Germany’s U-boats air defense systems

Much as they did towards the end of the Great War, in the early years of World War II German’s U-Bootwaffes roamed, almost with impunity, the sea trade routes of the Western Allies, engaging and sinking their much vital shipping at an alarming rate. It wasn’t until the Allies began to implement a sophisticated system of long rage, air patrol over the Atlantic that the tide of the submarine war finally began to turn in their favor.

Because most of Germany’s U-boat force was incapable of prolonged, submerge patrol time, they became easy targets for praying allied medium and heavy bombers covering the North Atlantic.

Engaging and hitting allied patrol airplanes became the sub’s main objective from late 1943 to the end of the war in May ’45. In an attempt to achieve this task, each boat was fitted with a vast array of defensive weapon systems.

The submarine’s main anti-aircraft weapon was the 2CM Flak Gun. Two basic designs of this uninspired looking, but tremendously effective flak system were employed. The first operating 2CM was the No. 30. The thirty was a single barrel weapon with a 360 degree traverse and capable of a two degree depression and 90 degree elevation. It fired a 0.32kg shell capable of reaching distances of up to 12,350 meters. What made this weapon so effective was it impressive cycle rate of 480 rounds per minute.

The second, improved version of the 2CM was Flak 38th. Similar to the 30th, but capable of reaching a cycle rate of 960 rounds per minute, the 38th was arguably the best German, light attack weapon of World War II.

Another light weapon use by U-Boats to fend-off attackers was the 3.7CM M/42 Flak Gun. In the bottom half of the war, most German submarines were fitted with the 42nd platform. It fired a .73Kg shell up to a distance of 15,350m. Maximum firing cycle was 50 rounds per minute.

Those two weapon systems accounted for almost 85 percentage of all hit allied aircraft. Official numbers regarding hit aircrafts varies from source to source, but the most reliable figure (coming from British-generated documents released in the mid 1950s) puts the amount at 247 from the spring of 1944 to April 1945. 

Although it was not a intended as a primary anit-aircraft weapon, the vaunted 8.8CM Schiffskanone Deck Gun was also use in that role, especially towards the end. This remarkable 8.8 gun employed by the German navy was not directly related to the more famous, 8.8 Acht-Acht flack gun utilized by the army as an anti-tank weapon. The CM was purely a naval gun develop in the waning days of World War One.

The gun was mounted on a low box forward of the conning tower. It could traverse through a field of 360 degrees. Its -4 degrees depressed parameter and 30 degree elevation capacity were two of the most impressive features of this remarkable weapon. The gun fired a 13.7kg high explosive shell at a 700m/sec muzzle velocity. It had a solid impact range of up to 12,350m.

Manned by a three men crew, the CM was a powerful, horizontal weapon that when use against sea-based platforms, it caused heavy damage. As the U-Boats began to sustain alarming losses to Allies praying bombers, German crews commenced utilizing their main armament on incoming enemy aircrafts. Although their use on that type of environment wasn’t tested before the war, the gun performed well.

Data on the numbers of downed allied aircraft hit by the 8.8CM is not reliable. But unofficial accounts put the numbers in the low 50s. Much of that amount was accounted for between the autumn of 1944 and the spring of 1945.

Aside from those three defensive weapons, German submarines carried a limited amount of small caliber fire arms including 9mm and 7.62mm hand guns. Nine mili-miters machine guns and some 7.92mm rapid fire rifles. No data on hit aircraft by these weapons are available.

Of course, no weapon can be effective if the enemy isn’t spotted. For long range detection, the U-boats employed the Funkmessorungsgerat (Fu) MO-29 Radar. The MO-29 was use primarily on Type IV boats as well as some Type VIIs. The 29 was simple to utilize thanks to its twin horizontal rows of eight dipoles on the upper front part of the conning tower.

On the top row laid the transmitters and in the lower one, the receivers. An improved version of the 29 was introduced in the summer of 1942. In that version, known as No. 30, the diploes were replaced by a retractable antenna which was housed in a slot in the tower.

Although relative powerful for the times, this system barely was able to detect surface vessels because of the low position of it’s mounting in respect to the horizon.

A more complex system, FuMB1 or the ‘Metox’ was introduced in the fall of 1942. This system was utilized in conjunction with a raw, wooden cross antenna strung with copper wire know as the ‘Biscay Cross’. But as with the early Fus platforms, this unit wasn’t that reliable. In fact, a case could be mad that their use was highly detrimental to the sub’s survival thanks to the Metox’s volatile emissions which were easy detectable by Allied radars.

By November 1943, the Germans had finally develop what would become the world’s first true, all around naval radar. Born out of desperation, FuMB7 combined Metox and Naxos emissions to give U-boat commanders a first rate, long range detection system. Further enhancements were performed (the FuMB24 and 25) to the base MB7 giving it an extended operational radius. 

Aside the radar, maybe the most ingenious defensive measure used by German submarines was the Focke-Achgelis. The ‘Focke’ was basically a manned rotary glider with a triple blade rotor. It was as simple to operate as it was to assemble. Housed in a storage cylinder on the afterdeck, the Focke was quickly armed and launched. It remained connected to the U-boat by an umbilical cord. From its advantageous position high above the sub (10-12,000 feet), the pilot could spot any target approaching the boat. Unfortunately for the Focke, if the U-boat came under direct attack, there was no time to reel it in, thus the sub cut the cord and left the pilot to defend himself until all was cleared to surface back again.

 More effective than the Focke-Achgelis was the Aphrodite. It was a basic devise consisting of a large (one meter diameter) hydrogen-filled balloon from which dangled small strips of metal foil. It was attached to the sub by way of an anchor weight. Its main purpose was to confuse allied aircraft utilizing radar navigational systems.

Cierva’s Autogyros

When the famous Spanish inventor, Juan de la Cierva, was only fifteen he designed and built his first glider. Three years later, in the summer of 1918, he was able to develop a three-engined aircraft. The goal of his experiments was to achieve the creation of an air platform that could sustain lift and land safely after an engine failure. With the development of helicopters still a thing more of the imagination rather than a practical concept, Cierva turned his attention to the idea of an airplane that utilized an unpowered rotor system for lift and a conventional propeller mechanism for propulsion. Does the concept sound familiar?

The term ‘autogyro’ was conceived by Cierva to describe his new aircraft idea, which featured a freewheeling main rotor providing lift for vertical flight. His idea would revolutionize the air industry thus paving the way for the full development of the helicopter. The main operational system of the autogyro was the articulated rotor hub. Its drag and flapping hinges allowed the individual rotor blades to rise and fall, and so even-out the plane’s lift. After a two year development program, Cierva’s first autogyro took to the air on a cool January morning in 1923. Called the C.4, the first unit flew a distance of 3 miles. By September 1928, Cierva’s C.81 design, powered by a 149kw (200hp) Lynx engine and based on an Avro 504 airframe, performed the always dangerous 25 mile crossing of the English Channel, bound for Paris.

Avro-built Cierva C.30P G-ACIN. (photo, via author)
Avro-built Cierva C.30P G-ACIN. (photo, via author)

After experimenting with a few ideas and systems, Cierva refined his autogyro concept into what would become the technology setter platform of C design and development: version number 19. Version 19 introduced a dedicated fuselage to the series. Prior Cierva models utilized existing aircraft fuselages. After the 19, all other versions were purpose-built. Sixty six 19s were licensed built by AV Roe and Co. Ltd. with headquarters in Manchester, England. France also got into the act. The famous Liore-et-Oliver produced twenty five units designated LeO C130. Even the German Focke-Wulf Corporation managed to built 19s (40 units are believed to have been produced by the venerable aviation company).

If version 19 was a leap forward in rotary wing development, then version 30 was the pinnacle of it. The C.30 was a two seat airplane that featured the pilot occupying the rear, open cockpit. The pilot was able to unlock and tilt the main rotor mechanism using the control column attached to the rotary head. The 30’s airframe structure was of Duralumin tubing with a fabric skin cover. The next evolution of the series, the C.40, was designed around a wooden skin cover over a metal internal frame. A seven cylinder, Armstrong Siddeley Genet Major I-a radial (139hp) engine gave power to the 40. The other main feature introduced in the 30 was folding rotor blades for easier hangar handling. It also possessed a reverse aerofoil section on the port tailplane in order to counter the anticipated rotor torque.

At the beginning, autogyro flying was deemed too dangerous for combat operations, thus not many air forces in the world were interested in Cierva’s revolutionary work. Early flying tests were plagued by accidents. In fact, the first three C designs failed to become airborne. It is worth remembering that the initial C.1, utilized a French Deperdussin fuselage that did not provide the aircraft with enough lifting area which impeded its ability to takeoff. It was number four in the series, C.4, which eventually broke that barrier and got airborne. Following the experiments of the C.1, Cierva went on to produce several other unreliable machines, including the C.4, until he designed the unit 6. With subsidies from the Spanish government, the ingenious Cierva developed the C.6 series utilizing an Avro 504K airframe. The new fuselage would give the series and its inventor a big boost with its lift-drag ratio and overall airframe performance. All other versions of the Autogyros will incorporate the same, basic layout of the 504K airframe.

By the mid 1930s, Cierva and his team were able to stabilize vertical takeoff to the point that air forces felt comfortable enough to invest heavily in the concept. Unfortunately, Juan de la Cierva died in an airline crash at Croydon in December 1936. By that time his ideas were more than accepted, it was becoming the ‘law of the land’ in rotary flying. At the time of his death, Cierva had formed his own aircraft company based in Great Britain. His design was being manufactured in England, France and Germany. The C.30 saw service in the Second World War, most of them with the British Royal Air Force (RAF). There were a commercial version of the C.30, chief among them the de Havilland’s C.24 developed in 1931, but the unit did not meet with much success. Nevertheless, the original Cierva concept would go on to become today’s helicopters platforms. Quiet the achievement for this distinguished Spanish inventor.

Specification for Cierva C.40
Powerplant: One 104kw (140hp) Armstrong Siddeley Genet Major I-A radial engine
Maximum Weight: 816kg
Main Rotor Diameter: 11.28m
Length: 6.01m
Height: 3.38m
Total Rotor Area: 99.89 m square
Top Speed: 117km/h
Operational Range: 459km
Service Ceiling: 5800m

– Raul Colon


More information:
The Encyclopedia of Modern Military Aircraft, Editor Paul Eden, Amber Books 2007
The men, machines and ideas that revolutionized war, from Kitty Hawk to Gulf War II, Stephen Budiansky, Penguin Books 2004
Concept Machines, Carl Thomas, Ispring Group 1972