Heinkel He 177 Greif
The Heinkel He 177 Greif was a long-range heavy bomber flown by the Luftwaffe during World War II. The introduction of the He 177 to combat operations was significantly delayed by problems both with the development of its engines and frequent changes to its intended role. Nevertheless, it was the only long-range, heavy bomber to become operational with the Luftwaffe during the conflict. The He 177 had a payload/range capability similar to that of four-engined heavy bombers used by the Allies in the European theatre.
Work on the design began in response to a 1936 requirement known as Bomber A, issued by the Reichsluftfahrtministerium for a purely strategic bomber. Thus, the He 177 was intended originally to be capable of a sustained bombing campaign against Soviet manufacturing capacity, deep inside Russia.
In contrast to its heavy payload and very wide, planform, the specifications called for the design to have only two very powerful engines. To deliver the power required, the He 177 needed engines of at least. Engines of this type were new and unproven at the time. The Daimler-Benz DB 606 power system that was selected, in conjunction with its relatively cramped nacelles, caused cooling and maintenance problems, such that the powerplants became infamous for catching fire in flight, and contributing to the He 177 gaining nicknames from Luftwaffe aircrew such as Reichsfeuerzeug or Luftwaffenfeuerzeug.
The type matured into a usable design too late in the war to play an important role. It was built and used in some numbers, especially on the Eastern Front, where its range was particularly useful. The He 177 is notable for its use in mass raids on Velikiye Luki in 1944, one of the late-war heavy bombing efforts by the Luftwaffe. It saw considerably less use on the Western Front, although the type played a role during Operation Steinbock against the British mainland in 1944.
Design and development
Background
During the mid 1930s, Generalleutnant Walther Wever, a longtime advocate of strategic bombing, pressed the Luftwaffe to develop a dedicated long-range bomber for the role of attacking the Soviet Union's factories in the Ural Mountain area. This concept was received with significant skepticism amongst many senior officials within the Luftwaffe and, by 1936, this "Ural bomber" program had delivered two rather uninspiring designs, the Dornier Do 19 and Junkers Ju 89.Wever continued to press for new designs for this role, and the Reichsluftfahrtministerium finally released a new specification for what they called Bomber A on 3 June 1936. This called for a significantly more advanced design with higher speeds, longer range and larger payloads. This was also the same day that Wever was killed in an air crash, and the design lost its only politically powerful champion.
The specification required the plane to carry a bomb-load of at least 1,000 kg over a range of 5,000 km, with a maximum speed of not less than 500 km/h at altitude. In addition to outperforming, by a considerable margin, any bomber then in service, the design's speed was intended to allow it to outrun any contemporary fighter, the so-called Schnellbomber concept.
On 2 June 1937, Heinkel Flugzeugwerke received instructions to proceed with construction of a full-scale mock-up of its Projekt 1041 Bomber A. Heinkel Flugzeugwerke's estimated performance figures for Projekt 1041 included a top speed of 550 km/h at 5,500 m and a loaded weight of 27,000 kg. In order to achieve these estimates, Ernst Heinkel's chief designer, Siegfried Günter, employed several revolutionary features.
Engines
The He 177 required at least a pair of 2,000 PS engines to meet performance requirements. No engine in the German aviation power-plant industry at that time developed such power. A four-engine version would have been possible with engines like the Daimler-Benz DB 601 but the four-engine layout would impose higher propeller drag to the detriment of performance in dive bombing. The use of only two counter-rotating propellers on a heavy bomber offered many advantages, such as a substantial reduction in drag, reduction of dive instability and a marked improvement in maneuverability. The eight initial V-series prototypes, and the larger number of A-0 pre-production models of the He 177, displayed an airspeed and maneuverability comparable to many heavy fighters of the time.For the He 177, Günter decided to employ two of the complex Daimler-Benz DB 606 "power system" setups for propulsion. He had already employed these engines on the record-breaking Heinkel He 119 reconnaissance aircraft prototypes. They consisted of a pair of DB 601 liquid-cooled 12-cylinder inverted-vee inline engines mounted side by side in a nacelle – for the He 119, centrally within the fuselage, just behind its heavily glazed cockpit enclosure – driving one propeller. The two engines were inclined inwards by 30° when mounted onto either side of their common, vertical-plane space-frame primary engine mount so that the inner cylinder banks were disposed almost vertically. A common gear-housing connected the front ends of the two crankcases, with the two crankshaft pinions driving a single airscrew shaft gear. The outer sides of each of the component engines' crankcases were connected to the nacelle firewall through forged mountings similar to what would be used for either a single DB 601 or DB 605 engine-powered aircraft installation. When combined with the central space-frame mount designed especially for the "power system" format, this resulted in a Daimler-Benz "coupled" twin-crankcase "power system" having a trio of engine mount structures within its nacelle accommodation. The starboard DB 601 component engine had to be fitted with a mirror-image version of its mechanically driven centrifugal supercharger, drawing air from the starboard side of the engine. Two of the DB 606s, each of which initially developed 2,600 PS for take-off and weighing some 1,515 kg apiece, were to power the He 177. The DB 606 — and its eventual replacement, the Daimler-Benz DB 605-based "DB 610" — were to be the only two production German aviation powerplants designed to surpass 2,040 PS of power, something that the Germans had considerable challenges in developing during the war into production-ready, combat-reliable aviation engines.
Surface evaporation cooling
For aerodynamic cleanliness, Günter intended to dispense with the usual system of drag-producing engine radiators and planned on using a surface evaporative cooling system instead. Such surface cooling, in the form of simpler surface radiators, had been used on British high-speed racing seaplanes as early as 1929. This sort of system was pioneered on the eight examples built of the Heinkel He 119 high-speed reconnaissance aircraft prototype series, already flying with the twin-crankcase DB 606 "power system" engine with success from the beginning, and was also intended for use on the He 100 high-speed fighter prototypes. The coolant water is pressurized, raising its boiling point, in this case to about 110 °C. As the superheated water leaves the engine it enters an expansion area where the pressure drops and the water flashes to steam. The steam is then cooled by running it in pipes along the outer skin of the fuselage and wings. Before the design of the He 177 was finalized, it was clear that such a system would be incapable of dealing with the vast amount of heat generated by each of the twinned DB 601-based powerplants, forcing the abandonment of the idea of using evaporative cooling systems, in favour of conventional annular radiators fitted directly behind each propeller. These resembled, but were larger in capacity, than those fitted to the Junkers Ju 88 A bomber, and added to the He 177's weight and drag.Defensive armament
Günter's original intention had been to equip the He 177 with three cockpit-controlled remote gun turrets, with two of them to come from the Junkers Ju 288 program, leaving one manned emplacement in the tail. Compared with the manned position, a remotely controlled, turreted defensive armament emplacement system traded technical complexity for reduction of size, weight and drag; it had the advantage that the gunner could be placed in a protected position, with the best possible view and with less risk of being blinded by the flash from his own guns. Although work on remotely controlled aircraft defensive systems had reached a relatively advanced stage in Germany in the late 1930s, progress in this field within Germany's aviation and armaments systems engineers and manufacturers was to prove insufficient to keep pace with the He 177. As a result, the He 177 had to be modified to accommodate larger and heavier manned positions, such as the manned rear dorsal turret usually fitted to almost all examples of the Greif, armed with a 13 mm MG 131 machine gun. That installation meant that the fuselage had to receive structural strengthening in several locations. Most of the later production aircraft did receive a remote forward dorsal turret, the Fernbedienbare Drehlafette 131Z, armed with two MG 131 machine guns, located at a point on the fuselage directly above the wing root's leading-edge, with its rotating hemispherical sighting station dome located a short distance forward of the turret and slightly offset to starboard, just behind the forward cabin area.A compact tail gun position was fitted for rearward defense, armed with one MG 131 machine gun but its streamlined glazing meant that the gunner lay prone, severely restricting his comfort on long missions. A revised tail gun position with a bulged upper glazing was fitted to the He 177A-3 and later models, which permitted the gunner to sit upright. The revised design required a reduction of the lower end of the rudder surface for clearance. The MG 131 gun would often be replaced with a 20 mm MG 151 cannon or in a few instances, a semi-experimental twin MG 131Z mount, with the twinned 13 mm calibre guns mounted one above the other, at the rear of the standard bulged upper glazing emplacement. Usually, a 7.92 mm MG 81 machine gun in a flexible mount was positioned in the upper starboard side of the cockpit nose glazing for defense against frontal attacks. The undernose, inverted-casemate Bola gondola, which was the full width of the fuselage where it emerged from under the nose and centered under the forward cabin, usually had a flexibly mounted, drum-fed 20 mm MG FF cannon at the front end as added forward defense and a flexibly mounted MG 81 machine gun in the rear, for the initial He 177A-1. An MG 151 cannon replaced the forward MG FF cannon in later production models, with an MG 131 usually replacing the MG 81, for rearwards ventral defense.