Bomber stopwatches of the Reichsluftwaffe WW2 Minerva, Vial, Lip, Longines, Breitling, Zenith, Leonidas, Universal Geneve
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The Bomber Stopwatches
Such special timepieces were used in multi-seat fighter aircraft in combination with a sighting device. Before World War II, the various types were primarily manufactured in Switzerland and used by the air forces of several countries. I have seen examples with signatures from the Italian, French, and British air forces. The manufacturers were several renowned Swiss manufacturers; I also heard of an IWC 'Bomb Timer' that was delivered from Austria to a Geneva auction house but never even made it to auction, disappearing into an IWC collection beforehand. The French examples, particularly those by Jean Vial, used Minerva movements. I don't know whether Longines had its own movement; I only have photos of the front. Of the last watch shown by Vial, I know of a version with a Minerva movement, and another that is externally identical but signed by Longines.
There's almost no information available from the manufacturers. Zenith no longer has any information about this watch, even in their archives, and I haven't learned anything about its variants from Jean Vial in Paris. I was only able to obtain information from Minerva in Villeret, as they still had a few examples in stock. As always, I'm happy to receive additional information about my publications.
There is no evidence that the German Air Force used watches separate from the bomb aiming device. The aiming device used by the Germans had a largely standard stopwatch built in. After the outbreak of the war, some national orders, including those from Italy and Yugoslavia, were no longer delivered, so that brand-new examples still appear among collectors. It is likely that Italy, as an Axis power, joined Germany in many areas of military aviation after the outbreak of the war. The striking feature of these watches is that the stopwatch hand runs backwards ('countdown' or 'ritorno') after the pusher is pressed twice.
How does a 'Chronografo a Ritorno' or 'Bomb Timer' work?
General principle:
The bomber squadron used an optical system to aim at the target. This device had three markings, for example, A, B, and C (see Figure 3). Normally, the distances between AB and AC are equal. Let the target be G. The problem is explained using the following diagram: Using a timer, the crew knows the time interval between A' = G and B' = G. For the bomb to reach the target at C' = G, the bomb's fall time must be known. We call it t. The bomb is released at (T' - t), measured from the moment when B' = G. With a normal timer, the bomb must be released at a time T + (T' - t). This calculation is possible, but too complicated and inaccurate if the bombardier is on target approach or under stress from defensive fire. With the countdown of the ritorno system, no calculation is necessary; one simply reads the time from the clock face.
The drop time and trajectory of the bomb were determined through practical throwing tests. In a vacuum, the point C' = G would be exactly perpendicular to the aircraft, but air resistance causes the actual impact point to be behind the aircraft and thus behind the theoretical point. The drop time varies with the size of the bomb.
Calculation of free fall:
We can calculate the fall time t of the bomb in a vacuum. The fall speed v is: v = g * t With the acceleration due to gravity g = 9.81 m/s2, in this case v = g * t = V 2 g * h It follows: t = V (2 g*h) / g2
= V (2 * h) / g For example, at an altitude of 2,000 m, the free fall time is t = 20.2 s. The actual time in the air is t = 22 s. At an altitude of 2,000 m, the crew can calculate this time of 22 s from a table. If the bombardier reads 22 s on the ritorno timer during the countdown, he drops the bomb. We don't know how the absolute speed, the wind, and the actual altitude difference affect the time. But this doesn't change the basic problem.
Versions:
On some 'ritorno' timepieces, e.g. Zenith, Leonidas, Minerva bracelet version, etc., the previously mentioned table (h, t in hm = altitude in 100 m (hectometers?)) is already printed.
Minerva's on-board 'ritorno' timepieces do not have this print on the dial. They represent an unusual type. The 'countdown' must be read on the side scale, and it is twice as slow as the 'countup'. This special feature shortens the time for the target distance AB, AB # BC, but AB = 1/2 BC. This was required to reduce the time of the sighting period and thus increase safety, as the aircraft was not allowed to perform any evasive maneuvers during the approach to the target. The scale for the 'countdown' on Minerva watches is 0 to 40 seconds. 40 seconds requires a bomb drop from an altitude of 6,000 meters. This was the maximum practical altitude for a bombing operation at that time.
There are few descriptions of this type of watch in the literature; I relied on a description of unknown origin included with the Minerva watch. Konrad Soppa helped me with the correct military designations.
Marvin Whitney describes a Wittnauer variant that appears identical to the Minerva bracelet version. I reproduce this description here in its original form:
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 models from the major manufacturers:
Universal wristwatch:
Reference: 39901, Building No. 742610, built around 1939 Manufacturer: Universal Watch Co. SA Geneve, Supplier: A.Cairelli Roma Purchaser: Royal Italian Air Force (Reale Aeronautica Italiana) Case: Diameter at the case 50 mm, solid construction in chrome-plated brass, movable lugs, push mechanism on the crown with thread for remote control, heavy case back hinged, case back signed on the inside with manufacturer's stamp, ref. and case no., strong leather strap Movement: 19"', gold-plated, column wheel control for the chronograph mechanism, movement signed Universal Geneve, monometallic ring balance, blued auto-compensating Breguet hairspring, index adjustment, no shock protection, chronograph lever finely ground and finished Dial: black with graduations from 5 to 60 units (hm), seconds scale designed over 360 degrees to 30 seconds, rotating bezel with luminous hands, inscription at 6: Universal Geneve, A. Cairelli Roma,
Literature: Auction House Dr. Fischer Heilbronn, Catalogue No. 86, p. 32
Leonidas wristwatch:
Case No. 155411, built circa 1937. Manufacturer: Heuer-Leonidas SA Biel. Purchaser: Italian Air Force. Case diameter: 55 mm, nickel-plated brass, movable lugs, push-button crown with thread for remote control, strong leather strap. Movement: Lever movement, stopwatch and counter controlled by a column wheel. When the stopwatch function is activated, the balance wheel stops. Dial: Black, signed 'Chronographo a Ritorno', scale 5-60 (hm), 0-30 sec., central 30-second stop hand and 15-minute counter by the crown, rotating knurled bezel with arrow.
Literature: K. Imai, Military Design, Kobunsho Bunko, p. 34 (US Air Force)
Minerva wristwatch:
Ref.: 20"', Case No. 291577, Year of manufacture approx. 1934 Manufacturer: Minerva SA, Villeret Purchaser: Italian Air Force (Reale Aeronautica Italiana) Case: Diameter of the case 55mm, nickel-plated brass, fixed wide lugs, case back hinged, engraved: R. AERONAUTICA with crown, strong leather strap Movement: signed Minerva, gold-plated, lever movement, 9 jewels, monometallic ring balance, flat hairspring, control of the stop and counter mechanism via ratchet wheel, when the stop function is activated the balance wheel is stopped. Dial: black enamel with white ring, signed: Minerva 'Chronographo a Ritorno', scale 5 - 50 hm and 0 - 30 (sec), central 30-second stop hand and 15-minute counter on a white background by the crown on the right, rotating Knurled bezel with arrow, numerals, hands and bezel arrow with luminous material
Literature: Whitney: Military Time Pieces, Awi Press, p. 85 (Royal Air Force)
Zenith wristwatch:
Ref.: 20"', Case No. 128197, Year of manufacture approx. 1933 Manufacturer: Zenith SA, Le Locle Purchaser: French Air Force Case: Diameter of the case 46mm, nickel-plated brass, fixed wide lugs, pressed case back, strong leather strap Movement: gold-plated unsigned, lever movement, control of the stop and counter mechanism via column wheel, when the stop function is activated, the balance wheel is stopped. Dial: black, signed: Zenith 'Chronographe a Retour', scale 5 - 40 hm and 0 - 30 (sec), central 30-second stop hand and central 15-minute hand, rotating knurled bezel with arrow
Breitling wristwatch:
Ref. 637, Case No. 484272, built circa 1939 (as good as new!) Manufacturer: G. Leon Breitling SA Biel Purchaser: Italian Air Force Case: Diameter of the case, wide lugs, large knurled crown on the right at 90 degrees, pusher for stopping the balance at 30 degrees, strong leather strap Movement: Cal. 637, nickel-plated, 2/3 plate lever movement, stop and counting mechanism controlled by a column wheel; when the stop function is activated, the balance wheel is stopped. Dial: black, signed 'Chronographo a Ritorno', scale 5-60 (hm), central 30-second stop hand and 15-minute counter by the crown, rotating knurled bezel with arrow, numerals and hands with luminous material
Literature: B. Richter: Breitling, Callwey, p. 141; Kahlert Muehe Brunner: Wristwatches, Callwey, p. 278
Minerva on-board instrument:
Ref.: Viro 40, built in 1939 (I received this watch in its original packaging and wrapped in a 1933 watchmaking magazine from Minerva). Manufacturer: Minerva SA, Villeret. Purchaser: Yugoslavian Air Force. Case: 72 mm diameter, aluminum without a glass cover. A second scale is engraved on the side of the case. The hand extension indicates a double time division. Movement: signed Minerva, gold-plated lever movement, 9 jewels, monometallic annular balance, flat hairspring. The stopwatch and counter mechanism are controlled by a ratchet wheel. When the stopwatch function is activated, the balance wheel is stopped. Dial: black, signed 'Minerva VIRO 40' and a stylized emblem with wings, scales 5-55 and 0-30 (sec), central 30-second stopwatch hand and 30-minute counter by the crown. Literature: -
Vial built-in clock:
Case No. 9237, built around 1937 Manufacturer: Jean Vial SA, Paris Purchaser: French Air Force Case: Case diameter 55mm, nickel-plated brass, 4 fixed, wide lugs for mounting in a cockpit bracket, hinged case back, Movement: 20", signed Minerva, 9 jewels, monometallic ring balance, flat hairspring, gold-plated, lever movement, stop and counting mechanism controlled by a ratchet wheel, when the stop function is activated, the balance wheel is stopped. Dial: Black enamel with a white crown, signed: 'brevete SGDG' 'Indicateur BCV', scale 5 - 50 hm and 0 - 30 (sec), central 30-second stop hand and 15-minute counter by the crown, rotating knurled bezel with red arrow,
Literature: Henry's Auction House, Catalogue 9/92
Longines built-in clock:
Year of manufacture approx. 1936 Manufacturer (supplier): Jean Vial SA, Paris Purchaser: French Air Force Case: Diameter of the case 60mm, nickel-plated brass, 3 rings for hanging in a cockpit holder, crown at the top, stop lever offset 90 degrees to the left Movement: probably Minerva, 9 jewels, monometallic ring balance, flat hairspring, gold-plated, lever movement, control of the stop and counter mechanism via a switching wheel, when the stop function is activated the balance wheel is stopped. Dial: black, signed: 'brevete SGDG' 'Indicateur BCV', scale 5 - 50 hm and 0 - 30 (sec), central 30-second stop hand and 15-minute counter by the crown, rotating knurled bezel with red arrow, Literature: Marozzi and Toselli: Longines, Giada Edizione, p. 99 (the other version of the Bomber stopwatch shown is also shown there)
