The early 30's
aviation design bureau, lead by K.A.Kalinin, was a capable aircraft
design team. One of their projects was a 3-engined passenger aircraft,
the K-7, also designated as "K-Tjazholij" (K-Heavy) for the new 500hp
German BMW "Hornet" engine. It was designed to carry 22 passengers. The
mock-up prototype was tested in the TsAGI wind tunnel in September
1928, and was approved for production by the Scientific-Technical
Commetee VVS in March following year.
However, production was not started, and the designation K-7 was used
again for a transcontinental aircraft project, begun in 1928. 'The idea
to built K-7, (K.A.Kalinin wrote later), came to me long ago, as early
as 1925. In 1929 the project was formulated and after two years of
refinement it started to come to life... With the larger engines
available it was possible to consider building aircraft capable of much
greater carrying capacity. This led to the design of a flying wing. The
new philosophy was "everything is in the wing"'.
The original aircraft design was all-wood with a 5-spar wing. But it
became clear that it would suffer from lack of strength, and after much
discussion, K.A.Kalinin decided to switch to an all-metal welded
construction with a 3-spar wing.
The K-7 was a gigantic elliptical wing of extremely thick airfoil (span
53m, area 452m2) with two triangle section tail booms, carrying tail
fin and tail plane with control mechanisms. The straight wing (width
6m, length 10m, height 2.33m) had room for payload and people.
Elliptical consoles with 14 fuel tanks were attached to the centroplane.
The centre was covered with duralumin, consoles with fabric. There were
no thick tubes able to comply with the requirements of the central spar
so it was made of two parallel tubes. The rest of the wing frame also
was made of welded steel tubes.
A nacelle protruded from the leading edge, with a cockpit for two
pilots, navigator, radio-navigator and chief mechanic. Seven more crew
members were located in other sections of aircraft and communicating by
The aircraft was
designed for 6 BMW engines, but a later decision was made to install
Russian made AM-34s. The six water-cooled engines lacked power, and
K.A.Kalinin was forced to add a 7th pusher engine on the trailing edge
of the wing between the tail booms. The designer was aware that this
engine would increase airflow turbulence and would lead to vibration of
the aircraft, but he had no other choice. Early AM-34 engines were not
equipped with gearboxes and had power of only 750hp.
The triangular tail booms protruded from the last spar to the tail. The
tail booms were equipped with tailwheels, protecting the tail from
contact with the ground. The original main gear design allowed the
aircraft to be horizontal while on the ground. The main gear consisted
of two widely placed gears of frame design with oil-pneumatic wheel
suspension, the first attempt in Soviet heavy aircraft.
Each main gear had 3 wheels and were covered with fairings - the left
gear had an entry door and stairs leading into the wing.
At the design stage it
became clear that it would be hard for the pilot to control the plane
because of the enormous load on the controls. For the original design,
K.A.Kalinin proposed electric 'boosters'. A contract was signed with
one of Moscow electro-technic institutions, which undertook to develop
and manufacture the new system. But they failed to do so, and the
control planes of the K-7 were equipped with control surfaces, placed
on light booms.
Theoretical problems of heavy aircraft control by use of control
surfaces were solved by the aerodynamic team lead by N.F.Freiman. On
the TsAGI wind tunnel more than 300 successful tests were performed,
and in 1932 the surfaces were tested on the K-5 flying laboratory. The
system workedperfectly, and it was recommended to install them on the
K.A.Kalinin also decided to use chromium-molybdenum tubes for the
aircraft frame - it was the very first use of such tubes in Russia.
Calculations for the frame were lead by Professor A.S.Balinsky - a
prominent scientist in the field of materials strength. Sadly, aircraft
was already overweight while still in the design phase, because
calculations had to take into account loss of strength of tubes near
Steel tube production started (at the same time as the aircraft design)
at the Dnepropetrovsk Metal Works. Until now, tubes were imported from
Sweden, and to buy tubes for a single K-7 could cost as much as 100,000
golden Roubles. The TsAGI commission noted in 1932, the K-7 represented
a big step forward in aircraft design being the first to employ the use
of chrome-molybdenum tubes in the USSR.
The K-7 was designed as a multipurpose civil and military aircraft. One
of passenger options was developed to carry 128 passengers up to
5,000km. Another design - "luxe" - was supposed to arrange 16 luxury
cabins for 64 passengers. The aircraft was supposed to have a
comfortable club-room, snack-bar and kitchen. This was the first
aircraft design allowing mechanics in-flight access to running engines.
A lot of effort was spent on the K-7 by an armament team lead by
D.I.Grigorov. The military version of aircraft was a real "flying
fortress", which emerged 10 years before the American Boeing B-17.
Defence of Kalinin's giant used as many as 12 gunner positions (8 20mm
cannons and 8 7.62mm machine guns). A special electric cart (running on
strings inside of tail booms) was designed for transportation of
gunners to two tail machine guns. The aircraft had no "blind spots",
and every direction was covered by 3 gunners, which greatly increased
its defence capability.
Bombs were located in wing bomb bays, and bomb bays frames were
included into wing structure. Depending on the required range, bomb
loads varied from 9,900kg to 16,600kg (25,550 - 42,840lb). External
fuel tanks allowed 2,400km flight with 6,000kg bombs.
Other military modifications could carry 112 fully equipped
paratroopers. Opportunities to carry an 8.4 ton tank or other parachute
droppable equipment between main gears was also under study.
The technical project was completed early 1932. In the yard of the
Kharkov Aviation Factory, a full size wooden mock-up of the aircraft
central part was built. The size of aircraft was too big for existing
manufacturing areas, and new assembly hall was built. In November 1932
assembly of a prototype aircraft started which completed just in nine
In the early August 1933 the aircraft was taxied on to the runway. Its
test pilot was M.A.Snegirev and the co-pilot was A.N.Giatsinsky. When
all of the engines were started a serious vibration throughout the
aircraft was noted so the affected parts were quickly strengthened. On
August 19 A.M.Snegirev made the first straight flight at 5m altitude.
More troubles were revealed - the elevators were vibrating with
amplitude up to 1m! In a few days the tail of the aircraft was
redesigned - it became a 'biplane' with an additional stabilizing
surface between tail fins. Surfaces from the tail fins were removed,
and those on the tail planes were moved closer to the elevators.
The day before the first flight P.I.Baranov (head of the Major
Management of Aviation Industry) together with test-pilot M.M.Gromov
arrived at Kharkov. At 6 a.m. August 21, 1933 K-7 was 'on the start'
with running engines. The weather was fair despite some light clouds.
Seven crew members took their positions and, as usual, at the last
moment, the co-pilot's seat was occupied by Kalinin himself.
After few taxis, the plane took off smoothly. As A.M.Snegirev said
later, 'the aircraft was very responsive. It was easy to control. Hard
to believe, but you just pull the yoke - and machine responds'.
The K-7 made a circle over Kharkiv, and safely landed on the factory
In the next flights it became clear, that vertical vibration was gone
with the new tail design, but vibration was still present in horizontal
surfaces. M.A.Snegirev, employing his experience as a test pilot, found
a way to suppress it by changing the engines power settings. Continuous
tests revealed fair flying characteristics. The last factory test
flight was scheduled for November 20, and the plane then had to be
flown to Moscow. It was planned to measure the maximum ground level
speed. This flight (10th one) was completed successfully, but the
ground team made a mistake which spoiled the measurements.
The next attempt was planned for November 21. The K-7 now had over 5
hours of flight time. The flight plan included approaching the
'measured kilometre' at an altitude of 1000m, then diving the aircraft
to 100m and exceeding the VNE.
At 2p.m. the K-7 took off with 20 crew members on board. Here is the
story of D.A.Chebyshev, one of ground engineers who participated in the
tests : "As scheduled, K-7 flew over us and the pilot saw that we were
ready for measurements. The aircraft made a turn, accelerated and
started the approach. At about 3 to 4kms from us it made a sudden dive
from 300-400M at full speed. First contact with the ground knocked away
the landing gear. The aircraft bounced and with the engines running it
hit the ground again. A fire started..."
What was recalled by one of the five surviving crew members,
P.I.Semerenko : "Approaching the 'measured kilometre' Snegirev gave
full throttle. The tail booms vibration increased. I counted 15-20
major shudders. Suddenly, to the noise of running engines was added the
sound of the left tail boom lower tube braking apart. Displaced tube
edges locked the elevators, and the K-7 was not able to get out of the
dive. Near the surface the aircraft went into a left pitch. I was
waiting for the end. Controls were still locked still dead. Smash..."
Technicians and designers suffered the destruction of their creation
and death of 15 crew members. K.A.Kalinin was ill for two months
because of heart problems. Despite this catastrophe the team did not
lose their confidence in the design. To investigate the causes of the
crash a committee was formed which included some of the most prominent
aviation experts of the country. It was concluded that the destructive
vibration was caused by the surfaces during some power settings of the
seventh engine. There was no hard evidence to prove this conclusion. A
few years later M.V.Keldysh found the reason for the crash and found a
solution to fight the flutter - weight balancing of elevators. But it
was too late...
After obtaining expert reports, revealing no mistakes in aircraft
design, the head of GUAP ordered Kalinin to start urgent production of
two new (passenger and military) K-7's with scheduled roll-out in the
beginning of 1935. Production was assigned to new base - The Voronezh
Changed views in the country's aviation leadership on production of
heavy aircraft did not allow the team to complete development of K-7.
Both planes were 'preserved', one of them half-built. Kalinin tried to
prove necessity of this aircraft type but failed to win over the
opposition to his project.