Shady Characters

This is the eighth in a series of twelve posts on 2023 Advent calendar. Start at PART 1, continue to PART 9 or view ALL POSTS in the series.

Welcome to day eight of the first ever Shady Characters advent calendar! I’m counting down to Christmas by way of a collection of beautiful, clever, important, and/or outright odd calculators and calculating devices. Some come from the pages of Empire of the Sum, some are part of my Calculator of the Day series, and some will be new to the blog. I won’t manage twenty-four posts, but I do plan to hit at least one every other day. I hope you enjoy the series!

Most slide rules are rectangular. (If you’ve never seen one before, there’s a good example of a rectangular slide rule belonging to the US astronaut Sally Ride in a previous post here at Shady Characters.) We won’t go into the mechanics of it all here,* but, in essence, a slide rule helps its user to multiply numbers simply by reading off a pair of distances, as on a conventional ruler. One distance plus another distance is a third, and all three can be read off a pair of logarithmic scales lined up next to each other. Really, a rectangle is all you need.

Yet in some cases, a circle is better. You can stretch a longer scale around the circumference of a circle than you can fit across its diameter, and a longer scale means more accurate measurements and more accurate multiplications.1 As such, the discerning slide rule enthusiast appreciates the accuracy of the circular slide rule as much as the portability of the rectangular version. And accuracy, it turns out, is very handy when navigating an aircraft. That’s where this slide rule, the Dalton E-6B dead reckoning computer, comes in.

Designed in the 1930s by an American naval pilot named Philip Dalton, the E-6B combined a circular slide rule with a “wind triangle” computer — a clever analogue mechanism for figuring out how the direction of an aircraft flying at a particular speed will be affected by the wind.2,3 The slide rule portion allowed a practiced pilot or navigator to compute air speed given a distance travelled and a time taken (or, indeed, to compute any one of those quantities given the other two); to calculate fuel burn rates; to convert between nautical miles and kilometres; to estimate how far off course an aircraft may have travelled; and to correct observed air pressure for the outside air temperature.2,3 Or even — *gasp* — to simply multiply or divide a pair of numbers.

The E-6B went on to become a standard part of the average US aviator’s training and tools, and although today it has been largely replaced by specialized electronic navigation computers (many of which look a lot like pocket calculators), the FAA’s own Weight and Balance Handbook still makes reference to the E-6B in its chapter on computing aircraft weight and balance.4

So useful was the E-6B in its day, in fact, that the professional prognosticators of the 1960s imagined that it would still be in use in the 23^rd century: Mr Spock, science officer of the USS Enterprise, is shown with an E-6B in hand in two separate Star Trek episodes.5 And if it’s good enough for Mr Spock, who am I to argue?

1.

Bruderer, Herbert. “How Does One Calculate With A Circular Slide Rule?”.

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2.

National Museum of American History. “Dalton E-6B Dead Reckoning Computer by Jeppesen”. Accessed December 2, 2023.

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3.

Safetech E-6B Computer Manual. Newtown, PA: Safetech, n.d.

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4.

Weight & Balance Handbook. Federal Aviation Administration, 2016.

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5.

Valerio, Pablo. “E6B Computer: Celebrating 75 Years Of Flight”. InformationWeek.

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…but you can buy my book to learn more! ↢