This large slide rule, the “Regolo aerofotografico Sbernadori”, helps you plan aerial photography flights. It computes, given the flight height, speed, plate (i.e. film) size and lens focal length, how to time the photo shooting to ensure proper coverage of the desired land area. It also helps you convert distances on the photo, on a topographic map, and on the actual terrain.
Aerial reconnaissance is a useful military art, and there were a variety of slide rules directed at this task, like the American one seen in the photo below. There is also my rare “Cornell F.24 photographic calculator”, described here, which achieves the same goal in a very different form factor.
The Pickett N700-T US Air Force Aerial Photo Slide Rule
The Sbernadori stands out in this class of slide rules because it has two slides rather than one, which allows complicated calculations to be concatenated in a sequence from slide to slide. The detailed instructions on the back of the rule are quite helpful… if you read Italian, that is.
Here is a translation:
For problems dealing with only one photograph, move only the upper slide.
Bring the value of F (focal distance) against the value of Q (altitude of the aircraft). Following the upper arrows you will find at the same time:
- The Sf (scale of photo) arrow will point to the Scale value of the photograph obtained,
- The value of C (field covered on the ground) corresponding to each value of L (length of plate),
- The value of Dt (Distance on ground) corresponding to each value of Df (Distance on photo).
You can operate in the opposite direction to obtain Q (or one of the other data) knowing F, Sf, C, L, and Dt.
For problems involving a series of photographs, after effecting the previous operation move the lower slide to:
- Bring the Aircraft Speed value against the arrow V and read on the lower stator the value of l” (time interval between exposures in seconds) which corresponds to the effective value of L (plate length) used.
- Bring the value of the Distance on the Ground against the arrow Dt and read on the lower stator the value of N (number of plates) which corresponds to the effective value of L (plate length) used.
- Bring the value of the Distance on the Photograph opposite the Df arrow and read on the lower stator the value of Dc (Distance on the map) which corresponds to the Sc (scale of map) used.
The three operations can be reversed to obtain the intermediate data of the problem.
To find out the minimum exposure time needed to keep the Image Definition ε within the limits of 1/10 to 1/30 of a millimeter, move both slides.
- First line up the value of Q (altitude of the aircraft) with that of F (focal distance), then the arrow with V (aircraft speed). Then read the value of the time T (exposure time in seconds) corresponding to the desired value of ε.
You can reverse the operation to obtain Q or V knowing the other data.
You can get an idea of the geometry involved from the diagram in my article about the Cornell F.24 calculator, whose purpose overlaps the Sbernadori’s to a large extent. Just keep in mind that some variable names are different due to the different language used.
The slide rule measures 32×7 cm and is quite well-built, with an aluminum back plate to which are screwed two stators and a central wooden slotted rail. These constrain the two slides, which like the stators are faced with celluloid. The four members allow three sliding interfaces, each carrying a number of separate scale pairs; this slide rule sure means business!
The rule comes in a leather case, marked “R. Aeronautica” (the Royal Italian Airforce), and also “Rossi – Roma”, which must be the company that made it. The connection to the Royal air force times it to the period 1923-1946.
So who is the Sbernadori who devised this sophisticated device? The slide rule gives no further clues, but my search found a U.S. patent granted to a Paolo Sbernadori of Rome in May 1950 for the invention of a slide rule for solving problems of quantities and prices of liquids, notably petroleum products. May well be the same Sbernadori, but can we be sure?
Well, now that I had a first name I could search the web and find several mentions of a Paolo Sbernadori, born in 1894, who had both an Engineering degree and a long career in aviation, from securing a pilot license in 1915 to retirement as an Air Brigadier General in 1967. In between, he served in the Italian air force in two world wars, and held a variety of positions, including Air Attaché to the Royal Italian Embassy in Washington DC in the 1930s. I also found a journal from 1948 that ascribes to him the invention of the petroleum slide rule. Putting all these snippets together we can be pretty certain that the inventor of the slide rule I have is this same engineer and aviator, Paolo Sbernadori. Here he is in the photo.
One last tidbit: we see in this state publication that in 1954 the Italian certification exams for aerial photographers included demonstration of proficiency on both the Sbernadori and Giovine slide rules (I also have Giovine’s slide rule in my collection… maybe I’ll write it up someday). So we know that the Sbernadori slide rule was looked on with favor in the Italian military. Looks like the same can be said of Paolo Sbernadori himself, given a career that spanned over half a century of aviation!
Exhibit provenance: Bought this from a collector in Italy.
More info:
Here is the patent for Sbernadori’s petroleum slide rule.