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The Turn (1993) (theatlantic.com)
146 points by karzeem on July 10, 2010 | hide | past | favorite | 32 comments

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:


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


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.

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.

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.

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


I do love how he mentions that the difficult part of the tea-pouring-during-a-roll was pouring the tea back-handed... I guess it all comes down to a matter of perception, as with so many things in life.

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?

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.

Think of the wings as the glass and the fuselage as the tea in the glass. The pilot's glass of tea is imitating what the wings and fuselage that contain the glass are doing.

If the pilot turns the plane at the correct rate, it's a 1 G maneuver. If he turns a little too fast (like spinning your arm too fast) the downward acceleration relative to the fuselage becomes greater than 1 G. If he turns too slowly, the downward acceleration is less than 1G and the tea might spill.

I use the term downward acceleration because "G Force" is not a term that physicists like to use.

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.

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.

The same reason a pail of water swung around your head doesn't spill. The path cut by the rolling plane in this case is a spiral, as it's a barrel-roll rather than an aileron roll.

So yes, if you stopped at the top of the roll, the liquid would indeed fall out. The reason it doesn't is that the plane is continuing to roll, and the acceleration of the plane away from its upside down position is greater than the acceleration due to gravity. The liquid is "falling" at 9.8 meters per second, but the plane - and hence the glass - is being pushed at a faster rate.

Both the plane and its contents experience the same acceleration due to gravity. Unless the plane is accelerating, the occupants will experience weightlessness (e.g. vomit comet). Of course the plane is accelerating - in straight and level flight this acceleration counteracts the acceleration due to gravity. This acceleration (lift) is provided by the wings, which don't move, so it is always in the same direction (in the plane's frame of reference).

So if you kept flying upside down the tea would eventually fall, right?

No. Eventually the plane would hit the ground, since it is not only falling but also generating "lift" in a downwards direction.

This idea was probably taken from Coca Cola the ad wherein a pilot of an F-16 had to do a full roll when the bottle got stuck in the console.

Bob was doing this before the designers of the f-16 were a sparkle in their parents eyes. Bob knew Orville Wright and Charles Lindbergh as friends.

youtube up the video of him flying the f-86 sabre if you want to be stunned.

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

or a 707:


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.

> 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.

> 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?

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)...

Not really, the UI works exactly like the thing it models. Bank the plane to the left, the horizon moves to the right. And so does the artificial horizon.

I think people get confused when they see the marking representing the planes wings not moving relative to the console, but they see the blue-and-red thing inside the instrument moving. So they assume that their control inputs are causing the artificial horizon to move, when in reality, they are moving the plane and the artificial horizon is remaining still, just like the real horizon would.

Presumably there is some sort of mental issue with the "size" of the horizon -- when you look out the window, it's really big, so of course you're not moving it. Newer cockpits make the artificial horizon bigger, perhaps to compensate for this effect: http://www.cockpitgps.com/VOR_magnetic_variation/G1000CBEAV-...

I know that as long as I remember "fly the little airplane, not the horizon", it's not a problem. And as the comment below mentions, you also have a turn indicator, which clearly shows "R" when you're turning right and "L" when you're turning left: http://www.unitedinst.com/images/TurnSlip9500Series.gif

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."

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

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.

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.

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.

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.

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.

Even with a centripetal force of 137N (2g, 70kg), you'd think passengers would notice earth's gravitational pull circling them.

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

Same here. Really liked the article!

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