In the second-half of the 20’s the Soviets decided to acquire an aircraft carrier. Realising that experience of design of catapults nor suitable aircraft was in the country, thel VVS turned to the German firm Ernst Heinkel.

On May 10, 1929. UVVS, which was hidden under the mask of foreign trade association “metal-import”, concluded with the Heinkel the official agreement about the acquisition of two catapults of the type K -3 and six flying boats HD.55. The pneumatic catapult K -3 could launch aircraft with a weight of up to 3,5 t to the speed of 130 km/h. Under its possibilities was designed the HD.55.

The prototype for the HD.55 became the HD.15, built 1927 to order of the German ministry of transport. Officially it was considered as the prototype of the post aircraft from transatlantic liners for accelerating the delivery of mail.

The preliminary design of new machine was carried out in several weeks and the HD.55 preserved the layout of its predecessor. A flying boat- biplane, built of wood. Wing and tail assembly were fabric covered. The HD.55 wingspan was increased in comparison to the HD.15. The upper and lower the wings were the same span – in HD.15 lower was shorter. Lengthening occurred due to the introduction to central horizontal section, to this transverse “V” began immediately from the centre of aircraft. The wings could be boxed during storage aboard the ship, decreasing the overall sizes of machine. Changes were made in the form of stabilizing floats and the outlines of the tail assembly. Horn balancing of the control surfaces were omitted.

The HD.15 was powered by the French gR9Af engine, while the HD.55 had a Siemens Sh20, but they differed little from each other since they were license copies of the Bristol Jupiter VI. The power plant was mounted on a frame of steel tubes, erected above the cockpit. Behind the engine, in a duralumin fairing, the oil tank and accessories was located.

The HD.15 did not have armament, but the HD 55 had two weapon emplacements: fixed machine gun from the front and ring mount (with the single or coaxial machine gun) after the wing. On the request of Soviets the Germans provided for ski landing gear, Heinkel did not provide the skis themselves.

After the visit to Germany by a Soviet delegation on September 28, 1929 it was decided to increase the order to 20 aircraft. Based on this, on October 11 they signed additional contract to 14 more boats. A large series cost somewhat less. Taking into account rigid delivery times and high quality requirements for weight, Soviet examiners checked the quality of each screw and rivet.

Nevertheless, the first aircraft proved to be on 15- 20 kg heavier than planned, and each excess kilogram was cost a penalty. Heinkel with the company began to develop the methods to deceive examiners. As Ernst Heinkel described the methods “For the first time we placed limiter inside the dial of weights. Finally, one young person, who with the aid of the underground lever by foot regulated weight…”.

On February 19, 1930, the first HD.55 was accepted. On July 25 five aircraft were shipped by steamship. At the beginning of autumn the aircraft arrived in the dismantled form to storage. In Leningrad the machines again subjected to thorough control. There were problems – the boats leaked, instruments malfunctions appeared, the control cables were entangled on several aircraft. They fixed 61 defects. Since to Germans did not in time send ring mount, the cameras and radio stations, the aircraft were collected without them. The machines were divided between the Baltic and Black Sea fleets.

On October 8 1930 installation of the catapult in the battleship “Commune of Paris” was finished and trials started, but the 12th and the third start the catapult was broken. All the remaining HD.55, even those that they were based on ships, took off from the water. On October 14 a taxiing the aircraft of Black Sea cruiser “Profintern (Red Trade Union International)” – pilot Chernukha damaged a left wing with a ladder.

In the Baltic region all HD.55 moved to Oranienbaume, in the Black sea to form the components of carrier-based aviation. Already cracks of bottoms and boards, and leaky fuel tanks appeared. In the Black sea in the n366 broke a propellor in air (a similar case occured during the acceptance tests in Germany). It became soon clear that declared data did not always correspond to those of the aircraft (which is not surprising with such tricks with the weight). On September 4 it was reported to Heinkel that there was no intention to buy more HD.55s.

The catapult on the “Commune of Paris” was repaired with a number of improvements and on February 17, 1931, four controled takeoffs were made. It was piloted by Ganulich, the second crew member was aircraft observer Valtonen.

In the Black sea modifications conducted at plant n45 in Sevastopol, to the HD.55, brought a redesignation to name KR-1 (shipborne reconnaissance aircraft – the first). On February 2, 1931 they there confirmed as the standard model, and plans were made to alter all machines. From the front on the brackets was a machine gun [with the ammunition box the same as the MR-1 aircraft.

It was a version 1924 Vickers machine gun, but the alteration of ammunition box, link collector and case ejection chute was required. The trigger button was located on the steering control, descent was accomplished with the aid of the cable. However, recharging occurred entirely simply – the lock of machine gun left into the cab. Pilot took aim through the simple annular folding sight.

Under the ring mount it was necessary to place 30 mm of wood so that it would move freely, so that the visor of rear cab it would not prevent rotation. It was moved forward by 60 mm. They altered the pilots visor.

Six bombs of 32 kg could be carried under the wings for dealing with submarines with an AP-2 bomb-sight.
The n45 plant altered three machines during February, the rest, which needed minor repair and strengthening it was decided to modernize on the completion of repair.

In spring 1931 for the first time they tried to place the KR-1 on the skis. On skis they made three flights with high crosswind, on the third landing they broke the right ski. They considered, that it is necessary to strengthen fastenings and to widen track of landing gear.

The new set of skis for KR -1 entered the tests in January 1932. Complete ski set weighed 93 kg, tested in January – February at Oranienbaume. Pilot Patsynko completed three tests, and then 14 additional successful flights. The new skis were accepted to the operation.

But it was in practice they proved to be weak, – the carburetors broke off from the shaking and the impacts. The plant n28 made one additional version of skis – with the rubber buffers. These skis began to enter in winter 1932 – 1933. In September 1932. on KR-1 tested the domestic propellers of the plant n28. These propellors not only proved to be more durable and more reliable than the original, but also they gave acceleration: one and the same aircraft began to develop 186 km/h instead of 173 km/h. Rate of climb deteriorated – the gain of altitude of 1000 m, took 3 min 10, instead of 2 min 30 s.

In autumn 1931 there was a rapid loss of combat efficiency in the KR-1. The construction of the lower wing led to water stagnation in it. Hence – swelling and the splitting of the structural assembly, the stratification of plywood, and mould. In January 1932 a verified state all KR -1 in the Baltic region revealed the cross cracks of bottoms, longitudinal cracks on the boards, warping of partitions, crack of screws and leak of fuel tanks. Furthermore, was revealed the weakness of the structural assembly of the deck of boat.

Repairs involved bleed holes bored in the wing, replacing part of the skin, glued ribs, and new varnish. All this charged to Heinkel, since the operational reliability was in detail in the agreement.

By this time in VVS had 19 KR-1. From them 12 were located in the Baltic region, 6 in the Black sea and one in OHM NII (Scientific Research Institute) VVS.

Already obsolete, there was no replacement to it in the Soviet fleet. To support the operational state of these wooden biplanes components were manufactured on the spot, and only the oil radiators and bakelite units were bought in Germany. Eroded Siemen engines were gradually replaced with domestic M-22 engines. The license for the M-22 was purchased not directly but from Gnome-Rhone. Soviet motors were the same power at 480 hp, but they were somewhat heavier than German.

They completed their last flights in 1938.

In June 1941 two or four) KR -1 were still stocked with the Black Sea fleet. According to German data, it was released by 41 HD.55. Different sources determine a quantity of those purchased by the USSR into 28, 30 and even 40 copies. These numbers are completely refuted.

Construction

The HD-55 flying boat was of completely wooden construction. Metal was used only in the fastenings, and the engine cowling was made of duralumin. The structural assembly of the fuselage was ashen stringers. Plywood skin varied from 2,5 to 6 mm, assembled with wood screws and nails. The entire external surface of boat had water-resistant varnish. The HD-55 had two-planing-steps.

To provide the possibility of takeoff and landing from the snow-covered surface in the housing before the first step were special links, through which passed fastens for basic skis to the tail section – load-bearing elements under the tail skid.

The navigation equipment made it possible to carry out flights at night, and in the adverse weather conditions.

The pilot’s seat was in the front open cockpit, and in the aft fuselage section was an open cockpit for the aircraft observer, whose seat could be removed to the starboard to fire the machine gun

Upper and lower the wings were of two-spar construction and cable braced. Upper wing consisted of center section and two removable panels, which could be removed for storage of the aircraft. The second longeron was equipped with rotary hinge.

Lower wing had analogous construction with the wing folding upward. Longerons are box, with pine and birch plywood. The wing ribs were pine and walls from plywood, and also several pairs of internal braces. Leading edge (to the first longeron) was faced by plywood, the rest with fabric. The center section had completely plywood covering on the leading edge of upper wing where generators (basic and reserve) were located.
To the lower wing were fastened the additional floats, which ensured stability on the water. With the installation of skis, the floats were removed and in their place were protective arcs.

The tail assembly was a semicantilever with fabric covering. The angle of stabilizer setting could be changed in flight with the aid of the coil- hoist from the cockpit.

Aircraft was equipped with the German engine “Siemens” Sh20 (by license copy of English “Jupiter” V1). Gradually on all KR -1 engines were replaced by the domestic M-22. Power plant was mounted on the frame from the steel tubes above the cockpit between the upper and lower wing. Behind the engine was a duralumin fairing, which held oil tank and fuel automation.

Propeller – wooden, two-bladed. Initially of German production, and since 1932 on KR -1 they began to use domestic propellors.

The armament of aircraft consisted of three machine guns. One machine gun PV-1 with 200 cartridges was before the cockpit position. With the aircraft observer was a ring mount “stage -5” or “stage -6” with the coaxial machine gun 500 cartridges.

HD.55
Engine, Siemens Sh.20, 480 hp
Wingspan, 14.00 m
Length, 10.35 m
Height, 4.28 m
Wing area, 56.90 sq.m
Empty weight, 1520 kg
Maximum takeoff weight, 2270 kg
Maximum speed, 194 km/h
Cruising speed, 175 km/h
Service range, 800 km
Service ceiling, 4800 m
Crew, 2
Armament: 2 machine gun