Sukhoi was instructed to convert his design into a two-seater. As in the Il-2, the rear gunner wielded an UBT (with 196 rounds), but his armour protection was better. The gunner enjoyed bottom and side armour as well as a rear plate, while the gun mount took the form of a rotating, armoured disc. This version was known as the Su-6(S2A) and was considered superior to the Il-2 Type 3, but still could not displace it from the production lines. Struggling to convert praise into production, Sukhoi then studied the M-82 radial before deciding to adopt the liquid-cooled AM-42 engine, to create a competitor for the Il-10. This engine was protected by plate 2 to 4 mm thick and the radiator was in a bath directly below it. During testing the prototype reverted to VYa cannon in the wings, in addition to two ShKAS guns. It was all to no avail.
The design and manufacture of the Su-6(A) single-seat attack aircraft by the Sukhoi Design Bureau started in 1940, after the bureau was given the manufacturing base in Podlipki. This work was conducted simultaneously with the testing of the Su-1 prototype and other extensive work on the development of various Su-2 versions. The aircraft, which was intended for operation against ground troops and enemy airfields, was powered by the new Shvetsov M-71 air cooled radial engine being developed at the same time.
The first Su-6(A) prototype was ready by the beginning of 1941, and it underwent its state trials from 28th August to 17th September. It was a single-seat monoplane of mixed construction. The main wing panels, wing centre section and empennage were metal, while the unarmoured fuselage rear section was a wooden semi-monocoque structure covered with veneer. The control surfaces had metal frames and were covered with fabric. Vital elements such as the cockpit and fuel tank were armoured. To improve manoeuvrability and stability at high angles of attack, the wing was fitted with automatic slats.
All of the gun armament, consisting of four 7.62mm high-rate-of-fire ShKAS machine guns and two 23mm VYa guns, was installed in the outer wing panels. The aircraft could carry up to 881lb (400kg) bombs in its internal bomb bays, and small calibre bombs could be stowed in bulk in the bays without carriers, greatly accelerating pre-flight preparation. Up to ten RS-82 or RS-132 rocket projectiles could be carried on external hardpoints.
The summary of the state tests, signed by the pilot, Major Dolgov, made the following points:
- The Su-6 with the M-71 is faster in horizontal flight than the 11-2 powered by the AM-38;
- without bombs and RS-82 rockets the Su-6 has a maximum speed of [300mph] 483km/h for 10 minutes in boosted mode. This makes the aircraft difficult to catch for enemy fighters with only a small speed advantage;
- it is expedient to consider the manufacture of a small series of Su-6 aircraft powered by the M-71; they are of interest because of their comparatively high maximum horizontal speed and powerful gun and rocket armament.
The Su-6(SA) back-up aircraft was manufactured with some modification and underwent flight testing at Molotov during February 1942. The new series of five aircraft for service trials were to be manufactured there as well. The Su-6(SA) was powered by an M-71 F enclosed in a NACA-type cowling. Its fuel tank was protected beneath, behind and at the sides by armour plate, and its void was filled by inert gas. The cockpit was protected by armourof 3/16 to 6/10in (4 to 15mm) thickness.
The tapered wing had a TsAGI ‘B’ aerofoil section of 15% thickness/chord ratio at the root and 9% at the tip, and was fitted with slats and Schrenk-type flaps. The wing centre section was all metal, while the outer wing panels were wooden with metal spars.
Six RU-235 launchers for RS-82 rocket projectiles were installed beneath the outer wing panels, and two 110 to 551lb (50 to 250kg) bombs could be carried under the wings. The normal bomb load was 440lb (200kg), while in overload the aircraft could carry up to 881lb (400kg) of bombs. Two 37mm OKB-16 guns with 40 rounds each and two 7.62mm ShKAS machine guns with 675 rounds apiece were installed in the roots of the outer wing panels. Cast iron counterbalances were mounted in the outer wing leading edges to delay the onset of wing flutter.
The controls were equipped with balances and aerodynamic compensation. The control system was duplicated, while the Frise-type aileron control was single, using rods. Rudder control was through duplicated cables, and the elevator control used duplicated rods.
Wartime operational experience made it clear that attack aircraft needed rear gun mountings to protect them from hostile fighters. In 1942, therefore, the Su-6(S2A) two-seat armoured attack aircraft was built on the basis of the Su-6(SA) single-seater, and was completed in December of that year. It was powered by a 2,200hp (1,641kW) M-71F air cooled radial driving a three-blade AV-5-4A propeller, replaced during testing by a four blade AV-9-4A unit.
Armament comprised two OKB-16 guns with 45 rounds, two ShKAS machine guns with 700 rounds per gun and a BLUB blister gun mounting with a UBT machine gun with 196 rounds. The aircraft had the VV-1 sight, which consisted of a ring on the armoured windshield and a fore sight on the engine cowling.
State tests were carried out from 19th June to 30th August 1943, and Colonel A Kabanov noted in his report dated 4th September 1943: ‘On the whole the aircraft creates a good impression, and would be a formidable weapon in engagements against ground troops and with enemy bombers …’. Colonel Pyotr Stefanovsky, the pilot who performed the flight tests, wrote in his report of 6th September 1943: ‘This two-seat attack aircraft… could replace the Il-2 because its armour is more effective and its performance is better’.
The report also stated: ‘The Su-6 two-seat aircraft was tested in aerial combat with the enemy’s Bf 109G-2 fighter without underwing guns, and with the He 111 H-11 bomber under daytime conditions … Enemy bombers such as the He 111, Fw200 and Ju87 could evade the Su-6 by climbing. Conclusions: In concert with the use of active, manoeuvring defence during the repulsing of enemy fighters attacks, the Su-6 is capable of higher horizontal speed than the Il-2 and its tail protection, using fire from the rear gun mounting, allows it to resist the enemy fighters while maintaining flight at maximum speed, especially at low altitude, and to turn to allow the gunner to fire from the rear mounting … Owing to the combination of high speed and powerful armament the Su-6 could effectively engage enemy bombers (such as He 111, Fw200 and Ju87), as well as the transports …’.
Sukhoi was awarded the Stalin Prize, First Degree, for the development of this aircraft. Unfortunately the lack of production M-71Fs decided its fate
AM-42 -powered Su-6(S2A)
The Su-6 was converted to have a liquid cooled engine at Plant No. 284, and then went for state trials, conducted from 28th April to 2nd July 1944. Only basic flight performance data were gathered during these tests, and the programme was not completed because a serious defect was discovered in the AM-42 engine with its AV-9L-172 variable-pitch propeller; this was the burning of the mixture in all of the branch pipes.
On this machine the slats were removed and the compensation between the controls and ailerons was increased. The tailwheel was enlarged, a new hydraulically actuated undercarriage with a piston of larger diameter was installed, and the undercarriage attachment points were reinforced. A standard fighter-type control column was fitted in the cockpit, and the oil cooler output ducts were positioned under the wing. The wing area was increased to compensate for the higher gross weight due to the heavier engine; the wings were now of all-metal construction. During the test flights the OKB-16 guns were replaced by lighter Volkov and Yartsev VYa weapons, allowing 1,322lb (600kg) of bombs to be carried at the same take-off-weight.
The Su-6 with the AM-42 was not put into production because the contemporary Il-10 attack aircraft was better and had superior performance. The Il-10, being similar to the Il-2 in its construction, was therefore introduced into the inventory.