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YAK-11

The YAK-1l is a single engine, tandem seat, low wing monoplane with conventional retractable main landing gear, fixed tail wheel and a single fin and rudder. This aircraft is normally used for training and is a counterpart of the USAF T-6 aircraft. It was built by the Soviets and used by their own and their satellites air forces. The aeroplane has a beefy and brutish fuselage with uncharacteristically thin wings and tail surfaces, its WWII heritage is unmistakable. The construction is a mixture of steel tubing and metal skinning for the forward fuselage, with fabric covering aft of the mainplane. The wings and tail surfaces are skinned in metal and the ailerons, rudder and elevator panels revert to fabric. The Yak sits on a wide-tracked, inwardly-retracting undercarriage and a small, non-retracting tailwheel. General improvements over earlier constructed aircraft were the fuselage is party covered by metal replacing plywood, the use of a flush mounted iron core antenna replacing the iron loop type, new instruments were incorporated into the pilots and copilots instrument panel, and provisions for a new gun sight ocmputing system was used.

As the end of the Second World War approached the Russian government announced plans for a new two-seater to replace its obsolete fighter trainers, the Yak 7 and U12. Part of the requirement for the new aeroplane was that it should have the same handling qualities as the fighter aircraft then in use. Yakolevs initially simple answer to this was to convert one of its most successful fighter aircraft, the small but agile Yak 3, into a two-seater. The Yak 3s big V engine was replaced with a lower powered radial and the forward fuselage was altered quite a bit to accept this change, but otherwise the original design was hardly altered. The new aeroplane flew for the first time in November 1945 and was a success although the government delayed signing an order straight away. Yakolevs response was to fiffle and refine the basic aeroplane until a deal was finally struck a year later. By this time the aircraft featured an increased wingspan over the Yak 3, a non-retractable tailwheel and an increase in max take off weight. The max level speed had dropped to 250mph, down quite a way from the 300mph prototype, and the name had changed to Yak 11.

The wing is constructed in two panels except for the detachable wing tips. It is a conventional two spar and rib constructlon with a metal stressed skin which is flush riveted. The ailerons are metal frames with a fabric covering. The flaps extend from the fuselage outboard to the ailerons. They are a split type with mechanical up locks linked to the pneumatically actuated push pull tube. The above locks are spring loaded locking the flaps in the up position.

Hinged metal inspection panels on both sides of the fuselage permit easy access to the controls and lines in both cockpits.

The main landing gear is a conventional type and retracts inward. In the retracted position this gear is housed in front of the main spar. The operation of extending, retracting and wheel braking is accozilished by pneumatic power. A mechanical up lock keeps the gear in the retracted position. The self-locking ating actu- cylinder and the geometric lock formed by the side brace in the extended position act as a positive downlock. A mechanical indicator which moves verticaly through the upper surface of the wings is a visual check for landing gear position.

The tail wheel is free to caster When the control stick is pushed forward. The mechanical lock mey be engaged by pulling back on the control stick.

All of the aluminum alloy sheet materials were equivalent to 17s alclad. In general, the sheet was fine-grained, good quality material with normal solution heat- treated structures. Al sheet stock in addition to being alclad was anodized and chromate sealed. Extrusions were also of 17s type material; rivets were equivalent to Al7s. The forgings were approximately the same as U. S. 14s alunim used in the solution heat-treated and artificially aged condition; wide variation in grain size was noted.

All of the low alloy steel samples from the airframe were of the 1% chromium, 1% manganese, and 1% silicon type varying only in carbon content. This steel was used for fuselage etructural tubing and engine mount, as well as highly stressed nuts and bolts. This type of steel called "Chromansil" is used widely in Soviet aircraft, e. g., MM1-15, and IL-10. Its chief advantage is its low critical alloy content; the disadvantages being its temper brittleness and low hardenability. Stainless steel similar to AISI321 was used in the engine cowling and the exhanst duct; the quality was satisfactory. Although the quality of weldments varied, they were considered adequate for the purpose intended.

The practice of using welded tubular fuselage structure is considered obsolete by American standards but is an advantage in the Soviet aircraft industry. The wing spars are buil in sections. In general, the plastic materials were largely used in non-critical applications, The bakelite, nitrocellulose, and transparent materials examined were adequate to do the job for shich they were intended. A practice not acceptable to US standards was the use of a flammable plastic for a main fuel line transfer hose cover. Four types of rubber parts were used: natural, polybutadiene, butadiene-styrene, and neoprene. Polybutadiene, used for the fuel transfer hose and an oil hose, was considered to be a poor choice because this type is not fuel resistant; neoprene used to some extent would have been a better material, although inferior to Buna N. Bitadiene-styrene (Buna S) rubber was used for wire insulation; this is considered a good choice of materials.

The ASh-21 engine is installed in a conventional manner but incorporates movable shutters on the nose section of the engine for controlling the air cooling inlet diameter.

From a manufacturing standpoint the most critical powerplant parts are gears, cylinder heads, and crankcase main sections. The gears had been ground with a Maag gear tooth grinder. The employment of a Maag gear tooth grinder has several tages advan- which are as follows: durability of the precision gears is enhanced through more effective lubrication, the probability of scrap durin manufacture is decreased since it is less like3y that the tooth surfaces will be burned during the grinding operation. In the ASh-21 counterpart of the Wright Cyclone 7, the casting method of fabri- cation of the cylinder heads was employed in favor of forging. This may be con- sidered to impose an optimum limit on engine power.potential. The same may be said for the employment of aluminum alloy instead of steel in the crankcase main sections.

The armament of the YAK-1 consists of one fixed 12.7mm Berezina machine gun. This machine gun is mounted to the left of the c rer line of the aircraft beneath the front fuselage panel. The Berezina is synchronized to the engine and fires forward through the propeller arc. The gun and sight head were missing from this air- craft. The camponent parts of the fire control system are similar to those used in a computing sight. Provision for carrying 2-110 or 220# bombs is provided by a bomb shackle on the underside of each wing. There was no provision for armor protection for the pilots since it is a training craft.

The radio navigation equipment consists of an RPK-IOM Direction Finding Receiver with a iron core, stationary, loop antenna flush mounted in the top of the fuselage behind the rear cockpit. The communication equipment consists of an RSI-6M-l Receiver, RSI-6K Transmitter and Interphone Amplifier. A wire antenna extending from a mast at the rear of the cockpit to the vertical stabilizer services the High Frequency Transmitter, High Frequency Receiver and the Direction Finding Receiver.

The cockpit arrangement is compact, simple, and similar to the YAK-9. The front instrument panel contains more instruments than the rear panel. A few of the controls and instruments are duplicated in the rear cockpit. Tho aircraft must be flown by a pilot from the front cockpit. A bucket type seat with belt and shoulder strap attachment is used. Forward and aft vision from the front cockpit is good. No provision is made for cockpit heating. Front cockpit ventilation is acconplished by means of an air scoop at the base of the front windshield which is controlled by turning a knob to the right of the sight mount. Rear cockpit ventilation is also accomplished by an air scoop. This scoop is located on the right side of the fuselage beneath the canopy guide rails. A lever on the right side of the cockpit controls the opening and closing of the scoop.

The control stick of the front cockpit contains a bomb release button on top, a trigger switch, and a brake handle. The brake handle is employed to meter air to the brake selected by the position of the rudder bar.

The engine controls are located on the left hand console which is the same as USAF practice in fighter aircraft. The rear cockpit control stick contains a button at the top of the pistol grip which the instructor may use to keep air from entering the brakes or bleed air from the brake system. This safety feature prevents the student from ground looping the aircraft.

The YAK-11 employs a pneumatic system for actuation of the landing gear, flaps, brakes, engine starting, shutters, gun chargers, and firing units. The source of air includes normal and emergency air storage tanks. The pressure in these tanks is maintained by an engine driven air compressor. Provisions are made for using an external source of air on the ground. The ground filler is located on the left side of the fuselage under the aft inspection panel. Plastic materials were found where its application was not critical and the choice of material could be dependent upon availability or ease of forming.

The airplane is well designed structurally and aerodynamically resulting in a maximum movement of the c. g. of 2.3 inches for the worst load condition studied. The air starting system employed is a positive simple method for sturting the aircraft engine. The pneumatic system is also sinPle, effective, accessible, and easily maintained. The TAK-11 pneumatic system incorporates filters but no desiccators for processing the air supplied to the system. For cold weather operation the pneumatic system encounters no added difficulties.

Aluminum alloys, low alloy steel, stainless steels, and resistance welds were all of good quality, adequate for the purpose intended. A major fire hazard was introduced when flammable cellulose materials were used in the manufacture of a fuel hose. The cellulose materials were used to give fuel resistance to a non-resistant rubber. The fuel and oil transfer hose was fabricated from a non-oil-resistant material, polybutadiene. However, in general the rubber material was of good quality.

The production breakdown of the airplane indicated that it is adaptable to mass production. The production can be done cheaply and easily with relatively unskilled labor. A greater variety of welding was used in the manufacture of the YAK-lI than had been found on other Soviet aircraft. The quality of the riveting was good.

Welded steel brackets were used to a great extent contrary to the American practice of forgings and castings. More attention was given to protective finishes on the YAK-11 than on other Soviet aircraft examined. In making a comparison regarding the manufacturing philosophy of the YAK-11 fighter trainer and the MiG-15 interceptor, it is strongly suggested that because of longer service life desired of the YAK-11 the Soviets placed emphasis on its overall quality. On the other hand the MIG-15 is a combat aircraft with a shorter service life; therefore, emphasis was placed on functional quality and only in those areas considered critical. This is further illustrated by the protection of alclad aluminum alloy sheet with anodic and chromate treatment.

The ASh-21 is wholly adaptable to modern quantity production methods and processes. The reason for retention of the casting method of fabrication of cylinder heads in favor of the forging method is unknown. The fact that the caseing is still practiced may be considered to impose a limit on optimum engine power potential.

In addition to the 3,800 aeroplanes eventually built by Yakovlev (big numbers youll agree) a further 700 were built as C11s under license in Czechoslovakia. The aeroplanes made their way into the majority of the Warsaw Pack countries in the role of advanced fighter trainer until the early 1960s. The Yak 11s ability to carry a 12.7mm machine gun in the nose plus two 220lb bombs made it desirable as a light attach aircraft and many third world countries used it as such. Egypt was one of these and this is where both Mark Jefferies and Eddie Coventrys aeroplanes originate. Both aircraft were bought from French aeroplane trader Jean Salis before being extensively restored. The saga of Marks own rebuild would make a feature length story in itself, the aeroplane finally being unveiled to the public at the 1994 PFA Rally at Cranfield where it won the prize for Concours de Elegance. And deservedly so.

YAK 11
Length 27ft 10in 8.2m
Height 10ft 5in 3.19m
Wingspan 30ft 10in 9.4m
Wing Area 166 sq ft 15.4m2
Empty weight 4,037lb 1,833kg
Max AUW 5,468lb 2,482kg
Useful load 1,431lb 649kg
Wing loading 32.9lb/sq ft 161kg/m2
Power loading 7.8lb/hp 3.83kg/kW
Fuel capacity 79 Imp gal 360 lit
Design G Loading +4.25/ 0G
EngineShvetsov Ash-21 air-cooled seven cylinder radial, producing 700hp (522kW) at 2,300rpm.
PropellerVIS-111-V20 metal two-blade constant speed
Vne 323kts 598km/h
Cruise 200kts 370km/h
Stall 73kts 135km/h
Climb rate 1,600ft/min 8.1m/s
Service ceiling 26,000ft 7,925m
Range 690nm 1,280km




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