Germany produced some spectacularly unique aircraft during WW2.
One of these creations was a giant transport glider called the Messerschmitt 321 Gigant,
but that isn’t the star of today’s video.
No, today we’re going to be looking at the aircraft that was built to tow it – the
equally strange Heinkel He 111 Zwilling.
During the early stages of the Gigant’s development, the Luftwaffe found itself in
a bit of an embarrassing position.
The Junkers Ju-90 – the intended towing aircraft – was not powerful enough to safely
tow the giant glider into the air.
A solution was found in the terrifying form of having the Gigant towed by three Bf110s
and assisted by rockets for take-off, an endeavour that was so complicated and dangerous that
only the most experienced pilots were allowed to take part.
Unsurprisingly the aerial equivalent of an overweight sleigh being towed by a trio of
explosive reindeer didn’t sit too well with the saner members of the relevant authorities,
one of whom was Ernst Udet, who was now the Luftwaffes Chief of Procurement and Supply.
Keen to not lose precious men and materials in numerous mid-air explosions, he approached
Heinkel in the hopes of finding a different solution that could be implemented quickly
and without great expense.
This would bear fruit when in early 1941 Udet paid a visit to Heinkel’s factory, and together
they stared longingly at a collection of He 111s that were under construction and thought…
“what if we stuck two of them together?”.
When put in such simple terms the idea sounds almost comical, but there were several strong
arguments for such a design.
Heinkel’s initial calculations showed that the project could be realised in a relatively
short timeframe as most of the design work was already done, and the only area that required
major re-design would be the central wing section that connected the two fuselages.
It would also be considerably cheaper to produce than a completely original design as the required
materials and equipment already had established production lines.
The cost-saving advantage meant that the project received almost universal approval, and the
design work was complete by August 1941.
Construction then immediately began on the prototype, which was built from a pair of
existing H-6 bombers.
This was all completed by Heinkel within two months, and it performed its maiden flight
in Autumn.
Despite looking highly unorthodox, The prototype was praised by test pilots for its surprisingly
good performance and handling, and it was ordered to Rechlin for official trials.
During this time as second airframe was also built for the trials, again from an existing
pair of H-6 bombers.
Both of them performed as expected, completing multiple tests, as well as performing a successful
take-off towing a Me 321 with a simulated payload.
Even a forced landing incident - when the first prototype was ditched on the Rechlin
Lake due to mechanical failure - did not do much to dampen spirits.
The prototype was quickly repaired, and a production order for 40 He-111 Z-1s was placed,
with deliveries expected to be completed by early 1943.
Compared with the prototype, there seemed to be little changes made to the design of
the production model.
Most of the units were built from H-6 airframes that were diverted from existing production
lines, however some were also built from the newer H-16 model as its production began to
get underway in 1942.
As mentioned earlier, the only completely new part of the assembly was the construction
of the central wing section that was used to connect the two He-111 airframes together.
Seeing as this was the only thing holding the two airframes together a lot of attention
was paid to its structural integrity, and for this reason its construction was kept
as simple as possible.
It maintained a constant section thickness and depth, and it was connected at the points
that would otherwise have connected a traditional wing to the He-111 fuselage.
When assembled the two fuselages were spaced 12.8 meters apart, and the new total wingspan
reached an impressive 35.2 meters.
In addition to providing the dual functions of added lift and necessary connectivity,
the re-designed centre section also served to accommodate a fifth engine.
Like many production He-111s, the Zwilling would use five F-2 versions of the Jumo 211,
which provided a total power output of 6700hp.
There was also talk of using the 41.8L BMW 801 as a fifth engine, however sources indicate
that this was only done as an experimental project, and it was never fitted to an operational
airframe.
As a result of the added engine, and the requirement for all engines to be running on higher power
to tow the heavy glider, the overall fuel capacity was increased.
Two fuel tanks were installed in the central wing, each with a capacity of 1025 litres,
and this brought the total internal fuel capacity up to 8570 litres.
The Zwilling was also designed to carry four 600 litre drop tanks to further improve its
range.
The landing gear component of the aircraft was incredibly simple, as it was just twice
that of a normal, He-111.
It consisted of four main landing gear legs combined with two tails wheels, which meant
you had a six-point landing configuration.
Though there was now essentially double the plane, it would not require double the crew.
The primary cockpit, navigation station, and most major instruments were concentrated in
the portside fuselage.
Only an additional set of flight controls and the most important instruments remained
in the starboard side, and the Zwilling was designed to operate with a crew of seven.
Four were in the port fuselage: a pilot, 1st engineer, radio operator, and a gunner.
Three were in the starboard fuselage: A secondary pilot (who was mostly an observer), a 2nd
engineer, and another gunner.
Due to the precarious personnel situation brought on by the surprise – or rather the
shock- of Operation Barbarossa, Heinkel did not manage to complete the first two production
aircraft until October 1942, but they eventually lumbered into service.
Not long after this it was found that the power of the five Jumo engines was only partially
sufficient to get a fully loaded Me 321 into the air (owing to the odd decision of only
testing the prototype with a partly loaded glider due to safety concerns).
For lighter missions it was perfectly capably, but for heavy lifting it was once again decided
to fall back onto rocket assisted take-off.
Six of these would be used in total: two under each fuselage, and one under each outer wing.
These would each add an extra 500kg of thrust during take-off, and once expended they were
jettisoned with parachutes for easy recovery.
The huge Me 321 was suspended from a 150 meter long, 16mm thick steel cable.
This branched out to connect to two couplings that connected it to the central wing section
of the Zwilling, this was done to both reduce total load stress and also reduce the risk
to the Zwilling’s control surfaces in the event of cable a failure.
If there weren’t any Gigants to tow, the Zwilling was also used to tow the smaller
– but still relatively chunky – Gotha 242 transport glider.
It would often tow two, and even on some occasions three at once, and it could do so for up to
10 hours at cruising speed thanks to the additional fuel tanks.
Though it was a singularly unique and impressive aircraft, the 111 Zwilling would not have
a particularly long service life, and this was for a couple of reasons.
The first was the cancellation of production orders.
Tests had been conducted on refitting the Me 321 glider with six powerful Rhone engines.
The subsequent testing phase of the newly re-designated Me 323 was so successful that
only 10 production models of the Zwilling were built before the order was cancelled.
The other reason was the Zwilling’s main theatre of operation.
Most of them were concentrated on the Russian front from January 1943, and due to a lack
of suitable landing sites thanks to a combination of ice, mud, and hostile bears, they only
saw limited use.
Some of this did involve running re-supply missions to the Kuban bridgehead which it
did pretty well, and sometimes to resupply Stalingrad, which it did less well, but these
were few and far between.
The Zwilling also saw some use over the skies of France and Italy.
But with allied air superiority growing by the day, the towing and resupply missions
went from dangerous to downright suicidal.
One aircraft was shot down over France, three over Italy, and the remaining eight were all
damaged on the ground at one point or another.
By September 1944, with only four aircraft remaining, the decision was made to pull them
from active service, and during the chaos of the final 18 months of the war they were
either cannibalised for parts or destroyed by allied bombing raids.
Though they were never built, there were also plans for two other versions of the 111 Zwilling.
The Z-2 version was to be a bomber capable of carrying a moderately heavy payload.
This would come in the form of either four 1800kg or six 1000kg bombs, or alternatively
it would carry four of the Henschel 293 anti-shipping missiles.
There was also talk of it carrying two large sea mines, but it doesn’t seem like this
was seriously considered.
To cope with the added loads, the Jumo engines would be supercharged, however initial calculations
would soon come out with an estimated range of just 1094 km and the project was ultimately
dropped.
The Z-3 would share the same fate, but its cancellation was mostly as a result of the
stoppage of construction on the Z-1 which thus robbed the project of a potential testbed.
It was designed as a long-range reconnaissance aircraft and was meant to have a range of
at least 5000km thanks to a combination of internal and external fuel tanks.
Further information on performance and armament however seems to be non-existent as the project
was cancelled very early on in the design phase.
In total only 12 of Heinkel’s weird twin-111s would be built, and none of them would survive
the war.
The aircraft they had been built to tow, the Me 321, would go on to be built in greater
numbers as the Me 323.
Though deeply impressive in size, it would also have an equally painful service life…mostly
thanks to the swarms of allied fighters.
But that’s a story for another day, as always thank you all very much for watching.
And I’ll catch you all next time, goodbye.