

The Turn (1993) - karzeem
http://www.theatlantic.com/past/docs/unbound/langew/turn.htm

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ggchappell
Very nice, but the description of Focault's pendulum, near the end, is
incorrect.

> ... though the pendulum appeared to change direction as it swung, in fact
> the plane of its swing remained constant ....

Nope. Unless you're at one of the poles, the period of the rotation of the
plane is not 24 hours; the plane can hardly be fixed.

Wikipedia says a bit about this:

[https://secure.wikimedia.org/wikipedia/en/wiki/Focault_pendu...](https://secure.wikimedia.org/wikipedia/en/wiki/Focault_pendulum)

And there was a nice article in the American Mathematical Monthly some years
back, which I found here:

<http://geomsymm.cnsm.csulb.edu/courses/451/foucault_pend.pdf>

But the basic idea is something like this: Say you're in the Northern
Hemisphere. As the Earth rotates, your path essentially pushes you to the
right (where "forward" is west). If you have a swinging pendulum with you,
then the plane of its swing will rotate just as if you were on a flat surface,
walking in a very large circle. The closer you are to the equator, the less
you are moving to the right, the larger is this theoretical circle, and the
slower the plane turns.

~~~
mechanical_fish
Sure, but this is just the Science Writing Uncertainty Principle: You can be
brief, or you can be precise, but rarely can you be both brief and precise.

Langewiesche doesn't have time for a discourse on the physics of the pendulum
-- which is basically just another kind of gyro. He has just a couple of
sentences for his aside, and his readers are not physicists, so he grabs for
the example of the Foucault pendulum, because that's a pendulum experiencing
gyroscopic forces that every science museum visitor has seen.

~~~
ggchappell
Ah, but the statement "the plane of its swing remained constant" is not
_imprecise_ ; it is _false_. And I don't mean false-but-kinda-true like "pi =
3.14" is false-but-kinda-true; I mean just plain wrong. The fixed-plane idea
seems to be a common misunderstanding, even among intelligent people, but it
ain't so. The plane turns, even with respect to a "fixed" frame of reference
that does not rotate with the Earth.

I would have been better just to say that the plane changes, showing that the
Earth rotates, and leave the explanations for another time.

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mmaunder
This is Bob Hoover (who he mentions) pouring iced tea during a roll.

<http://www.youtube.com/watch?v=pMWxuKcD6vE>

~~~
JesseAldridge
I don't really understand how this works. The article mentioned something
about inertia... So gravity moves the object in a downward direction... and
then, when the plane is flipped, it keeps moving in that direction -- which is
now up? I guess that the effect would "wear off" after awhile? So if you kept
flying upside down the tea would eventually fall, right?

~~~
JshWright
Pour a glass of water and hold it loosely between your thumb and index finger
(tightly enough that it won't fall, but loose enough that it can swing
freely). Then take it outside and swing it in a big vertical circle. You'll be
surprised how slowly you can swing it while still keeping the water in the
glass.

Just like the glass of water though, if the plane were to stop rolling, the
tea would immediately spill. It has nothing to do with "bending gravity",
simply the reactive centrifugal force imparted by the turn itself.

~~~
kokofoo
Centrifugal force is also used to achieve the concept called artificial
gravity, which is the reason why characters in Star Trek walk around like
normal in their starships.

~~~
dmoney
Some sci-fi has cylindrical ships or space stations spun for gravity, but in
Star Trek they have "gravity plating", so the ships don't have to spin. A
convenient plot device to cover the fact that they didn't have the budget to
shoot every episode aboard the vomit comet.

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cma
The article on rolls: "but if you ignore convention, you can fly them in any
airplane, including a Boeing 737"

or a 707:

[http://www.youtube.com/watch?v=Ra_khhzuFlE&feature=relat...](http://www.youtube.com/watch?v=Ra_khhzuFlE&feature=related)

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jeffcoat
I have Wolfgang Langewiesche's _Stick and Rudder_, a classic book for pilots
on how to fly airplanes, sitting on a shelf near me. According to Wikipedia,
the author of this article is his son.

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lisper
> The bank is a condition of tilted wings, and the turn is the change in
> direction that results. The connection between the two is inexorable: the
> airplane must bank to turn, and when it is banked, it must turn.

This is only true if the rudder is not being used. It is possible to turn a
plane without banking it, and to bank a plane without turning it using the
rudder. The former maneuver is called a skidding turn, and the latter is
called a slip.

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chime
> The artificial horizon is a gyroscopically steadied line, which stays level
> with the earth's surface. The airplane pitches and banks in relation to this
> steady line, which in spatial terms never moves. Of course, in airplane
> terms it does move -- which presents a problem, because pilots are part of
> the airplane: they fly it from within, strapped to their seats. In clear
> skies they would never misjudge a bank as the tilting of the earth, but with
> their view restricted to the abstractions of the instrument panel they
> sometimes do just that: when the airplane banks, they perceive the motion as
> a movement of the artificial horizon line across the face of the instrument.
> This causes them to "fly" the wrong thing -- the moving horizon line, rather
> than the fixed symbolic airplane. For example, as turbulence tilts the
> airplane to the left, the pilots, tilting with it, notice the artificial
> horizon line dropping to the right. Reacting instinctively to the indication
> of motion, they sometimes try to raise the line as if it were a wing.

Would this be considered an example of a dangerously confusing user interface?

~~~
rubyrescue
From personal experience, during instrument training this is something you get
used to and a trained pilot would not normally make such a mistake; the
interface is so simple it would be hard to simplify it further, and further,
it's not the only instrument you have at your disposal - you get a sense of
them working together (turn indicator; airspeed indicator; compass, GPS, and
VOR, all are secondary indicators indicating that you're turning in a
particular direction or pitching down or up)...

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joejohnson
The near-negligible Coriolis force can be felt (barely) at velocities which
planes reach. This could tip of the inner-ear to a bank, if ever so slightly.
However, this force is many orders smaller than the centrifugal force that the
authors is talking about.

However, I don't think it's technically correct to say the passengers "have no
way of guessing the airplane's degree of bank."

~~~
phreeza
If they had a gyroscope they could measure it, of course.

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sammyo
During an aerobatic glider ride for the initiation of the first maneuver all I
saw was the horizon rise then slowly drop below the edge of the cockpit. A few
moments later the pilot behind me said to look straight up. Directly above me
were cars on the highway. I had no sense of being upside down. Very cool.
Later maneuvers were more visceral.

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limist
Wonderful article, as it gives more reason to smile in appreciation on the
next flight. Human hubris, stubbornness and over-reliance on intuition add to
the article's lessons in physics and biology.

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javanix
_The airplane could be momentarily upside down and passengers would not know._

At some point I suspect gravity would override the masking sense of motion
that he talks about.

~~~
jacksoncarter
If the centrifugal force in the upward direction is 2g, that is, twice
gravity, then passengers will feel like the plane is flying exactly upright
and parallel with the earth.

~~~
asmithmd1
Or the plane could be on a zero g simulating parabolic arc and at the same
time doing a 1G barrel roll. A passenger with their eyes closed would think
they are flying strait and level.

We have a bubble level in our heads for sensor. Not a gyro.

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VictorHo
That was an amazing article. I've always been intrigued by flight but I think
I have a newfound appreciation.

~~~
ojilles
Same here. Really liked the article!

