The development of an aircraft to carry out operations from submarines was taken over by IV Chetverikov in 1931. In its design, Chetverikov decided to start from the already proven design used in the OSGA-101 light flying boat. The new model was known as SPL (Russian: Четвериков СПЛ), after the acronym for Samoliet Podvodnoy Lodki or Airplane for submarines.
As fundamental differences, the general dimensions were reduced in order to allow the aircraft to be accommodated in a sealed hangar of only 2.5 x 7.5 meters. Systems were also developed that would allow the wing, the power plant and the stabilizer consoles to be transformed, using quick-action fixing screws instead.
The decision to build a flying boat of minimum dimensions also had its cost. The tail had very little area, so the flying boat had little stability. The visibility of the cabin was poor, especially for the observer, since the crew was forced to position themselves towards the center of gravity of the aircraft in order to reduce the length. To reduce the weight, the flying boat did not have the possibility of carrying any type of armament. The resulting aircraft looked more like a sports competition model than a military aircraft.
The SPL was designed as a high-wing monoplane flying boat. The wing, similar to that used on the OSGA-101, featured a three-spar configuration with a trunk frame and wooden construction. The areas on the outside of the wings between the leading edge and the intermediate spar were covered with plywood. The rest of the wing presented fabric covering, sewn with thread to the ribs.
Unlike the OSGA-101, on the SPL there were no fasteners on the first and third stringers. In this way the force of the support was exerted only on the central beam. In the extended position, the wing was secured by a quick-release fastener located at the top of the center spar. At the bottom was a gimbal that was inserted into a cavity in the centroplane. To pick up the wing, it was enough to release the fixator, rotate the wing console around the axis of the spar and move them towards the sides of the hull.
The SPL wing was designed using the MOS-27 airfoil with a relative thickness of 18% at the root section and 12% at the tips. This fact, together with a considerable narrowing of the wing, made it possible to obtain a break in the flow at the wing tips when operating with large angles of incidence. For this reason, it was enough for the pilot to pull the stick a little hard on landing so that the plane began to dive on the wing. At that time, the phenomenon of wingtip vortices was little studied, so the fault for this behavior was attributed to the poor transverse stability of the model. Attempts were made to cure this problem by increasing the wing’s wing gradient to about 5°, but the effect was nil. According to Chetverikov himself the problem could be compensated thanks to the great effectiveness of the ailerons.
The large-area wings occupied the entire trailing edge of the consoles and featured two sections: an inner or root section and an outer section. The inner sections could be used as flaps during landing. The wings were made of wood and featured fabric-covered box-type spars. The leading edge was covered with plywood.
The underwing floats had a wooden structure with frames and braces, covered with plywood. In the SPL, unlike the OSGA-101, the construction of these floats was simplified.
The hull of the SPL was similar to that of the OSGA-101 and featured all-wood construction. The structure had several frames with stringers and braces and plywood sheathing. The hull design demonstrated excellent hydrodynamic qualities, which was necessary for the operation of the model in the open sea. The large angle of the keel reduced the overloads when riding the waves and the high rails ensured a smooth takeoff and landing in tidal conditions.
The three-sided tail beam was made of steel tubes and its rigidity was ensured by means of cable-stayed straps. The control cables for the rudders were routed inside the tubes of the structure. The empennage was built on a fabric-covered welded steel tube structure. The keel spars and stabilizers as well as the ribs and leading edges were constructed from thin steel tubing. Unlike the OSGA-101, in the SPL the stabilizers had a biplane structure in order to reduce their wingspan.
The SPL lacked its own landing gear and for operations on land, removable wheels were used whose axles were fixed to a tube that passed through the hull from side to side.
The power plant selected was the excellent 100 hp M-11 driving a two-bladed, fixed-pitch wooden propeller. This propeller had a diameter of 2.3 meters. Originally, an annular Townend-type hood made of aluminum was installed, but it was soon found that the aerodynamic improvements of its use on the M-11 engine they were not considerable, reason why it was eliminated. The engine was located on a high pile conceived of welded steel tubes and designed to prevent water splashing on the propeller during takeoff and landing operations. To facilitate its packing inside the narrow container, the engine lay down backwards. To achieve this in the SPL the drive location was slightly rearward, the strut could pivot and the rear mount base was moved rearward by the main tail tube. The motor control was carried out by sheathed cables that were collected along with it.
The optimum position on takeoff and landing ensured a wing angle of incidence of 6º, but during flight the optimum angle was 2º. For this reason, in flight, the nose of the flying boat was inclined slightly downwards, while the engine was at that moment totally in line with the flow of navigation.
In the motor gondola there was a 10-liter tank for oil and another 20-liter tank for fuel. The main fuel tanks were located in the hull.
The dimensions of the SPL once folded were only 7.45×2.12×2.35 meters. Flight readiness was achieved in 4-5 minutes and after the mission the wings, tail and engine could be folded in 3-4 minutes.
The SPL was fully painted silver. Originally it had no markings or symbols, but later on its nose and wings, above and below, the characteristic red stars were painted. The Gidro-1 example, which was shown at the Milan fair, featured Aeroflot symbols in blue on both gunwale and lacked the red stars.
The SPL was built in the workshops of the Sled and Hovercraft Building Department (OSGA), subordinate to the NII GVF. The prototype was finished by December 1934 and moved to Sebastopol to carry out sea trials, which were completed on August 29, 1935. Its realization was in charge of the test pilot of Factory No.45 AV Krzhizhevski.
The tests demonstrated the feasibility of the packing and unpacking operation of the aircraft in the submarine in a time of 4-5 minutes, its possible take-off and its effective recovery after the flight. The authors of the project managed to obtain a container only 7.45 meters long and with a diameter of 2.5 meters. In general, the SPL showed acceptable performance in the water, but its seaworthy conditions were not good, especially in the open sea. It was clear that very little more could be asked for an aircraft of this type, but in these conditions the aircraft was not very attractive to the military.
The Gidro-1 next to an MP-1bis with the colors of Aeroflot to participate in the fair in Milan in 1936.
In 1936 the SPL with the Aeroflot symbols and Gridro-1 denomination, participated in the Milan Fair, receiving a positive evaluation.
On September 21, 1937, in this plane, the pilot AV KrZhizhevski established a world speed record of 100 km, reaching 170.2 km/h, and on October 7 of that year, he established a new distance record for aircraft of its category. when reaching 480 km and high when setting 5400 meters. Despite these results, the model did not have further development.
Documentary references to a flying boat built by Chetverikov have been found at Sevastopol Factory No.45. It could be a second issue of the SPL, but this is just conjecture.
SPL
Power plant: М-11, 100 hp
Wingspan: 9.6m
Wing area: 13.4 m²
Length: 7.4m
Height: 2.72m
Empty weight: 592kg
Weight with normal load. 800kg
Maximum takeoff weight: 879kg
Fuel weight: 60kg
Oil weight: 10kg
Total load capacity: 208 kg
Wing loading: 59.7 kg/m²
Overloaded wing loading: 65.7 kg/m²
Power load: 8kg/hp
Overloaded power load: 8.8 kg/hp
Maximum speed at sea level: 186 km/h
Cruising speed: 174km/h
Landing speed: 85km/h
Endurance: 2 hours
Range: 480km
Practical ceiling: 5400 m
Time to 1000m: 3.9min
Time to 2000m: 8.7min
Time to 3000m: 15.3min
Time to 4000m: 25.5min
Time to 5000m: 50.0min
Accommodation: 2
All Aero 