In the experimental construction plan of the naval department of the TsKB a new task of development of a naval reconnaissance flying boat was introduced, which received the 11th number in the row of this institution. Based on its function, the new model was designated MDR-3 or TsKB-11 (Russian: Четвериков МДР-3 (ЦКБ-11)), according to the acronym for Morskoi Dalni Razvietchik or Long-Range Maritime Reconnaissance Aircraft.
The department, under the direction of IV Chetverikov, decided to use the wing, tail unit and other components of the Grigorovich TB-5 heavy bomber as a way to accelerate development. Despite this, the TsKB-11 included in its conception a series of novel ideas.
Chetverikov for the first time decided to build the hull similar to that of sharp-edged ships, which was a departure from the traditional flat shapes used in the USSR since the introduction of the Dornier Wal,
To verify the results of the new design, a scale model was created and for the first time in the USSR in January 1931, tests of the behaviour of an aeronautical design in water were carried out. These tests allowed to determine that the hull lines of the new model had been successfully designed.
The MDR-3 was built entirely of metal and had two pairs of liquid-cooled engines mounted in tandem, fixed above the wings by struts. It was conceived as a braced high-wing monoplane with the stabilization floats located in the midplane, quite close to the fuselage.
The wing was similar to that used on the TB-5 bomber, but with a slightly increased span. The construction was similar, but the covering of the centroplane was made with corrugated metal, so that it could be walked. In the midplane an exit hatch was opened and in the lower part the fixing points for the stabilization floats with a volume of 3 cu.m were created.
The hull was designed in a similar way to that of the ROM-2 but incorporated a transverse forward groove, new bulwarks and an external brace.
The tail unit had a characteristic configuration. A short empennage towered over the rear of the fuselage and supported the large area stabilizers, braced by N-studs to the structure. In the upper part there were two rounded-shaped keels with the rudders and a rigid joint support between them.
The power plant consisted of four BMW-VI engines located in two pairs in tandem. The selection of liquid-cooled linear motors significantly increased the weight of the aircraft, but Chetverikov knowingly decided to take the risk. The two engines of each installation shared a single radiator located in the lower part of the front-mounted one and were supported on a structure of rigid supports, facilitating access for service and maintenance. The fuel tanks were made of duralumin, being located in the centroplane.
The crew consisted of 7 people. The pilots were located in a closed cabin, with seats side by side. The navigator and gunners were located in positions similar to those of the TB-5. In the bow area, a watertight department was prepared to store cargo and equipment.
Defensive armament was generally similar to that of the bomber and consisted of eight Degtyaryov light machine guns for aircraft in four paired installations located in turrets. The MDR-3 could carry two 250 kg bombs hung on the rails, between the centerplane bracing supports.
The construction of the model took place in the spring of 1931 and the prototype was found finished for the month of December. Unarmed it was sent to Sevastopol, where the first flight was made on January 14, 1932, with the pilot BL Buxgolts at the controls. Engineer A. Dnieprov and Chetverikov himself also participated in this flight.
About this flight Chetverikov wrote: – “Let’s go” – said Buxgolts and accelerated. We moved forward, the plane lifts its nose easily, begins to glide without splashing, increases its speed easily and smoothly takes off from the water. All good!
At an altitude of 600 meters we make the first turn, then a circle over the bay, planing and a gentle landing. “I have no markings,” said Buxgolts. As we headed into the Dnieprov bay he began to look to see if there was any water on the hull. – “Surely everything is excellent – said Dnieprov upon returning – Just hurry up guys, quickly to the shore or we will sink! There are leaks at the bottom and the apartments are taking on water pretty fast. If we take longer than necessary, we will sink in front of the public.”
This was expected. The hull took 10 months to build and was finished without checking its tightness.
The next flights were delayed due to the sealing work on the hull. Taking into account that the production base was very far away, these jobs were not easy at all.
Finally the boat was ready and the flights could continue. Three – four short flights were made to define the small defects and familiarize the pilot with the model. On one of these flights we made the landing outside the bay, with considerable waves, and we verified that the plane did not deceive us in relation to its seaworthy behavior. We can start with the test flights.
The next flight was intended to define the flight ceiling, the rate of climb and the maximum speed. After a good takeoff the plane began to gain altitude. Initially it was found that the rate of climb was lower than calculated. After an hour of flight the plane had only managed to rise to about 2000 meters.
During this flight, creaking noises were observed in the rear area of the flying boat. The tail began to vibrate and move from side to side. Continuing the flight was dangerous, so it was decided to return.
Later tests showed the appearance of important vibrations that started from the tail towards the nose. The cause of this phenomenon was attributed to the action of the propellers of the tandem drive installations. After replacing the propellers with ones from a Dornier Wal, the vibrations practically disappeared, but the total solution of the problem required a radical redesign of the tail area.
The tests were carried out until March 25 and during their execution a speed of 210 km/h was achieved and excellent seaworthy conditions were demonstrated. As a positive aspect, the excellent autonomy and the great range of the model were pointed out. As negative aspects, the poor rate of ascent and the low ceiling, which only reached 2,200 meters, were pointed out.
The studies carried out showed that the fundamental cause of the poor performance lay in the inefficient engine configuration selected and the poor aerodynamics of the model: poor design of the underwing stabilizers with circular fronts, absence of aerodynamic fairings for the supports and structures, poor design of the radiators, located on the wing in a way that diminished its effectiveness, a terrible solution for the location of the 250 kg bombs on the sides. To all this were added the finishing problems and a series of details that affected the lines of the fuselage.
As a consequence of these deficiencies, the result of the tests was assessed as negative. The MDR-3 failed to meet the requested specifications.
Taking into account the great need for aircraft of this type, the commissioners of defense and heavy industry approved the continuation of the development of the model, correcting the deficiencies that affected the aerodynamics. A short time later the plans and calculations of the MDR-3 were delivered to AN Túpolev in the KOSOS TsAGI to work on the solution to the indicated problems. Tupolev decided not to waste time on modifications to the flying boat and to design a new model. Based on the MDR-3, in 1933 the brigade led by II Pogosski designed the flying boat MDR-4 (ANT-27). The new flying boat retained only the hull lines of the previous model. The new configuration included a high cantilever wing, three engines and a monoplane tail with a single tail.
MDR-3
Engines: 4 × 500/680 hp BMW-VI
Wingspan: 32.2m
Wing area: 153.00 m²
Length: 21.9m
Height: 7.0m
Empty weight: 8928 kg
Maximum takeoff weight: 13973 kg
Wing loading: 91.5 kg/m²
Power Load: 5.1kg/hp
Fuel load: 3300 kg
Total load capacity: 5044 kg
Maximum speed at sea level: 210 km/h
Landing speed: 110km/h
Practical range: 1600 km
Autonomy: 9 hours
Practical ceiling: 2200 m
Take off time: 36s
Accommodation: 7
Armament: 8 Degtyaryov 7.62 mm machine guns
Bombload: two 250 kg
All Aero 