The Magnus Effect

The Magnus effect is a name we've given to the phenomenon of an object curving away from a straight line because it is spinning while passing through a fluid. The most ready example is a curve ball. A pitcher achieves a curve ball by putting spin on the ball. The spin causes the curvature, and this is the Magnus effect. It can similarly be seen in table tennis, and it is responsible for long drives in golf (the curvature is upward due to backspin).

The name comes from Gustav Magnus even though Isaac Newton observed it almost 200 years earlier. That's fine; Magnus is a pretty cool name, and Newton get's plenty of recognition (and deserves it)

Let's try to understand how it works in the case of a golf ball with backspin. As the air encounters the ball, it would normally tend to part around it and meet on the other side. It may be stirred up into turbulence  but on average it's following about the same path it did before. The Magnus effect works by deflecting the path of the air. Because of the backspin, the underside moving forward and the top backward. The side on top is moving along with the air, and the air on that side is able to move farther along the face of the ball before meeting the air from the bottom side and continuing on it's way. Likewise, the bottom face is going forward  against the fluid and impeding it. On average the path of the air has been deflected downward, and by pulling the air down, the ball pulls itself up getting extra time in the air and extra flight distance. This helps us understand why golf balls are dimpled: that texture enhances the Magnus effect.

There have been attempts to use this effect to propel vehicles. Butler Ames made a prototype airplane that derived its lift from a horizontal spinning cylinder using the Magnus effect in 1910. It was able to lift off while being towed by a boat, but didn't make it any farther than that.

There have also been attempts to use a vertical spinning cylinder on a boat for propulsion. It would work as a sail replacement, interacting with the wind through the Magnus effect to push the boat in the desired direction. This was called a rotor sail, and the boats, rotor ships. These worked perfectly well, but it turns out the energy you use to spin the cylinder can be more efficiently used to turn a propeller in the water. However, there has been some limited use of smaller, horizontal spinning cylinders in the water used to stabilize boats against rocking.