The Physics of Boomerangs
The successful flight of a boomerang looks as though it never should happen. Its more or less circular flight path comes from the interaction of two physical phenomena: the aerodynamic lift of the arms of the boomerang and the spinning boomerang’s maintenance of angular momentum. Briefly put, the airfoil at the boomerang’s forward rotating edge provides more lift than its rearward rotating edge. This elevates one side of the boomerang. The spinning object maintains angular momentum by turning at a right angle to its axis of rotation. When the spin and the velocity of boomerang are just right, it flies away and returns in an aesthetically satisfying circle.
The boomerang’s distinctive flight starts with its aerodynamic properties. Boomerangs come in a variety of shapes. The traditional forms are variations on an L with equally long arms. There are also boomerangs with three or even four arms radiating from a center. And there are delta boomerangs.1 Whatever the configuration, every boomerang has airfoils at its extremities. Looking outward from the center of rotation of the boomerang, the left side of the blade is the leading edge of the air foil for right-handed throwers.2
As the boomerang spins, the airfoils at its perimeter create lift. Our text, Physics: A World View, discusses aerodynamics and Bernoulli’s principle. It explains the relationship between the velocity of a fluid and the pressure that the fluid exerts. A fluid gains energy when it speeds up as, for example when it goes through a constricting area in a pipe or passes around an airfoil. Since the gravitational potential energy is constant, the change is in kinetic energy. Since there is an acceleration of the fluid, the pressure behind the fluid at the constricted point must be greater than at the constriction. Putting it the other way, the pressure at the constricted point must be less than the pressure in the unconstricted part of the flow.3
This explanation is clear when discussing fluids in clearly defined areas of flow such as a pipe. But when a fluid encounters an obstruction in an open situation--a current in a river hitting a stick or an airfoil in the air--the same general rule applies. As the fluid accelerates around an object, its pressure decreases. If an airfoil is moving through the air, then the air accelerates as it goes over it. If the air foil were symmetrical, the air pressure would drop on both sides and the foil would have no net force acting on it. But if one side of a foil were curved and the other flat, then the pressure on the curved side would be less and the foil would be drawn in the direction of the lower air pressure (or the higher pressure on the flat side would push the foil in the direction of the curved side). For example, when rules allow, race cars have an
upside down foil along their bottoms to increase down force and with it, their cornering ability. Much more commonly, airplane wings and helicopter...