The K-7
was a gigantic elliptical wing of thick profile (span 53m, area 452m2)
with two triangle section tailbooms, carrying tailfin and stabiliser with
control mechanics. The wing had almost strait centroplane (widht 6m, length
10m, height 2.33m) where rooms for payload and people was arranged. Elliptical
consoles with 14 fuel tanks were attached to the centroplane. The centroplane
was covered with duraluminium, consoles with fabric. There were no such
a thick tubes to comply with requirements of central longerone, so it was
made of two parralel tubes. The rest of wing's frame also was made of welded
steel tubes. 
A nacelle was proturbed from the leading edge, with cocpite for two pilots,
navigator, radio-navigator and chief mechanic. Seven more crew members
were located in other sections of aircraft and communicated by internal
telephone.
The aircraft was designed for 6 BMW engines, but later decision was made
to install homemade AM-34.
Six water-cooled engines lacked power, and Kalinin was forced to add 7th
pusher engine on the trailing edge of the wing between tailbooms. Designer
was aware that this engine will increase airflow turbulence and can lead
to vibration of all aircraft, but he had no other choice. Early AM-34 engines
were not equipped with gearboxes and had power only 750hp.
Triangle tailbooms were proturbed from the last longerone to the tail.
Tailbooms were equipped wit pneumatics, protecting the tail from ocassional
contact with the groung.
Original main gear design allowed aircraft to have a horizontal position
while on the ground. Main gear consisted of two wide placed undercarriges
of frame design with oil-pneumatic wheel suspension, the first attempt
in Soviet heavy aircraft. Another innovation for this aircraft class were
tires with cameras "GoodYear". Soviet industry had no such tires
in production, and it had to start it in future for production aircraft.
Undercarriages had 3 wheels each and were covered with fairings - metallic
'pants' - left of which had an entry door and stair leading into the wing.
Yet on design stage it became clear that it will be hard for pilot to control
the plane because enormous load on the controls. Though for the original
design K.A.Kalinin proposed decision which later was widely used in aviation
- electric 'boosters'. Contract was signed with one of Moscow electrotechnic
institutions, which took a duty to develope and manufacture new system.
But developers failed to do so, and control surfaces of K-7 were equipped
with servosurfaces, placed on light booms.
Theoretical problems of heavy aircraft control by use of servosurfaces
was solved by aerodynamic team lead by N.F.Freiman. On the TsAGI
wind tunnel more than 300 successfull tests were performed, and in 1932
servosurfaces were tested on the K-5 flying laboratory. The functionality
was perfect, and it was recommended to install them on K-7.
Kalinin also decided to use chrom-molibdenium tubes for aircraft frame
- it was the very first use of those tubes in our country. Calculation
for the frame were lead by professor A.S.Balinsky - prominent scientist
in the field of materials strength. But aircraft was overweight still in
project, because calculation had to take into accont loose strenght of
tubes near wielded joints.
Steel tube production started (at the same time with aircraft design) at
the Dnepropetrovsk Metal Works. Until this similar tubes were imported
from Sweeden, and to buy tubes for single K-7 could cost as much as 100,000
golden roubles ($$ ? - 30's). As the TsAGI
comission noted in 1932, the K-7 deserve to be a big step forward in aircraft
design just because it stimulated practical use of chrome-molibdenium tubes
in the USSR.
K-7 was designed as a multipurpose civil and militery aircraft. One of
passenger options was developed to carry 128 passengers up to 5,000km.
Other design - "lux" - supposed to arrange 16 luxirily cabines
for 64 passengers. Aircraft supposed to have a comfortable club-room, snack-bar
amd kitchen. This was the first aircraft design allowing mechanics in-flight
access to running engines.
A lot of efforts were spend on K-7 by armament team lead by D.I.Grigorov.
Military version of aircraft was a real "flying fortress", which
emerged 10 years before the American Boeing B-17. Defenses of Kalinin's
giant reached as much as 12 gunner positions (8 20mm cannons and 8 7.62mm
machine guns). Special electric cart (running on strings inside of tailbooms)
was designed for "transportation" of gunners to two tail machine
guns. Aircraft had no "blind spots", and every direction was
under fire at least from 3 gunners, what greatly increased reliability
of defence.
Bombs were located in wing bomb bays, and bomb bay's frames were included
into wing structure. Depending on the required range, bomb load varied
from 9,900kg to 16,600kg (25,550 - 42,840lb). External fule tanks allowed
2,400km flight with 6,000kg bombs.
Other military modification could carry 112 fully equipped paratroopers.
Opportunities to carry 8,4tons tank or other parashute droppable equipment
between main gears was also under study.
The technical project was completed early 1932. In the yard of the Kharkov
Aviation Factory wooden mockup of aircraft central part was built in scale
1:1. The size of aircraft was too big for existing manufacturing areas,
and new assembly hall was built. In November 1932 assembly of experimental
aircraft started, and was completed just in nine months.
In the early August 1933 aircraft was taxied to the runway. It's test pilot
was M.A.Snegirev, co-pilot - A.N.Giatsinsky. First start of all engines
revealed serious vibration of different aircraft parts; those were urgently
strenghened. On Anugust 19 A.M.Snegirev made the first straight flight
on 5m altitude. More troubles were revealed - control surfaces were vobrationg
with amplitude up to 1m! In few days tail of the aircraft was redesigned
- it became a 'biplane' with additional stabilizing surface between tailfins.
Servosurfaces from tailfins were deleted, and ones on stabilisers were
moved closer to controls.
Day before the first flight P.I.Baranov (head of the Major Management of
Aviation Industry) together with test-pilot M.M.Gromov arrived to Kharkov.
At 6a.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 co-pilot's seat was occupied
by Kalinin himself.
After few taxis, plane took off smoothly. As A.M.Snegirev told later, 'aircraft
was very responsive. It was easy to control. Hard to believe, but you just
pull the yoke - and machine responds'.
K-7 made a circle over Kharkiv, and safely landed on the factory airstrip.
After pilot's report M.A.Snegirev received appretiations, and K.A.Kalinin
- symbolic punishment for illegal flight on the test aircraft.
In next flights became clear, that vertical vibration is gone with new
tail design, but horisontal is still there. But M.A.Snegirev, employing
his rich experience of test pilot, found way to suppress it by changing
of engines reginme. Continuous test revealed fair flying characteristics
of the plane. The last factory test flight was scheduled for November 20,
and plane had to fly to Moscow. It was planned to measure maximum ground
level speed. This flight (10th one) was completed successfully, but ground
team made a mistake which spoiled the measurements.
Next attempt was planned for November 21. For now K-7 had over 5 hours
in flight. Flight plan included approaching of the 'measuring kilometer'
on altitude 1000m, then lovering aircraft to 100m and tripple passing of
the 'ruler' on the maximum speed.
At 2p.m. K-7 took off with 20 crew members on board. Here is story of D.A.Chernyshev,
one of ground engineers participated in tests : "As scheduled, K-7
flew over us and pilot figured out that we are ready for measurements.
Aircraft made a turn, accelerated and started approach. In 3-4 km from
us it made a sudden dive at 30-40 degree at full speed. The first hit into
the ground knocked away landing gears. Aircraft jumped and with running
engines hit the ground again. Fire started..."
What was recalled by on of five survived crew members, P.I.Semerenko :
"Approaching the 'measure kilometer' Snegirev gave full trottle. Tail
booms vibration came up. I counted 15-20 hits. Suddenly to the noise of
running engines was added sound of breaking apart left tailboom lower tube.
Displaced tube edges locked vertical controls, and K-7 was not able to
get out of dive. I am notifying vibration, on eye estimate dive angle...
Near surface aircraft makes a left pitch. I'm waiting for the end. Controls
are still dead. Smash..."
Technicians and designers suffered destruction of their creation and death
of 15 crew members. Kalinin was out for two months because of heart problems.
But despite this catastrophe the team did not lose self-confidence. To
investigate causes of the crach few competent commetees were created, with
participation of the most prominent aviation experts of the country. It
was concluded that distructive vibration was caused by servosurvaces during
some regimes of the seventh engine. But there were no 'theoretical' proove
of this conclusion. Only few years later M.V.Keldysh explaned this (and
many other similar) crash and found a solution to fight the flatter - weight
balansing of control surfaces. But it was too late...
After obtaining expert conlusions, 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 Aviation Factory.
Changed views of country aviation leadership on production of heavy aircraft
did not allow the team to complete development of K-7. Both planes were
'conservated' (aka 'abandoned' - A.S.), one of them half-built. Few times
Kalinin tried to prove necessity of this aircraft class, but he failed
to overpower opposition to his project.
Modified April 22, 1996 by Alexandre Savine;