Tupolev MTB-1 (1934)

Patrol flying boat 1934-42: 15 built.

The Tupolev MTB-1 was initially started as the MDR-4 (internally by Tupolev, ANT-27) in 1933, a project of patrol flying boat answering a specification for the Soviet Naval Aviaiton. It was a refined version of the failed Chyetverikov MDR-3, not accepted. The revised design retained the latter hull, but Tupolev created a brand new, full-cantilever wing and new tail with a triple engine installation (two pulling, one pushing). Trials by March 1934 had the prototype destroyed at takeoff. A second prototype was tested in 1935 but redesignated MTB-1 to be torpedo-carrier. Poor performance in trials should had it shelved, but it was urgently needed to fill a niche in the Soviet Navy, and so accepted for production before even tests were complete. 15 were produced and despite structural failures, they remained in service until 1942.

Development

In February 1933, it was decided to revise the failed MDR-3, a derivative of the 1930 Grigorovitch TB-5, a quad-engine high wing monoplane bomber reminsicent of French designs, as a modern all metal bomber. It made 4 successful flights, but was not chosen for production. In 1931, the design was recycled as the MDR-3 floatplane to carry bombs and later designed with a floating hull to perform reconnaissance, but it was not further developed. This turned into the creation of a new model combining the features of Tupolev projects and the MDR-3's hull and general fuselage design. Tupolav found it lacked power and installed three-engines above the wing, two pulling and one pushing in the center on a brand new all-metal cantilever wing and single-fin tail.

The program called for a long-range reconnaissance aircraft that can carry a heavy load of bombs under wings or carry 14 passengers. On April 8, 1934, the newly-built ANT-27 prototype took off and managed to land after a short hope without issues. Other days were used to try the engines and test various systems. Pilots complained about the aircraft's behavior when taking off, reacting unusually to the then accepted takeoff rocking technique. This will have consequences later. On April 16, starting a longer test flight, when trying to take off, the prototype crashed into a wave. This claimed the lives of four.

To be more precise about this incident, its goes more into detail as an investigation. On 15 April, the ANT-27 initial prototype was scheduled for another test flight. But its pilots then were accustomed to fly the Dornier Do-J Wal (only military soviet flying boat in service back then) which had a flatter bottom. They had difficulty taking off, so to aid this, they used to pull the control on both sides (longitudinal rocking technique) to "unglue" the hull. However, this technique was applied by default on the ANT-27, which had a far narrower keel-like bottom. The habit was observed already by Andrei Tupolev, and he witnessed the subsequent incident.

Tupolev wanted to closely monitor the aircraft’s transition to the "step" redan, and when mid-way with engines full throttle, the rocking technique was applied, despite his frantic gestures not to do so. The crew past him and while at high speed in Kuglaya Bay (calm waters) the ANT-27 moved out of the bay and entered open sea, being pushed by a side wave while rocked the opposite side, and briefly lifted into the air before the nose plumitted and buried itself in the next wave. The impact broke the struts of the engines which collapses and fell, still spinning, onto the cockpit, killing the pilot instantly. The aircraft then toppled over and broke.

This ended with the death of KOSOS naval brigade head I. I. Pogossky, engineers G. S. Noskov and K. K. Sinelnikov, second pilot, A. A. Volynsky. Sources vary on who was piloting. Later in 1935, the ANT-27bis replaced the ANT-27. On October 29, the 27-bis made its maiden flight, which was successful. Factory tests went on until March 8, 1935 and from April 7 to June 1, still in Sevastopol, home of the Black sea fleet (in which it was intended to serve), the ANT-27 bis passed all state tests, with little margin. At the time, the Navy wanted a dual purpose model, one long-range reconnaissance aircraft, called the DR-4 and a heavy bomber capable of carrying torpedoes, called the MTB-1. In this second variant crew was reduced and navigation systems decreased. On September 23, the ANT-27bis took off but crashed again, when its linen cover tore off. The crew of 3 died in the crash.

Despite this disaster, after the loss of two prototype, decision was made to carry on with serial production, as no other model filled the niche the Navy was asking for. On April 29, 1936 production was setup at Tupolev, the MDR-4 was officially adopted by the Red Army Air Force, to serve with the Navy. But production was achieved only at slow pace and with great difficulty, so much so that only five had been completed by 1937. Only ten MDR-4 were registered in the plan for 1937. That small order was seen as a stopgap for a "passable" model whereas the USSR planned to acquire a license for the Consolidated 28 flying boat (PBY Catalina) for production in USSR, which was done later in wartime. The MDR-4 was in no way shape of form to its level. During the war, USSR obtained hundreds of PBYs, which influenced Beriev for a new generation of patrol seaplanes postwar.

Production in Taganrog plant number 31 stopped after ten were actually assembled in the end of 1937. Meanwhile the five 1936 models were re-equipped from the M-34R engines, to the new M-34RN. At least one received and tested the M-34FRN (with forced injection). The passenger version was dropped entirely. Performances were however average and pilots complained about worrying structural noises in flight, when manoeuvering and especially deafening noise when landing and taking off. There were scores of structural failures registered in 1937. However pilots reported good hydrodynamic qualities, ease of control and high flight characteristics. M-34N engines were recommended and after installation, top speed reached 266 km/h, service ceiling 6,550 m.

Design specifics

Plans 3 views of the Ant-27

The MDR-4 had the same all-metal hull with flat sides and reinforcement external beams fitted for extra structural strenght. The hull was the tallest forward with the fore hull, followed by two cutouts where the underhull met the tail. The model had a shoulder mounted main wing with a considerable wingspan ratio customary of Tupolav's models. The wing had a chord and elevated up with rounded ends. Floats were installed underwing at the junction of the flat core. The three engine nacelles were mounted at the center, with the pusher in the middle and the two puller engines further out forward on the wing, attached by N struts.

The rudder tail was quite high compared to the previous MBR-3 design. It supported a mid-low mounted cross tail, above the elevated position for the tail gunner. The latter had a fully enclosed small coscpit that could be folded back when in use (see below). The cockpit was narrow like the fuselage, only for a single pilot, and there was another MG position in the nose. The radio compartment was located between the cockpit and nose gunner. There was also a dorsal gunner inside a folding cockpit, the same way as on the nose.

The three engines were of the AM-34R radials, which developed 610 hp for 225 kph at best. The climbing rate was atrocious and th mdel was found sluggish on commands and took quite a large distance to take off. It was also unstable at low altitude and only climb (very slowly) to 4,470 m (14,650 ft), barely sufficient to dodge AA fire. The crew had five, a navigator (who also doubled as nose gunner), two pilots, a flight mechanic, and a rear gunner. In the bomber, they were joined by a radio operator-bombardier and a gunner.

The MDR-4 had a nose TUR-8 bow open mount for a flexible 7.62-mm ShKAS machine gun (with 1500 rounds of ammunition). There was a dorsal position with a simple hatch and a tail gunner position also with a ShKAS machine guns. None was able to impress a fighter. The MDR-4 was a heavy bomber and as such it carried racks under the inner wings (between the fuselage and floats for rigidity). They were flexible in tbeir payload, either two 1000 kg bombs (or torpedoes), four 500 kg, eight 250 kg, or twenty 100 kg. The bombardier controlled the drop thanks to ESBR-2 electric bomb release device or emergency mechanical system if the first failed.

⚙ Tupolev MTB-1 specifications
Empty Weight	10,521 kg (23,195 lb)
Max Takeoff weight	16,250 kg (35,825 lb)
Lenght	21.9 m (72 ft 10 in)
Wingspan	39.4 m (129 ft 3 in)
Height	8.6 m (28 ft 3 in)
Wing Area	177.5 m2 (1910.59 sq ft)
Engine	3 x AM-34R, 610 hp
Top Speed, sea level	225 km/h (140 mph)
Cruise Speed	196 kph
Range	2,000 km (1,243 miles)
Climb Rate	Unknown
Ceiling	4,300 to 4,470 m (14,650 ft)
Armament	Three flexible MG, 2000 kgs bombs underwing.
Crew	5 (pilot, navigator/radio, 2 gunners, bombardier).

Service

All manufactured flying boats entered the Black Sea Fleet Air Force, but they were not widely used. In documents they were called MTB-1. Only the 124th Naval Heavy Squadron based in Sevastopol flew them. In November 1937, a document instructed to prepare to swap from the MTB-1s within the next year as the unit was supposed to make the transition to the Ilyushin floatplane DB-ZPT as torpedo bomber. Still, unlike the DB-4T, the DB-ZPT was never produced.

From there, there are diverging sources. Some claimes the existing MTB-1 were indeed scrapped in 1938 or 1939 as no other mentions were made of them in service in subsequent years. Others, perhaps more generally accepted states they were still in service by 1942, retired when the PBY was delivered. It's possible that with the DB-ZPT failing to materialized, the order was not carried over to keep the 124th Naval Heavy Squadron its capability at least on paper. Nothing is said if the crews actually flew them, but they were never used as marine heavy bombers as intended.