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After the fall of France in 1940 German planners realized they would need to enhance the Luftwaffe’s airlift capability prior to the invasion of England. Messerschmitt designed an enormous glider designed to take a load the size of of a standard German railway flat car with a weight limit of 23 tons. This was the Me 321.
While the glider design was successful, the Luftwaffe lacked a suitable towing aircraft. The largest German transport in service at the time, the Ju 90, struggled with the Me 321. Three Bf 110 fighters were tried along with rocket assistance, but this arrangement proved extremely dangerous. At the behest of Ernst Udet, Heinkel designed the He 111Z as a towing aircraft.
The He 111Z Zwilling (Twin) was constructed by taking two He 111H-6 airframes and rebuilding them as a single twin-fuselage aircraft around a new common center wing section which contained three Jumo 211F engines, for a total of five engines.
The new design proved successful, and was reported to have good flying characteristics. It was able to remain flying with three of the five engines shut down as long as the remaining two powered engines were arranged symmetrically.
Total fuel capacity was 2,260 gallons (8,570 liters), and it could carry an additional 640 gallons (2,400 liters) in four external tanks. This gave the He 111Z an endurance of 10 hours while towing a glider.
A rare in-flight photograph of an He 111Z towing a Me 321 glider. This view gives an interesting size comparison of two enormous aircraft. Wingspan of the He 111Z was 115 feet 6 inches (35.2 meters), wingspan of the Me 321 was 180 feet 5 inches (55 meters).
The Zwilling carried a crew of seven, four in the port fuselage and three in the starboard. The pilot flew the aircraft from the port fuselage. The tow line attachment points can be seen in this picture.
Twelve He 111Z were built, two prototypes and ten production aircraft. These were converted from standard Heinkel He 111H-6 and H-16 airframes. The Me 321 gliders proved difficult to handle on the ground, and eventually they were converted into Me 323 transports by adding six Gnome-Rhone GR14N radial engines.
The Zwilling could be used to tow other types of gliders. Three Go 242 gliders were towed simultaneously during trials, but usually no more than two were towed at a time operationally.
A Go 242 at rest while a Zwilling takes off overhead. Like the larger Me 321, the Gotha glider was also converted to a powered transport using two Gnome-Rhone GR14N radial engines.
A poor photograph, but an interesting view of a He 111Z as seen from the second of two Gotha Go 242 gliders while under tow.
Due to a chronic shortage of transport aircraft, the Zwillings were used extensively as transports on the Eastern front. Here is one on a rather muddy airfield with white temporary camouflage.
Of the twelve Zwillings constructed, eight were lost during the war. None of the four remaining airframes were preserved, all being scrapped.