1672,1742,1852 - Magnus effect - The book of science

Magnus effect
Flettner rotor
Aft
Commentary

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.