Flight, May 1931

I.L.I.S. The Stockholm International Aero Show
May 15-31, 1931

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   The problem single-seater or two-seater fighter is a very complicated one, and does not come within the province of an article on the exhibits at an aero show. A brief reference must, however, be made to it in order to explain the raison d'etre of the machine shown by the Swedish company, A.B. Flygindustri, of Malmo. This is a two-seater fighter known as the type K.47, and is a Junkers design built under licence in Sweden by this company, who holds the building rights in Sweden for all Junkers types.
   From strategical and tactical considerations, this firm has come to the conclusion that a flight of five two-seater fighters is equivalent in offensive and defensive power to a flight of seven single-seater fighters. These considerations, into which it would take too long to go here, take into account the "blind areas" of both types, the increase in effective field of fire attained by the climbing or diving through 60 degrees from the horizontal, the inability of the rear machines in a flight to fire in certain forward directions owing to the positions of certain machines of the flight, the various formations, such as vee formation stepped up and stepped down, and so forth. The arguments advanced in favour of the two-seater fighter are interesting, and may be referred to on a future occasion. For the present, we must confine ourselves to stating that A.B. Flygindustri have gone into the subject very carefully, and that the K.47 has been produced as a result.
   The machine, by the way, is not unknown to English readers of FLIGHT, as it paid a visit to Heston Air Park in 1929, and a photograph of it was published in our issue of July 25, 1929. Like all Junkers aircraft, the K.47 is of all-metal construction, but it differs from most Junkers machines in that it is not a cantilever monoplane, but has a braced wing structure. It is stated, however, that the load factors are such that, even with a wing bracing strut shot away, the wing, as a cantilever, has a factor of 4-5, so that the machine should be able to return safely, even if not in a fit state to take any further part in a fight entailing violent manoeuvres.
   The structural design of the K.47 follows fairly closely usual Junkers practice, with the exception of the wing bracing already referred to. Duralumin is the chief structural material, with a few steel fittings for highly-stressed parts. The torsional strength of the wing depends mainly upon the duralumin covering, which, it is claimed, can be riddled with machine-gun bullets without the torsional strength being materially reduced.
   The pilot's cockpit of the K.47 is of normal layout, and his armament consists of the usual two fixed machine guns, firing forward. The gunner's cockpit, however, is the centre of interest in this machine, and is the key to the whole design. The fuselage is so shaped as to obstruct the view and field of fire as little as possible. To this end it is of narrow beam over the rear part, while the tail is set low, and the decking slopes down at a considerable angle. This has been done so as to enable the gunner to fire downward at a slight angle (some 3 degrees) below the horizontal. The upward angle of the gun is 90 degrees, so that the total elevation range is 93 degrees. Firing broadsides at the speeds attained by modern fighters is regarded by the designers as almost impossible in any case, and so no attempt to allow of this has been made. Instead, the lateral movement of the rear gun has been limited to 18 degrees on each side of the centre line of the machine, or a total of 36 degrees. This amount of traverse is entirely unobstructed, because, instead of the usual centrally-placed rudder, the K.47 has two rudders, placed near the tips of the tailplane. In this position they are outside the range of the gun, and there is thus no risk of the gunner shooting his rudders or fins away.
   The gun mounting is interesting, and is of a type known in Sweden as a "gunglavett," a "gung" being a rocker (of a rocking-horse, for example), and "lavett" meaning a mounting. This "rocker-mounting" consists in effect of a beam supported near its centre on a pivot, and carrying at one end the machine gun and at the other the gunner's seat. Its object is to enable the gunner to operate his gun even while the machine is making a very sharp turn, when otherwise centrifugal force would prevent him from moving about. Windage on the gun when it is swung outboard to the limits of its traverse is counteracted by a spring device, the strength of which is so proportioned to the angle of the gun as to keep the force required to traverse approximately constant.
   The gunner's seat has a tall backrest, tall enough to reach to head level and enable the gunner to rest his head firmly against it. A projection of the deck fairing between the cockpits acts as a wind-screen for the rear gunner and relieves his head and neck of most of the air pressure. The whole arrangement of the rear cock-pit is highly ingenious, but the "rocker mounting" would appear to be just a little complicated.
   The K-47, which is fitted with a Bristol “Jupiter" VII supercharged engine, has a length of 8.55 m. (28 ft. 1 in.). The wing span is 12.4 m. (40 ft. 8 in.) and the wing area 22.8 sq. m. (245.5 sq. ft.), the tare weight is 1,050 kg. (2,310 lb.) and the gross weight 1,635 kg. (3,600 lb.).
   When fitted with the Bristol "Jupiter" VII supercharged engine, the K.47 has a maximum speed at 3.500 m. (11,500 ft.) of 290 km./h. (180 m.p.h.). At 5,000 m. (16,400 ft.) the speed is 280 km./h. (174 m.p.h.). The speed at ground level is 245 km./h. (152 m.p.h.). The landing speed is about 100 km./h. (62 m.p.h.). The climb to 3,000 m. (10,000 ft.) occupies 6 minutes, and the service ceiling of 8,500 m. (28,000 ft.) is reached in about 35 minutes. The tankage is sufficient for 2 hours at full throttle and 16,400 ft. altitude.
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Flight, March 1934

A Swedish Two-Seater Fighter: The Junkers K.47

   UNLIKE the majority of Junkers types produced during recent years, the type K.47 built in Sweden by the A.B. Flygindustri was designed primarily as a military type. The aircraft is a braced low-wing monoplane, with the wing consisting of a centre section, which is integral with the fuselage and two outer sections of trapezoidal shape. Spherical screw joints connect the wings to the centre section, and a single strut braces each outer wing to the undercarriage.
   Structurally the wing is of typical Junkers multi-tube type, the torsional stresses being taken by corrugated sheet covering. A fuselage of oval section is used which consists of three parts, the engine section, the centre section containing the cockpits and the tail portion. The wing centre section and fuselage centre section are built up as one unit. Duralumin longerons are used, braced by transverse members of the same material. Unlike the wings, the fuselage has a covering of smooth sheet duralumin.
   Ailerons of high aspect ratio, statically and aerodynamically balanced, are used. The tail plane is adjustable in flight and is provided with a self-locking spindle adjustment controlled by a hand wheel in the pilot's cockpit. The elevators and tail plane are so large that the aircraft, when fully loaded and with the C.G. back, is still quite stable. Two fins with two balanced rudders are fixed to the extreme ends of the tail plane. This arrangement allows a very wide rearward field of fire for the movable gun. One disadvantage, however, is presented, in that the machine is not so easily manoeuvred on the ground. This handicap is overcome by the provision of wheel brakes and a tail wheel.
   The controls are of the conventional stick and rudder bar type, and all joints are provided with ball bearings. A cross axle type undercarriage, using two Vickers oleo pneumatic legs is fitted. The extremities of the axle are hinged by radius rods to two rigid Vee's interconnected by a cross strut. To this Vee is attached the main wing bracing. Palmer wheels and brakes, the latter being operated by pedals on the rudder bar, are usually provided.
   Almost any air-cooled radial engine with a dry weight of approximately 882 lb. and with a power of from 500 h.p. to 600 h.p. may be fitted. In the prototype a "Jupiter VII" engine was used, but later this was changed for a Bristol "Mercury IV S.2" of 540 h.p. with a corresponding increase in performance. Two electron fuel tanks of 37 and 31 gallons capacity are carried beneath the centre section, supplemented by one gravity tank of 10 pints capacity. There is one engine-driven fuel pump and one hand pump is provided.
   Like the majority of modern two-seater fighters, the K.47 is armed with two fixed machine guns and a movable gun over the rear cockpit. Space for about 1,000 rounds per gun, in standard metal clip belts, is provided behind the fireproof bulkhead. Any standard machine gun, such as the Vickers, Madsen or Colt, may be installed. The equipment of the gunner's cockpit differs greatly from that of conventional two-seaters. When standard ring mountings are used in high-speed aircraft, difficulties arise as a result of the air pressure against gunner and guns, and to the acceleration due to sharp turns or steep banks. In order to cope with present conditions, the K.47 is fitted with a "cradle mounting" This consists of a pivoted mounting carrying at one end the gun and the other the gunner's seat. The mounting is so balanced that it gives the gunner a slight excess of weight, thus enabling him by means of pressure with his feet to rock the whole device up and down. The performance figures given in the accompanying table apply to the K.47 fitted with the Bristol "Mercury IV S.2" engine. When fitted with the Pratt & Whitney "Hornet S.2-V1," the maximum speed at 7,874 ft. (2 400 m) is 192.6 m.p.h. (310 km/h.), and the cruising speed at 7,874 ft. (2 400 m) is 163.4 m.p.h. (263 km/h.).

JUNKERS K.47

   Span 41 ft (12 4 m)
   Length 27 ft. (8,8 m)
   Height 8 ft. 10 in. (2,9 in)
   Wing area 250 sq. ft. (23,5 m1)
   Weight, empty 2,540 lb. (1 150 kg)
   Disposable load 1,320 Ib. (600 kg)
   Wing loading 15-5 lb./sq.ft. (73,1 kg/m1)
   Power loading 7 lb./h.p. (3,2 kg/h.p.)
   Maximum speed at 13,120 ft. (4 000 m) 200 m.p.h. (324 km/h.)
   Cruising speed at 13,120 ft. (4 000 m) 171 m.p.h. (276 km/h.)
   Climb to 3,280 ft. (1 000 m.) 2 min.
   Climb to 13,120 ft. (4 000 m.) 8 min.
   Service ceiling 31,824 ft. (9 700 m)
   Absolute ceiling 32,808 ft. (10 000 m)
   Range 420 miles (675 km )