A German rarity – The Škoda-Kauba SK 257

by Chris Chant on 28/10/2013

In the spring of 1942, there came into existence at an address in the suburbs of Prague the Škoda-Kauba Flugzeugbau, a name which held no significance for any but the few members of a design bureau led by Otto Kauba, an Austrian engineer from Vienna, a few officials in the Reichsluftfahrtministerium in Berlin, and Reichsmarschall Hermann Göring. This small group knew that behind the windows of the house work was proceeding on the design of a pilotless flying bomb. Kauba had succeeded in interesting Goring in his ideas for a flying bomb, and Goring had, in turn, recommended his ideas to the RLM under whose auspices Škoda-Kauba had been created. The Austrian designer had chosen Prague as a centre for his activities, and it was decided that his prototypes would be built by the nearby Avia works, a part of the Škoda Werke Trust of Pilsen.

Škoda-Kauba-SK-257

Unorthodox concept

Kauba’s design was unorthodox. Featuring an abbreviated fuselage and elevons attached to supports well to the rear of the wing’s trailing edges, the flying bomb was to be powered in its definitive form by an Argus pulsating athodyd (ramjet). However, Kauba’s first task was to determine the flying characteristics of the unusual layout which he proposed, and he accordingly designed a strange low-wing cantilever monoplane powered by a Hirth HM 504A-2 air-cooled four-cylinder inverted inline engine rated at 105 hp (78.3 kW) and driving a two-blade tractor propeller, and featuring all-metal construction, fixed tricycle landing gear, and the unique trailing elevons. In its initial form, the aeroplane was to have an orthodox cockpit for a pilot, the intention being to replace this with an explosive charge after the machine’s flying characteristics had been validated. Guidance would then be effected by an autopilot combined with height and range-setting controls, the servo mechanisms controlling the elevons which were the only aerodynamic controls.

The RLM ordered Kauba to build four prototypes, and the first of these, the SK V1A, was completed and transported by road to the Prague-Gbely airfield, where a Messerschmitt test pilot was to undertake the initial flight trials. After only a cursory inspection of the SK V1A, the German pilot refused to fly the SK V1A. An instructor from the Luftwaffe’s flying school at Vyskov in Moravia was then asked to undertake the flight testing, but he too refused. In desperation, Kauba approached the management of the Škoda works to find him a pilot, and Petr Siroky, Avia’s chief test pilot and a well-known Czechoslovak aerobatic pilot, was ordered to undertake the initial flight trials.

Pilot refusals

After learning of the aeroplane’s eventual purpose, Siroky also refused the task, but veiled threats and suggestions of Gestapo interest in him and his family resulted in what appeared to be a change of heart. In fact, Siroky was already planning the deliberate destruction of the aeroplane, and after a few taxiing trials across the Prague-Gbely airfield, Siroky did actually take-off, but as soon as he was airborne he began to switch the ignition on and off so that onlookers would think that the engine was behaving erratically. Then, finally cutting the power completely and with the propeller windmilling, he put the nose down and bounced the aeroplane on the nose wheel. The SK V1A somersaulted several times and finally came to rest on its belly after losing its landing gear. Siroky was carried unconscious from the wrecked aeroplane, but Kauba was suspicious and immediately accused the pilot of sabotage. Fortunately for Siroky, a complicit Škoda director intervened and the charges were dropped.

After this setback RLM interest in Kauba’s proposals began to wane, but in 1943, as a result of representations made by a Nazi director of Škoda, work was resumed and the second prototype, designated as the SK V1, was finished. Essentially similar to its predecessor apart from having a rudder, the SK V1 was flown successfully by Rudolph Opitz, who had been flight testing the radical Messerschmitt Me 163 rocket-powered interceptor. The data for the SK V1 included a span of 19 ft 8.25 in (6.00 m), length of 14 ft 9.125 in (4.40 m), normal take-off weight of 1,323 lb (600 kg), and estimated maximum speed of 135 kt (155.5 mph; 250 km/h) at optimum altitude. The trials were conducted from Prague-Ruzyn airfield, and the aeroplane was found to be somewhat nose heavy. In an attempt to move the centre of gravity farther to the rear, the SK V2 third prototype featured appreciable wing sweep with the trailing elevons mounted closer to the wing’s trailing edges. The aeroplane was still nose heavy, however, and the lack of promise evinced during flight trials coupled with the more advanced work on pilotless flying bombs by that time being undertaken by other designers convinced Kauba to turn his attention to other projects, and all further work on his flying bomb was finally abandoned.

Continued efforts

In the meantime, salvaged components of the original SK V1A were used in the construction of a light sporting single-seater, the SK V3, with fixed tailwheel landing gear and an orthodox but very stumpy layout, but Kauba’s next major design, the SK V4, was a single-seat fighter trainer powered by one Argus As 10C-3 air-cooled inverted V-8 engine rated at 240 hp (179 kW). Characterised by exceptionally clean lines, the SK V4 was a low-wing cantilever monoplane with tailwheel landing gear including wide-track main units that retracted inward. Provision was made for the mounting of a single 0.312-in (7.92-mm) MG 17 fixed forward-firing machine gun, an electrically actuated variable-incidence tailplane was provided for rapid trimming of the aeroplane in the longitudinal plane, and the structure comprised a one-piece wooden wing built around a single box spar, and a welded steel-tube fuselage with plywood skinning. The data for the SK V4 included a span of 24 ft 11.25 in (7.60 m) with area of 90.42 sq ft (8.40 m²), length of 19 ft 8.25 in (5.60 m), height of 9 ft 6.25 in (2.90 m), empty weight of 2,205 lb (1000 kg), normal take-off weight of 2,756 lb (1250 kg), maximum speed of 226.5 kt (261 mph; 420 km/h) at optimum altitude, initial climb rate of 2,008 ft (612 m) per minute, service ceiling of 24,605 ft (7500 m), and range of 485.5 nm (559 miles; 900 km).

SK 257

So successful were the flight trials that the RLM awarded Škoda-Kauba a contract for the development of a more powerful fighter trainer of generally similar configuration and construction. Designated as the SK 257, the improved type was powered by the Argus As 410 air-cooled inverted V-12 engine rated at 485 hp (361.5 kW), and four prototypes were completed. Again, the type revealed exceptional handling qualities and performance, and the SK 257 trainer was ordered into quantity production for the Luftwaffe at a factory on the Biskupice airfield in Slovakia. In the event, only five production examples of the SK 257 trainer were completed, and the manufacturing standard of these was so low that they failed to pass the Luftwaffe quality control inspectors.

By this time, Kauba had several more ambitious projects on the drawing boards, one of which was the SK V5 single-seat fighter which was in essence a scaled-up development of the SK V4 intended to outperform the Focke-Wulf Fw 190 in every respect with the powerplant of one Daimler-Benz DB 603 liquid-cooled inverted V-12 engine rated at 1,750 hp (1305 kW) for an estimated maximum speed of 413 kt (475.5 mph; 765 km/h) at optimum altitude. The SK V5 employed an unusual form of wing construction, this being known as the Škoda-Kauba-Banweise tubular-spar structure in which normal ribs were built up on a single tubular spar extending from wing-tip to wing-tip with the forward bulkhead and engine firewall (to which the engine bearers were attached) constituting an integral part of this spar. This method of construction was claimed to offer both an increase in structural strength and a reduction in structural weight, and the Focke-Wulf organisation evinced interest at a late stage in the war, considering the introduction of the tubular spar in the Fw 190D and Ta 152 fighters. Unfortunately, although highly promising, the SK V5 progressed no further than a series of wind-tunnel models and a full-scale mock-up as the RLM felt the development of an entirely new piston-engined tighter to be wasted effort at a time when emphasis was being placed on turbojet-powered fighters.

Continued unorthodoxy

The next Škoda-Kauba design was the SK V6, a small single-seat monoplane of the twin-boom layout employing some of the structural components of the SK V1. The Hirth HM 504A engine, rated at 105 hp (78.3 kW), was installed as a pusher and drove a two-blade wooden propeller of the fixed-pitch type between the tail booms, and after a series of flight trials the prototype was modified into the SK SL6 at the request of Blohm und Voss to flight test the so-called ‘arrow wing’ arrangement evolved by Dr Richard Vogt and his chief aerodynamicist, Georg Haag. This arrangement called for the provision of short booms which, attached to the tips of a moderately swept wing, each carried a half tailplane and an elevator. This highly unorthodox layout was featured by several projected Blohm und Voss fighter projects, including the P.208, P.209, P.210, P.212 and P.215, and during 1944 the SK SL6 was flight tested to prove the feasibility of the control surface arrangement.

The SK V7 was a small research aeroplane of canard layout employing some of the structure of the SK V2, and was powered by a Walter Mikron II air-cooled four-cylinder inverted inline engine, rated at 60 hp (44.7 kW), installed as a pusher. The SK V7 was not completed, but the SK V8 light primary training and sporting aeroplane, with side-by-side two-seat accommodation, was completed and flown in the winter of 1944, flying with fixed landing gear carrying wheels or skis at various times. The SK V8 was powered by an HM 504A-2 engine, rated at 105 hp (78.3 kW) and had a wing spanning 26 ft 1 in (7.95 m), but although the aeroplane survived past the end of World War II, no further details were recorded.

There were many Škoda-Kauba projects, for this small company comprising only 40 German and 80 Czechoslovak employees was very prolific. These projects included the SK V9 miniature aeroplane which was to have been powered by an engine of only 15 hp (11.2 kW), the SK V10 trainer powered by two Argus As 411 engines, the SK V11 advanced trainer with a single Argus As 411 engine, and the SK V12 aerodynamic research aeroplane embodying a forward-swept wing and powered by an engine rated at 70 hp (52.2 kW).

Fanciful ending

It was Kauba’s last project which was his most spectacular. This was the SK P.14 ramjet-driven interceptor designed around a ramjet, some 31 ft 2 in (9.50 m) long, designed by Eugen Sènger and intended to operate on Diesel oil or powdered coal. Sènger had been working on ramjets for a number of years and in 1944, with some RLM support, Kauba began to evince interest in the possible application of this revolutionary powerplant as the prime mover of a fighter. The sheer size of the ramjet rendered its installation in a small fighter difficult, and Kauba elected to build the powerplant as an integral part of the airframe, housing the pilot, armament and all fuel in a superstructure above the ramjet. The pilot lay prone above the narrow neck of the ramjet, a single 30-mm MK 103 cannon being mounted above the pilot with one large fuel tank of 297-Imp gal (356.6-US gal; 1350-litre) capacity immediately to its rear and conforming in shape to the contour of the diffuser portion of the ramjet. The inlet was carried forward slightly beyond the nose, and the diffuser gradually tapered to the parallel-sided combustion chamber which possessed a diameter of 4 ft 11 in (1.50 m), a conventional wing spanning 25 ft 11 in (7.90 m) with area of 134.55 sq ft (12.50 m²) was fitted, and the diminutive tail surfaces were also conventional. For take-off it was intended that a three-wheel bogie would be used, this being jettisoned as soon as the SK P.14 was airborne and the landing being effected on an extending skid.

It was estimated that the Sènger ramjet would provide 9,678 lb st (43.05 kN) at sea level at 545 kt (627.5 mph; 1010 km/h) and 2,976 lb st (13.24 kN) at 32,810 ft (10000 m) at 475 kt (547 mph; 880 km/h). Loaded weight varied between a normal figure of 6,272 lb (2845 kg) and a maximum figure of 6,812 lb (3090 kg), and the performance estimate included an initial climb rate of 26,493 ft (8075 m) per minute, climb to 19,685 ft (6000 m) in 1 minute 42 seconds and to 32,810 ft (10000 m) in 6 minutes 18 seconds, service ceiling of 59,875 ft (18 250 m), and powered endurance of 28 minutes at 200 kt (230 mph; 370 km/h) at sea level increasing to 43 minutes at 307.5 kt (354 mph; 570 km/h) at 32,810 ft (10000 m).

The ramjet enjoyed little popularity at the RLM as a powerplant for manned aircraft, however, and with the demise of the research programme late in 1944 also came the end Kauba’s ambitious fighter. During the final months of World War II, the Škoda-Kauba facilities were occupied in adapting a number of Fw 190 fighters and Horten Ho VIII flying wings to take the Škoda-Kauba-Banweise tubular spar.

Shortly before the end of hostilities in May 1945, Kauba fled Prague. On 30 April 1945 all the company’s documentation and project drawings were burned and, just three years after its appearance, the Škoda-Kauba Flugzeugbau disappeared.

Specification

Škoda-Kauba SK 257

Type: fighter lead-in trainer aeroplane

Accommodation: pilot in the enclosed cockpit

Armament: none

Powerplant: one Argus As 410 air-cooled inverted V-12 piston engine rated at 485 hp (361.5 kW) for take-off

Fuel: internal not available; external none

Dimensions: span 24 ft 11.25 in (7.60 m); area 90.42 sq ft (8.40 m²); length 23 ft 3.5 in (7.10 m); height 5 ft 10.875 in (1.80 m)

Weights: empty not available; normal take-off 2,271 lb (1030 kg)

Performance: maximum speed 195 kt (217.5 mph; 350 km/h) at optimum altitude; no other data available


[Photo by kitchener.lord]

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