Initially, the plan was to build all-wood airplane with a 5-spar of the wing. But to build it did not go — not enough strength in the material of construction. After a certain reflections on the project, K.Kalinin changes the wood structure designed for welded all-metal aircraft with a 3-wing spar. K-7 left a huge elliptical wing dense profile, the magnitude of which was almost 53 meters with an area of 452 square meters. It marks the beginning of the tail boom 2 3-sided section that carried the vertical and horizontal tail surfaces and swivel mechanism. Self possessed straightened wing center section, where in fact and were placed space for passengers and cargo. To him docked elliptical in terms of the console, where the fuel is placed in store — 14 tanks. Center section lined with duralumin, and console — cloth. According to the project wing had to have a three-spar, but because the carriers under their is not found, the average side walls are 2-parallel tubes welded between a scarf. The wing ribs planned iron. Along the axis of avia forward positioned kbainu with 2 pilots, a navigator, radio operator and mechanic. The rest of the crew was placed in the hold of the aircraft, the connection is maintained by an internal telephone band.
According to the project K.Kalinina, the plane had to get a 6 German engines from BMW, but eventually took the Russian AM-34. Because of the small thrust main engines K.Kalininu have to be installed on the rear edge of the wing an additional seventh slider. He was aware that such a decision increment turbulence, and this can lead to vibration plane. But there was no way — AM-34 engines were no gears and provide power 750 hp From the last to the wing spar tail are part of the truss girders of triangular section with Dutikow that protect the structure against accidental contact with the ground. Own development of chassis design ensured the aircraft K-7 Webasto horizontal position. It consisted of a 2-spaced trolleys public for the first time the method used was oil-air cushioning wheels for languid avia. For the first time in Russian Union is set to plane balloon wheels "Goodyear." Every cart had on 3 wheels with fairings made of iron sheets in the left door wagon was equipped for passengers and crew to the stairs in the wing.
K.Kalinin realize how hard it will be to manage the pilot to that of the machine, because proposed to use to control the electric power-that have become widely used in the upcoming aircraft and received the name — the boosters. KB contracts at the ready these boosters with the Russian Institute of Electrical Engineering, but the institute can not cope with the order. Instead Electrobooster had to install the servo, which established on light poles.
Another thought K.Kalinina — for the creation of an airplane apply CrMo frame tube. It was the first case of the introduction of such pipes in our country. CrMo tubing in our country is not carried out, and buy abroad was a very, very expensive. This aircraft is its appearance gave impetus to the entire Russian aircraft — CrMo tubing began to manufacture and produce steel plant in Dnepropetrovsk. But still the application of chrome-molybdenum tubing is not brought into the project phase of readiness — the plane is still out of overweight and CB continued to find ways to solve this prepyadstviya.
"Kalinin — 7" was developed as a multi-purpose aircraft. Could be both passenger and military purpose. As a civilian version of the standard execution — had to provide flight 128 people at a distance of about 5 thousand kilometers. As a luxury option — provided the flight 64 people, with room for passengers were carried sleeping type. In the wing positioned 2 bunk cabin with eight people each. Apart from this, there were on board the mess-room, kitchen and a snack bar. Plane had for the convenience of passengers, window-windows. The mechanics of the crew could just go to work though which of the engines in flight. As a military option — it would be a real flying leviathan. Expected weapons — 12 firing points at which there are eight 20-mm guns and eight 7.62-mm machine guns. For sporting shooters move to the tail of the machine where the project has 2 pillboxes, designed the electronic spetstelezhku. Aircraft emplacements are set so that the dead zone it was not — it does not matter which point exposed to fire with at least 3 positions. Bombings department is included in the structural part of the aircraft structure. Weight bombing of supplies — a minimum of 9 tons, a maximum of 16 tons. With a payload of 6,000 pounds and extra fuel tanks with fuel rasschetnaja range of military K-7 was 2.5 thousand kilometers. In addition, other options were considered, and the K-7 for the armed forces: the troop-carrier with a capacity of 112 paratroopers, transport aircraft for the transport of the 1st tank between the landing gear, weighing up to 8.5 tons, or any other military equipment for the transport or airdrop.
Tekhproekt Design Bureau to develop graduated in 1932. It starts with the creation of the layout of the K-7 in a scale of 1:1. Because of the large size for the creation of an experienced reference build a separate workshop — in standard shops do not have the ability to assemble such aircraft. Experienced model is created not long in record time — less than 270 days. At the end of 1933 K-7 was first launched on the runway. First start engines and is the first glitch — some of the aircraft structure to vibrate. Work on strengthening the structure to reduce vibration acts. Bimbo is an attempt to take off in a couple of weeks — the plane makes podlet during a short period of time at a height of 5 meters. Revealed a huge vibration amplitude steering — begins reworking the tail. After a certain number of days of the K-7 was a biplane tail, a zener diode connected the tail fins. Vertical tail servo lost, and on the horizontal stabilizer moved to the handlebars of their control.
First test flight took place on 21 August 1933. The crew of the aircraft — seven people, located on the regular places, he sat in a chair K.Kalinin 2nd pilot. Warm up thoroughly engines, and making a couple of runs on the runway, the plane rose into the sky. In the sky, the plane was 14 minutes, having had during this period of time to make a circle over Kharkiv. The machine is perfectly obeyed control, right react to the light steering movements. Other missions have confirmed the first memories, the only thing that has directed attention to themselves — a small horizontal vibration. But experienced pilots picked up the desired operating modes engines and reduced it to a minimum. Penultimate, the tenth test flight took place on 20 November 1933. The main task for the flight — froze the maximum speed. Flying an airplane was fine, but were not able to correctly measure — were mistaken ground performers. The next day take a second test flight to measure velocities. At that time, the plane had flown a total difficulties over 5 hours. All known issues have been resolved. The task for the flight:
— a set of measuring height — one kilometer;
— podlet to dimensional kilometers above the runway;
— reduction of up to 100 meters;
— to make three dimensional clearance distances on very probable speed.
In this flight, on board were 20 people — the crew and the test team. The plane did take off, gained altitude and left on measuring distance. Having made the lumen of the ground crew and determined from the height of readiness to measurements at the start of the pilots went dimensional kilometers. After turning the pilots began to set speed. And here came the surprise. Before reaching distance of a few kilometers, the plane at one point began to fall at an angle of 40 degrees. Upon impact, the aircraft takes down the chassis, and it crashes into the ground running engines. Experienced standard lights. As survivors of the disaster have only 5 people. As later became clear from the words of the survivors — because of the speed dial greatest rate began vibration tail the structure. Breaks the lower left-hand side member of the tail boom. Torn pieces of the spar will block the elevator, and withdraw from the drop plane was unreal. In the course of an official investigation was launched version that the vibration could cause servo operation modes of high-speed engines, but the conclusion of professionals for sure — the calculations and design no mistakes. Only a couple of years to deal with the flutter did weight balancing rudders.
CB running K.Kalinina receives an order for urgent construction of 2-machines — civilian and military purpose. Order implementation in 1935 year. Manufacturer — aircraft factory in Voronezh. But the fast-changing eyes of higher management put an end to the construction of the transcontinental aircraft K-7. Creating a stop, the aircraft canned, some of which was by then half-built. The aircraft went down in history of aviation as the first transcontinental airplane purpose, because the plane of this type appeared only a decade later, during the second world war.
The main features:
— length of 28 meters;
— Weight empty / take-off 21/40 tons;
— DU — seven AM-34;
— speed of 200/230 km / h;
— 1 thousand km range;
— 5.5 km altitude ceiling.