MiG I-250, MiG-13

Purpose: To boost the speed of a piston – engined fighter.

Design Bureau: The OKB-155 ofAI Mikoyan.

In 1942 the Central Institute for Aviation Mo­tors (often abbreviated as TsIAM) began to develop an unusual method of boosting the propulsive power of fighter aircraft. Called VRDK (from Russian for ‘air reaction auxiliary compressor’) it involved adding a drive from the main engine to an auxiliary compressor for a flow of air rammed in at a forward-fac­ing inlet. The compressed air was then ex­pelled through a combustion chamber and propulsive nozzle. This scheme was worked on by a team led by V Kh Kholshchevnikov. In January 1944 the governments of the UK and USA announced their possession of jet air­craft. In a near-panic response, the GKO (State Committee for Defence) ordered all the main Soviet fighter OKBs to build jet air­craft. Stalin criticised designers for not al­ready having such aircraft. As the only Soviet turbojet (the Lyul’ka VRD-2) was nowhere near ready for use, MiG and Sukhoi were as­signed the urgent task of creating prototype fighters to use the VRDK booster system. Both quickly came to the conclusion that the VRDK method could not readily be applied to any of their existing fighters, and both designed spe­cial (quite small) fighters to investigate it. The MiG aircraft was called N by the OKB, and given the official designation I-250. The pro­ject was assigned to G Ye Lozino-Lozinskii. A mock-up was approved on 26th October

1944, and after frantic effort the ‘N’ Nol was rolled out painted white on 26th February

1945. OKB pilot A P Dyeyev began the flight – test programme on 3rd March. Soon the magic 800km/h mark was exceeded, and Mikoyan presented Dyeyev with a car. VRDK operation was generally satisfactory but deaf­eningly noisy. On 19th May a tailplane failed at low level and the ‘N’ Nol crashed. By this time ‘N’ No2 was almost ready to fly. Painted dark blue, with a yellow nose and horizontal streak, it was restricted to 800km/h to avoid a repetition ofthe failure. Stalin had meanwhile ordered that a ‘regiment’ of ten of these air­craft should fly over Red Square on 7th No­vember, October Revolution Day. ‘N’ No 2 was tested by LII pilot A P Yakimov, assisted by OKB pilot A N Chernoburov. This aircraft was written off in a forced landing in 1946. The hastily built ten further I-250s were tested by IT Ivashchenko. On 7th November nine were ready, but the flypast was cancelled be­cause of bad weather. In late 1946 Factory No 381 was given an order for 16 fully equipped fighter versions, designated MiG – 13. Factory testing of these took place in May-

July 1947,1 M Sukhomlin carried out NIl-WS testing between 9th October 1947 and 8th April 1948, and these aircraft were then deliv­ered to the A V-MF. They served with the Baltic and Northern Fleets until 1950.

Aircraft N bore little similarity to any previ­ous MiG design. Made entirely of metal, with a stressed-skin covering, it was smaller than most fighters, whereas its predecessors had been larger. The straight-tapered wing had a CAHI 10%-thick laminar aerofoil, with two spars and plate ribs. Movable surfaces com­prised two-part Frise ailerons and hydrauli­cally operated CAHI slotted flaps. The fuselage was relatively deep to accommodate the unique propulsion system. The engine was a VK-107, rated at l,650hp for take-off and l,450hp at 3,500m (12,470ft). At the front it was geared down to drive the AV-5B three – blade constant-speed propeller of 3.1m (10ft 2in) diameter. At the back it drove the en­gine’s own internal supercharger as well as a clutch which, when engaged, drove through 13:21 step-up gears to a single-stage axial compressor. This pumped air through a large duct from a nose inlet. Just behind the com­pressor was the engine’s cooling radiator. Be­hind this were seven nozzles from which, when the auxiliary compressor was engaged, fuel from the main tanks was sprayed and ig­nited by sparking plugs. The resulting flame filled the large combustion chamber, from which a high-velocity jet escaped through a two-position nozzle. Downstream of the burners the entire duct was refractory steel, and when the VRDK was in operation its walls were cooled by water sprayed from a 78 litre (17 Imperial gallon) tank, the steam adding to the thrust. At 7,000m (22,966ft) the VRDK was estimated to add l,350hp, to a total of 2,500hp. The oil cooler surrounded the pro­peller gearbox, with flow controlled by gills round the top of the nose. The engine was mounted on a steel-tube truss. Fuel was housed in three self-sealing tanks, one of 415 litres (91.3 Imperial gallons) in the fuselage and one of 100 litres (22.0 Imperial gallons) in each wing. The large central tank forced the cockpit to be near the tail, with a sliding canopy. The metal-skinned tail was repeat­edly modified, the small elevators having a tab on the left side. A unique feature of the main landing gear was that the wheels were carried on single levered-suspension arms projecting forward from the leg. The tail – wheel was fully retractable. Even the first air­craft, called ‘N’ Nol, was fully armed with three B-20 cannon, each with 160 rounds. The MiG-13 batch differed in having a larger verti­cal tail, larger fuel and water tanks, RSI-4 radio with a wire antenna from the fin to a mast projecting forwards from the wind­screen, and (temporarily) strange curved pro­peller blades in an attempt to reduce tip Mach number.

These aircraft performed as expected, but were a dead-end attempt to wring the last bit of performance from piston-engined fighters.

Photographs on the opposite page:

Top: I-250 Nol.

Centre: I-250 No 2.

Bottom: Production MiG-13 (straight propeller blades).