I
n the mid-1930s, the requirement arose
in Germany for an ocean-going medium-
range ying boat for armed maritime
reconnaissance. It would also have to
be able to land and take off in rough water
to support U-boats a long way from land.
In late 1934, a team led by Dr Richard
Vogt of Hamburger (Ha) Flugzeugbau, the
aircraft manufacturing subsidiary of Blohm
und Voss (BV), came up with three designs
all of which had the same wing construction,
namely a single main load-carrying tubular
spar located at the thickest point of the wing
section and sealed to form a fuel tank. The
preferred design had a gull-wing above a
hull that featured twin-tail booms and was
powered by two engines. Another design
had a bigger wing area and three engines,
while a nal one had four engines.
The preferred option, known as Projekt 8,
was chosen and in early 1935 a contract was
awarded for three prototypes, to be called
the Ha 138, powered by two Jumo 206 dieselengines, but with the Daimler Benz DB 600
being a possible option. It was now that it
earned the nickname, ‘Flying Clog’, owing to
the unusual shape of its fuselage.
As was commonplace with German
aircraft designed in the mid-to-late 1930s,
problems soon arose with the engines.
Production of the Jumo 206 fell well behind
schedule and with the rst prototype ready
by late 1936, Junkers announced that due
to yet more problems, production of this
engine would be terminated. Added to this,
the DB 600 was now being earmarked for
other aircraft types, such as the Heinkel 111
twin-engined bomber and Messerschmitt 110
twin-engined ghter, so the design team had
to remodel the prototype for three lower-
powered Jumo 205 engines, with the third
engine being mounted on a pylon directly
over the hull.
Prior to the rst test ight in July 1937,
water taxi trials were carried out and it was
noted that the hydrodynamic and yingcharacteristics were equally worrying. So
it was back to the drawing board for the
second prototype, which was already at an
advanced state of design with a deeper hull
and larger vertical tails. Also, it no longer
had the gull-wing design, the wings now
joining the fuselage at the top as opposed to
the middle. The mock-up of the bow turret
was removed, too, as it was found to throw
spray towards the windscreen on take-off,
making it difficult for the pilot to see.
The second prototype had an increased
gross weight, so had to be tted with
up-rated Jumo 205 engines. Production
started on the BV 138 A-01 (the pre x Ha
having now been dropped) in early 1939,
and more powerful engines could be tted
later. It was soon proven that the A-01 was
underpowered with a long take-off run from
calm waters but, once airborne, was now
more stable. The hull was strengthened and
stiffened so that it could meet the rigours
of Atlantic weather and eventually it wasTHE FLYING CLOG
32 Aviation News incorporating Jets October 2018
FLYING CLOGFLYING CLOG
The German aircraft manufacturer Blohm und Voss can lay claim
to designing and building a number of aerial oddities, such as the
BV 138 fl ying boat known to many as ‘Der Fliegende Holzschuh’
or the ‘Flying Clog’. Chris Goss relates its career.