The Grigorovich MR-5 (Russian: Григорович МР-5) flying boat was a development of the earlier DP Grigorovich MR-2 model with a new metal hull. Both the armament and the distribution were similar to the previous model but the power plant was modified to install a 500/680 hp BMW-VI engine.

The project was ready by 1927 and construction began the following year. The new seaplane was developed keeping the same scheme of the previous models. It was a flying boat with a liquid-cooled engine and propeller propeller. The descriptive memory that accompanied the model during the tests defined the MR-5 as “totally made of aluminum except for the wings and side floats” .

The hull was made up of seven watertight departments. In the bow section, a waterproof bottom was installed in an auxiliary way. Round hatches with hermetic hatches were installed to allow passage inside the hull, and a similar hatch was installed in the upper part of the fuselage to allow access to the entrance of the fuel tanks.
The fuel system consisted of two main tanks with a total capacity of 608 liters. There was a third tank of 64 liters. The oil tank was located below the engine cooling radiator, which gave the powertrain an almost triangular cross-section. The BMW-VI engine was fitted with a two-blade wooden propeller 3.18 meters in diameter.

The armament of the flying boat was located in two turrets located in the bow section and in the back (TUR-5 and TUR-6 respectively). Under the wings, on each side, a pair of DER-6 mounts was located, capable of fixing aircraft bombs in two combinations: 4 x 82 kg and 8 x 48 kg.

The MR-5 was completed at Factory No.22 in Fili by the summer of 1929. First flying in July 1929, the tests were carried out between July and August of that year in Taganrog by test pilot ST Rybalchuk. This pilot moved the model to Sebastopol in September, in order to carry out the acceptance tests there.

The tests showed that the flying boat had excellent behaviour in the air. Gone were the stability problems that characterized the earlier MR-2 and MRL-1 models.

The behaviour on the water did not have the same results. It was determined that during takeoff and landing operations, the flying boat raised large curtains of water to the sides due to poor design of the bow. The takeoff and landing runs were also extremely long. The tests of behaviour in the sea with waves were not carried out. The pilots also determined that the aircraft had poor manoeuvrability at a combat altitude of 3,000 meters.

The MR-5 flew at the time of the transfer of the OMOS from Leningrad to Moscow, that is why the plane concentrated all the problems of the moment in its history. Later in OPO-3 it would be confirmed that the work on the creation of the MR-5 was not carried out following the approved scheme: the conceptual project was never approved, the established tests for wind tunnels were not carried out, and in spite of all this the construction was approved in the OMOS and the construction began.

At the time of Grigorovich ‘s arrest, the MR-5 was in the process of being assembled. Builder DP Samsonov was given the task to continue the assembly and carry out the necessary modifications.

The commission to define the fate of the MR-5 was convened for December 1, 1929. The representative of the Technical Scientific Council of the Directorate of the VVS R. L. Bartini participated in this commission. The commission concluded that the flying boat had design problems in the bow section. It was decided not to recommend the MR-5 for entry into service with the RKKA VVS and it was decided that the model would be accepted only when the design of the bow rails was modified and it was submitted again to state tests.

To carry out the proposed modifications, the model was delivered to the OPO-4 directed by Frenchman Paul Aimé Richard, but by that time the benefits obtained had ceased to interest the VVS and despite the fact that two new hulls were built, the aircraft was not finished.

A short time later and in view of the growing need for reconnaissance naval models, the development was handed over to the TsKB at the Menzhinski Factory. There the subject remained without progress until 1931, when the task was assigned to the head of the naval department Chetverikov, who developed the model MR-5bis or TsKB-10.

Variation: Chetverikov MR-5bis

Several well-known Soviet aviation historians in their works refer to this Grigorovich model as the MR-3. This assertion has been later spread by numerous articles, publications and internet sites, which have used these authors as a reference. On the other hand, evidence has been introduced about the MR-5, also by Grigorovich and with such a similar story, that it suggests that it was the same plane. To increase the confusion, researchers, who realized the problem, present the model as MR-3 (MR-5), without clarifying the relationship between the two.

Stranger still it seems that the builder Grigorovich named his reconnaissance model for open sea ROM-1 MR-3 and its derivative ROM-2 MR-3bis. Taking into account that both models (ROM-1 and MR-3) coincide in time, it is very unlikely that the manufacturer used the same name in very different models with the sole purpose of creating confusion in future history.

The renowned aviation researcher and writer Mijail Maslov in the series of articles “Airplanes of Dmitri Grigorovich”, published in the specialized magazine Aviation and Cosmonautics between 2012 and 2013, expresses that from his study of the primary documents of the time in none appear references to this model as MR-3 and yes as MR-5.

In this way we will use the names MR-3 and MR-3bis to refer to the ROM-1 and ROM-2 reconnaissance flying boats and the MR-5 and MR-5bis designations to refer to the single-engine reconnaissance flying boat created as a development with the metal hull of the MR-2 and the improved version made by Chetverikov in 1931.

MR-5
Powerplant: 1 x 500/680 hp BMW-VI
Wingspan of upper plane: 15.6 m
Wingspan of the lower plane: 13.65 m
Length: 11.47m
Wing area: 53.0 m²
Empty weight: 2227 kg
Takeoff Weight: 3282 kg
Fuel weight: 440kg
Oil weight: 30kg
Total load capacity: 1055kg
Wing loading: 58.2 kg/m²
Power load: 6.2kg/hp
Maximum speed at sea level: 194 km/h
Cruising speed: 165km/h
Landing speed: 85km/h
Time to 1000m: 7.0min
Time to 2000m: 15min
Time to 3000m: 37min
Practical ceiling: 4000 m
Endurance: 4 hours
Range: 750km
Turning time: 29 sec
Accommodation: 2
Armament: Two 7.62mm machine gun
Bomb load: 4 x 82 kg or 8 x 48 kg

By 1924 the few M-5 and M-20 trainers still in flight condition had aged and needed constant efforts to keep them operational. The possibility of building new models was considered unproductive.

For this reason the 26 of June of 1925 Grigorovich was tasked to develop a flying boat training flight Soviet schools. An easy-to-build, stable in flight and cheap to operate two-seater hydrofoil was required. The new trainer was to replace the M-20 and MU-1 in service.

The Grigorovich MUR-1, (Russian: Григорович МУР-1, MUR – Morskoi Uchebnyi Rhône – seaplane trainer Rhône) was developed by the Grigorovich Design Bureau. In order to speed up the development process and facilitate the introduction into production, Grigorovich decided to use the hull of the M-5 (M-20) hydrofoil. The new design would only comprise the wing box and the tail unit. A 120 hp Le Rhône (M-2) engine was selected as the power plant. The MUR-1 differed from the Grigorovich M-5 in that it used a more powerful engine, single bay wings with thicker section, and stronger tail structure to reduce the twisting effect of this area during turns. Speed was increased, but other characteristics suffered. The new model introduced of ailerons on both planes.

The two crew members were located in an open cockpit, seated side by side and protected by a small windshield.

Built at GAZ Factory No.3 Krasni Liotchik of Leningrad, the prototype was completed in 1926. The tests, developed by the pilot TS Zhukov at the end of that year, showed an increase in speed, which was set at 120 km / h, but other indicators were lower than those of the M-5.

It was found that the center of gravity had been too far back, so that in hydrofoil it lacked stability in the air with a constant tendency to raise the nose. The structural reinforcement of the tail also contributed to this behaviour, which is why it was necessary to place additional weights of about 32 kg in the bow to restore balance.

It was clear that with these “adaptations” the MUR-1 could not reach production. Despite this, the need was such that it was decided to accept prototype in September 1926. In October of that year, TS Zhukov made the first training flight with pilot Rastyagayev on board as a student.

In 1929, when the TsAGI hydrochannel was created, the MUR-1 was used to carry out pressure studies on the hull. For this purpose, it was equipped with membranes with sensors attached to dynamometers with input from TsAGI engineer N.N.Podsevalov, which measured the forces of the water at different points. These experiments would continue with a slightly modified MUR-2 version.

Test flights were conducted 1929-1931 with the intent to test load condition during taxi, takeoff and landing.

Engine: 1 × Le Rhône M-2, 89 kW (120 hp)
Propeller: 2-bladed fixed-pitch wooden
Wingspan: 11.5 m (37 ft 9 in)
Wing area: 33.0 m2 (355 sq ft)
Length: 8.0 m (26 ft 3 in)
Empty weight: 700 kg (1,543 lb)
Gross weight: 1,000 kg (2,205 lb)
Fuel capacity: 124 kg (273 lb)
Total load capacity: 300 kg
Wing loading: 33.3 kg/m2 (6.8 lb/sq ft)
Power load: 8.4kg / hp
Maximum speed: 129 km/h (80 mph, 70 kn) at sea level
Alighting speed: 95 km/h (59 mph; 51 kn)
Range: 360 km (220 mi, 190 nmi)
Endurance: 3 hours
Service ceiling: 3,500 m (11,500 ft)
Take-off distance: 250 m (820 ft) / 22 seconds
Alighting distance: 170 m (560 ft) / 15 seconds
Time to 1,000 m (3,300 ft): 8 minutes
Time to 2,000 m (6,600 ft): 19 minutes
Time to 3,000 m (9,800 ft): 38 minutes
Crew: 2

In November 1925, the leadership of the VVS highlighted the difficult situation of naval aviation, considering it incapable of fulfilling the tasks set due to the lack of modern seaplanes. In the speech of November 10 and in relation to Grigorovich it was written:
“Of the national builders, only the engineer Grigorovich has the experience in the construction of seaplanes, however two of his last hydro-apparatuses have been unsatisfactory, because without laboratory research at the present time it is impossible to obtain good results.”

By November 1925 and on Grigorovich’s own initiative in the GAZ No.3 in Leningrad, work began on a second example of the MRL reconnaissance flying boat. The Aviatrust management protested against the constructor’s decision, which proposed not being able to fulfil the state order and nevertheless assumed a new design of its own, but finally ended up accepting to include it in the industry plan for 1925 / 1926 under the name MRL-2 (Russian: Григорович МРЛ-2 (МР-2)).

Initially the new model was also designed using a Liberty engine but later it was decided to use a Lorraine-Dietrich engine. For this reason the name used later was MR-2 according to the acronym Morskoi Razvietchik or Naval Explorer.

About this flying boat VB Shavrov wrote:
“Externally it was more beautiful than the MRL-1. The same hull coating, but with a larger area using pine and less mahogany, which at the bottom had a single layer. The rails of the forward section were modified and the upper wing received an increase in its span. The wing bracing cables were made with 5 mm steel cables with rounded ends. The main braces had three and the rest had two cables. The overall dimensions were slightly increased in relation to the MRL-1.”

The new model was developed as a flying boat with a liquid-cooled engine and propeller propeller.
The double reed hull was made of wood and had been lightened in relation to the predecessor MRL-1, maintaining a similar structure.

The wing had a sesquiplana configuration with the lower plane slightly smaller in span and with a certain positive dihedral. The planes were secured by uprights in pairs made of steel tubes with tension cables and had a marked offset. Small stabilization floats were located on the outer sections of the lower plane wing gondolas.

The tail was of the monoplane type with the stabilizers installed on a short keel and braced by simple struts. The keel ended in a small rudder.

The 450 hp Lorraine-Dietrich engine was attached to the upper midplane structure. The fuel system consisted of two main tanks located in the hull, with a third 64-liter tank installed in the midplane.

The armament of the seaplane was located in two turrets located in the bow section and in the rear part. Under the wings, on each side, the fixing of a certain number of bombs was foreseen.

On June 18, 1926 the prototype was finished in the GAZ No.3 workshops. In September it was submitted for testing. Among its main characteristics, the cheapening and simplification of construction stood out, which should be reflected positively in the characteristics of the seaplane and in the production process.

The MRL-2 flying boat was analyzed by a commission composed of II Mashkievich and Ye. K. Stoman of the Directorate of the VVS, and DP Grigorovich and VL Korvin for Factory No.3.

This commission noted that the new model differed from the MRL-1 in the shape of its wings, the improvements made to the hull, its engine covered by a well-designed hood, and the backward displacement of the firing point.

The first flight of the MR-2 took place on September 22, 1926 and lasted 15-17 minutes. The forward position was occupied by DP Grigorovich himself, while pilot AS Melnitsky sat next to VL Korvin at the controls. That same day a second flight was made in which, together with Melnitski, the motorcyclist Funtikov and the engineer Vigand flew. The third flight was made by NOA members with pilot VN Filippov at the controls. This pilot highlighted the long run on takeoff and landing, lack of stability in horizontal flight and during glide. Melnitski had pointed out that the control and transverse stability of the MR-2 were superior to those of the MRL-1 and only the longitudinal was somewhat worse.

It was precisely the longitudinal stability that caused the accident of the pilot FS Rastyagayev that occurred on October 19, 1926. Rastyagayev was not a naval pilot and had no flying boat experience, yet he was the person commanded from Moscow to conduct the first stage of the state tests. On October 18, he made a familiarization flight together with factory pilot Zhukov in a MUR flying boat (M-5 training flying boat with a 120 hp Le Rhône engine) and later he made a solo flight. In his opinion there was nothing special about naval aircraft:
“ – No problems. No difference.”

The next day Rastyagayev took off, circled over the bay, and then glided too steeply. Subsequently, the MR-2 dived, inverted and fell onto a sandbank covered only by a meter and a half of water. The pilot was pulled from the wreckage with a number of injuries and that night he died.

The commission investigating the event, chaired by NM Tulupov, originally concluded that the accident occurred due to the pilot’s lack of familiarity with naval models. In the official minutes of the commission presented on November 27, 1926 , it was written:
“The MR-2 is unstable in the longitudinal axis. At small angles of flight it does not reach the work of the rudders. When the pilot Rastyagayev went into glide he performed a ground pilot maneuver pulling the stick sharply towards himself which caused the plane to dive. On a second attempt the rudders did not respond, going into an inverted dive.”

A more detailed study of the catastrophe was later carried out. A scale model was urgently made and tests were carried out in the wind tunnel of the TsAGI . These tests showed that the cause of the accident was the lack of longitudinal stability of the MR-2 when flying with 48% cog and having the stabilizers set at a positive angle.
The commission established that if the aerodynamic tests had been carried out earlier, this accident could have been avoided.

It is noteworthy that the history of the MR-2 is coincident with that of Polikarpov ‘s Il-400 fighter, which was also destroyed during its first flight due to poor center of gravity selection. The cause of these accidents cannot be seen only in the inability of theoretical calculations. To a large extent, the self-sufficiency of its creators, Polikarpov and Grigorovich, who underestimated the scientific possibilities of institutions such as the TsAGI, played a role, considering that his experience was more than enough to achieve successful designs. This was not an isolated case. Many Soviet constructors presented important problems in the way of carrying out their constructions without the support of aeronautical science.

In any case, after the MR-2 accident, the aerodynamic centering of models was defined as one of the main aspects in aeronautical construction and wind tunnel testing was established as an obligation in the approval process of an aeronautical project.

MR-2
Engine: 1 × Lorraine-Dietrich 12E, 340 kW (450 hp)
Wingspan: 15.6 m (51 ft 2 in)
Wing area: 56.7 m2 (610 sq ft)
Length: 13.6 m (44 ft 7 in)
Empty weight: 1,770 kg (3,902 lb)
Gross weight: 2,770 kg (6,107 lb)
Fuel weight: 560kg
Total load capacity: 1000kg
Wing loading: 49.0 kg/m2 (10.0 lb/sq ft)
Power Load: 6.2kg/hp
Maximum speed: 179 km/h (111 mph, 97 kn)
Landing speed: 82km/h
Range: 900km
Endurance: 5 hours
Service ceiling: 4,200 m (13,800 ft)
Time to 1000m: 7min
Time to 2000m: 17min
Time to 3000m: 34min
Armament: 1-2 x machine guns
Crew: 2

The last representative of the M family of naval flying boats. In practice, it turned out to be a modification of the excellent M-9 with a more powerful power plant and some structural changes. Dmitri Grigorovich was active during the 20’s in the gas plant No.3 “Red Flyers” in St. Petersburg, where he began the development work on the M-24 (Russian: Григорович М-24) Soviet biplane flying boat in April 1922.

Despite the initial failures with the M-23, the modernization of the M-9 model continued its course. Grigorovich, knowing of the tests of the Renault – engined M-9bis model developed in 1921, decided to propose his improved version of the M-9, which comprised a major redesign. The VVF needed a quick result and for this reason he decided to proceed with as few changes as possible, keep the old hull, avoid structural modifications and concentrate his attention only on the powerplant and wingbox. Grigorovich, from the beginning he was opposed to this procedure, as he understood that in this way it would be impossible to achieve good seaworthy conditions, but the Main Directorate of the VVF in its efforts to achieve results was willing to accept certain limitations and finally the builder decided to satisfy his client.

The design was based on its successful M-9. Some of the differences in the M-24 was dual wings ailerons, and the total area of the hull was changed.

On July 19, 1922 at the State Aviation Factory No.3 “Krasni Liotchik in Petrograd, a commission of the aeronautical department of the Main Directorate of Military Industry (GlavBoyenProm) met and studied the capabilities of the factory and analysed the possibility of building eight new flying boats with the M-9 hull and Renault power plant. In August of that year, during the creation of the industry production program for 1922-1923, the construction of 56 M-9bis was approved.

By May 1, 1923, the first base model of the M-9bis series, with number 1717, was ready and by the beginning of June a commission chaired by the representative of the Scientific Committee of the VVF, the engineer Yermolaiev, developed the first tests of evaluation. The flights were made by naval pilot VG Chuxnovski.
Taking into account the differences between the model modernized by Grigorovich and the M-9bis, the commission decided to approve the new name M-24, classify it as a naval reconnaissance flying boat and gave a positive assessment of the aircraft’s seafaring and aeronautical characteristics. It was decided to authorize its serial production.

Several months later the problems would begin. The complex situation of heavy industry and the lack of supplies led to the decision to close the “Krasni Liotchik” factory as of October 1, 1923 and transfer the request for the production of the M-24 to Factory No.10 in Taganrog, decreasing the quantity requested to only 40 copies. In the midst of this complex situation, the Society of Friends of the Air Fleet (ODVF according to the acronym for Obshesctvo Druziei Vozduzhnovo Fleet) decided to support the aeronautical constructors of Petrograd, financing the creation of the “Krasni Baltiets” squadron (This term could be translated from inaccurately as Red Sailor of the Baltic, since the word Baltiets covers any inhabitant of this area, but in this case it is made with a direct allusion to the Marina – na) with the condition that the production was carried out in the GAZ No.3 “Krasni Liotchik”.

The decision to select the flying boat M-24 as a model for the new squadron was based on the positive report of the commission of the Scientific Committee of the VVF (NKVVF). Agreements with the management of GAZ No.3 to build the first 12 copies of the M-24 were signed on September 12, and by October 7 the GlavBoyenProm submitted to the factory a request for another 15 copies against the funds of the 40 approved.

By its construction the M-24 was similar to the M-9. Both were built with national types of wood. The structure was built with pieces of pine and ash. On top it was covered with birch plywood. The hull was divided into 10 watertight departments. All joints were secured by small screws and rivets. The submerged parts of the hull were covered with sheets of copper or brass. A flexible ash bow was attached to the rear of the keel piece, to which the rear ski could be attached in winter.

Although the hull of the M-9 had proven capable of withstanding landings on ice, the greater weight of the M-24 made such manoeuvres inadvisable. For this reason during the winter in the Baltic the M-24s were operated on skis with an installation scheme similar to that successfully used on the M-15 model.

The wings were made of wood, with some reduction in span relative to the M-9, and were attached by two pairs of pine supports. The lower plane had little positive dihedral and the stabilization floats were fixed externally. The tail unit was of typical Grigorovich design.

The M-24s were powered by 220-260 hp Renault water-cooled 12-cylinder engines entirely covered by an aerodynamic duralumin hood. These engines were installed in a wooden structure located in the space between the wing planes and had a cooling radiator at the front.

In the bow area of the boat an oval hatch was opened to locate the machine gun. In its central part and sitting side by side were the pilot and the mechanic.

The first series, of 12 copies, featured a single 400 liter fuel tank and was characterized by using the 200 hp Renault engine. When the construction began, the factory lacked the plans and technical specifications approved by the NKVVF. Neither did the Navy leadership. For this reason there were notable differences in the characteristics and completion of different copies of the M-24, fundamentally motivated by claims of their pilots or productive considerations. At that time, the poor seaworthiness of the model, announced from the beginning by Grigorovich, was revealed.

Soon a special commission would be appointed to investigate the problems that the model was presenting. The commission that reviewed the claims towards the first produced specimens concluded that: “the specimens differ in their flight characteristics, but due to their poor navigability they are not suitable for serious work… consider the aircraft requested by the ODVF available only for training flights.” It was decided to withdraw the model M-24 from service.

However, the situation for the VVF was extremely complex due to the lack of naval models. Apart from the M-24 the only aircraft of the VVF in 1924 were 30 Savoia S-16 flying boats and 20 Junkers Ju-20 in process of delivery. In this state it was decided to work on attempts to improve the existing M-24. By that time Grigorovich had already returned to Moscow and was beginning his work on GAZ No.1 (formerly Duks), so the modifications were made without his presence.

The second series, starting from issue 1730, featured a 280-litre main tank. The rest of the fuel was located in two tanks of 60 liters located in the wings or in the space between the wing supports.

At least 20 M-24 examples, related to the second series, used Renault engines with power increased to 260 hp by replacing the original pistons with aluminum ones. This series would present other minor differences, which is why in some literature they have been differentiated as M-24bis.

In the period between 1923 and 1924 at the Petrograd “Krasni Liotchik” 43 copies of the M-24 were manufactured (with serial numbers between 1718 and 1760). These would be the last flying boats of the M series.

In relation to the MR-4 the specialists of the Baltic Fleet at the end of July 1925 wrote:
The plane is three-seater, but when placing a man in the bow it tends to pitch and sink the nose. In the air it is more stable with the three crew members, with two you have to keep the lever as it tends to pull upwards. Most of the time you have to work with the right leg. It reacts poorly to spoilers. The plane handles turbulence poorly.
The plane does not have all the instruments. The hull is weak, the plywood is not of good quality. The plane easily loses control. During the replacement of parts, for example surfaces, they seldom coincide. The engine of the MR-4 (Renault 260 hp), but they did not arrive new, but assembled. Clearly, the material used in the distribution system does not have the necessary rigidity, which brings about constant problems. The water pump key also breaks… There have been few cases of engine stoppages in two years – two or three times, but when the damage does occur it is serious: broken connecting rods, deformed pistons…
…The MR-4 has the following problems: a) not very stable, b) the pilot carries out the bombing blindly, he cannot see the target.
…The MR-4 lacks a camera mount and its installation is impossible due to the fact that the camera lens during takeoffs and landings gets splashed with water and then, when being lowered or hoisted, it is necessary to hold the camera in the hands.
…The MR-4 lacks air navigation equipment. The compass is located between the legs of the crew or on the blackboard next to the moving metal parts, which means that it cannot be seen at times.

Of the 43 copies produced, only 28 would be accepted into service by the VVF. Most examples were used in the Baltic theatre, seeing service with the 1st and 2nd OMPAO (Otdielni Morskoi Razviedivatielni Aviaotryad or Independent Naval Air Exploitation Brigades).

The 2nd OMPAO would receive the 12 copies financed by the ODVF and two more. This unit had previously operated the M-9 and M-20 models. Deliveries were made in two groups of 4 aircraft and a final group of 6 between January and June 1924 and the unit would soon be renamed “Krasni Baltiets”. All its devices had their own names on the bow, many times related to the origin of the contribution of the funds. The names and numbers of most of these specimens are known:
M-24 — Factory number 1718 «Petrogradskaya Pravda» (Petrogradian Truth)
M-24 — Factory number 1719 «Volodarski»
M-24 — Factory number 1720 «Krasni Petrogurfin»
M-24 — Factory number 1721 «Krasni Putiloviets»
M-24 — Factory number 1722 «Volkhovstroi»
M-24 — Factory number 1723 «Leningradski Profsoyuci» (Leningrad Trade Unions)
M-24 — Factory number 1724 «Vladímir Ilich»
M-24 — Factory number 1726 «Pskovityanin»
M-24 — Factory number 1727 «Kommuna»
M-24 — Factory number 1728 «Cherepanin»
M-24 — Factory number 1729 «Krasnaya Karelia»
M-24 – Factory number ???? “Krym SSSR”
M-24 – Factory number ???? Bryansky Rabotnik
M-24 – Factory number ???? «Toplivnik»

Two M-24s were sent to serve in the Black Sea, entering service with the 3rd Air Reconnaissance Brigade, based in Sevastopol.

Originally the M-24 was called MR-1, but since the naval version of the Polikarpov R-1 received the same name, the flying boat began to be called MR-4 (acronym Morskoi Razviedchik or Naval Recognizer). By the beginning of 1925, 27 M-24 flying boats remained in service. Some were operated by the Finnish Air Force.

In 1925 the 1st OMPAO based in Peterhof began to receive the Junkers Ju-20 and the few received M-24s began to be withdrawn from service. The copies of the 2nd OMPAO began to be withdrawn, when this unit began to receive the Junkers Ju-20 also during the second half of 1925.

The 3rd Air Reconnaissance Brigade in the Black Sea was equipped with the Savoia S-16, so the M-24s were practically from the beginning destined for training functions.

After 1925, 17 copies of the M-24 were kept as trainers. A number served in the naval brigade of the Leningrad Military School of Pilots-Observers. The last copy in this center was discharged in December 1928.

Several flying specimens were offered free of charge to the “Doboliet” company, which rejected them. An M-24 would be transferred to the OSOVIAJIM around the month of May 1929, without its subsequent destination being known.

Although the M-24 basically served as trainers during their exploitation, they were frequently used in preparation for military tasks. For this reason many M-24s retained armament similar to that of the M-9: a machine gun in the bow.

M-24
Powerplant: One 220 hp Renault V-12
Upper plane wingspan: 14.40 m
Lower plane wingspan: 13.00 m
Wing area: 55.0 m²
Length: 8.8m
Height: 3.40m
Empty weight: 1240kg 1200kg
Maximum takeoff weight: 1700 kg 1650 kg
Wing loading: 30.0 kg/ m²
Power load: 7.5kg/hp
Speed at sea level: 130km/h
Ceiling: 3500m
Endurance: 4 hours
Range: 400km
Armament: 1 x 7.62 mm machine gun
Bombload: 100 kg
Accommodation: 2 – 3

М-24bis
Powerplant: One 260 hp Renault V-12
Upper plane wingspan: 14.40 m
Lower plane wingspan: 13.00 m
Wing area: 55.0 m²
Length: 8.8m
Height: 3.40m
Empty weight: 1280kg
Maximum takeoff weight: 1740 kg
Payload capacity. 500kg
Wing loading: 31.0 kg/ m²
Power Load: 6.6kg/hp
Speed at sea level: 160km/h
Cruising speed: 121km/h
ROC: 68m/min
Ceiling: 4500m
Endurance: 4 hours
Range: 400km
Armament: 1 x 7.62 mm machine gun
Bombload: 100 kg
Accommodation: 2 – 3

In 1922 and after a break of 5 years DP Grigorovich returned to aircraft construction with the development of more powerful versions of the successful Grigorovich/Schetinin M-9. Grigorovich considered that for a more powerful engine it would be necessary to increase the structural rigidity, which would entail a reinforcement of the fuselage and the need to modify the lines of the hull, but despite this he decided to continue the development of the already recognized model with the main objective to be able to interest new customers and quickly secure a new position in the aeronautical industry.

At that time, Grigorovich had not yet been able to put together a work team and lacked the facilities to develop it, so the design work began in his apartment in Moscow. He would soon invite old collaborators to his side, among them the engineers MM Shishmariov, Ye. I. Mayoranov and V. L. Korvin-Kerber.

The M-23 (Russian: Григорович М-23) was an interim development of the M-9 line with a new power plant. The initial model project was completed by mid-1923 but was soon abandoned in favour of a modified version designated M-23bis.

The M-23bis was built at the GAZ No.3 “Krasni Liotchik” in Petrograd in the middle of the summer of 1923. During the tests the model refused to take off from the water. The analyses showed that it was a design error of the bow and contour lines of the hull, so the flying boat was stored pending the necessary modifications, but at the end of the year it was destroyed during a flood that occurred in the hangars on Krestovsky Island.

M-23bis
Power plant: 1 Fiat, 280 hp
Wingspan: 12.5m
Wing area: 45.8 m²
Empty weight: 1165 kg
Takeoff weight: 1615 kg
Total load capacity: 450 kg
Wing loading: 35.0 kg/m²
Power Load: 5.8kg/hp
Accommodation: 2