What is the Magnus effect?

What is the Magnus effect?
Magnus Effect
They see me spinnin', they hatin'. 

Spinning Cylinders

In 1930, Plymouth A A 2004 flettner airplane used a bunch of spinning cylinders (flettner rotors) to achieve flight. This puts forth the question: CAN SPINNING CYLINDERS FLY?

By the Magnus effect, that is a possibility.

Magnus Effect

Almost a hundred years before this flettner aircraft took flight, in 1852, Heinrich Gustav Magnus was working on projectiles from firearms. The bullets seemed to take a deflected path rather than the destined path. Pissed off, he grabbed the bullet by the collar and asked it what the hell it was doing. “It’s the pressure difference man! Look I’ve got a family to feed ” said the bullet. Thus he proposed the Magnus effect. True story.

Fast forward to the 20th century, there came a revolutionary theorem barging into the abode of aerodynamics and that was the Kutta Jowkowski theorem. Kutta and Jowkowski sat together and whipped up some dank mathematics to prove our man Magnus’s effect.
As opposed to going into the mathematics of the Magnus effect, and analytically proving its existence, we’ll look into the physics of it and see what sort of sorcery it is.  
Magnus effect
Spinning cylinder
Suppose a cylinder is spinning in the clockwise direction. There’s going to be attached flow right above and below the cylinder creating a boundary layer. (layer where friction effects are felt). After this attached flow, there’s going to be flow separation in the aft of the cylinder (One could say after but Aircraft Nerds like cool words). This flow separation creates low pressure vortices. (The physics behind this attachment and separation has been elaborated in the article on Coanda effect linked below) Let’s step outside the boundary layer for a bit and examine the velocity above the cylinder. It’s going to be higher than the velocity below the cylinder. Why? Because it spins. Any spinning object rotates about it's axis by an angular velocity. This angular velocity is induced by the velocity acting tangent to the circle at every point. Over the top of the cylinder, this clockwise spin or rotation creates a tangential component of velocity which is added to the free stream velocity (velocity of the flow V which is a constant at a distance from the cylinder) creating a velocity Vtop. Why is it added? because the flow velocity and the spin velocity are in the same direction. And in the bottom of the cylinder the velocity is reduced from the free stream velocity (V) creating a lower velocity Vbottom. Why is it subtracted? Because the flow velocity and the spin velocity are acting in opposite directions. So the velocity over the top is greater than the velocity below. 

Magnus effect Angular velocity
Tangential velocity over top > Tangential velocity below

Bernoulli: “That makes the pressure over the top lesser than the pressure below the cylinder!”

What he said. So as mentioned in the linked article on Coanda effect, flow moves from high pressure to low pressure. This difference in pressure creates a force which pushes the flow in that direction. In our spinning cylinder, there’s high pressure below the cylinder and so the force acts upwards. This upward force is, drumroll, please…
Or Magnus force.
 Now look at the aircraft again, see those spinning cylinders separated by discs? They're flettner rotors. These flettner rotors take advantage of the Magnus effect to generate lift.
Random cricket fanboy: Curiouser and Curiouser..
Related image
Topspin on a cricket ball
Random cricket fanboy: This sounds an awful lot like topspin and backspin in cricket.
Right you are random fanboy, right you are.The Magnus effect explains the deviations of spinning balls in ball sports like golf, tennis, baseball and ofcourse cricket. In top spin the ball goes down because the Magnus force acts downwards. In backspin the ball rises.

Now you know how to throw a killer serve with physics.

Click here for more enlightenment: What is the coanda effect?
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