# 1672,1742,1852

## The book of science

Tom Sharp

 Isaac Newton, Benjamin Robins, Heinrich Gustav Magnus aerodynamics

## Magnus effect

• Considering then rejecting the idea that “Rays
• of light should possibly be globular bodies”
• to explain why a prism bends rays of light,
• Isaac Newton remembered seeing a tennis ball
• “struck with an oblique Racket,” describe a curve
• and explained why it turns in the air:
• “For, a circular as well as a progressive motion
• being communicated to it by that stroke, its parts on that side,
• where the motions conspire, must press and beat the contiguous Air
• more violently than on the other, and there excite a reluctancy
• and reaction of the Air proportionably greater.”
• *
• To show that a whirling motion of a bullet
• “deflects it from its course,” Benjamin Robins
• suspended a four-and-a-half-inch wooden ball
• from a long double string, twisted the string,
• and drew the ball away from the perpendicular.
• After he let it go, it whirled by the untwisting
• of the string and began to deviate from the vertical plane
• “and sometimes proceeded so far, as to have its direction
• at right angles to that, in which it began its motion;
• and this deviation was not produced by the action
• of the string itself, but appeared to be entirely owing
• to the resistance being greater on the one part
• of the leading surface of the globe than on the other.”
• *
• Guess who else studied the deflection of projectiles?
• The effect was named after him, and my main difficulty
• is that I have found his paper published only in German;
• however, my dogged attempt to translate it confirms
• that Gustav Magnus designed clever experiments
• to show that the effect results from a difference of pressure
• on opposite sides perpendicular to the direction of flight
• caused by rotation of a body while moving through the air.

## Flettner rotor

• Anton Flettner was not a mad scientist;
• that is, he was not mad. He invented
• the famous Flettner ventilator. He built
• the first schooner moved by the Magnus effect;
• two rotating cylinders propelled it
• across the Atlantic. He developed control devices
• for aircraft and boats in World War I.
• He designed helicopters in World War II.
• His wife was Jewish but the Germans
• escorted her and her family safely to Sweden.
• The Flettner airplane replaced wings
• with whirling cylinders
• and might still be the aircraft of the future.

## Aft

• Aft of the body,
• that is, opposite
• its direction of flight,
• the air produces
• a force on the body
• opposite to its spin.
• If in its wake
• it spins left,
• then it moves right.
• *
• An angular deflection
• in its turbulent tail
• in its direction of spin
• wants to move a body
• in the opposite direction
• of the deflection.
• *
• Since pressure
• is related
• to air speed
• and a spinning
• body moves
• the air around it,
• air flows faster
• where the spin
• is in the direction of movement
• and slower
• on the opposite side,
• causing a difference in pressure
• that moves the body
• from its high-pressure side
• to its low-pressure side.

Some sources say the effect is caused by the deflection of the wake, and others say it is caused by a difference in pressure perpendicular to the direction of flight.

If you could eliminate turbulence around a moving body, you would save a lot of energy. Turbulence at its tail slows a projectile more than the force of air at its head.