There were many attempts in various parts of the world to mount heavy-caliber cannons on airborne platforms, although designers rarely considered anything above 40 mm.
Grokhovsky G-52 (Object G-52)
If they did, the work hardly ever progressed beyond the initial design stage, as was the case with a German Ju-388 armed with a pair of 50 mm weapons. Very few types were actually built as prototypes, i.e. Mitsubishi Ki-109 with its 75 mm cannon. A small batch of the Ju-88 P-1s, another aircraft toting a 75 mm cannon, was actually manufactured and pressed into service.
In the Soviet Union the challenge was accepted by Grokhovsky design bureau. In 1934 a TB-3 bomber was modified to carry a regimental 76.2 mm M1927 gun in a ventral mount bolted to the fuselage. The arrangement was then successfully ground and flight tested with promising results.
The trials took place between December 15 and 18, 1934. During the first ground tests the crew stations were “manned” by dogs, but as soon as it became apparent that no threat to life existed, human crews took over. The truss gun mount worked well to transfer recoil energy onto the main attachment bolts, which protected the aircraft fuselage structure from significant damage. In fact, the only undesirable effects of test firings were a few torn fuselage skin rivets and slightly cracked panels near the gun’s muzzle. The damage was quickly repaired and the skin panels adjacent to the muzzle were strengthened to better withstand the blast.
The first live airborne firing of the gun took place on December 17, 1934 with the aircraft flying at 500 m and 150 km/h. During the test Grokhovsky, who often actively participated in flight testing of his own designs, took charge of the crew consisting of Cholobayev and Afanasiev (pilots), Shmidt (navigator) and Shamirov (gunner). Post-test examination of the aircraft showed no damage to the airframe and the official test report stated that “experimental use of 76 mm field guns in airborne applications is possible”.
Delighted by their initial success, the designers didn’t rest on their laurels and quickly converted a stock TB-3 powered by M-17 engines to carry a battery of no fewer than three guns (the original mounts – minus the wheels – and recoil mechanisms were retained). After a series of trials, a 76 mm anti-aircraft gun was mounted internally in the fuselage. Although the gun’s barrel was fairly long, the fuselage had to be shortened, which led to the elimination of the navigator’s station in the nose (it was moved further back into the mid-fuselage section of the aircraft). The barrel sat between the two pilots’ seats and protruded 250 mm beyond the nose. The breech was located just forward of the wing, reaching the aft main spar. The weapon was internally attached to the main spar boxes. To protect the flight deck crew from the gun’s muzzle gases, the barrel was encased in a steel tube between the gun port in the nose and the bulkhead just forward of the flight deck. (FOTOS 6,7)
The remaining two guns were mounted in the wings, just outside the propeller arcs of the two outboard engines. The wing-mounted weapons were short barrel variants of the 76.2 mm regimental gun, which were selected for their relatively compact size. The guns were placed in truss mounts and bolted to the wing’s main spars. The wing’s structure was reinforced in the area immediately around the weapons.
The modified aircraft, officially designated “Object G-52”, was ready for trials by mid-1935. Each gun was manned by a loader handling a supply of 12 rounds (for wing-mounted guns) or 20 rounds of ammunition (internally-mounted cannon). There was no integrated firing system. Instead, in front of each gun loader there was a panel with an indicator light that went on every time the aircraft commander deemed it necessary to fire. Synchronized firing of the guns was therefore impossible under those conditions. While the fuselage-mounted gun could be fired with a degree of accuracy, the wing-mounted weapons could only achieve one-half of that. Every time the wing guns were fired, the second round inevitably missed its mark due to a yawing moment caused by the recoil of the first cannon. In other words, if in a two-round salvo the first gun to fire was the internal cannon, followed by one of the wing guns, both rounds had a fighting chance of reaching their targets. If, on the other hand, the first gun to fire was one of the wing-mounted weapons, only one round would ever land on or near the target. The designers were hoping to remedy the situation by introducing an elaborate system of strings and pulleys which could be centrally activated by the gunner, but the work stalled and came to nothing.
The idea behind the “air cruiser” design dates back to the closing stages of World War 1, when it became apparent that newly-introduced heavy bombers required effective fighter cover along their entire route of flight. To perform the task effectively, a fighter design was needed with a speed at least matching that of the bombers, enough range to provide protection, heavy armament consisting of cannons and machine guns and, to top things off, agility to be able to successfully engage enemy fighters. It was also hoped that such a platform would be capable of performing reconnaissance missions, as well as engaging secondary targets, such as AAA positions protecting primary targets, or defeating enemy bomber formations. Although quite a few nations attempted to build such an aircraft, the task proved to be extremely complex and difficult.
How the Steel was Tempered, or the birth of ANT-7 (R-6)
Operational use of the Ilya Muromets bomber in World War 1 provided a wealth of lessons learned, which demonstrated that the heavily armed aircraft was extremely difficult to engage by enemy fighters due to its defensive firepower having been projected in all directions. As such, it could prove to be a formidable weapon against enemy fighters in the role of an “air cruiser”. Unfortunately, Russia’s mixed fortunes towards the end of the war meant the idea was not pursued. A.N. Tupolev, however, never quite gave up on it and it was revisited many years later.
In the USSR the trigger that ultimately led to the development of “air cruiser” was the introduction of the TB-1 (ANT-4) bomber. As the new aircraft was just barely rolling off the production line, A.N. Tupolev was already busy ordering his design bureau staff to begin work on the new design. The team consisted of N.I Petrov responsible for fuselage design, V.M. Petlyakov who would work on the wing and landing gear and E.I. and I.I. Pogosski delivering the powerplant.
The new aircraft was designed as an all-metal, cantilever monoplane with corrugated metal skin. It was to be powered by a pair of 500 – 600 hp engines and operated by a crew of three. According to the design documentation, the machine, designated ANT-7, was to be a multi-seat escort fighter. The aircraft was essentially a scaled-down version of the TB-1. When the design was first presented to the VVS (Voyenno-Vozdushnye Sily – Soviet Air Force), the brass liked what they saw and proceeded to compile official tactical and technical requirements for the new machine. The first draft of the document was released on August 9, 1927 and required the payload capacity of 588 kg. It also classified the aircraft as a long-range reconnaissance platform, hence the military designation R-6 (rozvedchik – reconnaissance aircraft). Tupolev relentlessly lobbied the Air Force to introduce changes to the document that would emphasize the fighter role of the design and to tweak certain technical requirements. Those were not extraneous efforts – Tupolev was hoping that the official requirements would eventually more closely match the project already sitting on the drawing boards. On October 27, 1927 the changes were officially approved and the new machine was defined as a “multi-seat frontline fighter”.
However, as was often the case in the USSR, that was not the end of changes to the technical requirements of the new aircraft. On October 26, 1927 a letter arrived at TsAGI offices, which contained a revised version of the document. The Air Force apparently went back to the idea of a four-seat, long-range reconnaissance platform carrying a payload increased to 725 kg. More changes were introduced on November 6, the scope of which rendered the existing design practically unusable. The required armament was increased from four to eight Lewis machine guns, while useful payload was once again raised – this time to 890 kg. The new draft of technical requirements called for gunner stations in engine nacelles, in addition to a pair of fuselage turrets. The crew was increased to five: pilot, co-pilot (doubling as a radio operator and rear gunner), navigator (also manning nose gun) and two engine nacelle gunners. The role of the future aircraft was described in the document as follows: “The key role of the proposed design would be to conduct reconnaissance deep behind enemy lines countering the threat of most modern and the best enemy fighters. In addition, the aircraft would have to be capable of performing escort missions on long-range bombing raids.” It was clear the new requirements would inevitably lead to increasing the overall size of the design and, what followed, degrading its performance. While the original estimates put the “cruiser’s” maximum speed at 215 – 220 km/h, that value was now described as “above 160 km/h”. Priority was given to the field of view and arc of fire, while maneuverability was way down as number six on the list. It corresponded to the requirement that the R-6 should be able to “…engage in a defensive battle with several enemy fighters at the same time”, using to its advantage spherical field of fire, rather than speed or agility.
Tupolev persevered in his lobbying tactics and eventually reached a compromise with the Air Force brass. In its final iteration the technical requirements defined the R-6 as an “air-defense fighter and army reconnaissance platform” (the “army” designation indicated “long-range” character of the mission). The aircraft once again became a four-seater with a payload of 700 – 725 kg. Gunner stations in engine nacelles were to be replaced with a retractable ventral turret. Most importantly, the overall airframe design was left largely unchanged, which was good news for TsAGI staff, who had already started, back in August 1927, manufacturing some of the components for the R-6 prototype.
The question of which powerplants would best suit the new design was still unanswered. The initial choice was a pair of Hispano-Suiza units rated at 450 and, later, 520 hp. Later the use of German BMW-VI motors was contemplated (680 – 730 hp, depending on the variant), while in January 1928 the British 480 hp Bristol Jupiter radials were eyed as a possible choice.
The work on the new design progressed at a good rate and by March 19, 1928 a full-scale wooden mockup was ready. After implementing several changes to the cockpit layout and equipment setup, the mockup was officially approved in July 1928. The Air Force had high hopes for the new aircraft, so they green-lighted full-scale production even before the prototype was ready for its first flight. The delays in prototype assembly were due mainly to problems with subcontractors and the VVS, who failed to deliver the TUR-6 gun turrets on time. Despite those hiccups, TsAGI team made up some of the lost time and finally delivered the R-6 prototype powered by a pair of BMW-VIE engines (500 – 730hp).
Flight test program
The maiden flight of the R-6 prototype was a short hop flown by M.M. Gromov. Three minutes into the sortie the starboard engine began to overheat and then seized up, forcing Gromov to put the aircraft down in a hurry. He came in perpendicular to the runway, which almost cost him his life. The root cause of the engine failure was a cracked radiator, which bled engine coolant at an alarming rate. Following Gromov’s advice, the rudder was enlarged to provide better authority. During the second sortie overcompensation of elevators became apparent and it wasn’t until the third flight that all flight controls were deemed to work fine.
On March 11, 1930 the machine was dispatched to perform state trials. Their results, to say the least, were not satisfactory. The official post-test report noted that the aircraft failed to meet the technical requirements in several areas: all-up weight was exceeded by 335 kg, while airspeed at sea level was 14 km/h less than required and 42km/h below expectations at 3,000 m. Time to climb to 5,000 m was 30 minutes instead of 15.3 minutes as per technical requirements. Additionally, demonstrated range of the machine (designed to be a long-range reconnaissance platform) was merely 268 km, while a much lighter and cheaper Polikarpov R-5 could reach 398 km! To add insult to injury, the report noted excessive vibrations of tail surfaces, fuselage and engines, leaking fuel tanks and poor damping characteristics of the landing gear. The navigator’s station allowed decent visibility only when the navigator stuck his head outside the dorsal gun turret. The part of the report summarizing the findings read: “Advantage over the TB-1 – marginal”. That must have hurt, when the hopes were that by scaling down the size of the aircraft and by reducing weight associated with standard bomb load, a design with superior flight characteristics could be achieved. To be fair, the report’s conclusions did mention some of the machine’s advantages: “The aircraft is simple and easy to fly” has “good stability and controllability”, in addition to a short take-off roll (100 – 120 m) and heavy armament.
State trials ended on March 30, and the aircraft returned to TsAGI where all the bugs revealed during tests were to be ironed out. The work took about four months and included raising the wing’s trailing edge by 175 mm, while leaving the wingtips in their original position. This was done to eliminate the vibrations of the fuselage and tailplane, which, as it turned out, were caused by turbulent airflow behind the wing. Engine coolant radiators, originally placed under the wings between the fuselage and engine nacelles (fully retractable in flight), where installed under the engines and fitted at an angle, in hopes of reducing drag. The engines received new cowlings and exhaust manifolds facing upwards. The pilot’s seat was raised 100 mm to improve visibility from the cockpit, while the navigator’s station was fitted with a celluloid porthole in the nose. The forward gun turret was replaced to match the rear one (both were now the TUR-5 models), with the latter receiving a celluloid fairing in front of it. A redesigned ventral turret was fitted to a larger cutout in the fuselage and the main landing gear received 1000x225 wheels and tires. Interestingly, all the changes introduced to the prototype not only didn’t increase its overall weight, but resulted in net savings of some 80 kg.
On 24 July, 1930 the R-6 prototype was once again delivered for further trials to NII VVS (Nauchno- Ispytatielnyj Institut VVS – Air Force Scientific Test Institute). The tests lasted until September 31 and, this time around, the results were much better. The aircraft’s speed at altitudes above 2,000 m increased, as did its service ceiling. However, there was also an increase in landing speed and the time to complete a turn and “figure eight” (by about 5 – 6 s). Test pilots reported better handling characteristics and improved visibility from the cockpit. In general, the aircraft’s performance was thought to “fulfill technical requirements”. This is not to say that the R-6 received a clean bill of. The report mentioned a host of issues: the aircraft was still overweight, engine mounts were not rigid enough causing excessive engine vibrations, upward-pointing exhaust stacks blasted exhaust gases straight into the cockpit, engines splashed oil all over radiators and main landing gear, the windshield was too flimsy and struggled under pressure of high-velocity airflow, the cockpit was lashed by howling wind caused by uninsulated fuselage design. In addition, the aircraft didn’t carry the required WOZ IV radio and had no provision for intra-crew communications.
Most of the issues were resolved fairly quickly, except the engine mounts whose design wouldn’t change until full-scale production began. As an added bonus, the designers decided to increase the forward-looking porthole in the navigator’s station and replaced celluloid with Triplex glazing. After the work had been completed, the machine was once again handed over to the NII VVS for follow-up testing. The trials lasted until October 17 and focused mainly on the tactical use of the R-6. Based on the test results, the Air Force commanders were keen to establish the machine’s role in a future war. The results must have been satisfactory, since several documents drawn in their wake offer statements like “dives well at 60 degree angle”, “heavy armament”, “easy to control in Immelman turn, stable, normal loads on flight controls”. The Air Force concluded that the R-6 could be employed as a long-range reconnaissance platform, bomber escort and a multi-seat fighter (the latter, unfortunately, only in a very limited scope). It is worth mentioning, however, that the question of what role exactly the R-6 was assigned by the Air Force, is not immediately clear from the available NII documents, which quite often contain contradictory statements. One of the documents clearly states that the machine was not suitable as a multi-seat fighter due to its “limited maneuverability and heavy weight”, but then goes on to say that the R-6 “…can be used as a multi-seat fighter to conduct long-range air-to-air engagements against heavy bombers”. The reason for those inconsistencies might have been the fact, that the R-6 would not stand a chance in a dog fight not only against single-engine fighters, but also typical light or medium bombers. It was only lumbering heavy bombers that could be potentially engaged by the “air cruisers”. Nonetheless, the overall conclusion of the report following state trials was summarized in one sentence: “The aircraft is cleared for full-scale production”.
Recommended - Armour
Versions armed with a 37 mm gun