavior.
That said, we can instead picture a boomerang spinning in the air like a set of helicopter blades. A properly thrown boomerang will look like a helicopter that has rolled over on its side about 70-ish degrees, so the spinning blades are almost but not quite spinning vertical. Also somewhat like helicopter blades, you want that boomerang to be spinning very fast as it flies.
Generally, the boomerang is doing the same thing helicopter blades do. Crudely speaking, the spinning blades act like a fan that blows enough air “downward” that it counteracts the force of gravity, allowing it to hover. In the boomerang’s case, it is tilted so sharply to one side that most of that lift being generated is being wasted out to one side. This is fine, because the boomerang doesn’t weigh that much. It’s still pointed a *little* downward, and that’s just enough lift to keep it airborne. Direct too much of the lift directly downward and it’ll probably fly away or flip over, neither of which you want. You want that sweet spot angle where the lift is just right.
The boomerang also needs to have a cross-section shape similar to helicopter blades as well. It should look something like two airplane wings chasing each other round and round as it spins.
The key feature about the boomerang that makes it return to its sender is the fact that, at any given time, one half of the boomerang (usually the top half) is spinning into the oncoming wind, and the other half is spinning away from the wind. This means the top half of the boomerang generates more lift than the bottom half. Imagine your mostly-tipped-over helicopter blades where the top half is being dragged “up” (with respect to the helicopter cab) while the bottom half is being dragged “down”. Essentially, the helicopter blades are being subtly yanked from 70 degrees tilted to completely 90 degrees tilted.
You might think that would just cause the helicopter to completely flip over. But it neglects the fact that the blades have a lot of spinning momentum. They’re not just going to flip over like that. If you try, you will instead cause the blades to precess, like a gyroscope.
Alternative picture: Imagine holding a bicycle wheel by the axle in both hands, with the tire standing up vertically. You use a motor to spin the tire up to a high speed to give it lots of spinning momentum. Once it’s up to speed, imagine trying to flip the axle over so one hand is above the tire and the other hand is below, and the tire becomes flat like a dinner plate. If you try to do that, the spinning momentum of the tire will fight it, and you will find the tire will actually try to turn to one side, as if the imaginary bike it were attached to were trying to turn a corner.
The same thing happens to the boomerang. The top half being yanked in one direction and the bottom half being yanked in the opposite direction is just like you trying to flip that spinning bicycle tire over. But because the boomerang has angular momentum, it resists that motion, and instead precesses on a different axis — it turns, like the bike wheel turning a corner. This causes the boomerang to slowly turn which way it’s “facing” while it flies. Allow it to fly long enough, and it will eventually make a full rotation, causing it to return to where it started. The full motion looks something akin to what you can maybe imagine a helicopter “drifting” might look like.
So, why the bend, then? Well, try this at home: find an object that you can safely throw, that has a sort of vaguely “cell phone” shape to it. That is, it’s flat, and longer one way that it is wide. If you were to try and throw such an object frisbee-toss style, you’ll find that it will quickly stop spinning that way and start tumbling end-over-end instead. ~~This is a principle in physics called the “intermediate axis theorem”, or sometimes the “[tennis racket theorem](https://en.wikipedia.org/wiki/Tennis_racket_theorem)”. Objects spinning in that manner are unstable, and will quickly find a different way to spin instead.~~ EDIT: This is false. I had the principles mixed up. The actual interaction is more complex. The general case though is that the object probably won’t be spinning on only this axis you want for long, particularly if it’s also generating lift.
The problem is that our boomerang, if we envision a straight one, is shaped *exactly* like that–it’s long, narrow, and flat. And frisbee-tossing it is *exactly* want we need to do if we want it to work (albeit we toss it almost vertical instead of horizontal). So a perfectly straight boomerang will probably stop spinning the way we need it to and start tumbling flat-end-over-flat-end instead, meaning it stops generating lift and simply falls out of the air. Simple solution? Bend it. The bend makes it so that tumbling flat-end-over-flat-end is not a favorable way for it to spin. It keeps the boomerang spinning the way we want it to spin.
All this comparison to helicopter blades is actually not coincidental. Helicopter blades themselves act just like boomerangs do when helicopters start flying in a particular direction. To counteract that, the blades of some helicopters will actually change their pitch in real time so that they’re shallower on the forward-spin and steeper on the back-spin.
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