Heinkel He 274

The Heinkel He 274 was a German heavy bomber design developed during World War II, purpose-designed for high-altitude bombing with pressurized crew accommodation.

Due to the Allied advance through North-west Europe, the prototypes were abandoned at the French factory where they were being built. They were completed after the war by the French and used for high-altitude research.

He 177 ancestry
On 17 November 1938, the owner of the Heinkel aviation firm, Ernst Heinkel, requested permission from the RLM that two of the requested eight prototype airframes for the nascent He 177 heavy bomber project, specifically the V3 and V4 airframes, be set aside for a trial installation of four separate Junkers Jumo 211 powerplants. Heinkel had foreseen that an individually engined version of his bomber would someday be preferred, quite unlike the requested fitment of the coupled pairs of Daimler-Benz DB 601 inverted V12 engines, each known as a DB 606 — weighing some 1.5 tonnes apiece — which ended up being fitted to all of the eight He 177 V-series prototypes at the request of the RLM, and the Luftwaffe High Command, with the concerned government agencies citing the desire for a dive-bombing capability to be present even with a heavy-bomber-sized offensive warplane, something Ernst Heinkel vehemently disagreed with.

By April 1939, interest in developing a high-altitude version of the He 177 had arisen, and on April 27, 1939, the first proposal for such an aircraft was presented to Heinkel by his firm's engineering staff. The aircraft was intended to have a reduced crew manifest of three people, with a fully pressurized nose compartment for the pilot and bombardier/forward gunner, and separate pressurized tail gun emplacement. The result, in December 1940, was the specification for the He 177A-2 high altitude bomber design, with a four-person crew manifest (pilot, bombardier, forward gunner and tail gunner) in the two specified pressurized compartments, and powered by the regular A-series pair of DB 606 coupled engines. The defensive armament had been reduced to a trio of Ferngesteuert-Lafette FL 81Z remote gun turrets, each with a twin-barrel MG 81 armament installation each in an upper nose mount, forward dorsal and (as part of the Bola casemate-style gondola under the nose) forward ventral location each, and a single MG 131 machine gun in an He 177A-1-style, pressurized manned flexible tailgun emplacement. The A-2 version had even been considered for a pioneering in-flight refueling capability, possibly using Ju 290 maritime patrol aircraft as the tankers - with such capability, the range of the A-2 would have been extendable to some 9,500 km (5,900 mi) of total flight distance.

The Heinkel firm had been working on practical cockpit pressurization methods and hardware for both the A-2, and slightly later A-4 versions (identical to the A-2, except for the fitting of a pair of the later DB 610 coupled engine "power systems") from 1940 through the late summer of 1941, when the DB 610-powered A-4's pressurized cockpit in provisional form, almost identical in external appearance to the standard "Cabin 3" He 177A-series production cockpit, was ready for tests and development.

By October 1941, a more developed "He 177H" specification for a high-altitude Heinkel-designed heavy bomber had emerged from the proposed A-2 and A-4 coupled-engine designs, with the intent of carrying a 2,000 kg (4,410 lb) bombload over a maximum range of some 3,000 km (1,895 mi), and accepted by the RLM for the first time, an individual four-engine installation was being considered for any He 177-based bomber airframe, with a quartet of either BMW 801 or DB 603 engines among the choices of powerplants being specified, with the same sort of reduced-armament defensive weapon format as the A-2 and A-4 were intended to have.

In conjunction with his request for help from then-Generalmajor Eccard Freiherr von Gablenz in May 1942 concerning the suitability of aircraft for the Amerika Bomber contract competition as that proposal first appeared, Generalfeldmarschall Erhard Milch also received von Gablenz's opinion on both the He 177 and its He 274 development, with von Gablenz stating that neither of the then-existing Heinkel "heavy bomber" designs had anywhere near the range to conduct a mission approaching the demands of the new contract.

A pair of the early He 177A-0 pre-production prototypes were redesignated the He 177 V10 and V11 for the purposes of high-altitude trials, and were to be the first to test the A-4 pattern pressurized cockpit design at altitude, but only the V11 was actively used for the needed research, and managed to achieve an altitude of 9,200 m (30,200 ft) with complete success on August 9, 1943, with further tests continuing through October of that year, before both the V10 and V11 were grounded in April 1944.

In February 1943, any further work on the coupled-engined A-2 and A-4 designs was halted by order of the RLM, as the four-engined He 177H high-altitude design had gained in importance from that time, evidenced by Reichsmarschall Hermann Göring's derisive "welded-together engines" complaints in August 1942, regarding the He 177 A-series unending engine problems from the choice of the DB 606 and 610 "power systems" for the A-series operational aircraft.

Development
The first proposal for what would become the He 274 started with six airframe orders for what had been known as the He 177H, which were requested from Heinkel as early as mid-October 1941, all to have four individual engines, and intended to use what were essentially production He 177A-3 fuselages mated to longer-wingspan, four-engined wings. These proposed aircraft were shortly thereafter officially given the project number 8-274 by the RLM, and due to the heavily preoccupied Heinkel factory design offices and aircraft manufacturing facilities, this new "He 274" high-altitude bomber was to have its prototypes built in France by the Societe des Usines Farman (Farman Brothers) in Suresnes.

Two He 274 prototypes were ordered built in France by the Farman Brothers and four pre-production prototypes by the Heinkel's Heinkel-Nord headquarters at its Rostock-Marienehe (today's Rostock-Schmarl) facility. Farman at Suresnes, began their prototype development.

Work on the requested half-dozen He 274 prototype airframes was leveraged off Heinkel aircraft production at AIA Breuget, Toulouse where French factories produced Heinkel components and Junkers aero engines. French production facilities at Toulouse for Heinkel aircraft were severely damaged by Royal Air Force(RAF) air raids on the night of 5/6 March 1944 and again by the US Eighth Air Force on 25 June 1944. This frustrated completion of the French prototypes as the design work in Germany and Austria had been ongoing from as early as February 1943, on what had emerged as the Heinkel-designed entry in the trans-Atlantic range Amerika Bomber strategic heavy bomber design competition, the Heinkel He 277, had been progressing at the Heinkel-Sud facility in Vienna, which had been cancelled by the German Air Ministry back in April 1944. The "Typenblatt" drawings for the never-completed He 277, with a heavy design influence for the fuselage's geometry from the smaller Heinkel He 219 night fighter, however show that it had also adopted many features from the He 274, especially its twin tail empennage design.

Characteristics
Major differences between the He 274 and the He 177 A were abandonment of the twin coupled "power system" engine arrangement in favor of four independent DB 603A-2 engine units, cooled by annular radiators nearly identical in appearance to those on the similarly-powered Heinkel He 219 night fighter, an extended rear fuselage with a pressurized double glazed cockpit of nearly identical external appearance to the 177A's standard "Cabin 3" nose, longer wingspan, a twin tail fin empennage and a more conventional set of twin-wheel main undercarriage, abandoning the cumbersome four-strut main gear system of the He 177A.

The He 274 also featured a pressurized compartment for a crew of four, this employing double walls of heavy-gauge alloy, hollow sandwich-type glazing and inflatable rubber seals, a pressure equivalent to that at 2,500 m (8,200 ft) being maintained at high altitude. Largely unnecessary defensive armament was restricted to a single forward-firing 13 mm (.51 in) caliber MG 131 machine gun and remotely controlled dorsal and ventral Fernbedienbare Drehlafette FDL 131Z gun turrets each containing a pair of MG 131s and with the dorsal turret operated from a slightly offset Plexiglas domed sighting station in the roof of the flight deck as most A-series He 177s were, with the ventral unit aimed from the rear of the ventral Bola gondola. The powerplants selected were the same type of Daimler-Benz DB 603A Kraftei "power-egg" unitized engine installations, complete with their He 219-style annular radiators that were placed on the wings of the quartet of ordered He 177B prototypes, but for the He 274's use, added DVL-designed TK 11 turbochargers, one per engine, for better power output at high altitude.

The significance of this design is that had this aircraft entered production and been used in operations over England it would have been impossible for Allied fighters to intercept over the target, owing to its extreme high altitude performance.

Abandoned prototypes
Construction of the two prototypes, the He 274 V1 and V2 did not commence until 1943. They were to have been built in France by SAUF at Suresnes, France, but the prototypes were not completed in time. The He 274 V1 was being readied for flight testing at Suresnes in July 1944 when the approach of Allied forces necessitated the evacuation of Heinkel personnel working on the project. Minor difficulties had delayed the flight testing and transfer of the aircraft to Germany, and orders were therefore given to destroy the virtually completed prototype. Only minor damage was actually done to the airframe of the He 274 V1, and repairs were begun after the Allied occupation.

The He 274 V1 was repaired by Ateliers Aéronautiques de Suresnes (AAS) and used by the Armée de l'Air (French Air Force) for several years as a high-altitude research plane. It was renamed the AAS 01A. The He 274 V2 was eventually completed as the AAS 01B, complete with the Hirth 2291 turbochargers.

Eventually the V2 flew exactly two years (on December 27, 1947) after the AAS 01A. By this time, the AAS organization had been absorbed into the French SNCASO (Société nationale des constructions aéronautiques du sud-ouest, or commonly, Sud-Ouest) aviation conglomerate. Both of the AAS 01 completed and airworthy versions of the He 274 were eventually broken up in late 1953, after serving as "mother ships" for aerial launching, in the manner of a composite or parasite aircraft, of a number of early French advanced jet and rocket test aircraft like the unpowered Sud-Ouest SO.4000 M.1, almost always launched from a strut-braced, above-fuselage position. The Leduc 0.10 and Leduc 0.16 also each had their first aerial tests from atop the pair of surviving He 274 prototypes. The slightly less powerful, French designed four-engined SNCASE SE.161 Languedoc airliner design took over this "mother ship" role later in the 1950s, for further air launch duties with French high-speed aerodynamic research prototypes.

Operators

 * French Air Force
 * French Air Force