The Hoffman airplane calculator

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Build your own airplane!
The Hoffman Airplane Calculator

Airplane related slide rules are rather common; in fact, the basic E6B style flight computer is one of the very few slide rules still being used today. However, these devices are mainly used to help you navigate your airplane in various weather conditions. By contrast, Hoffman’s airplane calculator allows you to design and build the airplane itself – to build a custom flying machine. From scratch. Like we all do when we get the itch to fly, right?

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The date on the device is 1918, back in the days when enterprising people could still aspire to build a flying machine in their garage; the image on the cover is of the 1903 Wright Flyer, no less.
Hoffman’s calculator consists of two parts bound in a rigid folding cover: a circular slide rule , and an instruction booklet that contains, besides the instructions for use, four pages of charts that facilitate calculation of horsepower and of gliding angle for various wing configurations.

The entire thing fits into a rectangle sized 15x13 cm (with the cover closed).

Illustration on the The Hoffman airplane calculator

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The instructions start with the following glowing description:

A Wonderful Mechanical Aid in All Airplane Calculations, Eliminating the Necessity of Tedious and Tiresome Figuring, Thereby Saving Time, Labor, Trouble and Worry.
Accurate and Reliable Calculations Made in a Few Minutes by a Simple but Valuable Device Which Is Indispensable for the Airplane Designer, Builder, Mechanic and Student.

See how easy it can be to build an airplane?

The various parameters indicated in the scale and chart captions – Altitude, Speed, Weight, Horsepower, Wing Area, Drag coefficient, etc. – do make sense individually, but I admit that I find the calculation instructions rather hard to follow. But then I am not an “Airplane Designer, Builder, Mechanic or Student”! If you are one of these, or just wish to tackle the challenge, feel free to do better; a scan of the instructions awaits you here. Be warned that the calculator I have seems to be missing a piece that is referenced in the instructions – the “Indicator”. It is unclear what this looked like, but it was used in connection with the charts, so it couldn’t have been a cursor that was part of the circular slide rule; indeed, the ad shown below shows the slide rule without any such cursor. A likely guess is that this indicator was a transparent celluloid ruler of some sort that was used to read the charts.
There are two charts, one titled “Horsepower calculation” and the other “Gliding angle calculation”. They address a number of different “wing curves”, cross sections of the wing, titled USA #1, USA #4, Eiffel #32 and RAF #6. It is interesting to note that the Eiffel in question is probably Gustave Eiffel of tower fame, who was also an aerodynamics pioneer. Each chart is followed by a blank version that one can use for drawing one’s own wing designs. You can see the charts at the end of the instructions.

A chart from the Hoffman airplane calculator

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The inventor of this calculator is listed as Mr. R. J. Hoffman, M. E. This is clearly Raoul J. Hoffman, a Hungarian-American airplane designer, who has copyrighted the device in 1918 and has commercialized it through his “Airplane Calculator Co.” in Chicago. The ad at the right appeared in the Feb. 15, 1919 issue of Aviation and Aeronautical Engineering.
Mr. Hoffman also produced a different circular slide rule called “The Hoffman Airplane Designer”, which is in Mike Frey’s collection; you can see it in the Oughtred Society rarities gallery, here.

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And let’s give a tip of the hat to B. J. Sacks, Mechanic, who has left his name penciled on the instructions cover. Now, there was a man who probably understood how to build an airplane!

		Exhibit provenance: A lucky find on eBay. More info: Here is a scan of the instructions section.

Build your own airplane!

The Hoffman Airplane Calculator