140. How does a boomerang work?
The correct way to throw a boomerang is overhand and, unlike a Frisbee, in a nearly vertical plane. (Usually the ideal angle is about 15° from vertical.) The boomerang is essentially a rotating airplane wing, and its shape produces lift using the Bernoulli effect in the same way an airplane wing does. But when it is thrown, notice that the top blade of the boomerang is moving faster through the air than the bottom blade, because of the rotation. This results in there being more lift on the top blade than on the bottom. From a right-handed thrower's perspective, there is a lift up and to the left, more so at the top than at the bottom. The upward lift is what keeps the boomerang in the air. You might think the leftward twist flips the boomerang over, but wait! The boomerang is also a flying gyroscope. Leaning the gyroscopic boomerang over results in its turning to the left, much the same way that leaning a moving bicycle leftward toward the horizontal causes the front wheel to turn and not fall over. (This is also why spinning tops start to slowly turn their axis of rotation when they lean, a process called "precession".) The boomerang doesn't flip over, but instead turns its axis of rotation around in a large horizontal circle, and it comes back to you.
After a moment's thought, you might wonder whether helicopters suffer the same effect. (How would a boomerang fly if thrown in a horizontal plane?) In fact, they do, and there is a tendency to pitch the helicopter upward (tip the nose up) precisely from this same effect, which the pilot instinctively corrects for.
(Thanks to Prof. Paul Draper, from the Physics Department of the University of Texas at Arlington, for writing this explanation.)