The Bomber Stop Watches
By Konrad Knirim of Düsseldorf Germany (prepared for the NAWCC Bulletin)
These special time pieces were used in multiseated aircrafts in combination with a 'visier' -Feature. The different types were mostly manufactured in Switzerland and implemented in the air forces of several countries. I saw items with markings of the Italian, French and British Air Forces. Suppliers were some well known Swiss manufacturers, so I heard about an IWC Bomb Timer' which was send from Austria to an auction house in Geneva, but instead of coming to the auction, it disapeared in a private collection.The French items especially of Jean Vial used movements of Minerva of Villeret. I don't know, if Longines had an own movement, I only know fotos of the front side. The last shown item of Vial I know in two identical versions from the outside, only one is marked Longines.I tried to get information of the suppliers, but there seem to be any. At Zenith in Le Locle they do not know there specimen even in their archive any more and Jean Vial in Paris couldn't as well tell me anything about their mades. Only at Minerva in Villeret I got some information, because they still did have some items of the 30th. As in any case I am happy to get mor information from anyone who has.For the German Air Force the use of these special pieces is not quite visible. The documented 'Bombenrichtgeraet' had just a normal pocket stop watch attached. During the world war II somme national orders of Italy or maybe Jougoslavia did not come to delivery, so there are some new production line items at collectors hands. Maybe that within the war the 'Aeronautica Italiana' a alliance to the axes forces did cooperate with the German military specifications and instruments.The very special feature of this watches is, that the stop hand turns backward after the second push of the button ('count up' or 'ritorno' or 'a retour'). How is the 'chronographo a Ritorno' or the 'Bomb Timer' working?: Look at figure 3: Time-Distance-Height-diagram of the bomb throwing scheme: H = height of the air craft, which is moving with the bomb operator and the instruments with the speed v over ground.
The bomber squadron used an optical system to visier the target. This instrument had three marks e.g. A, B and C (see figure 3). Normally the distance between AB and AC are equal. The target should be G. The problem is shown at the folowing scheme: By the time meter the crew knows the time between A' = G and B' = G. For the bomb reaching the target at C' = G, one has to know the duration of the fall of the bomb, we call it t. The bomb is thrown at (T' -t) measured from that moment, when B' = G. is. With a normal time keeper the bomb is to get away at the time point T + (T' - t). This calculation is possible but too complicated and unexact, when the bomber is under stress maybe in combat. With the 'count down' of the 'ritorno'-system no calculations is needed, the time is to be seen on the dial.The duration of fall of the bomb was investigated by practical testing. In vacuum the point C' = G would be vertical to the plane, but the air causes that the real point of touch is behind the plane and the theoretical point. The duration of fall depends on the type of the bomb.
Calculation of the free fall:
We can calculate the duration of the fall of the bomb in vacuum: The speed of falling is: V = g * t, with the geoid acceleration g = 9,81 m/s² is this case v = g * t = V 2 g * h. It follows T = V 2 g h / g² = V 2 h / g. E.g. is at a height of 2000 m the free falling time in vacuum t = 20,2 s. The real time maybe t = 22 s. The crew can take this time at the height of 2000 m from a table work. When the bomber personal reads the 22s during the 'count down' of the 'ritorno time piece', he gets off the bomb. We don't know, how the absolute speed, the wind and the real height-difference is influencing, but the basic problem is not changed.
The different versions:
On some 'ritorno' time pieces e.g. Zenith, Leonodas and Minerva wrist version the mentioned table (h, t in hm = hectometers) is already printed. The panel mounted version of Minerva had not this prints on the dial. This version is unusual, the count down has to be read on a lateral scale of it. The 'count down' scale is two times slower than the 'count up'. By this special feature the time for the target AB is shortend to increase the security, because the aircraft cannot fly moves when targeting. The scale for the 'count down' of the Minerva clocks is 0 to 40 s. 40 s is for a height of 6000 m. This was at that time the maximum practible height for bombing operations.
There are few descriptions of this species of watches in the literature. I did use a description lying at the Minerva clock of unknown origin. Konrad Soppa did help me with some military informations.
There is a short instruction of use at Marvin Whitney to an item of Wittnauer, which seams to be identical to the Minerva wrist version.
Here is the original text of M. Whitney:
British World War II Bombardier time recorder/countdown strap leg watch made by Wittnauer, ca 1941. The rotatable bezel is set with a luminous triangular pointer. The single hand makes 2 1/2 revolutions per minute and is started by a single depression of the crown, resulting in the hand moving clockwise. When countdown is to begin, the crown is depressed twice, causing the hand to retrogress (counterclockwise) to zero. Then 'bombs away'. The push piece, located at 1 o'clock, stops the hand and a depression of the crown will return it to zero.
Description of some Bomb Timers:
Universal Wrist Watch:
Referenz: 39901, Case No. 742610, Year of Make ca. 1939 Manufacturer: Universal Watch Co. SA Geneve, Supplier: A.Cairelli Roma Military Forces: Royal Italian Air Force (Reale Aeronautica Italiana), Case: diameter of case 50 mm, chromed brass, Movement: 19 "', golden surface, marked Universal Geneve, monometal ring, autocompensating Breguetspring, no shock prove, Dial: black with indication 5 to 60 units (hm), seconds 360 Grad on 30 sec, turning bezel with luminated hands, marked at 6: Universal Geneve, A. Cairelli Roma, Literature: Auktionshaus Dr. Fischer Heilbronn, Catalog Nr. 86, S. 32 Leonidas Wrist Watch: Case No. 155411, Year of Make ca. 1937 Manufacturer: Heuer-Leonidas SA Biel Military Forces: Italian Air Force Case: diameter 55mm, nickeled brass, handle for remote control Movement: monometal ring, autocompensating Breguetspring, no shock prove, ancre, Dial: black, marked 'Chronographo a Ri¬ torno', Scale 5 - 60 (hm), 0 - 30 sec, central 30-second-hand and 15 min-counter turning bezel with marking Literature: K. Imai, Military Design, Kobunsho Bunko, S. 34 (US Air Force)
Minerva Wrist Watch:
Ref.: 20 "', Case No. 291577, Year of Make ca. 1934 Manufacturer: Minerva SA, Villeret Military Forces: Italian Air Force (Reale Aeronautica Italiana) Case: diameter 55mm, nickeled brass, back is marked: R. AERONAUTICA Movement: marked Minerva, gold plated, 9 jewels, monometallic ring, flat coil, Dial: Emaill black with white part, signed: Minerva 'Chronographo a Ritorno', Scale 5 - 50 hm und 0 - 30 (sec), central 30-seconds-stop hand and 15 min-counter, turning bezel, indications and hands luminating Literature: Whitney: Military Time Pieces, Awi Press, S. 85 (Royal Air Force),
Zenith Wrist Watch:
Ref.: 20 "', Case No. 128197, Year of Make ca. 1933 Manufacturer: Zenith SA, Le Locle Military Forces: French Air Force Case: diameter 46mm, nickeled brass Movement: golden unmarked, Dial: black, signed: Zenith 'Chronographe a Retour', Scale 5 - 40 hm und 0 - 30 (sec), central 30-second-stop hand and central 15 min-hand, turning bezel
Breitling Wrist Watch:
Ref. 637, Case No. 484272, Year of Make ca. 1939 (like new!) Manufacturer: G. Leon Breitling SA Biel Military Forces: Italian Air Force Case: nickeled brass, handle on the right at 90 grade, Movement: Cal. 637, nickeled, 2/3 platine an¬ cre-movement, . Dial: black, signed 'Chronographo a Ri¬ torno', Scale 5 - 60 (hm), central 30-second-stop hand and central 15 min-hand, turning bezel Literature: B. Richter: Breitling, Callwey, S.141; Kahlert Muehe Brunner: Wrist Wat¬ chen, Callwey, S. 278
Ref.: Viro 40, Year of Make 1939 (I got this clock in the original packing in a watchmakers journals paper of 1933 von Minerva) Manufacturer: Minerva SA, Villeret Military Forces: Yougoslavian Air Force Case: diameter 72 mm, Aluminum withot glas cover, on the side case bezel is a second scale for duble markings Movement: marked Minerva, gold plated, 9 jewels, monometallic ring, flat coil, Dial: black, signed 'Minerva VIRO 40' and an emblem with wings, scale 5 - 55 und 0 - 30 (sec), central 30-secon¬ d-stopp hand and 30 min counter Literature: -
Vial Cockpit Instrument:
Case No. 9237, Year of Make ca 1937 Manufacturer: Jean Vial SA, Paris Military Forces: French Air Force Case: diameter 55mm, nickeled brass, 4 mounts to hang into the cockpit-plate, Movement: 20 "', marked Minerva, gold plated, 9 jewels, monometallic ring, flat coil, Dial: Email black with white ring, signed: 'brevete S.G.D.G.' 'Indicateur B.C.V., scale 5 - 50 hm und 0 - 30 (sec), central 30-secound-stop hand and 15 min-counter, turning bezel Literature: Auktionshaus Henry's, Katalog 9/92
Longines Cockpit Instrument:
Year of Make ca. 1936 Hersteller (Supplier): Jean Vial SA, Paris Military Forces: French Air Force Case: diameter 60mm, nickeled brass, 3 rings for mount to the cockpit, stop function turned by 90 degrees Movement: 20 "', marked Minerva, gold plated, 9 jewels, monometallic ring, flat coil, Dial: black, signed: 'brevete S.G.D.G.' 'Indicateur B.C.V.', scale 5 - 50 hm und 0 - 30 (sec), central 30-second-stop hand and 15 min counter, turning bezel Literature: Marozzi u. Toselli: Longines, Giada Edizione, S. 99